ii M m » - I}EraBUJ{YPOI(T PIIBiLI £ * Xi BE A'RY, FOUNDED, 1854 Received No Jill $£< PRESENTED 1!Y \ • I LECTURES ON NATURAL AMD EXPERIMENTAL CONSIDERED IN 1Tb PRESENT STATE OF IMPROVEMENT. DESCRIBING, IN A FAMILIAR AND EASY MANNER, THE PRINCIPAL PHENOMENA OF NATURE j AND SHOWING THAT THEY ALL CO-OPERATE IN DISPLAYING THE GOODNESS, WISDOM, AND POWER OF GOD. BY THE LATE GEORGE ADAMS, MATHEMATICAL INSTRUMENT MAKER TO HIS MAJESTY, &C. IN FOUR VOLUMES, Illustrated with forty-three co/ifier/ilates, elegantly engraved, THIS AMERICAN EDITION, PRINTED FROM THE LAST LONDON EDITION, EDITED BY WILLIAM JONES, MATHEMATICAL INSTRUMENT MAKER, IS CAREFULLY REVISED AND CORRECTED BY ROBERT PATTER SO JV, Professor of Mathematics, and Teacher of Natural Philosophy, in the University of Pennsylvania. TO THIS VOLUME IS SUBJOINED AN APPENDIX j Containing- "A BRIEF OUTLINE OF PHYSICS, OR NATURAL PHILOSOPHY, IN THE FORM OF A COLLEGIATE EXAMINATION :" BY THE AMERICAN EDITOR. VOL. IV. PRINTED FOR WILLIAM W. WOODWARD, N°. 52, CORNER OF CHESNUT AND SECOND STREETS, PHIL AD EL PHIA. 1807. TABLE OF CONTENTS. VOL. IV. LECTURE XXXVII. Page OF the Copernican system 1 Summary view of the solar system ..... 4 Tables of the diameters, distances, &c. of the planets 31 LECTURE XXXVIII. Explanation of the seasons, &c. on the Copernican system 39 Of the shape or figure of the earth 40 Of the diurnal motion of the earth 43 Of the phenomena occasioned by the diurnal motion 45 Of the annnal motion of the earth . . . . . 49 Of the apparent motion of the sun 50 Of the seasons of the year 55 LECTURE XXXIX. An explanation of the phenomena of the planets ac- cording to the Copernican system .... 64 Of the inferior planets 67 Of the superior planets 76 Of the secondary planets or satellites . . . . 79 Of the moon's motion 84 LECTURE XL. Of eclipses *..... 91 Of the eclipses of the moon ....... 92 Of the eclipses of the sun . 98 Of the limits of the solar and lunar eclipses . .104 ( f the period of eclipses 106 The darkness at our Saviour's crucifixion superna- tural .107 IV CONTENTS. LECTURE XLI. Page. Of parallax and refraction, &c Ill Of refraction 113 Of parallax Ill Of the aberration of light 119 Of the precession of the equinoxes . . . . 121 LECTURE XLII. Of solar and sidereal time, &c. ; . . . . . 1 23 Of mean and apparent time 128 Of the equation of time 130 LECTURE XLIII. On the planetarium, &c 134 Of the planetarium 136 Of the tellurian .147 Of the lunarium 155 Of the new terrestrial globe . . . . . . 161 Of a celestial globe 165 APPENDIX TO LECTURE XLIII. BY THE E, EDITOR. A further description of Martin's orrery . . 171 Comparative observations on globes . . . . 176 Advantages peculiar to the new mounted globes 178 Advantages peculiar to the common mounted globes 1 79 Description of the equatorial or universal sun-dial 180 LECTURE XLIV. Of the fixed stars . . . . . . . . . . 187 Herschei on the construction of the universe, &c. 192 Of the telescopic appearance of the planets . . 199 Of comets . . "... 207 Of a plurality of worlds 210 LECTURE XLV. Of physical astronomy .........214 Of the motion of the elements 215 Of the mathematical principles of astronomy . 225 Of deflecting forces 226 CONTENTS. Y Page. Of the gravitation of the moon - 230 Of the gravitation of the primary planets - 238 Of the centre of the solar system - - 241 Of the approach and recess of the planets - 245 Of the moon's irregularities ... 248 LECTURE XLVI. Of electricity - - - - . - 257 Electrical appearances - 261 Of electrical attraction and repulsion - - 263 Of the electrical machine - 267 Experiments on electrical attraction, &c. - 273 Of imitating the planetary motions - - 278 Electrical fluid universally disseminated - 280 Of the Franklinian theory - - - 282 Of the electric spark, and of the influence of points - - - - - - 285 Of the diffusion and subdivision of the electric fluid 292 LECTURE XL VII. Of the Leyden phial 293 Of the theory of the Leyden phial - - 296 Experiments on the theory of ditto - - 299 Experiments on the the contrary electricities of the Leyden jar 304 Of the electrical battery - - - - 313 LECTURE XLVIII. Of lightning and conductors, &c. - - - 317 Of conducting rods 326 LECTURE XLIX. Of the nature of electricity - - - - 331 Of animal electricity .... 354 Of the later experiments on animal electricity 362 LECTURE L. Of magnetism 374 To ascertain whether a body has any iron - - 377 Of the poles of a magnet - ' - - - 379 Of the action of the magnetic poles - - 380 VI CONTENTS. Page. Of magnetic centres 383 To render iron magnetic - 385 To touch a horse- shoe magnet - 387 To make a magnet with several poles - - 388 Of armed magnets - - - - - 391 Of the magnetism of the earth - 392 Of the directive power of a magnet - - 394 Of the variation of the needle - 397 Of the dip of the needle - 400 Of the influence, of the aurora borealis - 401 Similarity between electricity and magnetism 402 Of the theory of magnetism - - - 403 LECTURE LI. Of meteorology -.---- 405 Of the barometer 411 Of some of principal the requisites of a good ba- rometer ------ 413 To boil the quicksilver in a barometer tube - 414 Of the nonius - - 417 Of the portable barometer - - - - 418 Of the thermometer ----- 422 Of the rain-gage » 430 Of the hygrometer - - - - - 431 LECTURE LII. Of rain - - 4S9 Of the nature of clouds - 446 Of the duration of clouds - 448 Of hail 452 Of thunder 452 Of winds 455 Of trade-winds and monsoons - - - 458 Of the aurora borealis - - - - 465 Of the sources of heat - 466 Of the sources of cold - 468 Of evaporation 470 Of annual temperature - - - - 471 Of atmospherical electricity - 474 Of prognostic signs of the weather - - 477 Of prognostics by the barometer - 479 CONTENTS. . Vll Page. Further indications of the weather by the barometer 481 From the thermometer, &c. ... 482 From clouds - - - -- - 483 Jones on the superiority of the northern hemisphere 484 Conclusion - 487 APPENDIX TO LECTURE LIE BY THE E. EDITOR. Of a barometer to measure the heights of moun- tains, &c 489 Of de Luc's hygrometer . . . 491 Of Six's improved thermometer 495 Of the rain-gage 495 Of the wind-gage . . , 496 CONTENTS OF APPENDIX, BY THE A. EDITOR. Physics, or natural philosophy defined . . . 491 Rules of philosophizing 491 General properties of matter 492 Laws of motion ......... •» . 463 Impact of bodies 493 Central forces - . . . . 495 Attraction of cohesion 497 Attraction of gravitation 498 Projectiles 499 Centre of gravity 501 Mechanic powers . 502 The lever 503 The axis and wheel 504 The pulley and tackle 504 The inclined plane . .' 505 The wedge • . 505 The screw 505 Friction . : . . 506 Motion of bodies on inclined planes .... 506 Vlll CONTENTS. Page. Pendulums 507 Hydrostatics . . • 508 Specific gravities 509 Pneumatics 510 Sound 512 Hydraulics 514 Hydraulic machines 515 Optics 516 Catoptrics . . 517 Dioptrics 520 The eye and vision 522 The rainbow 523 Microscopes . - 524 Telescopes 426 Magnetism 529 Electricity . . , 532 Astronomy 537 Eclipses ........... 546 Table of the planets' motions, distances, &c. 548 Tides . . . 550 Winds . ' 552 Chronology 555 A TABLE OF JREFERENCES TO THE PLATES IN VOLUME IV. Fig. 1, p. 5. ASTRONOMY — Plate L ASTRONOMY— Plate II. Fig. 1, p. 42. Fig. 3, p. 47. Fig. 4, p. 54 Fig. 2, p. 42. ASTRONOMY.— Plate III. Fig. 3, p. 8, Fig. 4, p. 9. ASTRONOMY.— Plate IV. Fig. 1, p. 51. Fig. 3, p. 126. Fig. 5, p. 131. Fig. 2, p. 125. Fig. 4, p. 130. ASTRONOMY.— Plate V. Fig. 1, p. 56, 57, 59. Fig. 2, p. 66. ASTRONOMY.— Plate VI, Fig. 1, p. 61. Fig. 2, p. 67, 68, 69. ASTRONOMY— Plate VII. Fig. 3, p. 76. Fig. 1, p. 77. Fig. 2, p. 116. Fig. 3, p. 112. ASTRONOMY — Plate VIII. Fig. 4, p. 114. Fig. 1, p. 75. Fig. 2, p. 75. Fig. 3, p. 87. Fig. 4, p. 88. Fig. 5, p. 93. ASTRONOMY Plate IX. Fig. 6, p. 93. Fig. 7, p. 93. Fig. 1, p. 89. Fig. 2, p. 89. ASTRONOMY— Plate X. Fig. 3, p. 84o Fig. 1, p. 94. VOL. IV: Fig. 2, p. 95. A Fig. 3, p. 100, ASTRONOMY.— Plat_e XI. Fig. 1, p. 135, 172. Fig. 2. p. 142, 173. Fig. 3, p. 142, IT'S. ASTRONOMY.— Plate XII. Fig. 1, p. 135, 147, 172, Fig. 2, p. 135, 155, 174. 173. ASTRONOMY.— Plate XIII. Fig. 2, p. 161, 163, 176, Fig. 3. p. 165, 176, 178. Fig. 4, p. 164. 178. ASTRONOMY— Plate XIV. Fig, 2, p. 171, 181. ASTRONOMY — Plate XV. Fig. 1, p. 119.' Fig. 8, p. 249. Fig. 14,' p. 252. Fig. 2, p. 120. Fig. 9, p. 250. Fig. 15, p. 252. Fig. 3, p, 227. Fig. 10, p. 250. Fig. 16, p. 252. Fig. 4, p. 228. Fig. 11, p. 250. Fig. 17, p. 253. Fig. 5, p. 233, 235. Fig. 12, p. 251. Fig. 18, p. 253. Fig. 6, p. 245. Fig. 13, p. 251. Fig. 19, p. 253. Fig, 7, p. 248. ELECTRICITY— Plate I. Fig. 1, p. 265, 279. Fig. 8, p. 294. Fig. 14, p. 309. Fig. 2, p. 265. Fig. 9, p. 294. Fig. 15, p. 311, 316. Fig. 3, p. 266. Fig. 10, p. 302. Fig. 16, p. 313. Fig. 4, p. 266. Fig. 11, p. 303. Fig. 17, p. 296. Fig. 5, p. 267, 269. Fig. 12, p. 294. Fig, 18, p, 312. Fig. 6, p. 277. Fig. 13, p. 308. Fig. 7, p. 275. ELECTRICITY and MAGNETISM Plate II. Fig. 1, p. 280. Fig. 2, p. 288. Fig. 3, p. 329. Fig. 4, p. 289. Fig. 5, p. 291, Fig. 6, p. 289. Fig. 7, p. 380. Fig. 13, p. 386. Fig. 8, p. 381. Fig. 14, p. 391. Fig. 9, p. 382. Fig. 15, p. 386. Fig. 10, p. 382. Fig. 16, p. 378. Fig. 11, p. 383. Fig. 17, p. 401. Fig. 12, p. 383, 398. LECTURES ON NATURAL PHILOSOPHY. ON ASTRONOMY. LECTURE XXXVII. OF THE COPERNICAN SYSTEM. HAVING shown you the appearance of the hea- venly bodies as seen from the earth, it will be now pro- per to show you why the motions of the planets appear to us so different from what they really are. One of the ends for which man was formed, is to correct appear- ances and errors by the investigation of truth ; whoever considers him attentively from infancy to manhood, and from manhood to old age, will find him ever busy in endeavouring to find some reality to supply the place of those false appearances, by which he has hitherto been deceived. Thus, it is the business of the present lecture to correct those errors that arise from appearances in VOL. IV. 13 2 THE COPERNICAN SYSTEM. the heavens, and to prove the truth of the Copernican System, which is generally received, because it rationally accounts for, and accords with, the phenomena of the heavens. In this system, the sun is placed in the centre, and the earth and other planets revolve round him as their centre. There are, however strong reasons for believing, that some of the sages of antiquity were acquainted with the true solar system as revived by Copernicus. It was the universal doctrine of the Pythagorean school, and is clearly marked out as such by Aristotle : for these, says he, assert, that fire is in the midst of the world, and that the earth is one of the heavenly bodies. He after- wards speaks of a set of men, who held a system essen- tially similar to that of the modern Semitychonic. Eu- demus, in his history of astronomy, as cited by Anatolius, says, that Anaximander was the first who discovered the earth to be one of the heavenly bodies, and to move round the centre of the world. Aristar chits held, that the earth is carried round the sun, in the circumference of a circle, of which the sun itself is the centre ; and that the sphere of the fixed stars is so immense, that the circle of the earth's annual orbit bears no greater pro- portion to it, than the centre of any sphere bears to its whole surface. Pbilolaus, and others, declared the mo- tion of the sun round the earth to be only apparent. They saw and felt the importance of this globe over ours, and, supposing its influence to extend to much larger bounds than that of the earth, they placed it in the centre of the universe. Among the Romans we find, that Numa built a temple to represent, as Plutarch interprets it,* the system of the heavens, with a sacred fire in the centre of it. Thus also in the Jewish tabernacle, the seven lights had a reference to the seven chief lights of the hea- * Those who \v:mt forth- r ii if .nation on this head, muy consult the n^tes to Sydejiham's Trans) itym of the Rivals of Phito, IJu/en\s Inquiries fnt&the Origin of the D&crtvertes attributed to the Mbgerng; Jones Es- H iv < w mo First Principles oj N .»;ur«l Philos ,.;>hy ; Baih'ir lii that is, when their illuminated side is turned from us. The sun enlightens only half a planet at once ; the illuminated hemisphere is always that which is turned towards the sun, the other hemisphere of the planet is 12 OF THE SUN. dark. To speak with accuracy, the sun being larger than any of the planets, will illuminate rather more than half ; but this difference, on account of the great distance of the sun from any of the planets, is so small, that its light may be considered as coming to them in lines physically parallel. Like other opake bodies, they cast a shadow behind them, which is always opposite to the sun. The line in the planet's body which distinguishes the lucid from the obscure part, appears sometimes straight, sometimes Crooked. The convex part of the curve is sometimes towards the splendid, and the concave towards that which is obscure ; and vice versa, according to the situ- ation of the eye with respect to the planet, and to the sun which enlightens the planet. OF THE SUN. The sun is the centre of the system, round which the rest of the planets revolve. It is the first and greatest obj ^ct of astronomical knowledge, and is alone enough to stamp a value on the science to which the study of it belongs. The sun is the parent of the seasons ; day and night, summer and winter are among its surprising effects. All the vegetable creation is the offspring of its beams ; our own life is supported by its influence. Na- ture revives, and puts on a new face, when it approaches nearer to us in spring ; and sinks into a temporary death at its departure from us in the winter. Hence it was with propriety called by the ancients cor cali, the heart of heaven ; for, as the heart is the cen- tre of the animal system, so is the sun the centre of our planetary system. As, the heart is the fountain of the blood, and the centre of heat and motion ; so is the sun the life and heat of the world, and first mover of the mundane system. When the heart ceases to beat, the circuit of life is at an end ; and if the sun should cease to act, a total stagnation would take place throughout the whole frame of nature. OF THE SUN. 13 * Bv his magnetic beam he gently warms The universe, and to each inward part With gentle penetration, though unseen, Shoots invisible virtue." The sun is placed near the centre of the orbits of all the planets, and turns round his axis in 25\ days. It is inclined to the ecliptic in an angle of eight degrees. His apparent diameter, at a mean distance from the earth, is about thirty-two minutes twelve seconds. Those who are not accustomed to astronomical calcu- lation, will be surprized at the real magnitude of this luminary ; which, on account of its distance from us, appears to the eye not much larger than the moon, which is only an attendant on our earth. When look- ing at the sun, you are viewing a globe, whose diameter is about 890,000 English miles ; whereas the earth is not more in diameter than 7970 miles : so that the sun is ajDout 1,392,500 times bigger than the earth. As it is the fountain of light and heat to all the planets, so it also far surpasses them in its bulk. In proportion as science has advanced, and more accurate instruments have been made, the magnitude of this luminary has been found to exceed considerably the limits of former calculations. If the sun were every where equally bright, his rota- tion on his axis would not be perceptible ; but, by means of the spots which are visible on his pure and lucid sur- face, we are enabled to discover this motion. When a spherical body is near enough to appear of its true figure, this appearance is owing to the shading upon the different parts of its surface : for, as a flat cir- cular piece of board, when it is properly shaded by paint- ing, will look like a spherical body, so a spherical body appears of its true shape for the same reason that the plane board, in the present instance, appears spherical. But, if the sphere be at a great distance, this difference of shading cannot be discerned by the eye, and consequent- ly the sphere will* no longer appear of its true shape ; the shading is then lost, and it seems like a flat circle. 14 OF THE SUtt. It is thus with the sun ; it appears to us like a bright flat circle, which flat circle is termed the sun's disk. By the assistance of telescopes, dark spots have been ob- served on this disk, and found to have a motion from east to west : their velocity is greater when they are at the centre, than when they are near the limb. They are seen first on the eastern extremity, by degrees they come forwards towards the middle, and so pass on till they reach the western edge, they then disappear; and, after they have lain hid about the same time that they continued visible, they will appear again, as at first. By this motion we discover not only the time the sun em- ploys in turning round its axis, but also the inclination of its axis to the plane of the ecliptic* The page of history informs us, that there have been periods when the sun has wanted of its accustomed brightness, and shone with a dim and obscure light for the space of a whole year. This obscurity has been supposed to arise from his surface being at those times covered with spots. Spots have been seen that were much larger than the earth. The sun is supposed to have an atmosphere round it, which occasions that appearance which is termed the zodiacal light. This light is seen at some seasons of the year, either a little after sun-set, or a little before sun- rise. It is faintly bright, and of a whitish colour, re- sembling the milky way. In the morning it becomes brighter and larger, as it rises above the horizon, till the approach of day, wh : ch diminishes its splendour, and renders it at last invisible. Its figure is that of a flat or lenticular spheroid seen in profile. The direc- tion of its longer axis coincides with the plane of the sun's equator. But its length is subject to great varia- tion, so that the distance of its summit from the sun varies from 45 to 1 20 degrees. It is seen to the best * The observer may view the spots of the sun with a refracting tt lescope of two or three feet, or a reflecting till more surprising by the discoveries of Dr. Herschel, who finds that the planet Saturn has two concentric rings, of unequal dimensions and breadth situate in one plane, which is probably not much inclined to the equator of the planet. These rings are at a considerable distance from each other, the smaller being much less in diameter at the outside, than the larger is at the inside ; the two rings are en- tirely detached from each other, so as plainly to permit the open heavens to be seen through the vacancy be- tween them. Of the nature of this ring, various and uncertain were the conjectures of the first observers ; though not more perplexed than those of the latest. Of its use to the inhabitants of Saturn, we are as ignorant as of its nature. Saturn is not only furnished with this beautiful ring, but it has also seven attendant moons, the two first next his body were lately discovered by Dr. Herschel. OF THE GEORGIUM SIDUS. 9 From the time of Huygens and Cassini, to the disco- very of the Georgium Sidus by Dr. Herschel, though OF THE GEORGIUM SIDUS. 29 the intervening space was long, though the number of astronomers was increased, though assiduity in observ- ing was assisted by accurrcy and perfection in the instru- ments of observation, yet no new discovery was made in the heavens, — the boundaries of our system were not enlarged. The inquisitive mind naturally inquires, why, when the number of those that cultivated the science was increased, when the science itself was so much improved, in practical discoveries was it so defi- cient ? A small knowledge of the human mind will answer the question, and obviate the difficulty. The mind of man has a natural propensity to indolence ; the the ardour of its pursuits, when unconnected with self- ish views, is soon abated, small difficulties discourage, little inconveniencies fatigue it, and reason soon finds excuses to justify, and even applaud this weakness. In the present instance, the unmanageable length of the telescopes that were in use, and the continual exposure to the cold air of the night, were the difficulties that astronomer had to encounter ; and he soon persuaded himself, that the same effects would be produced by shorter telescopes, with equal magnifying power ; here- in was his mistake, and hence the reason why so few discoveries have been made since the time of Cassinu A similar instance of the retrogradation of science oc- curs in the history of the microscope, as I have shown in my Essays on that instrument. The Georgium Sidus was discovered by Dr. Hers- cbel, in the year 1781 ; for this discovery, he obtained from the Royal Society the honary recompence of Sir Godfrey Copley* s medal. He named the planet in honour of his Majesty King George III. the patron of science, who has taken Dr. Herschel under his patronage, and granted him an annual salary. By this munificence he has given scope to a very uncommon genius, and ena- bled him to prosecute his favourite studies with unre- mitted ardour. In so recent a discovery of a planet so distant, many particulars cannot be. expected. Its year is supposed to be more than 80 of ours ; its diameter 34,299 miles ; distance from the sun about 1,832 millions of miles; 30 OF THE GEORGIUM SIDUS. the inclination of its orbit 43 minutes 35 seconds ; its diameter, compared to that of the earth, as 431,769 to 1 ; in bulk it is 8 049,256 times as large as the earth. Its light is of a bluish white colour, and its brilliancy between that of the Moon and Venus. Though the Georgium Sidus was not known as a planet till the time of Dr. Herschel, yet there are many reasons to suppose it had been seen before, but had then been considered as a fixed star. Dr. Herschefs attention was first engaged by the steadiness of its light; this induced him to apply higher magnifying powers to his telescope, which increased the diameter of it : in two days he observed its place was changed ; he then concluded it was a comet ; but in a little time he with others determined that it was a planet, from its vicinity to the ecliptic, the direction of its motion, being station- ary in the time, and in such circumstances as correspond with similar appearances in other planets. With a telescope which magnifies about 300 times, it appears to have a very well-defined visible disk ; but, with instruments of a smaller power it can hardly be distinguished from a fixed star between the sixth and seventh magnitude. When the moon is absent, it may also be seen by the naked eye. Dr. Herschel has since discovered, that it is attended by two satellites : a discovery which gave him conside- rable pleasure, as the little secondary planets seemed to give a dignity to the primary one, and raise it into a more conspicuous situation among the great bodies of •our solar system.* As the distances of the planets, when marked in miles, are a burden to the memory, astronomers often express their mean distances in a shorter way, by sup- posing the distance of the earth from the sun to be di- vided into ten parts. Mercury may then be estimated * Four additional satellites to this planet have been discovered by Dr. Hascinl, so that it now appears to have six. The planes of their orbits form such large angles with that oi the planet, and conseque nth the ecliptic, as to be almost perpendicular to it ; and another more singular dissimilari- ty to that of the old planets is, that they move in a retrograde direction. For further particulars, see Philos. Trans. 1798, p. 47. — E. Edit. C ^ 1 at four of such parts from the sun, Venus at seven, the Earth at ten, Mars at fifteen, Jupiter at fifty-two, Saturn at ninety-five, and the Georgium Sidus at one hundred and ninety. TABLES OF THE DIAMETERS, DISTANCES, &C. OF THE PLANETS: Accompanied with various comparisons in order to render the ideas of these distances, &c. clearer to the mind. When you endeavour to form any idea of distance, magnitude, or duration, by numbers only, you soon ex- ceed the limits of conception, and find your faculties of reasoning as finite as your senses. Hence astrono- mers are frequently obliged to have recourse to mixed ideas, and make things of different natures and proper- ti >s assist each other, to excite more adequate ideas of what they would express. Some of these methods I shall now lay before you, to assist your immagination in forming ideas of the vast distance and size of the planets. Diameters In diam. Proportion Proportion of in English of the >f surface bulk with re- miles. earth. with re- spect to .he earth. spect to the earth. Sun 893,522 113 12,719 1,434,400 Mercury . . 3,261 2 3 j 6 l Venus .... 7,699 32 T3 near I 9 10 Earth .... 7,920 1 1 1 Moon .... 2,161 3 TT above T ^ 1 Mars .... 5,312 1 4 9 T 3 T Jupiter ... 90,255 ■4 129 3 1,479 Saturn • . . 80,012 10 102 1,030 Georgium > Sidus ? 34,217 a m .81-1 C 32 ] MEAN DISTANCES IN MILLIONS OF MILES. The distances being very great, the nearest million only is inserted, that it may be the easier remembered. Distances from the sun. Millions of miles. Sun Mercury . . 37} Venus . . . 69| Earth . . . .96 Moon . . . 96 Mars .... 1464 Jupiter . . . 4991 Saturn . . . 9161 Georgium ? 1,832 Sidus \ Difference between the greatest and least distance from the earth, in millions of miles. 139 33,570 miles. Millions. 192 192 192 192 PERIODS ROUND THE SUN ACCORDING TO OUR YEARS AND MONTHS. Progressive mo- tion in their orbits, miles years. days, hours 3 | min. sec. per hour. Mercury . . — 87 23 15 37 110,680 Venus . . . — 224 16 49 12 80,955 Earth . . . — 365| 5 48 45 68,856 Mars .... — 686 1 23 30 63 55,783 Jupiter. . . 11 314 1 12 — — 30,193 Saturn . . . 29 167 5 — — 22,298 Georgium > Sidus 5 80 — — — — 16,411 days, hours, min. Moon *s periodical revo-> _, 45 lution round the earth J 2,299 Synodicalrevolv.or from } ori 10 AA change to chan S e V" I A *•* [ 33 ] Tlie following Table from Mr. Vince's Plan of a Course of Lectures, may be considered as more accu- rate ; it is deduced from M. de la Lande's work. meandis. Sid Rev. Nod.inl75C Incl. 1,786 Aphelia 1750. d. h. ' " s ° ' " oft' s. ° ' " Mercury. 38710 87 23 15 14 1 15 20 43 TOO 8 13 33 58 Venus. . . 72333 224 16 49 11 2 14 26 lb 3 23 35 10 7 46 42 Earth . . . 100000 365 6 9 12 3 8 39 34 Mars . . . 152369 686 23 30 36 1 17 38 38 1 51 5 1 28 14 J liter.. 520279 4332 14 27 11 3 7 55 32 I 18 56 6 10 21 4 Saturn .. 954072 10759 1 51 11 3 21 32 22 2 29 50 8 28 9 7 Ge» Sidus 1, 9Q81 80 83yr. I57d. 18h. 3 12 33 31 46 20 U 17 6 44 Revolution en its own axis according Motion on its axis, to our da) s. miles per hour. days.jhouis naio. s^c. 1 Sun 25 J 6 3,957 Mercury . . unknown. unknown. Venus . . . 23 22 1,065 Earth .... ^3 56 4 -1,042 Mars .... 24 39 556 Jupiter . . . 9 56 25,920 Saturn . . . unknown. unknown. Georgiuni > unknown. unknown. Sidus ) Mo on ... . 27 7 |4f I near LOJ The rotation of Saturn, agreeable to Dr. Usher's computations, is 10 hours, 12£- minutes. A different result was however obtained, by taking the density of Saturn, as stated by M. de la Lande. Dr. Herschel has settled the rotation of Saturn's ring at 10 hours, 32 minutes, 16 seconds. VOL. IV. [ 34 ] Light and heat in proportion to what the earth receives. Mercury .... 7 times more Venus double Earth 1 Mars half Jupiter one 27th. Saturn one 91 Georg. Sidus . one 364th Moon 1 Appearances of the sun in proportion to what it appears on the earth. 7 times greater twice as great 1 half as great one 27th one 91 one 364th 1 DISTANDES AND APPARENT DIAMETERS OF THE SUN AND PLANETS. Distances from the earth. Sun . . . Mercury Venus . Earth . . Mars • • Jupiter . Saturn . Geor. Sid. Moon Greatest, mill, of miles Least. | mill, of miles m 94 58j 261 59 5> 4034 1.0m 82 q| 1.928 1736 miles. miles. 256.785 223.211 Mean. 96 96 96 146-1 4991 9161 1132 miles. 240.000 I Apparent diameter viewed from the earth. Greatest Least. Mean. 52 38 11 1 22 46 18 0.3.9 31 3« t H. f 4 3 H 3.9 33.36 28.55.30 52 5 7 17 7 37 27 16 3.9 31 15 It has been found that a cannon-ball moves about 8 miles in one minute, or 704 feet in a second ; and that sound moves about 13 miles in one minute, or 1144 feet in a second. A very high wind may make sound move one mile in 4,4 seconds, that is, in about one-twentieth less time than in calm weather. The most violent storm does not move above one mile in a minute, or 88 feet in a second. DISTANCE OF THE PLANETS. 35 From hence it has been computed, that a body issu- ing from the sun with the swiftness of a cannon-ball, that is, eight miles in a minute, would employ the fol- lowing times in reaching Mercury Venus Earth Mars Jupiter Saturn Gtrorgium Sidus . . . Any fixed star that') has been accurate- I ly observed J years. 8 «5 JB «-> c o £ 10 35 6 CO S- 3 O \3 16 6 8 20 22 10 4 21 34 9 19 16 118 9 8 16 217 10 2 19 435 4 24 7,600,000 c 'i 18 58 20 35 40 16 40 A ray of light comes from the sun to the earth in 8 min. 13 sec. moves therefore 11,693,462 miles in one minute, or 194,891 miles in one second. A ray of light comes from the moon in 1,23 seconds. From the very accurate observations of Dr. Bradley, it is inferred, that no fixed star, of the great numbers ob- served by him, can be at a less distance from the earth than about 400,000 distances of the sun from the earth ; so that a ray of light, which comes from the earth in 8 minutes, 13 seconds, issuing from such a star, must re- quire 6 years and 3 months to reach the earth. The following may therefore be considered as propor- tional distances of the celestial bodies from the sun. Mercury 28 yards. Venus 52 ditto. llarth 79 ditto. Mars 109 ditto. Jupiter 273 ditto. Saturn • 684 ditto. (ieorgium Sidus .... 1357 ditto. Moon 6* inches from the earth, Syrius 8410 miles, [36 ] PROPORTIONAL MAGNITUDE. Sun 2 feet in diameter. Mercury yj of art inch. Vemn y of an inch. Mara iV ol iin well- Jupiter 25 inches. Saturn 2Jg inches. Georgium Sidus about 1 inch. The distance of Syrius has been computed at not less than 18,717,442,690,526 miles. A cannon ball, going at the rate of 19.05 miles per minute, would therefore only reach it in about 1 ,868,307 years. The circumfe- rence of its orbit would be 1 17,605,162,638,454 miles; if the star moved through this space in 24 hours, it must go at the rate of 361,170,863 miles per second. Such is the immense distance even of a star of the first magnitude, that, supposing the world to have existed 6000 years, and the distance to be reduced to 31 1^- inch- es, or 25 feet 1 1 inches, then a cannon ball, going at the rate of 1143 miles per hour, and set in motion at the creation of the world, would now have passed only one inch of that reduced space, because one inch bears the same proportion to 31 1.3 as the distance which the ball would go in 6000 years does to the whole distance of the star. For, If a cannon ball go 1 inch in 6000 years, how far will it go in 1,868,307? — Answer , 311.3o3, or 26 feet, 11 inches. Or, If a cannon-ball go 18,717,442,690,526 miles in 1,868,307 years, how far will it go in 6000? — Ans. 60,110,386,645, which number is to 18717, &c. as 1 inch is to 311.333. By the same rule, if we suppose a star of the second, third, tenth, one-hundredth magnitude, its distance will be proportionably great, and the space gone through pro- portionably small; that is, in the same time the ball would have. gone only one-halt, one- third, one-tenth, one-hun- dredth, &'_• of an inch. With respect to a star of the third magnitude, it would have passed through a space PROPORTIONAL DISTANCES, &C. 37 equal to one barley-corn ; with respect to one of the four-hundredth, not the space corresponding to one hair's breadth, reckoning 400 hairs equal to one inch. This supposes that a star of the third, tenth, four- hundredth, &c. magnitude, is 3, 10, 400 times as dis- tant as one of the first ; and we may also suppose that a star, which cannot be seen but with a power of 100, 1000, &c. is 100, 1000, &c. times more distant than one which the naked eye can just discover. Upontheforegoingsupposition,thedistanceofastarofthe second magnitude would be 37,434,885,38 1, 053 r miles; the diameter of its orbit, equal to 74,869,770,762, 1 07, and the circumference of its orbit, equal to 235,210,325,276,- 908.7; a degree of this is 653,362,014,658; a minute, 10,889,366,910.75; and a second, 181,489,448^. A cannon ball would therefore require 65216 years, 86 days, 6 hours, 17 minutes, 13 seconds, to go through one de- gree of this orbit; 1086 years, 342 days, 2 hours, 30 mi- nutes, 17 seconds, to pass through one minute; and 18 years, 42 days, 4 hours, 50 minures, 30 seconds, to go over one second of it. The distance between the stars marked & and « in Orion's belt, is 1 degree, 23 minutes, 1 2 seconds. Between « and „ SATELLITES. j £ (~ C discovered | thn 1789, by SATURN'S Z(Df '^ cU SEVEN *{ ^ SATELLITES- I 5 j 6. ::::::::::::::.:::::::::: \j Distance Synodical Re- from its volutions Primary. round its Primary. g X |S CD English '< 2 p miles. j - 269,10.5 428,312 683,071 1 j 7 1 8 13 3 28 17 5<5 36 54 36 1,201,386 It 18 5 7 126,000 22 40 46 162,000 1 8 53 c 195,671 1 21 18 2? 250,631 2 17 44 22 350,099 4 12 25 12 811,610 1£ 22 M 38 2,365,222 rs 7 47 — Proportion of bulk with re- spect to the Earth. i About ± Ring of Saturn 21,000 miles broad, and 21,000 miles distant from his Rodv on all sides. Thickness of the Ring- unknown. GEORGIAN'S SATELLITES discovered" 11 Jan. 1787, bv Dr. Hersc. About [289,1181 387,5051 I 8 i lr 13 11 n * The following discoveries respecting this pi un-t and its satellites by Dr. Herschel, it may be proper tq acquaint the reader with, in addition to what I have given in the note to page 30, and the author's annexed table. The reader must observe, that they are the two old satellites u that move in a retrograde direction. Whether the motion of the four new ones be direct or retrograde, appears not yet determined." The two old satellites were formerly found to revolve, the first in 8 days, 17 hours, 1 min. 17 sec. at the distance of 33" from its primary ; the second in 13 d. 11 h. 5 m. 1.5 sec. at the distance of 44.23". " Tiie new satellites revolve as follows: the periodical times being inferred from their greatest elongations ; the interior satellite in 5 d. 21 h. 25 m. at the distance of 25.5" ; a satellite intermediate between the two old ones in 10 d. 23 h. 4 m. at the distance of 38.57" ; the nearest exterior satellite at about double the distance of the farthest old one, and consequently its periodical lime 38 id. 1 h. 49 m. and the most distant satellite full tour times as far from its primary as the old second satellite, whence it will take at least 107 d. 16 h. 40 m. to com- plete its revolution. The disk of the Georgium Sidus he finds to be flat- tened, and therefore it. must revolve with considerable rapidity on its axis. From the very faint light of the satellites, they are observed to disappear in those parts of their orbits, which bring them apparently nearer to the C 84 ] OF THE MOON'S MOTION, You have seen, that four of the primary planets are attended in their revolutions by secondary planets ; we, as one of these, are attended by the moon, she enligh- tens our nights, by reflecting the light she receives from the sun, as the other satellites enlightens the planets to which they severally belong. If you imagine the plane of the moon's orbit to be extended to the sphere of the heavens, it would mark therein a great circle, which may be called the moon's apparent orbit ; because the moon appears to the inhabitants of the earth to move in that circle, through the twelve signs of the zodiac, in a periodical month. This position is illustrated by plate 9, Jig, 3 ; let E F G H I be the orbit of the earth, S the sun, abed the orbit of the moon, when the earth is at E : let A B C D be a great circle in the sphere of the heavens, in the same plane with the moon's orbit. The moon, by going round her orbit according to the order of the let- ters, appears to an inhabitant of the earth to go round in the great circle A B C D, according to the order of those letters : for when the moon is at a, seen from the earth at E, she appears at A ; when the moon is got to b, she appears at B ; when to c, she will appear at C ; when arrived at d, she will apear at D. It is true, when the moon is at b, the visual line drawn from E, through the moon terminates in L ; as it does in M, when the moon is at d ; but the lines, L M and D B, being pa- rallel, and not farther distant from each other than the distance of the earth's orbit, are as to sense coincident, their distance measured in the sphere of the heavens being insensible ; for the same reason, though the earth moves from E to F, in the time that the moon goes round her orbit, so that at the end of a periodical month the moon will be at a, and is seen from the planet. Tins dors not ai ise from an atmosphere, for the effect is the same whether the satellite be within or beyond the planet." His observations were made with reflecting telescopes of 7,10, and 20 feei in length, and the powers from lib to 2400.— K. Kdit* f For particulars of four others, recently discovered, bee the note in the preceding- page. — E. Edit. THE MOON'S MOTION. 8,5 earth at F, in the line F N ; the moon will notwith- standing, appear at A, the lines, FN and E A, being parallel, and as to sense coincident : in like manner, in whatever part of her orbit the earth is, as at H or I, the moon, by going round in her orbit, will appear to an inhabitant of the earth to go round in the great circle ABCD. The plane of the moon's orbit produced till it cuts the -plane of the ecliptic, makes an angle with it of about 5° : this angle is sometimes more, sometimes less than 5°. The points where the moon's orbit produced cuts the ecliptic, are called the moon's nodes ; her as- cending Q the dragon's head, and her descending node £5 the dragon's tail. The moon's nodes have a slow motion of 19° 22' in a year, which carries them round the ecliptic, contrary to the order of the signs, in 19 years. The line of the moon's nodes is a line drawn from one node to the other. The extremities of the line of the nodes are not al- ways directed to the same point of the ecliptic, but con- tinually shift their places from east to west, or contra- ry to the order of the signs, performing an entire revo- lution about the earth, in the space of something less than nineteen years. The moon appears in the ecliptic only when she is in one of her nodes : in all other parts of her orbit she is either in north or south latitude, sometimes nearer to, sometimes farther removed from the ecliptic, accord- ing as she happens to be more or less distant from the nodes. When the place, in which the moon appears to an in- habitant of the earth, is the same with the sun's place, she is said to be in conjunction. When the moon's place is opposite to the sun's place, she is said to be in opposi- tion. When she is a quarter of a circle distant from the sun, she is said to be in quadrature. Both the conjunc- tion and opposition of the moon are termed syzygies. The common lunar month, or the time that passes between any new moon and the next that follows it is called a synodical month, or a lunation. This month con- tains 29 days, 12 hours, 44 minutes, 3 seconds. 86 THE MOON'S MOTION. The moon's motion in her orbit is considered either absolutely, or with relation to the sun. The moon's motion in her orbit, which is also her motion in longi- tude, is sometimes swifter, sometimes slower ; her mean motion is 13 degrees, 10 minutes, 35 seconds, in a day, which carries her round the zodiac in 27 days, 7 hours, 43 minutes. The time wherein the moon is carried round the zodiac, called a periodical month, is the time in which the moon performs one entire revolution about the earth, from any point in the zodiac to the same again ; and contains 27 days, 7 hours, 43 minutes. The moon's motion considered with relation to the sun is called her elongation from the sun. ' The moon's motion from the sun is the excess of the velocity of the moon's motion, above the velocity of the sun's appa- rent motion in the ecliptic ; this excess is sometimes more, sometimes less. The moon's mean motion from the sun is 1 2 degrees, 1 1 minutes, 26 seconds in a day, which carries the moon from one conjunction with the sun to another in 29 days, 12 hours, 44 minutes, 3 se- conds. The time between any conjunction and the con- junction immediately following, as before observed, is called a synodical month, or a lunation, wherein the moon appears in all her phases. If the earth had no revolution round the sun, or the sun no apparent motion in the ecliptic, the periodical and synodical months would be the same ; but as this is not the case, the moon takes up a longer time to pass from one conjunction to the next, than to describe its whole orbit ; or the time between one new moon and the next, is longer than the moon's periodical time. The moon going round our earth in an orbit, whose ssmidiameter is less than the nearest distance of any planet, may come between our eye and any planet or star that is near the ecliptic. The time when the moon appears to touch a planet or star, is called its appulse, which being instantaneous, serves to determine the lon- gitude of aifferent places where it is observed. The moon revolves round the earth from west to east, and the sun apparently revolves round the earth the same way. Now at the new moon, or when the sun and moon are in conjunction, they both set out the moon's motion. 87 from the same place, to move the same way round the earth ; but the moon moves much faster than the sun, and consequently will overtake it ; and when the moon does overtake it, it will be a new moon again. If the sun had no apparent motion in the ecliptic, the moon would come up to it or be in conjunction again, after it had gone once round in its orbit ; but as the sun moves forward in the ecliptic, whilst the moon is going round, the moon must move a little more than once round, before it comes even with the sun, or before it comes to conjunction. Hence it is, that the time be- tween one conjunction, and the next in succession, is something more than the time the moon takes up to go once round its orbit ; or a synodical month is longer than a periodical one. In plate 8, fig. 3, let S be the sun, C F a part of the earth's orbit, M D a diameter of the moon's orbit when the earth is at A, and m d another diameter parallel to the former, when the earth is at B. While the earth is at A, if the moon be at D, she will be in conjunction ; and if the earth were to continue at A, when the moon had gone once round its orbit, from D through M, so as to return to D again, it would again be in conjunction. Therefore, upon the supposition that the earth has no motion in its orbit, the periodical and synodical months would be equal to one another. But as the earth does not continue at A, it will move forward in its orbit, du- ring the revolution of the moon from A to B : and as the moon's orbit moves with it, the diameter, M D, will then be in the position m d ; therefore, when the moon has described it's orbit, it will be at d, in this diame- ter m d ; but if the moon be at d, and the sun at S, the moon will not be in conjunction, consequently, the pe- riodical month is completed before the synodical. The moon, in order to come to conjunction, when the earth is at B, must be at e, in the diameter e f ; or besides going once round in its orbit, it must also describe the arc d e. The synodical month is, therefore, longer than the periodical, by the time the moon takes up to describe the arc d e. This may be also explained in another manner, by considering the motion of the sun ; a view of the sub- 88 the moon's motion. ject, that may render it more easy to some young minds than the foregoing. Thus, let us suppose the earth at rest at E, plate &>Jig. 4, M the moon in conjunction with the sun at S, while the moon describes her orbit, ABC, about the earth at E, let the sun advance by his apparent annual motion from S to D. It is plain, that the moon will not come in conjunction with the sun again, till, besides describing her orbit, she hath described, over and above^ that, the arc M F, corre- sponding to the arc S D. As the moon goes round the earth in a much smaller orbit than that in which the earth revolves round the sun, sometimes more, sometimes less, and sometimes no part of her enlightened half will be towards us ; hence she is incessantly varying her appearance ; sometimes she looks full upon us, and her vissage is all lustre ; sometimes she shows only half her enlightened face ; soon she appears as a radiant crescent ; in a little time all her brightness vanishes, and she becomes a beamless orb. The full moon, or opposition, is that state in which her whole disk is enligntened, and we see it all bright, and of a circular figure. The new moon is when she is in conjunction with the sun ; in this state, the whole surface turned towards us is dark, and she is therefore invisible to us. The first quarter of the moon she appears in the form of a semicircle, whose circumference is turned towards the west. At the last quarter, she appears again under the form of a semicircle, but with the circumference turned towards the east. The moon is generally invisible a day or two before and after conjunction, and the obscure light, visible in the moon a little before and after conjunction, is reflect- ed upon her from the earth. These phases may be illustrated in a very pleasing manner, by exposing an ivory ball to the sun, in a va- riety of positions, by which it may present a greater or smaller part of its illuminated surface to the observer. If it be held nearly in opposition, so that the eye of the observer may be almost immediately between it and the sun, the greatest part of the enlightened side will be THE MOON'S MOTION. 89 seen ; but if it be moved in a circular orbit, towards the sun, the visible enlightened part will gradually decrease, and at last disappear, when the ball is held directly to- wards the sun. Or, to apply the experiment more im- mediately to our purpose ; if the ball, at any time when the sun and moon are both visible, be held directly be- tween the eye of the observer and the moon, that part of the ball, on which the sun shines, will appear exactly of the same figure as the moon itself. The phases of the moon, like those of Venus, may also be illustrated by a diagram ; thus m plate 9, Jig 1, let S be the sun, T the earth, ABCDEFGH the orbit of the moon. The first observation to be deduc- ed from this figure, is, that the half of the earth and moon, which is towards the sun, is wholly enlightened by it ; and the other half, which is turned from it, is totally dark. When the moon is in conjunction with the sun at A, her enlightened hemisphere is turned to. wards the sun, and the dark one towards the earth ; in which case, we cannot see her, and it is said to be new moon. When the moon has removed from A to B, a small portion of her enlightened hemisphere will be turned towards the earth ; which portion will appear of the form represented at B, plate 9, Jig. 2, a figure which exhibits the phases as they appear to us. As the moon proceeds in her orbit according to the order of the letters, more and more of her enlightened parts is turned towards the earth . When she arrives at C, in which position she is said to be in quadrature, one half of that part towards the earth is enlightened, appearing, as at C, among the phases ; this appearance is called a half moon. When she come to D, the great- est part of that half which is towards us is enlightened ; the moon is then said to be gibbous, and of that figure which is seen at D, in j£g. 2. When the moon comes to E, she is in opposition to the sun, and consequently turns all her illuminated sur- face towards the earth, and shines with a full face, for which reason she is called a full moon. As she passes through the other half of her orbit, from E by F, G, VOL. IV. N 90 THE and H to A, she again, puts on the same phases as be- fore, but in a contrary order or position. As the moon, by reflected light from the sun, illumi- nates the earth, so the earth does more than repay her kindness, in enlightening the surface of the moon, by the sun's reflex light, which she diffuses more abun- dantly upon the moon, than the moon does upon us ; for the surface of the earth is considerably greater than that of the moon, and, consequently, if both bodies reflect light in proportion to their size, the earth will reflect much more light upon the moon than it receives from it. In the new moon, the illuminated side of the earth is fully turned towards the moon, and the Lunarians will have a full earth, as we, in a similar position, have a full moon. And from thence arises that dim light which is observed in the old and new moons, whereby, be- sides the bright and shining horns, we can perceive the rest of her body behind them, though but dark and ob- scure. Now, when the moon comes to be in opposi- tion to the sun, the earth, seen from the moon, will ap- pear in conjunction with him, and its dark side will be turned towards the moon, in which position the earth will be invisible to the Lunarians ; after this, the earth wiil appear to them as a crescent. In a word, the earth exhibits the same appearance to the inhabitants of the moon, that the moon does to us. The moon turns about upon its own axis in the same time that it moves round the earth ; it is on this account that she always presents nearly the same face to us : for by this motion round her axis, she turns just so much of her surface constantly towards us, as by her motion about the earth would be turned from us. This motion about her axis is equable and uniform, but that about the earth is unequal and irregular, as being performed in an ellipsis ; consequently, the same precise part of the moon's surface can not be shown constantly to the earth : this is confirmed by a telescope, by which we often observe a little segment on the eastern and west- ern limb appear and disappear by turns, as if her body Iibrated backwards and forwards ; this phenomenon is OF ECLIPSES. 91 called the moon's libration. The lunar motions are subject to several other irregularities, which are fully discussed in the larger works on astronomy.* LECTURE XL. OF ECLIPSES. 1 HOSE phenomena, that are termed eclipses, were in former ages beheld with terror and amazement, and looked upon as prodigies that portended calamity and mi- sery to mankind. These fears, and the erroneous opinions which produced them, had their source in the hierogly- phical language of the first inhabitants of the earth. We do not, however, imagine, that even the most ancient of these knew any more of the laws and motions of the hea- venly bodies, than what could be discovered from imme- diate sight ; or that they knew enough of the lunar sys- tem to calculate an eclipse, or even that they ever at- tempted it. The word eclipse is derived from the Greek, and sig- nifies dereliction, a fainting away, or swooning. Now, as the moon falls into the shadow of the earth, and is de- prived of the sun's enlivening rays, at the time of her greatest brightness, and even appears pale and languid before her obscuration, lunar eclipses were called luntz labores, the struggles or labours of the moon ; to relieve * See a Complete System of Astronomy, by the Rev. 5. Vince , 4to. E. j&»aT. 92 OF ECLIPSES. her from these imagined distresses, supertition adopted methods as impotent as they were absurd. When the moon, by passing between us and the sun, deprived the earth of its light and heat, the sun was thought to turn away his face, as if in abhorrence of the crimes of mankind, and to threaten everlasting night and destruction to the world. But thanks to the advance- ment of science, which, while it has delivered us from the foolish fears and idle apprehensions of the ancients, leaves us in possession of their representative knowledge, enables us to explain the appearances on which it was founded, and points out the perversion and abuse of it. Any opake body that is exposed to the light of the sun, will cast a shadow behind it. This shadow is a space deprived of light, into which, if another come, it cannot be seen for want of light ; the body, thus falling within the shadow, is said to be eclipsed. The earth and moon, being opake bodies, and deriving their light from the sun, do each of them cast a shadow behind, or towards the hemisphere opposed to the sun. Now, when either the moon or the earth passes through the other's shadow, it is thereby deprived of illumination from the sun, and becomes invisible to a spectator on the body from whence the shadow comes ; and such a spec- tator will observe an eclipse of the body which is passing through the shadow ; while a spectator on the body which passes through the shadow, will observe an eclipse of the sun, being deprived of his light. Hence there must be three bodies concerned in an eclipse; 1. The luminous body; 2. The opake body, that casts the shadow ; and 3. The body involved in the shadow. OF ECLIPSES OF THE MOON. As the earth is an opake body, enlightened by the sun, it will cast a shadow towards those parts that are oppo- site to the sun, and the axis of this shadow will alway be in the .plane of the ecliptic, because both the sun and the earth are always there. % ECLIPSES OF THE MOON. 93 The sun and the earth are both spherical bodies; if they were, therefore, of an equal size, the shadow of the earth would be cylindrical, as in plate 8, fig. 5, and would con- tinue of the same breadth at all distances from the earth, and would consequently extend to an infinite distance, so that Mars, Jupiter, or Saturn, might be eclipsed by it ; but as these planets are never eclipsed by the earth, this is not the shape of the shadow, and consequently the earth is not equal in size to the sun. If the sun were less than the earth, the shadow would be wider, the farther it was from the earth, see plate 8, fig. 6, and would therefore reach to the orbits of Jupiter and Saturn, and eclipse any of these planets when the earth came between the sun and them ; but the earth never eclipses them, therefore this is not the shape of its shadow, and consequently the sun is not less than the earth. As we have proved, that the earth is neither larger than, nor equal to the sun, we may fairly conclude that it is less ; and that the shadow of the earth is a cone, which ends in a point at some distance from the earth, see plate 8, fig. 7. ^ The axis of the earth's shadow falls always upon that point of the ecliptic that is opposite to the sun's geocentric place ; thus, if the sun be in the first point of Aries, the axis of the earth's shadow will terminate in the first point of Libra. It is clear, therefore, that there can be no eclipse of the moon but when the earth is interposed between it and sun, that is, at the time of its opposition, or when it is full ; for unless it be opposite to the sun, it can never be in the earth's shadow : and if the moon did always move in the plane of the ecliptic, she would every full moon pass through the body of the shadow, and there would be a total eclipse of the moon. We have already observed, that the moon's orbit is in. dined to the plane of the ecliptic, and only coincides with it in two places, which are termed the nodes. It may, therefore, be full moon* without her being in the plane * A planet may be in opposition to, or conjunction with, the sun, without being in a right line that passes through the sun and the earth. Astronomers term it. in conjunction with the sun, if it be in the same part of the zodiac; in opposition, if it be in a part of the zodiac, 180 degrees from the sun. 94 ECLIPSES OF THE MOON. of the ecliptic ; she may be either in the north or the south side of it ; in either of these cases, she will not enter into the shadow, but be above it in the one, and below it in the other. To illustrate this, let H G, plate 10, Jig. 1, represent the orbit of the moon, E F the plane of the ecliptic, in which the centre of the earth's shadow always moves, and N the node of the moon's orbit ; A B C D four places of the shadow of the earth in the ecliptic. When the shadow is at A, and the moon at I, there will be no eclipse ; when the full moon is nearer the node, as at K, only part of her globe passes through the shadow, and that part be- coming dark, it is called a partial eclipse; and it is said to be of so many digits as there are twelfth parts of the moon's diameter darkened. When the full moon is at M, she enters into the shadow C, and passing through it becomes wholly darkened at L, and leaves the shadow at O ; as the whole body of the moon is here immersed in the shade, this is called a total eclipse. But when the moon's centre passes through that of the shadow, which can only happen when she is in the node at N, it is called a total and central eclipse. There will always be such eclipses, when the centre of the moon, and the axis of the shadow, meet in the nodes. The duration of a central eclipse is so long, as to let the moon go the length of three of its diameters totally eclipsed, which stay in the earth's shadow is computed to be about four hours ; whereof the moon takes one hour from its beginning to enter the shadow, till quite im- mersed therein ; two hours more she continues totally dark ; and the fourth hour is taken up from her first be- ginning to come out of the shadow, till she is quite out of it. From the magnitude of the sun, the size of the earth, their distance from each other, the refraction of the at- mosphere, and the distance of the moon from the earth, it has been calculated, that the shadow of the earth ter- minates in a point, which does not reach so far as the moon's orbit. The moon is not, therefore, eclipsed by the shadow of the earth alone. The atmosphere ? by re- fracting some of the rays of the sun, and reflecting others. ECLIPSES OF THE MOON. 95 casts a shadow, though not so dark a one as that which arises from an opake body ; when, therefore, we say that the moon is eclipsed, by passing into the shadow of the earth, it is to be understood of the shadow of the earth, together with its atmosphere. Hence it is that the moon is visible in eclipses, the shadow cast by the atmosphere not being so dark as that cast by the earth. The cone of this shadow is larger than the cone of the earth's shadow, the base thereof broader, the axis longer. There have been eclipses of the moon, in which the moon has entirely disappeared : Hevelius mentions one of this kind which happened in August 1647, when he was not able to distin- guish the place of the moon, even with a good telescope, although the sky was sufficiently clear for him to see the stars of the fifth magnitude. All opake bodies, when illuminated by the rays of the sun, cast a shadow from them, which is encompassed by a penumbra or thinner shadow, every where sur- rounding the former, and growing larger and larger as we recede from the body ; in other words, the penumbra is all that space surrounding the shadow, into which the rays of light can only come from some part of that half of the globe of the sun, which is turned towards the planet, all the rest being intercepted by the intervening body. Let S, plate 10, fig. 2, be the sun, E the planet, then the penumbral cone is F G H. The nearer any part of the penumbra is to the shadow, the less light it receives from the sun ; but the farther it is, the more it is enlightened ; thus, the parts of the penumbra near M are illuminated only by those rays of light, which come from that part of the sun near to I, all the rest being intercepted by the planet E : in like manner, the parts about N can only receive the light that comes from the part of the sun near to L, whereas the parts of the penumbra at P and O are enlightened in a much greater degree ; for the planet in- tercepts from P only those rays which come from the sun near L, and hides from Q^only a smali part of the sun near L The moon passes through the penumbra before she enters into the shadow of the atmosphere ; this causes 96 ECLIPSES OF THE MOON. her gradually to lose her light, which is not sensible at first, but as she goes into the darker part of the penum- bra, she grows paler ; the penumbra, where it is conti- guous to the shadow, is so dark, that it is difficult to dis- tinguish one from the other. If the atmosphere be serene, every eclipse of the moon is visible at the same instant to all the inhabitants of that side of the earth to which she is opposite. If we imagine a plane parallel to the base of the earth's conical shadow, to pass through the shadow at the dis- tance of the moon's centre from the earth, there will be projected upon the plane the circle of the earth's shadow, surrounded with the circle of the penumbra : the centre of those circles is always in the plane of the ecliptic. The circle of the earth's shadow, when the earth is at the same distance from the sun, is greater the nearer the moon is to the earth. The circle of the earth's shadow is greater, w r hen the moon is at the same distance from the earth, the farther the earth is from the sun. The apparent semi- diameter of the moon in her syzygies is about 15 minutes: the semidiameter of the circle of the earth's shadow is about three times as great as the semidiameter of the moon. If the moon in opposition be in the node, the eclipse of the moon will be total and central ; if very near the node, total, but not central ; if so far from the node, that only pan falls into the shadow, the eclipse is partial; if so far from the node, that the distance of her centre from the centre of the circle of the earth's shadow is greater than the semidiamer of the shadow added to the semidiameter of the moon, she will not be eclipsed at all. The moon passes through the penumbra before she falls into the sha- dow ; this makes her gradually lose her light, and grow paler, a little before she begins to be eclipsed. The moon is sometimes in the middle of a total eclipse invisible in some places, and not in others, because of the different constitution of the air ; but generally she ap- pears of a dusky red colour, especially towards the edges, being more dark about the middle of the shadow : this reddish colour is owing to the rays of the sun, or to the ECUPSES OF THE MOON. 97 light of the sun's atmosphere refracted through the earth's atmosphere, or to the light of the stars and planets ; most probably to the first of these. The sun or moon seen from the earth, or the earth seen from the sun or moon, though spherical, on account of their distance appear like circular planes i these circular planes are called the disks of the sun, earth, or moon. The apparent diameter of the disk of the sun or moon is by astronomers divided into 12 equal parts, which are called digits ; each digit into 60 parts, which are called minutes : as many of these digits and minutes as are co- vered by the shadow in the middle of a partial lunar eclipse, so many digits and minutes of the moon are said to be eclipsed. In a total eclipse of the moon without continuance, the moon is eclipsed 1 2 digits ; in a total eclipse with continuance, she is eclipsed more than 12 ; thus, if her whole disk be immersed so deep within the shadow, that if her diameter contained 15 such parts as now it contains 12 of, the whole 15 would be eclipsed, the moon is then eclipsed 15 digits. Sometimes the ap- parent diameter of the moon is observed near the time of the eclipse, and the greatness of the eclipse expressed in minutes of degrees and seconds. The motion of the moon in her orbit being eastward, the beginning of a lunar eclipse is when the eastern limb or edge of the moon's disk touches the western limb of the shadow; the end of a lunar eclipse, when the western limb of the moon's disk leaves the eastern limb of the shadow ; in a total eclipse, the time the whole disk is in the shadow is called the stay, or time of total immer- sion. The beginning or end of a lunar eclipse, being instan- taneous, serves to discover the longitude, but not accu- rately without a telescope ; for, by reason of the penum- bra, the beginning appears too soon, the end too late to the naked eye, and not at the same time to all eyes': for this reason, the longitudes of places, and the places of the moon, determined by eclipses, before the invention of telescopes, cannot be depended upon. The moderns, that they may have a greater number of opportunities of detei mining the longitude than the beginnings and end- ings of eclipses would afford, do it by observing the im- VOL. IV. O 98 ECLIPSES OF THE SUN. mersions of the most remarkable spots of the moon into the shadow, or by their emerging out of it. The quantity of a lunar eclipse depends, 1. Upon the largeness of the circle of the earth's shadow, whose dia- meter may be different. 2. Upon the apparent diameter of the moon, which may be different. 3. Ceteris paribus, upon the distance of the moon from her node, at the mo- ment of her being at the full. The duration of a lunar eclipse depends partly upon its quantity, partly upon the velocity of the moon's motion across the shadow, which is the same as her motion from the sun. The moon's motion from the sun is swiftest when she is in her perigee, and the duration of a central eclipse will then be shortest, though the moon's diame- ter and the diameter of the circle of the earth's shadow be then greatest ; because the excess of the moon's way through the shadow is more than compensated by the greater velocity of the moon's motion. The longest du- ration of a central lunar eclipse, u e. when the earth is in aphelion, and the moon in apogee, is about 3 hours, 57 minutes, 6 seconds. The shortest duration of a central lunar eclipse, /'. e. when the earth is in perihelion, and the moon in perigee, is 3 hours, 37 minutes, 26 seconds. OF ECLIPSES OF THE SUN. The moon, when in conjunction, if near one of her nodes, will be interposed between us and the sun, and will- consequently hide the sun, or a part of him, from us, and cast a shadow upon the earth : this is called an -eclipse of the sun ; it may be either partial or total. An eclipse of any lucid body is a deficiency or diminm- tion of light, which would otherwise come from it to our eye, and is caused by the interposition of some opake bod/. The eclipses of the sun and moon, though expressed by the same word, are in nature very different ; the sun, in reality, loses nothing of its native lustre in the great- est eclipses, but is all the while incessantly sending forth streams of light every way round him, as copiously as be* ECLIPSES OF THE SUN. 99 fore. Some of these streams are, however, intercepted in their way towards our earth, by the moon coming be- tween the earth and the sun : and the moon having no J^ight of her own, and receiving none from the sun on that half of the globe which is towards our eye, must appear dark, and make so much of the sun's disk appear so, as is hid from us by her interposition. What is called an eclipse of the sun, is therefore in re- ality an eclipse of the earth, which is deprived of the sun's light by the moon coming between, and casting a shadow upon it. The earth being a globe, only that half of it which at any time is turned towards the sun, can be en- lightened by him at that time ; it is upon some part of this enlightened half of the earth, that the moon's sha- dow or penumbra falls in a solar eclipse. The sun is always in the plane of the ecliptic ; but the moon being inclined to this plane, and only coin- ciding with it at the nodes, it will not cover either the whole or a part of the sun ; or, in other words, the sun will not be eclipsed, unless the moon at that time be in or near one of her nodes. The moon, however, cannot be directly between the sun and us, unless they be both in the same part of the heavens ; that is, unless they be in conjunction. There- fore the sun can never be eclipsed but at the new moon, nor even then, unless the moon at that time be in or near one of the nodes. The moon being much smaller than the earth, and having a conical shadow, because she is less than the sun, can only cover a small part of the earth by her sha- dow ; though as we have observed before, the whole body of the moon may be involved in that of the earth. Hence an eclipse of the sun is visible but to a few in- habitants of the earth ; whereas an eclipse of the moon may be seen by all those that are on that hemisphere which is turned towards it. In other words, as the moon can never totally eclipse the earth, there will be many parts of the globe that will suffer no eclipse, though the sun be above their horizon. An eclipse of the sun always begins in the western, and ends on the eastern side ; because the moon mov- ing in her orbit from west to east, necessarily first ar- 100 ECLIPSES OF THE SUN. rives at and touches the sun's western limb, and goe\ off at the eastern. It is not necessary, in order to constitute a central eclipse of the sun, that the moon should be exactly in the line of the nodes, at the time of its conjunction ; for, it is sufficient to denominate an eclipse of the sun central, that the centre of the moon be directly between the centre of the sun and the eye of the spectator ; for to him, the sun is then centrally eclipsed. But, as the shadow of the moon can cover only a small portion of the earth, it is obvious this may happen when the moon is not in one of her nodes. Further, the sun may be eclipsed centrally, totally, partially, and not at all, at the same time, to different parts of the earth. A total eclipse of the sun is a very curious spectacle : Clavius says that, in that which he observed in Portu- gal, in .1650, the obscurity was greater, or more sen- sible than that of the night : the largest stars made their appearance for about a minute or two, and the birds were so terrified, that they fell to the ground. Thus in plate 10, fig. 3, let A B C be the sun, M N the moon, fi 1 g part of the cone of the moon's shadow ; f d the penumbra of the moon : from this figure it is easy to perceive, 1. That those parts of the earth that are within the circle represented by g h, are covered by the shadow of the moon, so that no rays can come from any part of the sun into that circle, on account of the interposition of the moon, 2. In those parts of the earth where the penumbra falls, only part of the sun is visible ; thus between d and g, the parts of the sun near C cannot be seen, the rays coming from thence towards d or g being intercepted by the moon : whereas at the same time the parts be- tween f and h are illuminated by rays coming from C, bat are deprived by the moon of such as come from A. 3. The nearer any part of the earth, within the pe- numbra, is to the shadow of the moon, as in places near g, 1, or h, the less is the portion of the sun visi- ble to its inhabitants ; the nearer it is to the outside of the penumbra, as towards d, e, or f, the greater is the portion seen. ECLIPSES OF THE SUN. 101 4. Out of the penumbra, the entire disk is visible. The quantity of a solar eclipse in general is according to the size of the moon's shade projected upon the earth ; this shade is largest when the earth is in aphe- lion, the moon in perigee. The quantity of a solar eclipse to those within the line, which the centre of the moon's shade describes upon the earth, depends upon the apparent diameters of the sun and moon ; if they be exactly equal, the eclipse will be barely total \ if the diameter of the moon be greater than that of the sun, it will be more than total ; if the diameter of the moon's shade be less than the sun's, the eclipse will be annular, /. e. the sun's disk will not be entirely covered, but there will be a ring of his light visible round the disk of the moon. Eclipses may be also total or annular, in places a little distant from the way of the centre of the shade, but not central. More than total eclipses appear great- est in those places which are nearest the path of the centre of the shadow. Partial eclipses appear greatest in those places which are nearest the way of the moon's shadow upon the earth. The quantity of a solar eclipse in any place is estimated by the number of digits of the sun's diameter covered by the disk of the moon, in the middle of the eclipse : in an eclipse barely total, the sun is eclipsed 1 2 digits : when the eclipse is more than total, he is eclipsed so much more than 12 digits, as the distance between the limbs of the disks of the sun and moon amounts to in those points where those limbs are nearest to each other. The shape of the moon's shadow projected upon the earth in the middle of the eclipse, depends upon the moon's distance from her node. If the moon be in her node, the centres of the sun, moon, and earth, are all in a straight line, which is perpendicular to the spherical surface of the earth, and therefore the projection of the moon's shadow upon the disk of the earth will be a circle. When the moon has latitude, the axis of her shadowy cone makes an oblique angle with the spherical surface of the earth, and therefore the projection of the shadow upon the earth's disk will be an ellipsis, 1 whose excentri- city will be greater, the greater the moon's latitude is. 102 .ECLIPSES OF THE SUN. The largeness of the moon's shade projected upon the earth, depends upon the following circumstances : the conical shadow of the moon is longer, and similar sec- tions at equal distances from the moon are larger, the greater the moon's distance is from the sun. There- fore, the projection of the moon's shadow upon the earth is largest, when the earth is in aphelion and the moon in perigee ; least, when the earth is in perihelion and the moon in apogee, at the same time. In a solar eclipse that is central and barely total, the vertex of the moon's shadow does but just reach the surface of the earth ; in an annular eclipse, the conical shadow of the moon does not reach so far as the earth. The way of the moon's shadow upon the earth is gene- rally from west to east ; inclining towards the north pole, when the moon is near her ascending node ; towards the south pole, when she is near her descending node. The way of the shadow upon the earth may sometimes, but rarely, be from east to west, and the sun appear to be eclipsed first near his eastern limb. The way of the centre of the moon's shadow is a straight line, only when it describes a diameter upon the earth's disk; otherwise it is an elliptic curve, but so near a straight line, that it may without any sensible error be repre- sented by one. The duration of solar eclipses depends on the follow- ing circumstances. If the moon be in her node, the centre of her shadow passes over the centre of the earth's enlightened disk, and describes a diameter, u e. the longest line which can be taken in a circle ; if the moon have latitude, the centre of her shadow describes a chord in the circular disk of the earth, /. e. a line less than a diameter. The whole time the penumbra of the moon is passing over the disk of the earth, is called the time of the general eclipse ; because all that time the sun appears eclipsed in some place of the earth or other. The be- ginning of the general eclipse is when the moon's pe- numbra enters upon the disk of the earth ; the end, when the penumbra of the moon leaves it. The dura- tion of the general eclipse depends upon the length of the line described upon the earth's disk by the centre ECLIPSES OF THE SUN. 103 of the shadow, the velocity of tihe moon's motion from the sun, and the largeness and shape of the projection of the shadow and penumbra. The beginning of the solar eclipse in any place upon the earth, is when the penumbra which surrounds the moon's shadow first touches the place ; the end of the eclipse is when the penumbra leaves the place ; the du- ration of the eclipse is while the penumbra passes over the place. In any place upon the earth where the eclipse is more than total, the beginning of the total darkness is when the shadow of the moon first touch- es the place ; the end, when it leaves it. Eclipses more than total are said to be total with stay ; the time of stay, viz. of total darkness, in any place, is the time of the shadow passing over that place. The time the shadow is in passing over any place, (and the same is true of .the penumbra, which is always similar to the shadow) is variable, 1st, from the velo- city of the moon's motion from the sun ; 2d, from the length of a shadow measured in a line parallel to the way of the shadow, and drawn through the place ; Sd> from the proximity of the place to the centre of the earth's disk. The circumference of the moon's orbit is 60 times as great as the circumference of the earth ; and therefore^ each degree of the moon's orbit is equal to 60 degrees of a great circle on the earth's surface. And as one degree of such a circle on the earth contains 69^ Eng- lish miles, a degree of the moon's orbit contains 4155 miles. The moon's motion in her orbit, considering it as from the sun to the sun again, or from change to change, is through all the 360 degree thereof in 29- days ; and therefore she moves about half a degree, or 2077 miles from the sun in one hour ; and with the same velocity her shadow moves over the earth, name- ly, at the rate of 34^ miles in a minute ; which is more than four times as swift as the motion of a cannon-ball. The moon goes round the earth, not in a circular, but in an elliptical orbit ; and the earth's centre is in one of the foci of that orbit. Hence the moon's dis- tance from the earth is continually varying : at a mean it is 240,00Q miles. 104 THE LIMITS OF ECLIPSES. When the moon changes at her least distance from the earth, her dark shadow may cover a spot 1 TO miles broad on the earth's surface, if the time be about noon ; but much more if the time be in the morning or even- ing : and to all who are within that spot, the sun will appear to be totally eclipsed ; but to no place without it, although he will be partially eclipsed to several hun- dred miles around. But, as the moon's motion is then the swiftest that it can be, the dark shadow will be car- ried quite over that spot in five minutes at most, al- though the diurnal motion of the earth is the same way the moon's shadow goes ; and therefore the longest du- ration of a total eclipse of the sun can never be more than five minutes, even if it happen at noon. In the morning and evening, the earth's motion contributes very little toward the duration of a solar eclipse, because the dark shadow falls so obliquely on the earth ; and indeed, in such an eclipse, the darkness will be over in less than five minutes, although the shadow then ^covers more of the earth's surface than it can do about noon. When the moon changes at her mean distance from the earth, the point of her dark shadow does but just reach the earth : and, to the places where it goes suc- cessively over, the sun will be totally eclipsed only for an instant of time. When the moon changes at her greatest distance from the earth, her dark shadow does not reach the earth at all : and therefore the sun is not then totally hid from any part of the earth, but appears like a luminous ring, all round the dark body of the moon, to each part of the earth where the point of her shadow is successively di- rected it while she is passing between the earth and the sun. Thus it is plain, that the sun can never be eclipsed but at the time of new moon, nor the moon but when she is full. OF THE LIMITS OF SOLAR AND LUNAR ECLIPSES. The earth goes round the sun every year in an orbit called the ecliptic ; and therefore the sun, as seen from the earth, appears to go round the ecliptic once a year. THE PERIOD OF ECLIPSES. 105 If the moon's orbit lay quite even with the ecliptic, or, as it is commonly expressed, in the plane of the ecliptic, the sun would be eclipsed at the time of every new moon, because the moon would then be di- rectly between the earth and the sun : and the moon would be eclipsed at every time she was full, because the earth would then be directly between her and the sun. But one half of the moon's orbit lies on the north side of the plane of the ecliptic, and the other half on the south side of it. Therefore the moon's orbit in- tersects the plane of the ecliptic only in two opposite points, which are called the moon's nodes. The angle which the moon's orbit makes with the plane of the ecliptic is 5 degrees, 18 minutes ; so that, when the moon is in the northmost point of her orbit, she is 5 degrees, 1 8 minutes, north of the ecliptic ; and as far south of it, when she is in the southmost point of her orbit. Hence it is plain, that the moon can never be in the ecliptic but when she is in one or other of her nodes. When the moon is any more than 1 8 degrees from either of her nodes, at the time of her change, she does not pass between the sun and any part of the earth ; but goes either above or below the sun, according as she is then north or south of the ecliptic ; and there- fore she cannot then hide any part of the sun from any part of the earth. But when she changes within 18 degrees of either node, she will hide the whole or part of the sun from some part of the earth. And if she be in either of her nodes at the time of change, the sun will be centrally eclipsed to that point of the earth's surface which is then in a straight line between the sun's centre and the earth's. At all other places, which the centre of the moon's shadow goes over, the sun will likewise be centrally eclipsed. When the moon is any more than 12 degrees from either of her nodes at the time of full, she passes clear of the earth's shadow ; and therefore she cannot be eclipsed at that time. But when she is within 12 de- grees of either node, at the time of her being full, she vol. iv. p 106 THE PERIOD OF ECLIPSES. is eclipsed. And when she is full in either of her nodes, she goes through the middle of the earth's shadow, and is totally eclipsed with the longest continuance, which may be above an hour and a half. OF THE PERIOD OF ECLIPSES. The ecliptic is divided into twelve equal parts, called signs ^ and each sign into 30 equal parts, called degrees. If the moon's nodes had no motion through the signs of the ecliptic, there would be just half a year between the times of the sun's conjunctions with the nodes ; and then, in whatever signs the sun and moon were eclipsed in any given year, they would be eclipsed every year after. But the eclipses fall so much sooner every succeeding year than they did on the year before, as to prove, that the nodes move backward, or contrary to the motion of the moon, 19| degrees every year, from the consequent towards the antecedent signs. And, therefore, they go backwards through all the signs and degrees of the ecliptic in 1 8 years and 225 days. If, in that time, there were any exact number of courses of the moon from change to change, without any fraction, there would be an exact period or restitu- tion of eclipses in the same time. But, during this re- volution of the nodes, there are 230 courses of the moon, and a quarter of a course more : so that there can be no exact period of eclipses in any complete revolution of the nodes. But in 1 8 years, 1 1 days, 7 hours, and 43y minutes, in which time there are just 223 courses of the moon, from change to change, there is a conjunction of the sun and moon with the same node as before ; and, con- sequently, a period or restitution of all the eclipses of the sun and moon. And, therefore, if to the mean time of any eclipse, either of the sun or moon, you add 18 years, 1 1 days, 7 hours, 4>3j minutes, you will have the mean time of the return of that eclipse. Only note, that when the last day of February, in leap-year, comes but four times into this period, you are to add the abovs SUPERNATURAL DARKNESS, &C. 107 number of days, hours, and minutes: but when it comes five times, as it will sometimes do, you must add one whole day less. And thus, any one, who has a set of al- manacks for 1 9 years, in which all the eclipses are not- ed for that time, may very easily calculate the time of any future eclipse. This is called the Chaldean Saros, or period of eclipses. As the nodes go backwards at the rate of 1 9 T de- grees every year, which, for the sake of round numbers, we may call 19 degrees ; these 19 degrees are nearly equal to 19 days of the sun's motion, and the half of of 19 is 9i; subtract 9l days from 182^ days, which make half a year, and there will remain 173 days for the time between the sun's being in conjunction with either of the nodes till the time of his being so with the other. Now, as the sun can never be eclipsed when he is more than 1 8 degrees from either node, nor the moon, when she is more than 12, as already mentioned, it is plain, there must be an eclipse of the sun at the time of every new moon that falls within 18 days before or after the time of his being in conjunction with either of the nodes ; and that the moon must be eclipsed at every time of her being full within 12 days before or after the time of the sun's being in conjunction with either of the nodes. And, consequently, if we can tell on what days of the year these conjunctions fall, we can easily tell at what new and full moons there must be eclipses ; seeing the days of new and full moons are so generally known. In some years there are six eclipses, four of which are of the sun, and two of the moon : in other years there are only two, and when that happens, they are both of the sun : but the most common number is four ; name- ly, two of the sun, and two of the moon. THE DARKNESS AT OUR SAVIOUR'S CRUCIFICTION, SUPERNATURAL. From the account I have given you of eclipses it plain- ly appears that the sun can never be eclipsed, in a natu- ral way, but at the time of new moon, nor the moon bufc 108 SUPERNATURAL DARKNESS AT when she is full ; and that, when the sun is totally eclipsed, the darkness can never continue above five mi- nutes at any place on the earth. But the three evangelists, St. Matthew, St. Mark, and St. Luke, mention a darkness that continued three hours, at the time of our Saviour's crucifixion. If their account of that darkness had been false, it would have been contradicted by many who were then present ; es- pecially as they were great enemies both to Christ and his few disciples, as well as to the doctrine he taught. But as none of the Jews have contradicted the evangel- ists' account of this most extraordinary phenomenon, it is plain that their account of it is true. Besides, the evangelists must have known full well, that it could not be their interest to palm such a lie upon mankind ; which, when detected, must have gone a great way to- wards destroying the credibility of all the rest of the ac* count they gave of the life, actions, and doctrine of their master : and, instead of forwarding the belief of Christi- anity, it would have been a blow at the very root thereof. We do not find that they have bestowed any panegyric on the life and actions of Christ, or thrown out an in- vective against his cruel persecutors ; but, in the most plain, simple, and artless manner, have told us what their senses convinced them were matters of fact : so that we have as good reason to believe, that there was such dark- ness, as we have to believe that Christ was then upon the earth : and that he was, has never been contradict- ed, even by the Jews themselves. But there are other accounts of Christ, besides those which the evangelists have left us. It is expressly af- firmed, by the two Roman historians, Tacitus and Sue- tonis, that there was a general expectation spread all over the eastern nations, that out of Judea should arise a per- son who should be governour of the world. That there lived in Judea, at the time which the gospel relates, such a person as Jesus of Nazareth, is acknowledged by all authors, both Jewish and Pagan, who have written since that time. The star that appeared at his birth, and the journey of the Chaldean wise men, is mentioned by Chal- cidius the Platonist. Herod's causing the children in our saviour's crucifixion. 109 Bethlehem to be slain, and a reflexion upon him on that occasion by the emperor Augustus, is related by Macro* bius. Many of the miracles that Jesus wrought, parti- cularly his healing the lame, and curing the blind, and casting out devils, are owned by these inveterate and im- placable enemies of Christianity, Celsus and Julian, and the author's of the Jewish Talmud. That the power of the heathen gods ceased, after the coming of Christ, is acknowledged by Porphyry, who attributed it to their being angry at the setting up of the christian religion, which he calls impious and profane. The crucifixion of Christ under Pontius Pilate is related by Tacitus, and the earthquake and miraculous darkness attending it were recorded in the Roman public registers, commonly ap- pealed to by the first christian writers, as what could not be denied by the adversaries themselves ; and are in a particular manner attested by Pblegon, the freed man of Adrian. Some people have said, that the above mentioned darkness might have been occasioned by a natural eclipse oi the sun ; and, consequently, that there was nothing miraculous in it. If this had been the case, it is plain, that our Saviour must have been crucified at the time of new moon. But then, in a natural way, the darkness could not possibly have continued for more than five minutes : whereas, to have made it con- tinue for three hours, the moon's motion in her orbit must have been stopped for three hours, and the earth's morion on its axis must have been stopped as long too. And then, if the power of gravitation had not been suspended during all that time, the moon would have fallen a great way towards the earth. So that nothing less than a triple miracle must have been wrought to have caused such a long continued darkness by the in- terposition of the moon between the sun and any part of the earth : which shows, that they who make such a supposition are entirely ignorant of the nature of eclipses. But there couid be no natural or regular eclipse of the sun on the day of Christ's crucifixion, as the moon was full on that day, and, consequently, in the side of the heavens opposite to the sun. And, 110 SUPERNATURAL DARKNESS, &C. therefore, the darkness at the time of his crucifixion was quite supernatural. The Israelites reckoned their months by the course of the moon, and their years, after they left Egypt, by the revolution of the sun, computed from the equal day and night in the spring to the like time again. For we find, they were told by the Almighty, Exod. xii. 2, that the month Abib, or Nisan, should be to them the first month of the year. This was the month in which they were delivered from their Egyptian bondage, and includes part of March and part of April in our way of reckoning. In several places of the Old Testament, we find, that the Israelites were strictly commanded to kill the pas- chal lamb on the evening, or, as it is in the Hebrew, between the evenings, of the fourteenth day of the first month ; and Josephus expressly says, " The passover was kept on the fourteenth day of the month Nisan, according to the moon, when the sun was in Aries." And the sun always enters the sign Aries, when the day and night are equal in the spring season. They began each month on the first day of the moon's being visible, which could not be in less than twenty-four hours after the time of her change ; and the moon is full on the fifteenth day reckoned from the time of change. Hence, the fourteenth day of the month, according to the Israelites' way of reckoning, was the day of full moon : which makes it plain, that the passover was always kept on a full-moon day ; and at the time of the full moon next after the equal day and night in the spring ; or, when the sun was in Aries. All the four evangelists assure us, that our Saviour was crucified at the time of the passover : and hence it is plain, that the crucifixion was at the time of full moon, when it is impossible that the moon could hide the sun from any part of the earth. St. John tells us, that Christ was crucified on the day that the passover was to be eaten ; and we likewise find, that some re- monstrated against his being crucified " on the feast day, lest it should cause an uproar among the people."* * Ferguson's Astronomical Lecture on Eclipse?. [ in 3 LECTURE XXXIX. OF PARALLAX AND REFRACTION, AND THE ABERRATION OF LIGHT, &C. ASTRONOMY is subject to many difficulties, besides those which are obvious to every eye. When we look at any star in the heavens, we do not see it in its real place ; the rays coming from it, when they pass out of the purer etherial medium, into our coarser and more dense atmosphere, are refracted, or bent in such a manner, as to show the star higher than it really is. Hence we see all the stars before they rise, and after they set ; and never, perhaps, see any one in its true place in the heavens. There is another difference in the ap- parent situation of the heavenly bodies, which arises from the station in which an observer views them- This difference in situation is called the parallax of an object. OF REFRACTION. As one of the principal objects of astronomy is to fix the situation of the several heavenly bodies, it is neces- sary, as a first step, to understand the causes which oc- casion a false appearance of the place of those objects, and make us suppose them in a different situation from that which they really have. Among these causes re- fraction is to be reckoned. By this term is meant the bending of the rays of light as they pass out of one me- dium into another. The earth is every where surrounded by a hetero- geneous fluid, a mixture of air, vapour, and terrestrial 112 OF REFRACTION. exhalations, that extend to the regions of the sky. The rays of light from the sun, moon, and stars, in passing to a spectator on the earth, come through this medium, and are so refracted in their passage through it, that their apparent altitude is greater than their true altitude. Let A C, plate 7, Jig. 3, represent the surface of the earth, T its centre, B P a part of the atmosphere, H E K the sphere of the fixed stars, A F the sensible horizon, G a planet, G D a ray of light proceeding from the planet to D, where it enters our atmosphere, and is re- fracted towards the line D T, which is perpendicular to the surface of the atmosphere ; and as the upper air is rarer than that near the earth, the ray is continually en- tering a denser medium, and is every moment bent to- wards T, which causes it to describe a curve as D A, and to enter a spectator's eye at A, as if it came from E, a point above G. And as an object always appears in that line in which it enters the eye, the planet will appear at E, higher than its true place, and frequently above the horizon A F, when its true place is below it at G. This refraction is greatest at the horizon, and de- creases very fast as the altitude increases, insomuch that the refraction at the horizon differs from the refraction at a very few degrees above the horizon, by about one third part of the whole quantity. At the horizon, in this climate, it is found to be about 33 minutes. In climates nearer to the equator, where the air is purer, the refraction is less ; and in the colder climates, nearer to the pole, it increases exceedingly, and is a happy provision for lengthening the appearance of the light at those regions so remote from the sun. Gassendus re- lates, that some Hollanders, who wintered in Nova Zembla, in latitude 75 degrees, were agreeably surpriz- ed with a sight of the sun seventeen days before they expected him in the horizon. This difference was owing to the refraction of the atmosphere in that lati- tude. To the same cause, together with the peculiar obliquity of the moon's orbit to the ecliptic, some of these very northern regions are indebted for an unin- terrupted light from the moon much more than half Of PARALLAX* 113 the month, and sometimes almost as long as it is capa- ble of affording any light to other parts of the earth. Through this refraction we are favoured with the » sight of the sun, about 3 l T minutes before it rises above the horizon ; and also as much every evening after it sets below it, which in one year amounts to more than 40 hours. It is to this property of refraction that we are also in- debted for that enjoyment of light from the sun, when he is below the horizon, which produces the morning and evening twilight. The sun's rays, in falling upon the higher part of the atmosphere, are reflected back to our eyes, and form a faint light, which gradually augments till it becomes day. It is owing to this, that the sun illu- minates the whole hemisphere at once ; deprived of the atmosphere, he would have yielded no light, but when our eyes were directed towards him ; and even when he was in meridian splendour, the heavens would have ap- peared dark, and as full of stars as on a fine winter's night. The rays of light would have come to us in straight lines, and the appearance and disappearance of the sun w r ould have been instantaneous ; we should have had a sudden transition from the brightest sun- shine to the most pro- found darkness, and from thick darkness to a blaze of light. Thus, by refraction, we are prepared gradually for the light of the sun, the duration of its light is prolonged, and the shades of darkness softened. To it we must also attribute another curious phenome- non, mentioned by Pliny; for he relates, that the moon had been eclipsed once in the west, at the same time that the sun appeared above the horizon in the east. Masti- Hnusy in Kepler, speaks of another instance of the same kind, which fell under his own observation. OF PARALLAX. The parallax of a celestial object is the difference be- tween the places that the object is referred to in the ce- lestial sphere, when seen at the same time from two dif- ferent places within that sphere. Or, it may be considered as the angle under which any two places in the inferior VOL. I\. q 1H OF PARALLAX. orbits are seen, from a superior planet, or from a fixed star. The parallaxes principally used by astronomy, are those which arise from considering the object as viewed either from the centre of the earth and the sun, or from the surface and centre of the earth, or from all three compounded. t The difference between the place of any planet as seen from the sun, and that of the same planet as seen from the earth, is called the parallax of the annual orbit; in other words, it is the angle at any planet subtended be- tween the sun and the earth. The diurnal parallax is the change of a celestial body's apparent place, arising from its being viewed from two different stations, one on the surface, and the other at the centre of the earth. The necessity of this distinction is obvious, for you know, that an object will change its apparent situation with respect to another, according to the station from which it is viewed ; hence celestial objects, viewed from different parts of the earth's surface, will appear in dif- ferent situations. To facilitate and give certainty to cal- culation, astronomers refer all celestial appearances to the centre of the earth ; of course they are obliged continu- ally to calculate parallaxes, in order to reduce the ob- served places of the objects to that where they would be situate, if seen from the centre of the earth. Let a line, A B, plate 7, jig* 4, be drawn perpendicu- lar to the distance B C, between an adjacent object C, and any given station B : the apparent places of the object, when viewed from the extremities of the line A B, will be different. 1. The perpendicular line, A B, is called the base* *?. The extremities, A, B, of the base are called stations. 3. The angle A C B, subtended by the extremities of the base at the object, is called the angle of the parallax. 4. The base is to the lesser of the two diftances of the object from the extremities of the base, as the tangent of the angle of parallax to radius ; and to the greater, as the sine of the same angle to radius. Suppose lines to be drawn from the two stations to an object : one of the angles contained by these lines, as in OF PARALLAX. 1 1,5 the figure, being a right angle, the other will be the com- plement of the parallax to 90 degrees. If the angles at the stations terminating a given base be known, it is easy, by trigonometry, to determine the dis- tance of the object. N. B. We here suppose one of the angles at the base to be 90 degrees. When the distance of an object is greater than 100,000 times the base, the angles at the two stations will not sen- sibly differ from two right ones ; and, consequently, the lines drawn from the object to the stations, are, physi- cally speaking, parallel. Now the angle, whose tangent is to radius, as 1 to 100,000, is very little more than a second ; and the most accurate instruments constructed for the mensuration of angles can scarce be depended upon to 2 seconds. Hence, the parallax of an object, whose distance is above 100,000 times greater than that between the two stations of observation, is insensible. We may therefore conclude, that if the parallax of an object, observed with an instrument sufficiently exact to measure an angle of 2 seconds, be insensible, the distance of it from either station cannot be less than 100,000 times the base, from the extremities of which it is observed. But you are to observe, that although the distance of the object cannot be less than 100,000 times the base, it may be greater in any assignable ratio. Lines drawn from any given point in a base, to an object, may in practice be esteemed parallel, without sensible error, if the distance of the object be more than 100,000 times the base. ' Having laid down these few general principles, we may now proceed to explain the parallaxes used by astrono- mers, which are principally those which arise from consi- dering the object as viewed from the centre of the earth and sun, from the surface and the centre of the earth, and from these compounded. The diameter of the earth is the longest straight line we can accurately obtain, and is, in general, the base used for determining the distances of celestial objects by their parallaxes. The change in the apparent place of a planet, or fixed 116 OF PARALLAX. star, or any celestial body arising from its being viewed on the surface, or from the centre of the earth, is called its diurnal parallax. To explain the parallaxes with respect to the earth, I shall use the diagram, plate 7, Jig. 2, where H S W re- presents the earth ; T its centre ; ORG part of the moon's orbit ; P r g a part of the planet's orbit ; Z a A part of the starry heavens; ZS a line which passes through the zenith. Now it is plain, from the inspection of the diagram, that a planet, P, situate in the zenith-line, always an- swers to the same point of the heavens, whether it be re- garded from the centre T, or from the point, S, on the surface: so that a celestial body in the -zenith has no parallax. If the planet, instead of being in the zenith, be in the horizontal line S A, perpendicular to the line Z S, its dis- tance, T, from the centre of the earth is the same as its distance T P. But the place of the planet, seen from the centre of the earth, is at d, while its place seen from S, or the surface, is at A ; — the difference between these two situations is their parallax. Let us now compare these two points or situations with the point Z, where the planet is seen, when in the zenith of the observer. The c\ng\e Z Sg, formed by the vertical line 8 Z, and the line S A, in which the planet appears, is the apparent distance of the planet from the zenith : but if you were at the centre of the earth, the angle, Z Tg, would show the true distance from the zenith. The apparent distance, ZS g, is greater than the true distance Z Tg, in the right angled triangle gT S. Geo- metry proves, that the angle, Z Sg, is equal to the two angles S Tg, S g T. It is therefore greater than the angle S Tg, by the quantity Sg T. Thus the apparent distance of the planet from the zenith, is greater than the true dis- tance ; and the difference between these two angles SgT, is the parallactic angle, which is in this case called the ho- rizontal parallax, the line S T being the base. The parallax of a celestial body is then the angle formed at the centre of the body by two lines, one of which pro- ceeds to the centre of the earth, the other to its surface ; or it is the inclination of two lines which proceed the one OF PARALLAX, 117 from the centre, the other from the surface of the earth, to unite in the centre of the planet ; or still, in other words, it is the angle under which the semidiameter of the earth would appear, if seen from the centre of the planet. The triangle, T S £, is called the parallactic triangle ; it is always situate vertically, because the line, S T, is a vertical line ; thus the whole effect of parallax is made in a vertical circle ; indeed, as the centre of the earth is under your feet, it is in the plane of all the vertical cir- cles. Therefore, parallax is always reckoned on these circles, making the object appear lower, but never to the right or left of a vertical circle ; consequently, the paral- lax does not change the azimuth of a planet. I have hitherto spoken only of the parallax when the planet is in the horizon, that is, when Z S g is a right angle, and I have called this the horizontal parallax. If the planet be nearer the zenith, as at r, the parallactic angle becomes smaller, and is called the parallax of alti- tude. It is evident by the diagram, that the horizontal parallax is the greatest of all, and that as the planet rises above the horizon, it gradually diminishes until it come to the zenith, where it vanishes, or becomes equal to no- thing. Thus the parallax, A G D, of the object G, is greater than the parallax, a R B, of the same object when at R ; but when it is at O, in the zenith, there is no pa- rallax. The parallax of a planet is smaller in proportion as it is more distant ; for the nearer g is to S, the greater is the angle S g T ; hence mathematicians prove, that when the altitudes are the same, the parallax of altitude is in the inverse ratio of the distance. The horizontal parallax of the moon, which exceeds that of all the other planets, scarce ever amounts to a de- gree. The parallax of a planet increases also with its appa- rent diameter ; in fact, the farther a planet is off, the less is its apparent diameter, and the diameter diminishes like the parallax in an inverse ratio of the distance ; therefore the parallax is as the diameter. If the parallax were les- sened one half, the diameter would also be one half less ; and the same relation subsists, whatever be the distance. 118 OF PARALLAX. Thus, the apparent diameter of the moon is always A. of its parallax, and the cube of this fraction, marks its size with respect to the earth. When the horizontal parallax of a celestial object is known, it is easy to discover, by the rules of trigonome- try, the distance of the object ; for in the right-angled tri- angle S T G, you have the semidiameter of the earth, ST, known, the angle, S T D, 90 degrees, and the parallactic angle, TGS, given, from whence it is easy to obtain the rest. It is, indeed, difficult to determine the hori- zontal parallax with accuracy, on account of the effects of refraction. But the parallax of an object at any al- titude being observed, its horizontal parallax may be computed. The diurnal parallax of an object according to the different situation of the ecliptic and equator in respect to the zenith, will sometimes cause an apparent change or parallax'of the latitude, longitude, declination, and right ascension thereof. In finding the parallax of the sun, or, which is the same thing, the angle under which the earth's semidia- meter would appear at that distance, the angle is so very small, that a mistake of one second would occasion an error of about seven millions of miles in the distance ; from whence you may judge of the exactness necessary in finding the parallax of any celestial object. The annual parallax is the change in the apparent place of an object, which is caused by its being viewed from the earth in different parts of its orbit. The annual parallax of all the planets is very consi- derable, that of the fixed stars insensible. The sun's parallax being so small as to be scarcely Sensible to the best observers, when using the most ac- curate instruments, various indirect methods have been proposed : of these, that suggested by Dr. Halley is al- lowed to be the most perfect. It was to observe the transit or passage of Venus over the sun's disk ; a phe- nomenon which happened in the years 1761 and 1769, and by which this difficult problem was resolved with an accuracy unlooked for by astronomers of ancient times. t 119 ] OF THE APPARENT MOTION OF THE FIXED STARS, OCCASIONED BY THE ABERRATION OF LIGHT. The astronomers of the last century, in their endea- vours to discover the parallax of the fixed stars, found annual variations in the stars, following a law contrary to what would have happened, had it arisen merely from the earth's situation in its orbit. These variations threw them into great perplexity, from which they were not relieved till Dr. Bradley, re- applying himself to observe accurately these variations, at last discovered the true cause thereof; and has given rules for calculating the changes, and shown what al- lowances are to be made in consequence thereof, in ob- servations of the stars. He has also proved clearly, that this aberration of the fixed stars, or the motion which makes them appear to describe ellipses of 40 seconds diameter, arises from the motion of light combined with the annual motion of the earth. This I shall now endeavour to explain, and place in as clear a point of view as possible, desiring you only to recol- lect the idea of the decomposition of forces into parallelo- grams, as explained in our Lectures on Mechanics. Let E, plate 15, fig. 1, be a star darting a ray of light, which I shall consider here as a single particle, going from E to B. Let A B be a small portion of the earth's orbit, of 20 seconds, for example ; and C B the space that the ray of light has passed through, while the earth moved from A to B ; thus, the particle was at C, when the earth was at A, and arrives at B the same time as the earth. Hence C B and A B express the velocity of light and the earth respectively during 20 seconds. Draw C D parallel to A B, and finish the parallelo- gram DBA; now, according to the known principle of the composition and decomposition of forces, we may consider the velocity, E B, of the light, as resulcing from the two velocities in the directions CD, CA ; the velo- city, C D, being the same in quantity and direction as the velocity, A D, of the earth, cannot be perceived, and 120 OF THE APPARENT MOTION is therefore destroyed with respect to us ; since the eye cannot see by a ray moving in the same direction and with the same velocity as the eye itself. So that the part, C A, only of the velocity of the light will subsist to us, and the ray coming to the eye in the direction C A, we shall perceive the star in the line A C, or, ac- cording to B D, which is parallel thereto ; the angle, C BD, is that termed the aberration; it is the quantity that a star appears out of its true place, in consequence of the motion of light and the earth. Perhaps another way of considering this may render it more clear to your apprehension. Suppose a tube to be erected perpendicular to the horizon at a time when it rains, the drops to fall in a perpendicular direction, and the tube to be of such a diameter as to admit but one drop at a time ; now it is plain, that if a drop of water enter the orifice of the tube, it will fall down without touching the sides. But if the tube be moved along, still preserving its perpendicular direction, any drop that enters the tube will strike against the sides, and none could pass freely through while the tube is. in motion, unless the tube have such a direction as will compensate the motion. Thus, let AB, plate 15, fig. 2, represent the hori- zon, C D the perpendicular tube, and G D the course of a drop of rain ; then, if C D be moved towards A, while the drop is falling within the tube, it is evident, that the inner surface of the tube, which is situate towards B, will be carried against the drop, and prevent its arriv- ing at the bottom without touching. But if the inclined tube be moved with a similar motion to that of the drop from E to D, in the same time that the drop moves from C to D, the lower orifice of the tube and the drop will be found at the same instant at D, and the velocity of the drop will be expressed by C D, and that of the tube by E D. The same reasoning holds good, if instead of drops of rain we suppose particles of light, and a telescope instead of a tube. For to an observer, who, through the tube C D, views the vastly distant object C, if the motion of light be instantaneous or infinitely swift, no finite mo- OF THE FIXED STARS. 121 tion of C D, its position being unaltered, can prevent its being visible ; because by the supposition the light, which enters at C, will arrive at D before C D can have moved at all. But if light be propagated in time, and the observer be carried by a motion similar as to acceleration to that of light, the tube must be inclined in an angle, whose sine is to the sine of C E D, as the velocity of the ob- server is to the velocity of light. By this theory, which is established by numerous ob- servations of stars of different magnitudes and situa- tions, it appears, that the small apparent motion, which the fixed stars have about their real places, which is called their aberration, arises from the proportion which the velocity of the earth's motion in her orbit bears to that of light. This proportion is found to be as 10210 to 1 ; from whence it follows, that light moves or is propagated from the sun to the earth, in 8 minutes, 12 seconds. This discovery of the aberration of light by Dr. Bradley ', is a direct proof of the motjion of the earth in its orbit. The motion of light, combined with the motion of the earth, produces an apparent difference in the places of the fixed stars ; and as this motion is found to affect all the stars differently, according to their situ- ations, it fully proves the truth of the cause upon which they were supposed to depend, and shows that the Co- pernican system is conformable to the nature and order of things. OF THE PRECESSION OF THE EQUINOXES. The stars, which compose the constellations, are found to increase their longitude continually. The whole starry firmament appears to have a slow motion, from west to east, about the poles of the ecliptic, so that the constellations seem to have deserted the places first ap- propriated to them ; insomuch that the first star in the constellation of Aries, which appeared in the vernal in- VOL. iv.' "~~ ' R *22 PRECESSION OF THE EQUINOXES. tersection of the equator and ecliptic in the time of Me- ton, the Athenian, upwards of 1900 years ago, is now removed above SO degrees from that point ; so that Aries is now where Taurus was, Taurus where Gemini was, &c. The discovery of this motion is due to Hip- parchus of Rhodes, one of the most celebrated astrono- mers of ancient times. Hence the constellations on the zodiac of a celestial globe, do not agree in figure and character, the signs or constellations of the zodiac being to the east of those signs, or arcs of the ecliptic, which are called by the same names ; for, in order to avoid confusion, astrono- mers thought proper to let the several portions of the ecliptic, where those constellations were first observed to be, retain their old names, consequently, the vernal equi- nox is still considered as the first point of Aries. The spaces formerly occupied by the zodiacal con- stellations, retaining their ancient names, are called ana* stra, or without their former stars; whereas the spaces they now possess are called stellata. This slow motion of the stars forwards, is really caus- ed by a like slow motion of the equinoxial points back- wards; and this is owing to the revolution of the axis of the equator about the axis of the ecliptic ; the pole of the equator describing in the heavens a circle about the pole of the ecliptic. By this precession of the equinoxial points from east to west, they meet the sun every year 50 seconds of longitude before a complete revolution has been made. The time, in which the sun appears to revolve from tro- pic to tropic, is called a tropical year ; this, with the time he has yet further to go to complete the revolution, namely, 50 seconds, is called the siderial year. Sir Isaac Newton attributes this motion to the spheroidal figure of the earth, deducing from this figure the revolution of the poles of the world round those of the ecliptic. This motion carries the stars about 1 degree, 20 mi- nutes, 23 seconds, in 100 years ; so that the total revo- lution of the fixed stars eastward, back to the equinoc- tial points again, will be completed in 25972 years. C 123 ] LECTURE XLIL OF SOLAR AND SIDERIAL DAYS ; OF MEAN TIME ; THE EQUATION OF TIME, &C. IHE rotation of the earth about its axis being uni- form, it necessarily follows, that the apparent diurnal revolution of the stars about the earth must be also uni- form, that is, made in equal times ; they therefore will form a very proper measure to denote time. But then, as they turn successively with a constant motion, one must be selected, by whose revolutions time may be measured ; we must also fix a term from whence to commence our reckoning. The sun being the most conspicuous object, was fixed upon by the astronomers of early ages, as the most pro- per measure for the parts of time. But when more ac- curate observations were made, the sun's motion was found not to be uniform, and consequently the time measured thereby would be neither regular nor equal ; they were therefore obliged to find out a mean or regular time for the basis of their calculations. An astronomical or solar day is divided into 24 hours, reckoning them in numeral succession, from 1 to 24. The first twelve hours are sometimes distinguished by the mark P. M. for after noon ; the other twelve are distinguished by A. M. for before noon. Astronomers generally reckon through the 24 hours from noon to noon ; and what is by the common way of reckoning called morning hours, is by them reckoned in succes- 124 SOLAR AND SIDERIAL DAYS. sion from noon to midnight. Thus 5 o'clock in the morning of April the 10th, is by astronomers called April 9, 17 hours. If the sun had no other apparent motion but that of its diurnal revolution, it would every day describe the same parallel, and be accompanied by the same stars. But it has also an apparent annual motion, by which it seems to be carried through the zodiac every year, from west to east, that is, in a direction contrary to that of its diurnal revolution. Hence, if on any day the sun and a star pass the me- ridian at the same instant, on the next day when the star returns to the meridian, the sun will have departed towards the east, as much space as in that interval it has passed over by its annual motion, and will there- fore arrive at the meridian some moments after the star ; the day following it will be still later, so that at the end of six months, it passes 12 hours after the star, which has therefore gained 1 2 hours on the sun ; and at the end of the year the star will have passed 366 times over the meridian, while the sun has only passed 365 times. In this view we have considered the sun's apparent motion ; the result is the same if you consider the earth's real motion. If, indeed, the earth had no real motion, and consequently the sun no apparent motion, the length of a natural day would be about 23 hours 56 minutes, for in that time a revolution of the earth is completed, as appears by an easy observation ; for any fixed star that is on the meridian at a given hour of the night, will, after 23 hours 56 minutes, be on the meri- dian again the night following. This interval of time is called a siderial day. Thus you see that there is a distinction between a solar day and a siderial day. A solar or astronomical day is the space of time that intervenes between the sun's departing from any one meridian, and its return to the same again. The side- rial day is the space of time that elapses between the departure of a star from a given meridian, and its return to the same again. SOLAR AND SIDERIAL DAYS. 125 I shall now endeavour to show you, why these days differ in length ; that is, why the sun takes up more time to complete one revolution than a star. This difference arises from the sun's annual motion. The sun does not continue always in the same place in the heavens as the fixed stars do : but if it be seen at M, plate 4, jig. 2, one day, near the fixed star R, it will have shifted its place the next day, and will be near to some other fixed star L. This motion of the sun is from west to east, and one entire revolution is completed in a year. Suppose, therefore, that the sun, when it is at M, near to the fixed star, R, appears in the south of any particular place, S ; and then imagine the earth to turn once round upon its axis from west to east, or in the direction S T V W, so that the place may be returned to the same situation. After this rotation is completed, the star R, will be in the south of the place as before ; but the sun having, in the mean time, mov- ed eastwards, and being nearer to the star L, or to the east of R, will not be in the south of the place S, but to the eastward of it : upon this account the place, S, must move on a little farther, and must come to T be- fore it will be even with the sun again, or before the sun will appear exactly in the south. This may be illustrated by an instance : the two hands of a watch are close together, or even with one another at twelve ; they both turn round the same way, but the minute-hand turns round in a shorter time than the hour hand ; when the minute-hand has completed one rotation, and is come round to twelve, the hour- hand will be before it, or will be at one ; so that the minute-hand must move more than once round, in order to overtake the hour-hand, and be even with it again. As this subject is of some importance, we shall en- deavour to render it more clear, by placing it in a dif- ferent point of view. The diameter of the earth's orbit is but a physical point, in proportion to the distance of the stars ; for which reason, and the earth's uniform motion on its axis, any given meridian will revolve from any star to the same star again, in every absolute turn of the earth 126 SOLAR AND SIDERIAL DAYS. upon its axis, without the least perceptible difference of time being shown by a clock which goes exactly true. If the earth had only a diurnal, without an annual motion, any given meridian would revolve from the sun to the sun again, in the same quantity of time as from any star to the same star again ; because the sun would never change his place with respect to the stars. But, as the earth advances almost a degree eastward in its orbit, in the time that it turns eastward round its axis, whatever star passes over the meridian on any day with the sun, will pass over the same meridian on the next day, when the sun is almost a-degree short of it, that is, 3 minutes 56 seconds sooner. If the year contained only 360 days, the sun's apparent place, so far as his motion is equable, would change a degree every day, and then the siderial days would be just four minutes shorter than the solar. Let ABCDEFGH, plate 4, fig. 3, be the earth's orbit, in which it goes round the sun every year, ac- cording to the order of the letters, that is, from west to east, and turns round its axis the same way, from the sun to the sun again, in every 24 hours. Let S be the sun and R a fixed star, at such an immense distance, that the diameter, G C, of the earth's orbit bears no sensible proportion to that distance ; N m n, the earth is in different points of its orbit. Let N m be any particular meridian of the earth, and N a given point or place lying under that meridian. When the earth is at A, the sun S hides the star R, which would always be hid if the earth never moved from A ; and, consequently, as the earth turns round its axis, the point N would always come round to the sun and the star at the same time. . But, when the earth has advanced through an eighth part of its orbit, or from A to B, its motion round its axis will bring the point N an eight part of a day, or three hours, sooner to the star than to the sun. For the star will come to the meridian in the same time as though the earth had continued in its former situation at A, but the point N must revolve from N to n, before SOLAR AND SIDERIAL DAYS. 127 it can have the sun upon its meridian. The arc, N n, being therefore the same part of a whole circle, as the arc A B, it is plain, that any star which comes to the meridian at noon with the sun, when the earth is at A, will come to it at nine o'clock in the forenoon, when the earth is at B. When the earth has passed from A to C, one-fourth part of its orbit, the point, N, will have the star upon its meridian, or at six in the morning, six hours sooner than it comes round to the sun ; but the point, N, must revolve six hours more, before it has mid-day by the sun : for now the angle, A S C, is a right angle, and so is N C n ; that is, the earth has advanced 90 degrees on its axis, to carry the point N from the star to the sun ; for the star always comes to the meridian, when N m is parallel to R S A ; because C S is but a point in respect to R S. When the earth is at D, the star comes to the meridian at three in the morning at E, the earth having gone half round its orbit ; N points to the star at midnight, it being then directly opposite to the sun ; and, therefore, by the earth's diurnal motion, the star comes to the meridian twelve hours before the sun ; and then goes on, till at A it come to the meridian with the sun again. Thus it is plain, that one absolute revolution of the earth on its axis, which is always completed when any particular star comes to be parallel to its situation at any time of the day before, never brings the same meridian round from the sun to the sun again ; but that the earth requires as much more than one turn on its axis, to finish a natural day, as it has gone forward in that time, which at a mean, is a 365th part of a circle, that is 59 minutes 8 seconds ; for, as 365 days are to 1 day, so are 360 degrees to 59 minutes 8 seconds. Hence, in 365 days the earth turns 366 times round its axis, and consequently, as one revolution of the earth on its axis completes a siderial day, there must be one more siderial day in a year than there are solar days. L 128 ] OF MEAN AND APPARENT TIME. Further and more accurate observations showed, that the solar days were not equal to each other ; after inves- tigating this subject, astronomers were under the neces- sity of distinguishing two sorts of time, one they called apparent time, the other mean time. Apparent time is that reckoned since the sun's centre was last on the meridian of the place, and is that shown by a sun-dial, which marks the hours every day in such a manner, that every hour is a 24th part of the time be- tween the noon of that day and the noon of the day im- mediately following. Mean time is that shown by. a clock, which goes uni- formly. The time shown by a sun-dial, and the mean time, or that shown by a well-regulated clock, agree only four times in the year ; viz. on the 1 5th of April, the 1 6th of June, the 3 1st of August, and the 24th of December. The clock, if it go equably and true all the year round, will be before the sun from the 24th of Decem- ber to the 15th of April ; from that time, to the 16th of June, the sun will be before the clock ; from thence, to the 31st of August, the clock will be again before the sun ; and from the 31st of August to the 24th of December, the sun will be- faster than the clock. On any other day, if you would set a clock by a sun-dial, you must make use of an equation-table, which shows, for every day in the year, how many minutes and se- conds the sun is before or behind the clock : the dif- ference between the sun and the clock is called the equation of time.* Both the solar and mean days are divided into 24 hours, or 86400 seconds. Three hundred and sixty degrees of the equator pass under the meridian in a mean day, and 59 minutes 8 seconds, which is that part of 360 degrees of the sun's * See my pamphlet u Metnods of finding a true Meridian Line for placing Sun Dials, setting Clocks, &c. — E. Edit. THE EQUATION OF TIME. 12$ annual motion corresponding to the time of a mean day. In a solar or true day, the 360 degrees of the equator pass under the meridian and an arc thereof answering to the ecliptic arc described the same day, called the sun's motion in right ascension. When the sun is farthest from the earth or in apogee, his motion in right ascension in a day, is 1 degree, 2 minutes, 6 seconds ; therefore 361 degrees, 2 minutes, 6 seconds, pass the meridian in a solar day. By work- ing this proportion, as 360 degrees, 59 minutes, 8 se- conds, is to 24 hours, so is 361 degrees, 2 minutes, 6 seconds, we find 24 hours, O minutes, 12 seconds. Consequently, when the sun is in apogee, the solar day is 12 seconds longer than the mean day. From hence it follows : 1 . That in every second of a clock well-regulated to mean time, an arc of 15 minutes 28 seconds of the equator passes the meridian ; for this is the quotient of 360 degrees, 59 minutes, 8 seconds, divided by 86400 seconds. 2. That a star's revolution answers to 360 degree of the equator, while the mean day answers to360 degrees, 59 minutes, 8 seconds. This difference of 59 minutes 8 seconds, being reduced to time, gives three minutes 56 seconds : therefore, the stars anticipate 3 minutes 56 seconds, every day on mean time ; or, which is the same thing, a star's diurnal revolution is made in 23 hours, 56 minutes, 4 seconds. To find whether a clock be well regulated to mean time, observe if it show exactly 23 hours, 56 minutes, 4 seconds, from the instant of any star's passage through a fixed point, to that of its return to the same point. By what the clock exceeds this, it is faster, by what it wants thereof, it is slower than mean time. OF THE EQUATION OF TIME. I have already observed to you, that the equation of time is the difference between mean and apparent time, or that pointed out by a good clock, and by a sun-diai respectively. Vol. IV. S 130 THE EQUATION OF TIME. You will soon perceive that there would have been no difference, and consequently no need for any equation, 1st, if the earth's orbit had been a perfect circle with the sun at the centre ; 2dly, if the earth had moved through an equal part or portion of that circle every day; and 3dly, if the axis of her diurnal motion were always perpendi- cular to the plane of her orbit. But neither of the fore- going suppositions is true ; for, 1. the orbit of the earth is an ellipse ; 2. her motion therein is not equable ; and 3. her axis is inclined to the plane of her orbit : the mea- sure of time therefore, as far as it depends on these cir- cumstances, must be unequal and subject to an equation. The equation of time may then be considered as aris- ing, 1. from the obliquity of the ecliptic to the equator ; 2. from the unequal progression of the earth through her elliptic orbit. Of the first cause of 'inequality ', or that arising from the obliquity of the equator to the ecliptic. The motion of the earth on its axis is perfectly equable, or always at the same rate; and, the plane of the equator being perpendicular to its axis, it is evident, that in equal times equal portions of the equator will pass over the meridian ; and so also would equal portions of the ecliptic, if it were either pa- rallel to, or coincident with the equator. But, as the ecliptic is oblique to the equator, the equa- ble motion of the earth carries unequal portions of the ecliptic over the meridian in equal times, the difference being proportionate to the obliquity ; and, as some parts of the ecliptic are more oblique than others, those differ- ences are unequal among themselves. If, therefore, we should suppose two suns to start from the beginning either of Aries or Libra, and continue to move through equal arcs in equal times, one in the equator, the other in the ecliptic, the equatorial sun would always return to the meridian in 24 hours time as measured by a good clock, but the sun in the ecliptic would return to the meridian sometimes sooner, sometimes later, than the equatorial sun, and only the same instant with him on four days in the year. To render this plainer, we shall have recourse to a dia- gram, plate 4, fig. 4. This figure is to be considered as THE EQUATION OF TIME. lSt a view of part of the concave sphere of the heavens, where- in D E represents a part of the celestial equator, F G a part of the ecliptic, A the intersection of the two circles at the vernal equinox, A B a degree upon the equator. If we imagine the plane of the meridian to pass from the situation M M, into the situation N N, in going through the arc A B, one degree of the equator, it will also go through the arc, A C, more than one degree of the eclip- tic. For in the triangle ABC, the angle, at B, is a right one, consequently, the hypothenuse, A C, is the longest side. At the solstices the obliquity of the ecliptic has a con- trary effect, and helps to lengthen the natural days : this will be easily comprehended by viewing the diagram, plate 4, Jig. 5, where T T is part of the tropic of Capri- corn, C D part of the ecliptic, which may be considered as coincident with the tropic for some distance on each side of the solstitial point, as from A to B ; and therefore meridians, which are perpendicular to the tropics, may be considered for that space as perpendicular also to the ecliptic. This being supposed, a meridian, in going from A towards B, will go through as large an arc in the tro- pic as the ecliptic : but the tropic not being a great cir- cle, any arc, as a b, taken in both these circles, will mea- sure more minutes in the tropic than in the ecliptic, and that in the ratio as the ecliptic exceeds the tropic in di- mensions : now, the circumference of the ecliptic is to that of the tropic, nearly as 60 to 55 ; and therefore the arc ab, of 55 minutes in the ecliptic, will be 60 minutes in the tropic. But every meridian passes in the same time through similar arcs in the celestial equator, and all circles parallel to the equator, as the tropic's arc : con- sequently, at the solstices every arc of the ecliptic passed through by any meridian in a given time, will be to the arc of the equator passed through in the same time, as 55 to 60. The second cause of the difference in the time shown by a well-regulated clock, and a true sun-dial, arises from the inequality of the sun's apparent motion, which is slow- est in summer, when the sun is farthest from the earth, and swiftest in winter, when he is nearest thereto ; where- 132 THE EQUATION OF TIME. asthe earth's motion on its axis is equable all theyear round. If the sun's apparent motion in the ecliptic were equa- ble, the whole difference between the equal time as shown by the clock, and the unequal time as shown by the sun, would arise from the obliquity of the ecliptic. But this is not the case, for the sun's motion sometimes exceeds a degree in 24 hours, though it is generally less. And when his motion is slowest, any particular meridian will return and revolve sooner to him than when his motion is quickest, for it will overtake him in less time when he advances through a less space, than when he moves through a larger one. On the 1st of January, the daily motion of the sun in the ecliptic is nearly 1 degree, 1 minute, 13 seconds; but on the 1st of July, the daily motion is 57 minutes, 13 seconds; the medium of these is 59 minutes, 13 se- conds. The sun's place in the ecliptic, calculated on the supposition of a daily motion of 59 minutes, 13 seconds, will be behind his observed place from the beginning of January to the beginning of July, and will be before k from the beginning of July to the beginning of January £ the greatest difference is about 1 degree, 55 minutes, 32 seconds, which is observed about the beginning of April and of October, at which times the observed daily motion is 59 minutes, J 3 seconds. It is necessary for an astronomer to know both true and mean time ; the first, to ascertain the time of observation; the second, because the tables of the planets, &c. are cal- culated in conformity thereto. The relation between true and mean time is discovered by observing the time marked by your clock, at the in- stant when the centre of the sun passes the meridian, and adding what it wants of 12 hours, or subtracting the ex- cess above it. It is obtained for any other hour besides 12, by taking the difference between the times, per clock, of the sun's passing the meridian on the given day, and on the foregoing or following day ; and applying a part of this difference proportional to the given time past noon. * Example: March 3, when the sun's centre passed the meridian, the clock was 12 hours, 17 minutes, 49 se- THE EQUATION OF TIME. 133 conds ; the clock was therefore 17 minutes, 49 seconds, faster than true or apparent time. On the 4th of March, it was 1 2 hours, 1 7 minutes, 42* seconds; the difference is 6 v seconds, or about one-fourth of a second per hour. Now on the 3d. the planet Mars passed the meridian at 14 hours, 27 minutes, 32 seconds; the clock was therefore S\ seconds more advanced than at noon, which gives its advance for that hour, 1 7 minutes, 45\ seconds, and this, subtracted from 14 hours, 27 mi- nutes, 32 seconds, gives 14 hours, 9 minutes, 46* se- conds, for the true or apparent time of the transit of Mars. From what I have now explained to you, it appears, that there is no body in nature, whose motion is perfectly uniform and regular ; that whenever we look for com- mensurabilities and equalities in nature, we are always dis- appointed. The earth is spherical, but not perfectly so ; the summer is unequal when compared with the winter ; the ecliptic disagrees with the equator, and never cuts it twice in the same equinoctial point, the orbit of the earth has an eccentricity, more than double in proportion to the spheroidity of its globe ; no number of the revolutions of the moon coincide with any number of the revolutions of the earth in its orbit; no two of the planets measure one another; and thus it is wherever we turn our thoughts, so different are the views of the Creator from our narrow conceptions of things ; where we look for commensura- tion, we find variety and infinity.* It is scarce possible to refrain here from joining with an elegant moralist in observing, that all the appearances of nature uniformly conspire to remind us of the lapse of time, and the flux of life. The day and night succeed each other, the rotation of the seasons diversifies the year, the sun attains the meridian, declines and sets, and the moon every night changes its form. The day may be considered as an image of the year, and a year as the representation of life. The morning an- swers to the spring, and the spring to childhood and youth ; the noon corresponds to the summer, and the * Jones'* Physiological Disquisitions. 134 THE EQUATION OF TIME. summer to the strength of manhood ; the evening is an emblem of autumn, and autumn of declining life. The night, with its silence and darkness, shows the winter, in which all the powers of vegetation are benumbed; and the winter points out the time when life shall cease, with its hopes and pleasures. He that is carried forward, however swiftly, by a mo- tion equable and easy, perceives not the change of place, but by the variation of objects. If the wheel of life, which rolls thus silently along, passed on in undistinguishable uniformity, we should never mark its approaches to the end of the course. If one hour were like another ; if the passage of the sun did not show its wasting ; if the chan- ges of the seasons did not impress upon us the flight of the year ; quantities of duration, equal to days and years, would glide away unobserved. If the parts of time were not variously coloured, we should never discern their de- parture or succession ; but should live thoughtless of the past, and careless of the future, without will, and perhaps without power, to compare the time which is already lost, with that which may probably remain. LECTURE XLIII. ON THE PLANETARIUM, TELLURIAN, AND LUNARIUM, J O represent by machines the motions and various aspects of the heavenly bodies, the parallelism of the earth's axis, together with its annual and diurnal motions, and by these means to explain the beautiful variety of sea- sons, and other terrestrial and celestial phenomena, has THE PLANETARIUM, &C. 133 ever been considered as one of the noblest efforts of me- chanical genius. Among the variety of machines contri- ved for these purposes, that before you, and its parts, plate 1 1, fig 1, and plate 12, fig. 1 and 2, is best adapted for representing the celestial motions. It seems highly probable, that the ancients were not un- acquainted with planetary machines, but that the same powers of genius, which led them to contemplate and rea- son upon the heavenly bodies, induced them to realize their ideas, and form instruments for explaining them ; and we may fairly presume, that these were carried to no small degree of perfection, when we consider, that of which Archimedes was the maker, and Cicero the encomiast. A planetarium may be considered in some sort as a dia- metrical section of our universe, in which the upper and lower hemispheres are suppressed. The upper plate is to answer for the ecliptic ; on this are placed, in two opposite, but corresponding circles, the days, of the month, and the signs of the ecliptic, with their respective characters ; by this plate you may set the pla- netary balls so as to be in their respective places in the ecliptic, for any day in the year. Through the centre of this plate, you observe a strong stem, on which is a brass ball to represent the sun; round the stem are different sockets to carry the arms, by which the several planets are supported. The planets are repre- sented by ivory balls, having the hemisphere which is next the sun white, the other black, to exhibit their respective phases. I can with ease either take off, or put on, any of the planets, as occasion may require. About the primary planets are placed the secondary planets or moons, which, are in this instrument only moveable by the hand. I turn the handle, and all the planets are put in motion, moving round that ball which represents the sun. Now, if you take the earth's motion as a standard, they move with the same relative velocities and periodical times that they observe in the heavens. I scarcely need observe, that it is impossible to give an idea of the proportion and dis- tances of the planets in the compass of an instrument so small as that before you, or indeed of any instrument whatsoever. 136 SOLAR SYSTEM EXPLAINED. The motions are carried on by a train of wheel-work concealed in the brass box, ABC, under the ecliptic* GENERAL EXPLANATION OF THE SOLAR SYSTEM, BY THE PLANETARIUM. As the centre of the solar system is the only place from which the motion of the planets can be truly seen, let us suppose ourselves situate at the centre of the ball repre- senting the sun. In this situation the heavens would ap- pear perfectly spherical, the stars being so many lucid points in the concave surface of the sphere. Having attentively considered the stars for a long time, you will remark two sorts, the one scattered throughout the heavens unequally luminous, perfectly at rest, and therefore called fixed stars ; the other sort, moving round the sun with unequal velocities, called planets. By taking one of the fixed stars for a point to set out from, or for this purpose in our instrument, using any of the points into which the ecliptic is divided, it will be easy to deter- mine the motions of the planets. > Thus, by observing the earth as I turn the winch, you may perceive, that it continually approaches nearer and nearer to the more eastern signs ; in a certain space of time, it will return to the place from whence it set out. Thus you see how readily the periods of the planets' revolutions may be obtained, by observing the time that elapses between their setting out from any fixed point, and returning to the same again. The annual motion of the earth is the basis or standard, with which the motions of the other planets are compared ; and this is one' of the reasons, why the months and days of the months are en- graved on the ecliptic circle of the planetarium. The curves, which the planets describe in their revdlu- tions, are called their orbits. If the paths of the planets were in one plane, as in this instrument, they would all be referred to one circle in the * This complrte instrument was contrived by the late Mr. B. Martin; as it, in my opinion, deserves a fuller explanation, I shall give one in my Appendix to this leciiux* E. Edit. BY THE PLANETARIUM. 137 heavens ; but this is not the case, for their paths cross each other in different parts of the heavens. When you consider the motions of the little system before you, while you are supposed to view it from the sun, all is regular ; but when you view it from the earth, many of the appearances become intricate and perplexed. When the works of God are examined from a proper point, there is nothing but uniformity, beauty, and precision, and the heavens present you with a plan inexpressibly magnificent, and yet regular beyond the power of invention. When properly examined and looked into, you will always find the volume of the universe perfect like its Author, containing mines of truth for ever opening, fountains of good for ever flow- ing, being an endless succession of brighter and still brighter exhibitions of the glorious Godhead, always answering the nature and idea of infinite fulness and perfection. In the centre of the system is the sun, placed in the heavens by that Almighty Power, who said, " Let there be light, and there was light,'* to be the fountain of light and heat to all the planets revolving round him. In this machine, his situation is pointed out by this brass ball. The nearest planet to the sun is Mercury; observe the part of the ecliptic he is at, and also the place where the earth is situate. I now turn the handle, Mercury is arrived at the place from whence he set out, and our earth has gone over 88 days of the ecliptic ; the velocity we here give the planet is inconsiderable, but in his course in the heavens he is supposed to move with a ve- locity equal to 100,000 miles in an hour. Venus is the next planet in the system ; in the heavens she is distinguished by the superiority of her lustre, ap- pearing to us the brightest and largest of all the planets. By observing her course through the ecliptic, and com- paring it with the days passed over by the earth in the same time, you will find, in our instrument, Venus re- volving round the sun in 2 C 25 days ; in the heavens she moves at the rate of 80,955 miles in an hour. The third planet in the solar system is the Earth; di- minutive as it appears before you in this instrument, its VOL. IV. T 13H SOLAR SYSTEM EXPLAINED real diameter is near 8000 miles ; it revolves round the sun in the space of 365 days, into which number the brazen ecliptic is divided ; this revolution constitutes our year, while its revolution round its axis forms day and night. The little ball, close and annexed to the earth, repre- sents the Moon, of which I shall say nothing at present, as there is a part of the instrument for explaining more particularly her phenomena. The planet Mars is the next in order, being the first above the earth's orbit ; he revolves round the sun in about 686 days ; so that our earth, as you will observe by the instrument, goes nearly twice round, while he is performing his revolution ; he is supposed to move at the rate of 55,783 miles in an hour. To this planet our earth and moon will appear like two moons, some- times half or three quarters illuminated, but never full. Jupiter ', the largest of all the planets, is next beyond Mars; and our earth must have gone nearly twelve times round the ecliptic for one revolution of Jupiter ; yet so far is its path removed from the sun, that to go round it in this space of time, it moves at the rate of 30,193 miles an hour. Though larger than the earth, it appears but small in the heavens, because, as you know, objects decrease in their apparent magnitude in proportion to their real distance. It is attended by four satellites, here represented by these four balls; they are invisible to the naked eye, but appear beautiful through a telescope. Saturn, the next planet, is still higher in the system, performing its circuit in about thirty years of our time ; so that in this instrument its motion is scarcely sensible, while in the heavens it goes at the rate of 22,298 miles an hour, it is accompanied by five satellites, and a large luminous ring, here represented by this ivory circle, and which is one of the most curious phenomena of nature. The Georgium Sidus, or Georgian planet, so called in compliment to his Majesty, King George the Third, the Royal patron and promoter of the arts and sciences, is the seventh planet in our system ; it is near twice the distance of Saturn from the sun, round which it revolves BY THE PLANETARIUM. 139 in about eighty years. Dr. Herschel has discovered six satellites to this planet. To explain, by the planetarium, why the sun, being a fixed body, appears to pass through all the signs of the zodiac in one year : also showing, that this phenomenon is occa- sioned by the annual motion of the earth. As the general phenomena of the planetary system will be best understood by an induction of particulars, I shall remove all the planets but those whose motions I am going to explain ; for instance, I shall leave only the earth and sun, and place the earth over Libra, and it is plain, that the sun will then be transferred by the eye of • a spectator on the earth to Aries, in which sign it will appear at the latter end of March : move the earth on its orbit to Capricorn, and the sun will appear at Cancer in June, seeming to have moved from v to 25, though it has not stirred, the real motion of the earth having caused the spectator to trasfer the sun to all the intermediate points in the heavens, and thus given it an apparent mo- tion. Continue to move the earth till it arrive at Aries, and the sun will be seen in Libra in the month of Sep- tember : moving the earth on to Cancer, the visual ray of the spectator defers the sun to Capricorn, as it ap- pears in the month of December. Lastly, continue moving the earth, and it will arrive at Aries, where we set out. Thus I have shown, that it is the motion of the earth which causes the sun to appear in all the dif- ferent signs of the zodiac. Custom, indeed, has taught us to say, the sun is in Aries, when it is between us and Aries, and so of any other sign ; whereas it would have been more proper to say, that the earth is in Libra. To show why, at different times of the year, we see the heavens decorated with an entire different collection of stars. This phenomenon is occasioned by the earth's pro- gressive or annual motion : while the earth is traversing 140 PHENOMENA OF THE PLANETS. its course under the vast concave of fixed stars, we are gradually carried under the different constellations. From hence it is evident, that at night, when the earth is turned from the sun, we shall in succession have the opportunity of viewing from time to time all the stars in the zodiac, and consequent!) a different constellation will present itself every month. Thus, the Pleiades in Taurus are not visible in the summer ; but in the winter the earth is between the sun and them. These stars are observable at night, because they are not intercepted from our sight by the sun's rays ; and in this manner they appear during the whole winter, only they seem to get more westerly every night, as the earth moves gradually by them to the east. To make this more clear, place the earth in the planetarium between the sun and any of the signs, that side towards the sun will be day, and that towards the sign night : it follows, that at night we are turned towards the stars, which in that sign (suppose, as before, the Pleiades in Taurus) will then be conspicuous to us ; but as the spring approaches, the earth withdraws itself from between the sun and the Pleiades, till at length, by its progressive motion, it gets the sun between it and them, which then lie hid behind the solar rays : after the same manner, while the earth performs its annual tract, the sun, which always seems to move the contrary way, effaces, by his splendor, the other constellations succes- sively ; but the stars opposite to those hid by the sun, are at night presented to our view. GENERAL PHENOMENA OF THE PLANETS. I shall now place the earth, Mars, and Venus, on the planetarium, and as each planet moves with a different degree of velocity, they are continually changing their relative positions. Thus, on turning the handle of the mechine, you find, 1st, That the earth moves twice as fast as Mars, making two revolutions while he makes one ; and Venus, on the other hand, moves much faster than the earth. Secondly, that in each revolution of the earth, these planets continually change their relative PHENOMENA OF THE PLANETS. 141 positions, corresponding sometimes with the same point of the ecliptic, but much oftner with different points. To explain the conjunction, opposition, elongation, and other phenomena of the inferior planets. We may now proceed to make some observations on the motions of Venus, as observed in the planetarium. If considered, as viewed from the sun, we shall find that Venus would appear at one time nearer to the earth than at another ; that sometimes she would appear in the same part of the heavens, and at others in opposite parts thereof. As the planets, when seen from the sun, change their position with respect to the earth, so do they also, when seen from the earth, change their position with respect to the sun, being sometimes nearer to, at others farther from, and at other times in conjunction with him. But the conjunctions of Venus or Mercury, seen from the earth, not only happen when they are seen together from the sun, but also when they appear to the solar spectator to be in opposition. To illustrate this, bring the earth and Venus to the first point of Capricorn ; then by applying a string from the sun over Venus and the earth, you will find them to be in conjunction, or on the same point of the ecliptic. Whereas, if you turn the handle till the sun is be- tween Venus and the earth, a spectator in the sun will see Venus and the earth in opposition ; but an inhabi- tant of the earth will see Venus not in opposition to the sun, but in conjunction with him. In the first conjunction, Venus is between the sun and the earth ; this is called the inferior conjunction. In the second, the sun is situate between the earth and Venus ; this is called the superior conjunction. After either of these conjunctions, Venus will be seen to recede daily from the sun, but never departing beyond certain bounds, never appearing opposite to the sun ; and when she is seen at the greatest distance from him, a line joining her centre with the centre of the earth, will be a tangent to the orbit of Venus. 142 PHENOMENA OF THE PLANETS. To illustrate this, I take off the sun from its support, and the ball of Venus from its supporting stem, and place this wire, plate 1 l,y%. 2, so that one part P, may be on the stem, that supports the earth, and a similar socket, Jig. 3, on the pin which supports the ball of Venus ; the wire, F, is to lie in a notch at the top of the socket, which has been put upon the supporting stem of Venus : then will the wire represent a visual ray going from an inhabitant of the earth to Venus. By turning the handle, you will now find that the planet never departs farther than certain limits from the sun, which are called its greatest elongations, when the wire becomes a tangent to the orbit, after which it ap- proaches the sun, till it arrive at either the inferior or superior conjunction. It is also evident from the instrument, that Venus, from her superior conjunction, when she is farthest from the earth, to the time of her inferior conjunction, when she is nearest, sets later than the sun, is seen after sun- set, and is, as it were, the forerunner of night and darkness. But from the inferior conjunction, till she come to the superior one, she is always seen westward of the sun, and must consequently set before him in the evening, and rise before him in the morning, foretelling that light and day are at hand. Bring Venus and the earth to the beginning of Aries, when they will be in conjunction ; and turn the handle for nearly 225 days, and as Venus moves faster than the earth, she will arrive at Aries, and have finish- ed her course, but will not have overtaken the earth, who has moved on in the mean time ; and Venus must go on for some time in order to overtake her. There- fore, if Venus should be this day in conjunction with the sun, in the inferior part of her orbit, she will not come again to the same conjunction till after 1 year, 7 months, and 12 days. It is plain, by inspection of the planetarium, that though Venus does always keep nearly at the same distance from the sun, yet she is continually changing her distance from the earth •> her distance is greatest PHENOMENA OF THE PLANETS. 143 when she is in her superior, and least when she is in her inferior conjunction. To explain the phases, the retrograde, direct, and stationary situations of the planets. As Venus is an opake globe, and only shines by the light she receives from the sun, that face which is turn- ed towards the sun will always be bright, while the op- posite one will be in darkness ; consequently, if the situation of the earth be such, that the dark side of Ve- nus be turned towards us, she will then be invisible, except she appear like a spot on the disk of the sun. If her whole illuminated face be turned towards the earth, as it is in her superior conjunction, she appears of a cir- cular form ; and, according to the different positions of the earth and Venus, she will have different forms, and appear with different phases, undergoing the same chan- ges of form as the moon. These different phases are seen very plain in this instrument, as the side of the planet which is opposite to the sun is blackened ; so that in any position, a line drawn from the earth to the planet, will represent that part of her disk which is visible to us. The irregularities in the apparent motions of the pla- nets, is a subject that this instrument will fully elucidate; and the pupil will find that they are only apparent, taking their rise from the situation and motion of the observer. To illustrate this, let us suppose the fore- mentioned wire, when connected with Venus and the earth, to be the visual ray of an observer on the earth ; it will then point out how the motions of Venus appear in the heavens, and the path she appears to us to de- scribe among the fixed stars. Let Venus be placed near her superior conjunction, and the instrument in motion, the wire will mark out the apparent motion of Venus in the ecliptic. Thus Venus will appear to move eastward in the ecliptic, till the wire become a tangent to the orbit of Venus, in which situation she will appear to us to be stationary, 144 OF THE SUPERIOR PLANETS. or not to advance at all among the fixed stars ; a cir- cumstance which is exceedingly clear and visible upon the planetarium,.. Continue turning, till Venus be in her superior con- junction, and you will find by the wire or visual ray that she now appears to move backward in the ecliptic, or from east to west, till she has arrived at that part where the visual ray again becomes a tangent to her orbit; In which position, Venus will again appear sta- tionary for some time : after which she will commence anew her direct motion. Hence, when Venus is in the superior part of her orbit, she is always seen to move directly, according to the order of the signs ; but when she is in the infe- rior part, she appears to move in a contrary direction. What has been said concerning the motions of Venus is applicable to those of Mercury ; but the conjunctions of Mercury with the sun, as well as the times of his be- ing direct, stationary, or retrograde, are more frequent than those of Venus. OF THE SUPERIOR PLANETS, AS SEEN FROM THE EARTH. If you extend your observations on the instrument to Mars, you will find by the visual ray that Mars, when in conjunction and when in opposition, will ap- pear in the same point of the ecliptic, whether it be seen from the sun or the earth ; and in this situation only is its real and apparent place the same, because then only the ray proceeds as if it came from the centre of the universe. You will find, that the direct motion of a superior planet is swifter the nearer it is to the conjunction, and slower when it is nearer to quadrature with the sun ; but that the retrograde motion of a superior planet is swifter the nearer it is to opposition, and slower the nearer it is to quadrature ; but at the time of change from direct to retrograde, its motion becomes insensible. [ 145 ] TO PROVE BY THE PLANETARIUM THE TRUTH OF THE COPERNICAN, AND ABSURDITY OF THE PTOLE- MAIC SYSTEM. Of all the prejudices which philosophy contradicts, there is none so general as that the earth keeps its place unmoved. This opinion seems to be universal, till it be corrected by instruction, or by philosophical specu- lation. Those who have any tincture of education, are not now in danger of being held by it ; but yet they find at first a reluctance to believe that there are antipodes, that the earth is spherical, and turns round its axis every day, and round the sun every year. They can recollect the time when reason struggled with pre- judice upon these points, and prevailed at length, but not without some efforts.* The planetarium gives ocular demonstration of the motion of the earth about the sun, by showing that it is thus only that the celestial phenomena can be explain- ed, and making the absurdity of the Ptolemaic system evident to the senses of young people. For this pur- pose, I take off the brass ball which represents the sun, and put on a small ivory ball, jig. b, in its place to re- present the earth, and place a small brass baling, a, for the sun, on that arm which carries the earth. The instrument in this state will give an idea of the Ptolemaic system, with the earth immoveable in the centre, and the heavenly bodies revolving about it in the following order : Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. Now, in this disposition of the planets, several circumstances are to be observed, that are contrary to the real appearances of the celestial mo- tions, and which therefore prove the falsity of this sys- tem. It will appear from the instrument, that on this hy- pothesis Mercury and Venus could never be seen to go behind the sun, from the earth, because the orbits of Reid's Essays on the Intellectual Powers of Man. VOL. IV. U 146 TO RECTIFY THE PLANETARIUM, both of them are contained between the sun and the earth ; but these planets are seen to go as often behind the sun as before it ; we may, therefore, from hence conclude that this system is erroneous. It is also apparent from the planetarium, that on this scheme these planets might be seen in conjunction with, or in opposition to the sun, or at any distance from it. But this is contrary to experience ; for they are never seen in opposition to the sun, or on the meridian of Lon- don, for instance, at midnight ; nor do they ever re- cede from the sun beyond certain limits. Again, on the Ptolemaic system all the planets would be at an equal distance from the earth, in all parts of their orbit, and would therefore necessarily appear al- ways of the same magnitude, and moving with equal and uniform velocities in one direction ; circumstances which are known to be repugnant to observation and experience. To rectify the planetarium^ or place the planets in their true situations ^ as seen from the sun. The situation of the planets in the heavens are accu- rately calculated by astronomers, and published in al- manacks appropriated to the purpose, as the Nautical Almanack, White's Ephemeris, &c. An ephemeris is a diary or daily register of the motions and places of the heavenly bodies, showing the situation of each pla» net at 12 o'clock each day. These situations it exhibits both as seen from the sun, and from the earth ; but, as the former, or the heliocentric, is the only one of any use for this purpose, I shall here explain so much of that part of Mr. White's Ephemeris, for the year 1790, as will enable you to rectify the planetarium. SI Day Length Helioc. Helioc. Helioc. Helioc. Helioc. Helioc. increa of Day. long. long. long. long. long. long. h % * 1 e 9 s 1 7 4 14 48|27X 35 2«JU4 5J7|^16 11 til U 8 ^ 35 18 18 7 7 24 15 8 27 47 2 42 29 57 17 2 18 7 26rJ53 13 7 44 15 2827 59 3 2=£=3921 52 7 37 3 SI 4 19 8 15 44 28 11 3 37 5 20128 36 7 V$ 7 4^15 25 8 10 16 0|28 23 4 5 8 3) 4/22 16 3fy0 ^ TO USE IT AS A TELLURIAN. 147 In the foregoing table for May, 1790, you have the heliocentric places calculated to every six days of the month, which is sufficiently accurate for general pur- poses. Thus on the 19th, you have Saturn in 28° IT of Pisces, Jupiter in 3° 37' of Virgo, Mars in 5° 20' of Libra, the Earth 28° 36' of Scorpio, Venus 7° 7' of Ca- pricorn, and Mercury 4° 1 3' of Virgo ; to which places on the ecliptic of the planetarium, the several planets are to be set, and they will then exhibit their real situa- tions, both with respect to the sun and the earth for that day. To use the instrument as a tellurian^ plate 12, jig. 1. The sun, the earth, and the moon, are bodies, which, from our connection with them, are so interesting to us, that it is necessary to enter into a minute detail of their respective phenomena. To render this instrument a tellurian, all the planets are first to be taken off, the piece of wheel-work, A B, is to be placed on in their stead, in such a manner that the wheel, c, may fall into the teeth that are cut upon the edge of the ecliptic. The milled nut, D, is then to be screwed on, to keep the wheel-work firmly in its place. It is best to place this wheel-work in such a manner, that the index, E, may point to the 21st of June, and then to move the globe, so that the north pole may be turned towards the sun. The instrument will then show, in an accurate and clear manner, all the phenomena arising from the an- nual and diurnal motions of the earth : as the globe is of three inches diameter, all the continents, seas, king- doms, &c. may be distinctly seen ; the equator, the ecliptic, tropics, and other circles, are very visible, so that the problems relative to peculiar places may be sa- tisfactorily solved. The axis of the earth is inclined to the ecliptic in an angle of 66j degrees, and preserves its parallelism during the whole of its revolution. About the globe there is a circle to represent the terminator, or boundary between light and darkness, dividing the enlightened from the dark hemisphere. At N O is an 148 TO EXPLAIN THE CHANGES OF hour-circle, to determine the time of the sun's rising or setting. The brass index, G, represents a central solar ray ; it serves to show when it is noon, or when the sun is upon the meridian at any given place : it also shows what sign and degree of the ecliptic on the globe the sun describes on any day, and the parallel it describes. The plane of the terminator, HI, passes through the centre of the earth, and is perpendicular to the central solar ray. The index, E, points out the sun's place in the ecliptic of the instrument for any given day in the year. To explain the changes of seasons by the tellurian. The first thing to be done, is to rectify the tellurian ; or, in other words, to put the globe into a position si- milar to that of the earth for any given day. Thus, to rectify the tellurian, for the*21st of June, turn the han- dle till the annual index come to the given day ; then move the globe by the arm K L, so that the north pole may be turned towards the sun ; and adjust the termi- nator, so that it may just touch the edge of the arctic circle. The globe is then in the situation of the earth for the longest day in our northern hemisphere, the an- nual index pointing to the first point of Cancer and the 21st of June ; brings the meridian of London to co- incide with the central solar ray G, and move the hour- circle, N O, till the index L, points to XII ; we then have the situation of London with respect to the longest day. Now, on gently turning the handle of the machine, the point representing London will, by the rotation of the earth, be carried away towards the east, while the sun seems to move westward ; and when London has arrived at the eastern part of the terminator, the index will point on the hour-circle to the time of sun-setting for that day ; continue to turn on, and London will move in the shaded part of the earth, on the other side of the terminator ; when the index is again at XII, it is mid- night at London : by moving on, London will emerge SEASONS BY THE TELLURIAN. 149 from the western side of the terminator, and the index will point out the time of sun-rising, the sun at that instant appearing to rise above the horizon in the east to an inhabitant of London. It will be evident by the instrument, while in this po- sition, that the central solar ray, during the whole revo- lution of the earth on its axis, only points to the tropic of Cancer, and that the sun is vertical to no other parts of the earth, but those which are under this tropic. By observing how the terminator cuts the several parallels of the globe, we shall find that all those be- tween the northern and southern polar circles, except the equator, are divided unequally into diurnal and noc- turnal arcs, the former being greatest on the north side of the equator, and the latter on the south side of it. In this position the northern polar circle is wholly on that side the terminator which is nearest the sun, and therefore altogether in the enlightened hemisphere, and the inhabitants thereof enjoy a continual day. In the same manner, the inhabitants of the southern polar cir- cle continue in the dark at this time, notwithstanding the diurnal revolution of the earth ; it is the annual mo- tion only which can relieve them from this situation of perpetual darkness, and bring to them the blessings of day and the enjoyments of summer. While in this state, the inhabitants in north latitude are nearest to the central solar ray, and consequently to the sun's perpen- dicular beams ; and of course, a greater number of his rays will fall upon any given place, than at any other time : the sun's rays do now also pass through a less quantity of the atmosphere, which, together with the length of the day and the shortness of the night, are the reasons of the increase of the heat in summer to- gether with all its other delightful effects ; the season when the Lord pours forth his blessings upon every living creature in the greatest abundance. While the earth continues to turn round on its axis once a day, it is continually advancing from west to east, according to the order of the signs, as is seen by the progress of the annual index, E, which points suc- cessively to all the signs and degrees of the ecliptic ; 150 TO EXPLAIN THE CHANGES OF the sun in the mean time seems to describe the ecliptic also, going from west to east, at the distance of six signs from the earth ; that is, when the earth really sets out from the first point of Capricorn, the sun seems to set out from the first point of Cancer, as is plain from the index. But as, during the annual revolution of the earth, the axis always remains parallel to itself, the situation of this axis with respect to the sun must be continually changing. As the earth moves on in the ecliptic, the northern polar circle gets gradually under the terminator ; so tnat when the earth is arrived at the first point of Aries, and the annual index is at the first point of Libra, on the 22d of September, this circle is divided into two equal parts by the terminator, as is also every other parallel circle, and consequently the diurnal and nocturnal arcs are equal : this is called the time of equinox ; the days and nights are then equal all over the earth, being each of them twelve hours long, as will be seen by the horary index, L. The central solar ray, G, having successively pointed to the parallels that may be supposed to be be- tween the equator and the tropic of Cancer, is at this period perpendicular to the inhabitants that live at the equator. By continuing to turn the handle, the earth advances in the ecliptic, and the terminator shows how the days are continually decreasing, and the diurnal arcs shorten- ing : till by degrees the whole space contained by the northern polar circle is on that side of the terminator which is opposite to the sun ; this happens when the earth has got to the first point of Cancer, and rhe annu- al index is at the first point of Capricorn, on the 21st of December. In this state of the globe, the northern polar circle, and all the countries within that space, have no day at all ; while the inhabitants that live within the southern polar circle, being on that side of the termina- tor which is next the sun, enjoy perpetual day. By this, and the former situation of the earth, you will ob- serve that there are nations to whom a great portion of the year is darkness, who are condemned to pass weeks SEASONS BY THE TELLURIAN. 151 and months without the benign influence of the solar rays. The central solar ray is now perpendicular to the tropic of Capricorn ; the length of the days is inversely what it was when the sun entered Cancer, the days be- ing now at their shortest, and the nights longest in the northern hemisphere : the length of each is pointed out by the horary index. The earth being again carried on till it enter Libra, and the sun Aries, we shall again have all the pheno- mena of the equinoctial seasons. The terminator will divide all the parallels into two equal parts ; the poles will again be in the plane of the terminator ; and conse- quently as the globe revolves, every place from pole to pole will describe an equal arc in the enlightened and ob- scure hemispheres, entering into and going out of each exactly at six o'clock, as shown by the hour-index. As the earth advances, more of the northern polar circle comes into the illuminated hemisphere, and conse- quently the days increase with us, while those on the other side of the equator decrease, till the earth arrive at the first point of Capricorn, the place from which we first began to make our observations. To explain the phenomena that take place in what is called a parallel^ direct , and right sphere. , Take off the globe and its terminator, and put on in its place the globe which accompanies the instrument, and which is furnished with a meridian, horizon, and quadrant of altitude ; the edge of the horizon is gradu- ated from the east and west, to the north and south points, and within these divisions are the points of the compass to the under side of this horizon ; but at 1 8 degrees from it another circle is affixed, to represent the twilight- circle : the meridian is graduated like the meridian of a globe ; the quadrant of altitude is divided into degrees, beginning at the zenith, and finishing at the horizon. This globe, if the horizon be differently set with res- pect to the solar ray, will exhibit the various phenomena arising from the situation of the horizon with respect to the sun, either in a right, a parallel, or an oblique sphere ; 152 PHENOMENA IN A PARALLEL, or having set the horizon to any place, you will see by the central solar ray how long the sun is above or below the horizon of that place, and at what point of the compass he rises, his meridian altitude, and many other curious particulars, of which we shall give a few examples. Set the horizon to coincide with the equator, and place the earth in the first point of Libra ; then will the globe be in the position of a parallel sphere, and of the inhabitants of the poles at that season of the year, which inhabitants are represented by a pin at the upper part of the quadrant of altitude ; the handle being turned round gently, the earth will revolve upon its axis, and the solar ray will coincide with the horizon, without deviating in the least to the north or south ; showing, that on the 21st of March the sun does not appear to rise or set to the terrestial poles, but passes round through all the points of the compass, the plane of the horizon bisecting the sun's disk. Now place the horizon so, that it may coincide with the poles, and the pin representing an inhabitant to be over the equator, the globe in this position is said to be in that of a right sphere ; the equator, and ail the pa- rallels of latitude, are at right angles, or perpendicular to the horizon ; by turning the handle till the earth has completed a year, or one revolution about the sun, we shall perceive all the solar phenomena as they happen to an inhabitant of the equator, which are, 1. That the sun rises at six, and sets at six, throughout the year, so that the days and nights there are perpetually equal. 2. That on the 21st of March, and 22d of September, the sun is in the zenith, or exactly over the heads of the in- habitants. 3. That one half of the year between March and September, the sun is every day full north, and the other half, between September and March, is full south of the equator, his meridian altitude being never less than 66- degrees. If the pin, representing an inhabitant, be now removed out of the equator, and set upon any place between it and the poles, the horison will not then pass through either of the poles, nor coincide with the equator, but cut it DIRECT, AND RIGHT SPHERE. 153 obliquely, one half being above, the other half below the horizon ; the globe in this state is said to be in that of an oblique sphere, of which there are as many varieties as there are places between the equator and both poles. But one example will be sufficient ; for whatever appearance happens to one place, the same, as to kind, happens to every other place, differing only in degree, as the lati- tudes differ. Bring the pin, therefore, over London, then will the horizon represent the horizon of London, and in one revolution of the earth round the sun, we shall have all the solar appearances through the four seasons clearly exhibited, as they really are in nature ; that is, the earth standing at the first degree of Libra, and the sun then entering into Aries, the meridian turn- ed to the solar ray, and the hour-index set to XII, you will then have the globe standing in the same position towards the sun, as our earth does at noon on the 21st of March. If the handle be turned round, when the so- lar ray comes to the western edge of the horizon, the hour-index will point to VI, which shows the time of sun-setting ; London then passes into, and continues in darkness, till the hour-index having passed over XII hours, come again to VI, at which time the solar ray gains the eastern edge of the horizon, thereby defining the time of sun-rising ; six hours afterwards the meri- dian again comes to the solar ray, and the hour-index points to XII ; thereby evidently demonstrating the equality of the day and night, when the sun is in the equinoctial. You may then also observe, that the sun rises due east, and sets due west. Continuing to move the handle, you will find, that the solar ray declines from the equator towards the north, and every day at noon rises higher upon the graduations of the meridian than it did before, continually approaching to London, the days at the same time growing longer and longer, and the sun rising and setting more and more to- wards the north, till the 21st of June, when the earth gets into the first degree of Capricorn, and the sun ap- pears in the tropic of Cancer, rising about 40 minutes past 3 in the morning, and setting about 20 minutes past 8 in the evening, and after continuing about seven hours VOL. iv. x 154 PHENOMENA IN A PARALLEL SPHERE. in the nether hemisphere, appears rising in the north- east, as before. From the 21st of June till the 22d of September, the sun recedes to the south, and the days gradually decrease till the autumnal equinox, when they again become equal. During the three succeeding months, the sun continues- to decline towards the south pole, till the 21st of Decem- ber, when the sun enters the tropic of Capricorn, rising on the south-east point of the compass about 20 minutes past 8 in the morning, and setting about 40 minutes past 3 in the evening, at the south-west point upon the hori- zon ; after which, the sun continues in the dark hemi- sphere for 17 hours, and then appears again in the south- east, as before. From this chill solstice the sun returns towards the north, and the days continually increase in length till the vernal equinox, when all things are re- stored in the same order as at the beginning. Thus all the varieties of the seasons, the time of sun rising and setting, and at what point of the compass ; as also the meridian altitude and declination every day of the year, the duration of twilight, and to what place the sun is at any time vertical, are fully exemplified by this globe and its apparatus. Before we quit the phenomena particularly arising from the motion and position of the earth, let the globe, with the meridian and horizon, be removed, and the ivory ball, which fits upon a pin, be placed thereon, to repre- sent the earth. As the axis of this globe stands perpendicular to the plane of the ecliptic, you will find, that the solar ray con- tinually points to the equator of this little ball, and will never deviate to the north or south ; though, by turning the handle, the ball is made to complete a revolution round the sun. This shows, that the earth in this position would have had the days and nights equal in every part of the globe, all the year long ; there would have been no difference in the climates of the earth, no distinction of seasons ; an eternal summer, or never-ceasing winter, would have been our portion ; an unvaried sameness, that would have limited inquiry, and satiated curiosity; and that the variety of the seasons is owing to its axis being inclined to the plane of its orbit. [ 155 ] of the lunarium, plate 12, Jig. % Having thus illustrated the phenomena, which arise particularly from the inclination of the earth's axis to the plane of the ecliptic, from its rotation round its axis, and revolution round the sun ; we now proceed to explain, by this instrument, the phenomena of the moon. But, in order to this, it will be necessary to speak first of the instrument, which is put in motion like the preceding one, by the teeth on the fixed wheel ; it is also to be placed upon the same socket as the tellurian, and con- fined down by the same milled nut, D, fig. 1 . The sloping ring, P O, represents the plane of the moon's orbit, or path round the earth; so that the moon, in her revolution round the earth, does not move paral- lel to the plane of the ecliptic, but on this inclined plane; the two points of this plane, that are connected by the brass wires, are the nodes, one of which is marked &, for the ascending node, the other t3 , for the descending node. The moon is therefore sometimes on the north, and sometimes on the south side of the ecliptic, which deviations from the ecliptic are called her north or south latitude ; her greatest deviation, which is when she is at her highest and lowest points, called her limits, is 5 de- grees, 18 minutes ; this, with all the other intermediate degrees of latitude, are engraved on this ring, beginning at the nodes, and numbered both ways from them. At each of the nodes, and at about 18 degrees distance from them, we find this mark o, and at about 22 degrees this 3 , to indicate, that when the full moon is got as far from the nodes as the mark y , there can be no eclipse of the moon ; nor any eclipse of the sun, when the new moon has passed the mark g ; these points are generally termed the limits of eclipses. The nodes of the moon do not remain fixed at the same point of the ecliptic, but have a motion contrary to the order of the signs. T V is a small circle parallel to the ecliptic ; it is di- vided into 12 signs, and each sign into 30 degrees; this circle is moveable in its socket, and is to be set by the hand, so that the same sign may be, opposite to the sun 156 PHENOMENA OF THE MOON. that is marked by the annual index. These signs always keep parallel to themselves, as they go round the sun, but the inclined plane, with its nodes, go backwards, so that each node recedes through all the above signs in about 19 years. R S is a circle, on which are divided the days of the moon's age ; X Y is an ellipsis, to represent the moon's elliptical orbit, the direct motion of the apogee, or the line of the apsides, with the situation of the ellip- tical orbit of the moon, and place of the apogee in the ecliptic at all times. To rectify the lunarium. Set the annual index, E, on the large ecliptic to the first of Capricorn ; then turn the plate, with the moon's signs upon it, until the beginning of Capricorn point di- rectly to the sun ; turn the handle till the annual index come to the first of January ; then find the place of the north node in an ephemeris, to which place, among the moon's signs, set the north node of her inclined orbit, by turning it till it be in its proper place in the circle of signs; set the moon to the day of her age. GENERAL PHENOMENA OF THE MOON. Having rectified the lunarium for use, on putting it into motion, it will be evident, 1. That the moon, by the mechanism of the instru- ment, always moves in an orbit inclined to that of the ecliptic, and consequently in an orbit analogous to that in which the moon moves in the heavens. 2. That she moves from west to east. 3. That the white, or illuminated face of the moon, is always turned towards the sun. 4. That the nodes have a revolution contrary to the order of the signs, that is from Aries to Pisces ; that this revolution is performed in about nineteen years, as in nature. 5. That the moon's rotation upon her axis is effected and completed in about 277 days ; whereas it is 29j days from one conjunction with the sun to the next. PHASES OP THE MOON. 157 6. That every part of the moon is turned to the sun, in the space of her monthly or periodic revolution. To be more particular. On turning the handle, you will observe another motion of the earth, which has not yet been spoken of, namely, its monthly motion about the common centre of gravity between the earth and moon, which centre of gravity is represented by the pin Z. From hence we learn, that it is not the centre of the earth which describes what is called the annual orbit, but the centre of gravity between the earth and moon, and that the earth has an irregular, vertical, or spiral motion about this centre, so that it is every month at one time nearer to, at another time farther from the sun. It is evi- dent, from this instrument, that the moon does not regard the centre of the earth, but the centre of gravity, as the centre of her proper motion ; that the centre of the earth is farthest from the sun at new moon, and nearest at full moon ; that in the quadratures the monthly parallax of the earth is so sensible, as to require a particular equation in astronomical tables. These particulars were first ap- plied to the orrery by the late ingenious Mr. Benjamin Martin. To explain the phases of the moon. The moon assumes different phases to us, 1. On ac- count of her globular figure. 2. On account of the mo- tion in her orbit, between the earth and the sun : for whenever the moon is between the earth and the sun, we call it new moon, the enlightened part being then turned from us ; but when the earth is between the sun and the moon, we call it full moon, the whole of the enlightened part being then turned towards us. The phases of the moon are clearly exhibited in this instrument ; for we here see that the half which is oppo- site to the sun is always dark, while that which is next to the sun is white, to represent the illuminated part. Thus, when it is new moon, you will see the whole white part next the sun, and the dark part turned towards the earth, showing thereby its disappearance, or the time of its con- junction and change : on turning the handle, a small por- tion of the white part will begin to be seen from the earth, 158 TO EXPLAIN THE PHASES which portion will gradually increase, and towards the end of the 7 th day, you will perceive, that half of the light, and half of the dark side, is turned towards the earth, thus exhibiting the appearance of the moon at the first quarter. From hence the light side will continually show itself more and more in a gibbous form, till at the end of fourteen days the whole white side will be turned towards the earth, and the dark side from it, the earth now standing in a line between the sun and moon ; and thus the instrument explains the opposition, or full moon. On turning the handle again, some of the shaded part will begin to turn towards the earth, and the white side to turn away from it, decreasing in a gibbous form till the last quarter, when the moon will appear again as a crescent, which she preserves till she has attained ano- ther conjunction. In this lunarium the moon has always the same face or side to the earth, as is evident from the spots deline- ated on the surface of the ivory ball revolving about its axis in the course of one revolution round the earth ; in consequence of which, the light and dark parts of the moon appear permanent to us, and the phases are shown as they appear in the heavens. As the earth turns round its axis once in 24 hours, it must in that time exhibit every part of its surface to the inhabitants of the moon, and therefore its luminous and opake parts will be seen by them in constant rotation. As the half of the earth which is opposed to the sun is al- ways dark, the earth will exhibit the same phases to the lunarians that they do to us, only in a contrary order, that is, when the moon is new to us, we shall be full to them, and vice versa. But as one hemisphere only of the moon is ever turned towards us, it is only those that are in this hemisphere who can see us ; our earth will appear to them always in one place, or fixed in the same part of the heavens ; the lunarians in the opposite hemi- sphere never see our earth, nor do we ever view that part of the moon which they inhabit. The moon's apparent diurnal motion in the heavens is produced by the daily revolution of our earth. OF THE MOON, &C. 159 If we consider the moon with respect to the sun, the instrument shows plainly, that one half of her globe is always enlightened by the sun ; that every part of the lu- nar ball is turned to the sun, in the space of her monthly or periodical revolution ; and that, therefore, the length of the day and night in the moon is always the same, and equal to 14- of our days. When the sun sets to the lu- narians in that hemisphere next the earth, the terrestrial moon rises to them, and they can therefore never have any dark night ; while those in the other hemisphere can have no light by night, but what the stars afford. OF THE PERIODICAL AND SYNODICAL MONTH. The difference between the periodical month, in which the moon exactly describes the ecliptic, and the synodi- cal, or time between any two new moons, is here ren- dered very evident. To show this difference, observe at any new moon her place in the ecliptic, then turn the handle, and when the moon has got to the same pojnt in the ecliptic, you will see that the dial shows 27y days, and the moon has finished her periodic revolution. But the earth at the same time having advanced in its annual path about 27 degrees of the ecliptic, the moon will not have got round into a direct line with the sun, but will re- quire 28 days and 4 hours more, to bring it into con- junction with the sun again. OF ECLIPSES OF THE SUN AND MOON. There is nothing in astronomy more worthy of our contemplation, nor any thing more sublime in natural knowledge, than rightly to comprehend those sudden obscurations of the heavenly bodies, that are termed eclipses, and the accuracy with which they are now fore- told. " One of the chief advantages derived by the pre- sent generation from the improvement and diffusion of philosophy, is delivery from unnecessary terror, and ex- emption from false alarms. The unusual appearances, whether regular or accidental, which once spread con- 160 ECLIPSES OF THE SUN AND MOON. sternation over ages of ignorance, are now the recreations of inquisitive security. The sun is no more lamented when it is eclipsed, than when it sets ; and meteors play their coruscations without prognostic or prediction." We have already observed, that the sun is the only real luminary in the solar system, and that none of the other planets emit any light but what they have received from the sun ; that the hemisphere, which is turned to- wards the sun, is illuminated by his rays, while the other side is involved in darkness, and projects a shadow, which arises from the luminous body. When the shadow of the earth falls upon the moon, it causes an eclipse of the moon ; when the shadow of the moon falls upon the earth, it causes an eclipse of the sun. An eclipse of the moon, therefore, never happens but when the earth's opake body interposes between the sun and the moon, that is, at the full moon ; and an eclipse of the sun never happens but when the moon comes in a line between the earth and the sun, that is, at the new moon. From what we have already seen by the instrument, it appears, that the moon is once every month in conjunc- tion, and once in opposition ; from hence it would ap- pear, that there ought to be two eclipses, one of the sun, the other of the moon, every month ; but this is not the case, and that for two reasons; first, because the orbit of the moon is inclined in an angle of about five degrees to the plane of the ecliptic ; and secondly, because the nodes of this orbit have a progressive motion, which causes them to change their place every lunation. Hence it often happens, that at the time of opposition or conjunction the moon has so much latitude, or what is the same thing, is so much below or above the plane of the eclip- tic, that the light of the sun will in the first case reach the moon without any obstacle, and in the other the earth; but as the nodes are not fixed, but run successively through all the signs of the ecliptic, the moon is often, both at the times of conjunction and opposition, in or very near the plane of the ecliptic ; in these cases an eclipse happens, either of the sun or moon, according to her situation. The whole of this is rendered clear by the lunarium. OF A NEW TERRESTRIAL GLOBE. 161 where the wire projecting from the earth, shows when the moon is above, below, or even with the earth, at the times of conjunction and opposition, and thus when there will be, or not, any eclipse. The distance of the moon from the earth varies sen- sibly with respect to the sun ; it does not move in a cir- cular, but in an elliptic orbit round us, the earth being at one of the foci of this curve. The longest axis of the lunar orbit is not always directed to the same point of the heavens, but has a movement of its own, which is not to be confounded with that of the nodes ; for the mo- tion of the last is contrary to the order of the signs, but that of the line of apsides is in the same direction, and turns to the same point of the heavens in about nine years. This motion is illustrated in the lunarium by means of the brass ellipsis X Y, fig. 2, which is carried round the earth in less than nine years ; thus showing the situation of the elliptical orbit of the moon, and the place of the apogee in the ecliptic. OF A NEW TERRESTRIAL GLOBE,* plate 13, fig % 2, And of a new apparatus adapted thereto, for solving, in an easy and natural manner, the several phenomena of the sun, moon, and earth. Though globes have ever been considered as the best instruments for conveying general ideas of astronomy and geography, yet they have always been mounted in a way that must perplex and confuse the learner, and furnish him with ideas that are altogether false, and con- trary to the nature of things. That you may clearly perceive the great advantages of a globe mounted like that before you, I shall first * The terrestrial globe was first improved by my father, and placed in a fixed position, &cc. The floating meridian -and horizon were added by Mr. Newman. VOL. IV. Y 162 OF A NEW TERRESTRIAL GLOBE. point out a few of the imperfections of globes mounted in the common way.* Now, in the first place, what is rectifying a globe thus mounted, but a continual absurdity ? For to rectify the globe to any particular latitude, the axis of the earth is continually shifted from one false position to another, the mind of the pupil is confused, and he with difficulty conceives, that the axis of the earth never varies its po- sition, but always preserves the same inclination to the plane of its orbit. The broad paper circle of the common globes is de- signed to represent the ecliptic and the horizon ; but on examination you will find, it represents neither the one nor the other. Now the ecliptic is the apparent path of the sun, with which the earth's axis always makes an angle of 66 1 degrees ; but by shifting the axis of the globe, to rectify for the latitude, this circle can never be in its position as ecliptic, except when the axis is at 66± degrees, and consequently, cannot be used as the ecliptic. Now, let us consider it as the horizon. Every place is in the zenith of its horizon, and the place and horizon always move together ; but in the common globes, the broad paper circle is the horizon in one situation only, that is, when the place is in the zenith ; after having rectified the globe to the latitude, the moment you move the globe, the broad paper circle is no longer the hori- zon. Thus it is plain, that this circle cannot with pro- priety be considered either as a horizon or an ecliptic. As if it were to multiply confusion, a circle is laid down on the terrestrial globe to represent the ecliptic, and used as such in solving problems upon the common globes, though it involves the pupil in numerous absurdities : thus, having marked the sun's place in the ecliptic, and rectified the globe to the latitude, then turn the globe, and the sun and earth have a diurnal motion together ; of course, if you have day when you begin, you will * As I do not agree entirely with our Author, in what he considers the imperfections oi gl bes moun t d in the common manlier; in my Appen- dix to this lecture I have added a few explanatory observations thereon* E. Edit. OF A NEW TERRESTRIAL GLOBE. 163 have the same daring the whole twenty-four hours. Ma- ny other errors might easily be pointed out, but these are sufficient to show you that no one can be properly taught by globes so mounted. The globe before you, plate 1 S^Jig. 2, does not hang in a frame like the common globes, but is mounted on a pedestal, and is supported by, and moveable on, an axis, which is enclined 66\ degrees to the ecliptic, and of course is always parallel to the axis of the earth, sup- posing the path of the globe to be parallel to the eclip- tic*. On the pedestal, but under the globe is a gradu- ated circle C D, marked with the signs and degrees of the ecliptic ; adjoining thereto is a circle of months and days, answering to every degree of the ecliptic ; within this circle is the sun's declination for every day of the month. There is a moveable arm A B, which, being set to the day of the month, immediately points out the sun's place in the ecliptic, and his declination. On this moveable arm, but nearer the index, you observe a pillar E ; on the top of the pillar is fixed a small ball, through which a steel wire passes, to represent a ray proceeding from the centre of the sun. Round the globe is a brass circle F, to represent the horizon of any place ; and at right angles to this horizon, is fixed a semicircle G, to answer for a general meridian. The middle point of the semicircle answers for the situ- ation of any inhabitant on the earth, for which reason a steel pin is fixed over the middle point of this semicircle. One supposition only is necessary for performing every problem with this globe ; namely, that a spherical lumi- nous body will enlighten one half a spherical opake bo- dy, and, consequently, that a circle at right angles to the central solar ray, and dividing the globe in halves, will be a terminator, showing the boundaries of light and darkness for any given day. * Globes of 9 or 12 inches in diameter, when mounted in the i bove man- ner, are ot the most convenient dimensions. If much accuracy be want- ed, and no limited expense given, an 18 inch globe, so mounted, makes a very useful and illustrative instrument. — E E«it. 164 OF A NEW TERRESTRIAL GLOBE. For this purpose, at the end of the moveable arm opposite to the sun, there is a pillar H, from the top of which projects a piece to carry a circle I, that surrounds the globe, and always divides it into equal portions, sepa- rating the enlightened from the dark parts. Eighteen degrees behind this circle, but parallel thereto, is another circle, to represent the limits of twilight. There are two brass plates, K and L, under the globe, which are turned by the diurnal revolution thereof ; each of them is divided into twice twelve hours and parts of an hour, to answer for the hours of day ; on the out- side are laid down the degrees of longitude for every hour ; so that these circles give you at sight the hour of the day or night, at any two places on the globe, and the difference of longitude corresponding thereto. There is one thing more relative to the globe, which renders it a planetary globe ; for, by setting this pillar, plate 13,y%. 4, to the place of any planet in the eclip- tic, and the ball to the latitude of the planet, it will solve all problems relative to that planet, or the moon ; as it does for the sun, by means of a central solar ray. To rectify this globe, set the division under the repre- sentative inhabitant over the given place, set the solar index, L, to the day of the month ; then turn the globe round on the axis till the meridian coincide with the central solar ray, and the hour-index under the globe to XII ; and this globe is then in the position of our globe, with respect to the sun and that place, &c. Place the inhabitant on the western side of the termi- nator, and he will see the sun or the central solar ray rising in the horizon, and this ray will mark thereon the sun's amplitude, and the hour-circle gives you the hour and minute of the sun's rising on that day and place ; turn the globe gradually till the meridian coincide with the central solar ray, and the point will mark out the sun's meridian altitude for that day. As the globe goes on, the altitude decreases, and when the inhabitant is arrived at the other side of the terminator, the solar ray- is in the horizon, points out the sun's amplitude, and you have the time of his setting on the hour-circle. If you proceed to turn the globe till the inhabitant be under OF THE CELESTIAL CLOBE. 165 that circle which is behind the terminator, the hour-circle will give the time that twilight finishes. The sun's alti- tude for any given time of the day, is obtained by stopping the globe when the index points out that hour, and the quadrant of altitude over the inhabitant, and then bring- ing it to the central ray, which will point out thereon the altitude for that hour. In this manner you may solve the same questions for any other place, or any other day, always observing, 1. To fix the inhabitant over the given place. 2. To set the sun's annual index to the given day of the month. 3. To bring the meridian to the central solar ray, and the hour- index under the globe to XII. By placing the small pillar to the moon's place in the ecliptic, and the ball to her latitude, the same problems may be solved at the same time for the moon ; and so respectively of any other planet. By this globe, a person entirely unacquainted with as- tronomy may, in a few hours, acquire a competent and natural notion of the principal phenomena, and be en- abled to solve the greatest part of the most interesting problems concerning the sun, moon, and planets. OF THE CELESTIAL GLOBE, Jig. 3, plate 13. As the terrestrial globe js mounted to correspond ex- actly with the globe of our earth, and every problem answered as the phenomena are really occasioned by the annual and diurnal motion of the earth ; so the celestial globe, to be comfortable to nature, should be as nearly as possible an exact imitation of the heavens, and their situation with respect to the earth ; which is far from being the case with the common globes. To make the celestial globe thus comfortable to na- ture, it should have no motion ; the appearance of motion in the firmament arises from the diurnal motion of the earth ; it is plain, therefore, that whatever gives a true representation of the heavens will have no motion. The celestial globe before you, is therefore fixed on an axis, making, like that of the terrestrial globe^ an angle \66 OF THE CELESTIAL GLOBE. of 66* degrees with the plane of the ecliptic ; and the ecliptic on this globe exactly coincides with the sun's apparent path round the earth. All problems concern- ing the sun, moon, and planets, are performed by the terrestrial globe. This globe needs only be used for the stars, and one or two problems will give you a suffi- cient idea of the manner of solving all that relates to the stars. Tojind the latitude and longitude of a given star. Find the star on the globe, and then place the index and clip, A, on the ecliptic-plate, slide the siderial index till it be exactly over the star ; then the latitude is shown on the arc, and the longitude, by the index, on the ecliptic. Tojind the rising, setting, amplitude, and meridian alti- tude of the same star. Take the clip from the celestial globe, and put it to the same degree of longitude on the terrestrial ecliptic plate ; turn the globe on its axis, and the time of its rising and setting is immediately pointed out by the hour-index ; its amplitude is shown on the horizon ; its meridian altitude, by the meridian ; and its azimuth and altitude for any hour, by applying the quadrant of altitude under the siderial index for that hour. I shall conclude my lectures on these instruments with some observations that naturally arise from consi- dering the art and ingenuity with which they are con- structed. For, when we see materials working with an art and contrivance that is not in their nature, we are at once convinced that a superior intelligence has been concerned in their arrangement, &c. Thus also in nature, whatever bears the marks of a wisdom not belonging to the known causes produc- ing it, may be properly stiled providential : for, when agents void of wisdom act wisely, it is plain there must be some hand to conduct them, though we may not be able to perceive by what springs or channels of communication it operates. There wants, therefore, no long train of reasoning to lead us into the knowledge of a Providence. Penetration and closeness of thought have no further use in this case, than to discover the fal- lacy of those sophisms, wherein infidel writers endea- CONCLUSIVE OBSERVATIONS. 167 vour to overcloud the most apparent truths. The plain man needs no assistance here from the philosopher, but may say to him as Diogenes did to Alexander •, " Only please to stand out of my sun-shine." Intelligence* is manifested two ways, either by means supplied to answer the endwe conceive to have been had in view, though we do not discern the method by which they were prepared ; or else by the contrivance appa- rent in productions, though we do not see what end they answer : the former more particularly gives us the dis- play of providence ; the latter, of the wisdom where- with it is administered. If you saw a house stored with furniture, utensils, and victuals ; the gardens planted with herbs and fruit trees ; the ground stocked with cows, horses, deer, and poultry, all in a manner fitted for the entertainment and convenience of a family ; you would certainly con- clude, there was some master who had taken care to pro- vide for the uses whereto they were respectively proper. Or, if an ignorant person went into a room where, among scales, weights, compasses, rules, and other things of common use, he should find quadrants, theodolites, armillary spheres, planetariums, tellurians, &c. of whose use, as well as of the figures upon them, he was entire- ly ignorant ; yet he would know, without being told, that they were the work of some artificer proceeding with skill and contrivance, and who made them for purposes worthy the care with which they were finished. In this manner we constantly reason upon common occasions, and there wants only a proper attention to lead us into the like train of thinking upon the pheno- mena of visible nature. For there you may perceive ample provision made in vast variety for the numerous family of Adam ; corn, fruit, herbs, cattle, and fowl, for our sustenance ; wool, flax, and cotton, for our cloathing ; drugs and simples for our relief ; air for our breathing ; timber, stone, lime, and brick-earth for our habitation ; wood and coal for our firing ; beasts of Tucker's Light of Nature, vol. iii. part 1, page 192. 168 CONCLUSIVE OBSERVATIONS. burden for our assistance ; winds to purify our atmos- phere, to re fresh our heats, and waft us from shore to shore ; variety of climes and soils to bear us a produce of every kind ; dews and rains to make them yield us their increase. The sea, that original source of water, so necessary to us for many uses, serves likewise to as- sociate distant nations by opening the communication of commerce. The sun diffuses his warmth and light to cherish us. The distant stars guide us over the boundless ocean and inhospitable desert, extend the fields of science to an immensity of space, and turn the rugged brow of night into a cheerful scene of contem- plation. Even within the narrow compass of our own bodies, we carry about no inconsiderable stores, without which we could not receive benefit from those arround us. We have engines of digestion and secretion, springs and channels of circulation, limbs for instruments of action, bones for our support and protection, organs of speech for our mutual intercourse. What a multitude of ves- sels, glands, and ducts, to concoct and distribute our aliment ! What artificial structure and excellent dispo- sition of muscles and joints, to serve for instruments of action ! What amazing nicety in the organs of sense ! The eye, with her humours and coats mathematically arranged, and duly proportioned one among the other ; the ear in winding and modulating the vibrations of air into sounds ; the nerves in imperceptible threads running every where through the fleshy parts, yet returning their notices without impediment from the farthest ex- tremities of our limbs ! And all this complicated machine, containing an infinitude of multiform works, is bound up in a small compass, yet with such stupendous skill, that they do not interfere with each others operations, nor fall into discord upon our motions ! We have appetite to stimulate, senses to inform, the faculties of comparing, distinguishing, judging, to en- lighten, and reason to direct us. In the capacity of our senses and affections, we have sources of pleasure, en- joymentj, and innocent mirth. CONCLUSIVE OBSERVATIONS. 169 In the multitude of the objects of creation, we find a provision made and suited to our various organs, tastes, and faculties, a fund for bodily support, sub- jects for intellectual inquiries and mental gratification. " Which shall we admire most, the multitude of our organs, their finished form and faultless order, or the power which the mind exercises over them ? Ten thou- sand veins are put into her hands, and yet she manages and conducts them all without the least perplexity or irregularity ; with a promptitude, a consistency^ that nothing else can equal ; touching every spring of the human machine with the most masterly skill, though she knows nothing of the nature of her instrument, or the process of her operation. If you turn your eyes upon the vegitable tribes, you perceive them, in countless multitudes of trees, shrubs, weeds, mosses, &c. each growing, spreading, and flou- rishing, by laws adapted to its own kind ; and all work- ed with such exactness and nicety of art, as the greatest human ingenuity could not imitate ; their sap-vessels curiously woven within the stem, and dispersed among the roots and branches ; their leaves wrought finer than needle-work. The finest works of the loom and the needle, when examined with a microscope, appear so rude and coarse, that a savage might be ashai;;ed to wear them : but, when the work of God is brought to the same test, we see how fibres, too minute for the naked eye, are composed of others still more minute; and these again of others ; till the primordial threads, or first principles of the texture, are utterly undiscern- able ; while the whole substance presents a celestial ra- diance in its colouring, as if it were intended for the cloathing of an angel. Yet are these wonders of the vegetable world sur- passed by those of the animal, whose frame contains a more complicated machinery, capable of more admira- ble play : for, besides the engines of growth and nutri- ment analogous in both, the animal is furnished with organs of sensation, and instruments of activity. What a richness of invention is displayed in the variety of VOL. iv. z 110 CONCLUSIVE OBSERVATIONS. their forms, and the diversity of their cloathing. Nor can we help remarking those surprising instincts that Severally guide them to their harbours, their fo<. ds, their ways of breeding and preservation, instruct them to build their nests, to make their comb, to spin their webs, and provide for the future without knowledge of their wants. Nor must we omit the uses and qualities assigned to animals, that we can turn commodiously to our advan- tage : we have not to seek our wool from the fierce lion, nor want the untameable tyger to plow our grounds ; but the ox, the horse, and the sheep, have docility and manageableness given them for their cha- racteristics. Creatures saleable in the market are more prolific than those of the savage kind. Poultry and rab- bits keep within their accustomed purlieus ; but nobo- dy knows where to find the coarse-grained heron, or the worthless cuckoo.* " O Lord, how manifold are thy works ; in wisdom hast thou made them all ; the earth is full of thy riches! All creatures wait upon thee, that thou mayest give them their meat in due season. When thou givest it them they gather it, and when thou openest thine hand they are filled with good." How great and beautiful is this idea ! The hand of man scatters food to the few crea- tures that are about him ; but when the hand of God is opened, a world is fed and satisfied.! Rev. W, Jones's Sermons, vol, ii. p. 63. f Ibid. p. 104. [ 171 ] APPENDIX TO LECTURE XLIII. BY THE E. EDITOR. CONTAINING A PARTICULAR DESCRIPTION OF THE BEST SIMPLE PORTABLE ORRERY; COMPARATIVE OBSERVATIONS ON THE DIFFERENT MODES OF MOUNTING GLOBfcS ; AND A DESCRIPTION OF THE PORTABLE EQUATORIAL INSTRUMENT. Plate 14, Jig- 2. J\S our author has not given a full reference to the several parts of the orrery, as noticed in page 135, and its being of the kind the most portable and complete hitherto made, I have thought it better to add here a further description of the machine, and some observa- tions on globes and the equatorial. The words, orrery and planetarium, are frequently used indifferently to signify the same instrument. Among instrument-makers, the term, orrery, is generally given to a large and complete machine, showing all the mo. tions of the planetary bodies in the most perfect man- ner possible by wheel-work. The term, planetarium, is given to an instrument showing chiefly the periodical revolutions of the primary planets ; the word, tellurian, to an instrument showing completely the annual and diurnal motions of the earth only ; and lunarium, to an instrument showing the motions and various appearances of the moon only. These instruments have been made of various de- grees of magnitude and perfection, and the completest sort are those that exhibit by wheel-work the periodical revolutions of the satellites, as well as of the primary 172 martin's orrery described. planets and their diurnal motions. The quantity of wheel-work, as well as other machinery, to produce such motions, creates an expense unavoidably great. Rowlefs grand orrery, as made of late years by Wright, appears to be the most expensive one ever made in this country ; the expense of which, I am informed, is not less than 5001. It will be unnecessary to enter here into a detail of the very considerable varieties of inferior orre- ries since made by Ferguson, Martin, and myself; we now make them from 100 guineas downwards to one guinea, and therefore suitable to the different purposes and purses of all students and amateurs of astronomy. The orrery now selected by our Author, see plate 1 1 , Jig. ], and plate 12, Jig. 1 and 2, was contrived by the late learned and ingenious instrument-maker Mr. B. Martin, about the year 1770, and in a small pamphlet published by him in 1771, was announced as an orrery of a new construction, representing in the various parts of its machinery all the motions and phenomena of the planetary system. For a portable instrument, it is in my opinion the most complete and elegant ever made ; excepting the ivory balls representing the planets, it is all made of brass. The brass box, ABC, plate 11, fig. 1, is about 1 1 inches in diameter, and instead of being supported upon three short feet, as represented in the plate, we make the box to be supported on a brass pillar and claw feet, like the stand of the tellurian, plate IS, fig. 3, as originally done by the inventor : in this manner, it is found to be more commodious and conspicuous when in use. To represent all the various motions of the pla- nets by wheel-work in one machine, occasions it to be of great bulk and weight ; Mr. Martin, by constructing his machine in different parts, certainly diminished that unity and magnificence of one great and elegant ma- chine, but at the same time decreased very considerably the expense, and rendered the apparatus, notwithstand- ing, very simple, complete, and instructive. The component parts of this orrery are as follows : 1 . A planetarium, plate 1 1 , fig. 1 , exhibiting the or- der, motion, and aspects, of all the primary planets of martin's orrery described. 173 our system. The new Georgian planet is not represent- ed in the figure, but is usually applied by us to the instru- ment. The planets are easily put on or taken off from their respective sockets occasionally. Their heliocentric places in the ecliptic below, for the day, are to be first set by the astistance of White's Ephemeris or the Nautical Ephemeris for the year ; then, by turning the winch or handle they will all move from west to east, with the same respective motions and periodical times as they have in the heavens ; thus representing in a just manner the Pythagorean, and Copernican or Solar System. For the representation of the Ptolemaic, or vulgar sys- tem, our Author has already given directions at page 145, et seq. The apparatus, plate 1 1, Jig* 2 and 3, is used to ex- emplify the apparent retrograde and direct motions, and sometimes stationary positions, of the planets. You take off the ivory earth ®, and place the socket, P, upon the wire in its stead ; the socket,^. 3, is to be applied upon either the arms of Mercury $ , or Venus 9 , instead of the ivory balls. The retrograde arm or wire, Jig. 2, about the part F, is then to be placed on the nut-piece, as shown at Jig. 3 ; this wire represents a ray of light coming from the planet to the object ; and the small ball, the planet as it appears among the stars in the hea- vens. When the winch is turned, the direct motion, stationary position, and apparent retrograde motion of the inferior planets, as seen from the earth, will be clearly shown agreeable to what our Author has before related, see page 142, and plate \\^ Jig. 2. 2. The tellurian, plate 12 , jig. 1, is the second part of this orrery. When the planetary arms are all taken off from their central sockets, this part is to be applied ; it is made fast on the socket by the brass screw-nut, D. The globe is three inches in diameter, by which is accu- rately and evidently shown all the phenomena arising from the annual and diurnal motions of the earth. For this purpose, the axis of the earth keeps a perfect paral- lelism and constant inclination to the plane of the eclip- tic : the circle of illumination or terminator, H, divides the globe into its enlightened and dark hemispheres, and 174 martin's orrery described. and by the dial-plate, N O, under the globe, it will appear at what hour the sun rises and sets to every country and on every day of the year. There is an index, G, to show when it is noon, or when the sun is upon the meridian of any particular place ; by the same index is also shown what sign or degree of the ecliptic the sun is in for every day, the parallel of declination it describes, and the length of the diurnal and nocturnal parts of that parallel in the light and dark hemispheres. At times, it is convenient to have a slower motion of the earth, for which there is in the box a provision made, and you have only to shift the winch from one hole in the side of the box to the other. When the tellurian is used, it is to be put upon the socket of the earth, and placed exactly over the degree of the ecliptic which the earth possesses that day, or so that the index, E, at the end of the arm, may point to the sun's place in the ecliptic ; and then screwing it fast upon the socket by the nut D, and turning the winch, the proper motions will commence. To explain more completely the various phenomena of the earth, another three- inch globe is made, furnished with a brass divider, meridian, horizon, and quadrant of altitude, similar to those shown at plate 13, Jig. 2, all graduated ; and also a circle to represent the twilights. By this, all the chief problems on the terrestrial globe may be performed and illustrated in a very natural man- ner, and has been described by our Author at page 151. 3. The lunarium\ plate 1 2, fig. 2, is placed upon the same socket, -fig. 1 , instead of the tellurian. Its machi- nery is also put into motion by the teeth of the large fixed ecliptic plate P Q^ The motions shown by this part an- swer all the phenomena of a satellite or moon revolving about its primary, while that moves about the sun. These motions and appearances are as follows : 1 . The menstrual motion of the earth and moon about the common centre of gravity, at z,fig. 2, between them. 2. The circular motion of this centre of gravity about the sun, which describes the true annual orbit, and in which the earth has a very irregular, or rather vermicu- martin's orrery described. 175 lar motion, from one side to the other, being in each month nearer to and farther from the sun. 3. The monthly motions of the moon, viz. the peri- odical month, in which the moon describes exactly the ecliptic ; and the synodical month, which shows the space or time between two moons or conjunctions. 4. The annual parallel motion of the ecliptic showing the place of the moon ; by the ring, T Q, the place of the nodes ; by the oval ring, X V, the apogee and peri- gee, as also the geocentric motions, places, and aspects of the sun and all the planets. 5. The retrograde motion of the nodes, with the in- clination of the lunar orbit, and the degree of her latitude from the ecliptic in every part. 6. The direct motion of the apogee, or the line of the apsides, with the situation of the ecliptic orbit of the moon, and the place of the apogee in the ecliptic at any time. 7. The mechanism of the wheels, &c. being such as to show the phases of the moon, and always to show the same face to the earth, every way similar to the face of the moon in the heavens at the same time. By such a variety of motions in the lunarium, it will be easy to see the rationale of most of the lunar irregu- larities and variable phenomena ; also every thing relative to the nature and doctrine of eclipses, both solar and lunar, total, annular, and partial. A small brass lamp, to be lighted with oil and cotton, or a wax-taper, usually accompanies the instrument, which is fitted to the stem where the sun is applied. In a dark- ened room, with the lamp lighted, the several phenomena, exhibited by the parts of the machine, are more illustra- tive and striking. A machine, or secondary planetarium, is sometimes made, either to correspond with that of plate 1 1, fig. 1, or to be occasionally connected with it, and is called the Jovian system. This part shows the motion of Jupiter's four moons or satellites, as nearly corresponding to their motions in the heavens as the latest observations upon them will admit of. The distances of the satellites have here the same proportions as in the heavens, which are expressed in semidiameters and decimal parts of Jupiter's 176 OBSERVATIONS ON GLOBES. globe. In a similar way, machines are made to repre- sent the motions of the satellites of Saturn, and ot the Georgian planet. To illustrate the nature and manner of the precessions of the equinoxes, and the phenomena of the transirs of Mercury and Venus over the face of the sun, apparatuses might easily be adapted ; but, like the Jovian and Satur- nian machines, they are only made from particular orders, as not being the most material parts of the orrery. COMPARATIVE OBSERVATIONS ON GLOBES, MOUNTED IN THE COMMON MANNER, AND THOSE MOUNTED in the improved method, see platel3, fg. 2 and 3. As I do not agree in opinion with our late Author in his assertion, that the mounting of globes in the common way, renders them unfit for the intended purposes of in- struction, and as conveying unnatural and confused ideas to the learner, see page 162, 1 will only trouble the reader with a few remarks, and leave him to judge which he con- ceives to be the most perspicuous and useful for his pur- pose ; both methods, in many cases, have their peculiar advantages. Globes are not designed as instruments of accurate cal- culation, or universal illustration ; they are made for the assistance, in a familiar way, of beginners, in the perform- ance of a variety of useful and instructive problems in the sciences of astronomy, geography, navigation, spherical trigonometry, dialling, chronology, &c. &c. The more simple the manner of mounting globes for these purposes, the more easy and familiar will the operation be found by the learner. On this account, 1 always preferred the me- thod of a well-divided horary circle at the north pole, to the semicircular wires and sliding points, adopted origi- nally by our late Author's father. In the new 18 inch globes, noticed in my note in page .501, vol. iii. I have constructed the hour circles with-such clear and fine divi- sions, as to show the time to five minutes of a degree, which is quite sufficient for ail the material problems, and is as accurate as the fitting up of a globe to be depended upon, can be made. OBSERVATIONS ON THE GLOBES. 177 No terrestrial globes convey just representations of the position of our earth and the heavens, for any particular day, till they be rectified completely, and duly set by the compass. In respect to the shifting of the inclination of the axis or the north pole, considered by our Author as an absurdity, it must be observed, that when properly set, the horizon is a true representation of the place it is rec- tified for. The relative position of the horizon to the axis is the same in both methods of mounting. In the globe mounted, as shown in plate 13, Jig. 2, the moveable horizon is shifted, as it would naturally shift, should an inhabitant move from one latitude to an- other. In the common globe you elevate the pole to the same effect; and, though in London, should you elevate the pole for the latitude of Jamaica, the axis at London would not point to the north pole, or polar star; yet, supposing you vvere at Jamaica, it would then point justly. The axis of the earth, with respect to the horizon, may therefore be said, relatively to the horizon, to be conti- nually shifting, as an inhabitant changes his latitude. This can be easily understood by the learner, and cannot be the means of any confused ideas. The horizon on the broad paper-circle upon the stand of the globes, containing the ecliptic, divided into signs and degrees, and a contiguous calendar, is not designed to represent more than the horizon of a place. The gra- duated circle of the ecliptic is useful for finding the place of the sun for any day in a ready manner, without the trouble of referring to an ephemeris ; the sun's place being found, the ecliptic circle upon the globe is imme- diately used to have the requisite mark set on it. The sun's place being thus fixed on the globe, the perform- ance of a problem, by turning the sun and globe round together, is certainly not strictly agreeable to nature, but in part so, and is quite sufficient for the result of the problem. In the inspecting of particular countries on the terres- trial globe, it is proper to have it quite clear, and with- out being obliged to push away any contiguous appenda- ges. For the performance of the problems on the pla- nets, and tracing the paths of comets, &c. the celestial VOL. IV. A 2 178 OBSERVATIONS ON THE GLOBES. globe, mounted in the common way, will be found more convenient. Upon the whole, therefore, I judge the fol- lowing particulars to be the chief advantages in the im- proved mounted globes, and those mounted in the com- mon manner respectively. Advantages peculiar to the new-mounted globes y as repre- sented in plate 13, fig. 2 and 3. 1. The axis always retains a natural and permanent position, directed to the north pole in the heavens, when duly set by the compass. 2. The sun upon the stem E, jig. 2, moon, or planets, being in a fixed position, and the globe turning about its axis, is the natural and just representation of the cause of all the various phenomena respecting their rising, cul- minating, setting, lengths of days and nights, &c. result- ing from the diurnal motion of the earth, and the respec- tive situations of the planets in their orbits. 3. The moveable horizon, meridian, &c. applied to the globe, are to have their positions naturally changed, according to the motion or situation of an inhabitant of the globe. 4. The globe being placed in a darkened room, and a candle or lamp being placed at a proper distance, in a line with the centre of the sun ; the globe will be divided, as in nature, into the enlightened and darkened hemi- sphere, and show in what degree the various countries enjoy the presence of the sun or day, and the lengths, at that time of the year, of the day and night. 5. By the wheel- work, contained underneath the plate C D, the relative position of the axis to the terminator of light and darkness is shown, consequently, the state of the light and darkness of the globe at any season of the year. This change of the position of the axis is not agreeable to nature, but sufficient to explain the phenomena. In this instance it makes a complete tellurian upon a large scale; but, in respect to the permanent parallelism of the axis, it will be best explained by the tellurian of the improved orrery before described : see plate 1 2, jig. 1 . OBSERVATIONS ON THE GLOBES. 179 Advantages peculiar to the globes, mounted in the common manner, 1. Their having no brass circles, or other appendages, contiguous to their surfaces, renders them easy and accu- rate for inspection, and in the performance of a variety of problems. 2. The external appendages are less in quantity than in the globe shown at plate 1 3, jig. 2, consequently more easy and ready to the learner. 3. In many cases, where the use of a quadrant of alti- tude is necessary, they are done in a ready manner, which could not be the case with the new-mounted one. 4 Ail great circles of the sphere, being imaginary, and referred to in the heavens, the position of right, pa- rallel, and oblique, to the inhabitants of the globe, can only be represented by the common mounted globe, where there is a contrivance of the horary circle being under the meridian, or to shift away occasionally from the pole. 5. Globes of large dimensions, such as 1 8 inches in diameter and upwards, are much less expensive and port- able, mounted in the common way, than in the other way. These observations are quite sufficient to give the reader a general idea of the merits, and unavoidable defects of both mountings. In the performance of a variety of pro- blems upon the common globe, he will find many others that are not necessary to be noticed here ; and to con* vince him, that a rational knowledge of the celestial phe- nomena can only be obtained by joining observations of the heavenly bodies with the portions of his studies on globes, orreries, and other astronomical instruments. Besides the methods of mounting globes just described, other mountings are applied for the following purposes, 1. A globe to show the phenomena of the transits of Mercury and Venus over the sun. 2. A globe to show the phenomena of solar and lunar eclipses on all places of the terrestrial globe, called an eclipsareon. 180 DESCRIPTION OF THE EQUATORIAL. 3. A globe to show the nature and manner of the pre- cession of the equinoxes, and thereby the difference be- tween the sidereal and tropical years, as also the apparent and direct motion of the fixed stars. 4. A celestial globe, with a telescope to fix on the north pole of the globe occasionally, with a divided arc, &c. &c. An apparatus of my invention for observing any celestial body, and thereby, in an instantaneous manner, obtain- ing all the particulars of any phenomena presenting them- selves in the heavens. 5. A lunar globe, or the selenegraphia, invented by Mr. John Russel, R. A. forming an apparatus for exhi- biting the phenomena of the moon, and the useful pur- poses to which it may be applied. A DESCRIPTION OF THE EQUATORIAL, OR UNIVER- SAL SUN-DIAL. In the former edition of this Work, our Author omit- ted the description of the equatorial, as represented in plate 14, fig, 2. A description, with a variety of pro- blems to be performed with it, he published in his Astro- nomlcal and Geographical Essays , 8vo. 4th edit. 1794, and to which I must refer the reader, as the limits of this work will admit but of a concise description and use of the instrument. An equatorial instrument is the most general and com- prehensive of all instruments made for the purposes of practical astronomy. It is the best of any to exercise the beginner in the science. For when he has obtained a per- fect knowledge of the management of this, there will be no other construction of an astronomical machine, but what he will comprehend the use of in a ready manner. An equatorial is considered by astronomers as an in- strument useful in taking the following particulars of the heavenly bodies: their altitudes and azimuths ; their right ascensions and declinations ; to determine the latitudes and longitudes of places ; to find, by observations on the sun and stars, the hour of the day and night ; to mea- sure angles universally, or the distances of objects on land, and to take angles in general for the resolution of DESCRIPTION OF THE EQUATORIAL. 181 many problems in practical astronomy, trigonometry, &c. &c. Plate 14, jig. 2, represents one of the smallest dimen- sions usually made. Its equatorial circle, M N, is about four inches in diameter, and the rest in proportion. The instrument consists of the following parts : a horizontal circle E F, divided into four quadrants of 90° each, with a fixed nonius or vernier scale at N, and the circle itself may be turned by the hand on its centre or axis. A strong pillar is fixed to the centre of the horizontal circle, sup- porting the centre of a vertical semicircle A B, divided into two quadrants of 90° each. This is called the semi- circle of altitude, as it is used to take angles of altitude and depression. There is a vernier scale at K. At right angles to the plane of this semicircle, the equa- torial circle, M N, is fixed, representing the equator of the globe, and divided into 24 hours, or twice 12 hours, each hour being subdivided into five minutes. Upon this cir- cle moves another with a chamfered edge, carrying a no- nius, by which the divisions of the equatorial are further subdivided, and are read off to single minutes. At right angles to this circle is fixed the semicircle of declination, D, divided into two quadrants of 90° each. The brass bar, that carries the sight O P, is fixed to an index moveable on the semicircle, and carrying a nonius at Q^ The sight O, to which the eye is to be applied, has two small holes, and a dark glass for screening the eye from the sun ; and the sight, P, has two small pieces screwed on, the lower with a small hole to admit the rays from the sun, and the upper two cross wires for observa- tions by their intersection. There are two spirit-levels, L, L, fixed on the horizontal circle, at right angles to each other, which, with the three adjusting-screws, I, G, H, are useful for duly levelling the instrument. A small telescope is sometimes applied in place of the two sight-pieces P, O. I shall here insert our Author's directions for adjusting the instrument, and one -problem, as an example of its use; for further information, the reader may consult his Astronomical Essays. 182 ADJUSTMENT OF THE EQUATORIAL. Probl m I. To adjust the equatorial for observation. Set the instrument on a firm support. First, to adjust the levels, and the horizontal or azimuth circle. Turn the horizontal circle till the centre-line, or o, of the divi- sions coincide with the middle stroke of the nonius, or near it. In this situation, one of the levels will be found to lie either in a right line joining the two foot-screws, which are nearest the nonius, or else parallel to such a right line. By means of the two last-mentioned screws, cause the bubble in the level to become stationary in the middle of the glass ; then turn the horizontal circle half round, by bringing the other o to the nonius ; and if the bubble remain in the middle, as before, the level is well adjusted ; if it do not, correct the position of the level, by turning one or both of the screws which pass through its ends, by means of a turn-screw, till the bubble have moved half the distance it ought to come to reach the middle ; and cause it to move the other half, by turning the foot-screws already mentioned. Return the horizon- tal circle to its first position, and if the adjustments have been well made, the bubble will remain in the middle ; if otherwise, the process of altering the level and the foot-screws, with the reversing, must be repeated till it bear this proof of its accuracv. Then turn the horizon- tal circle till 90° stand opposite to the nonius; and by the foot-screw immediately opposite the other 90°, without touching the others, cause the bubble of the same level to stand in the middle of the glass. Lastly, by its own proper screws set the other level, not yet attended to, so that its bubble may occupy the middle of its glass. Secondly, to adjust the line of sight. Set the nonius on the declination-semicircle at o, the nonius on the horary circle at VI, and the nonius on the semicircle of altitude at 90°. Look through the sights towards some part of the horizon, where there is a diversity of remote objects. Level the horizontal circle, and then observe what object appears on the centre of the cross-wires. Reverse the semicircle of altitude, so that the other 90° may apply to the nonius ; taking care, at the same time, that the other ADJUSTMENT OF THE EQUATORIAL. 183 three nonii continue at the same parts of their respective graduations as before. If the remote object continue to be seen on the centre of the cross-wires, the line of sight is truly adjusted ; but if not, unscrew the two screws which carry the frame of the cross- wires, and move the frame till the intersection appear to lie on a new object, half-way between the object first observed, and that to which the wires are applied in the last position. Return the semi- circle of altitude to its original position : if the intersex tion of the wires be then found to be on the object to which they were last directed, the line of sight is truly adjusted ; but if not, the frame must be again altered as before: and the same general operation must be repeat- ed, till the cross- wires in both positions apply to the same object. Besides this adjustment of the centre of intersection, it is necessary that one of the wires should be in the plane of the declination-semicircle, and the other at right an- gles to that plane. As the wires are fixed at right angles to each other, the adjustment of one of them will be sufficient. For this purpose, observe any small object on one of the wires ; if it be the vertical wire, move the index of the semicircle of declination ; or if the other, move the last-mentioned semicircle on the axis of the equatorial circle. In either case, the object will coincide with the wire during its motion, if the position be right ; if not, alter that position, taking care not to displace the centre from its adjustment. To adjust the piece which carries the hole for forming the solar spot, direct the sights to the sun, so that the centre of the luminous circle, formed by the aperture which carries the cross- wires, may fall precisely on che upper sight-hole. Then move the frame, with the small perforation, till the solar spot fall exactly on the lower $ighNhole. Thirdly, to find the correction to be applied to obser- vations by the semicircle of altitude. Set the nonius on the declination-semicircle to o, and the nonius on the horary circle to XII ; direct the sights to any fixed and distant object, by moving the horizontal circle and semicircle of altitude, and nothing else \ note the degree and minute 184 TO OBSERVE BY THE EQUATORIAL"! of altitude or depression ; reverse the declination-semi- circle, by directing the nonius on the horary circle to the opposite XII ; direct the sights again to the same object by means of the horizontal circle and semicircle of alti- tude, as before. If its altitude, or depression be the same as was observed in the other position, no correction will be required ;' but if otherwise, half the difference of the two angles is the correction to be added to all observa- tions or rectifications made with that quadrant, or half of the semicircle, which shows the least angle; or to be sub- tracted from all observations or rectifications made with the other quadrant, or half. When the levels and cross- wires are once truly set, they will preserve their adjustment a long time if not de- ranged by violence ; and the correction to be applied to the semicircle of altitude is a constant quantity. For the observations on the sun and the rest of the hea- venly bodies, a suitable fixed position in a room should be determined for the instrument. Problem II. To measure angles, cither of azimuth, alti- tude, or depression. Set the middle mark of the nonius on the declination at o, and fix it by means of the milled screw behind. Set the horary circle at XII, on the equator, and the instru- ment, previously adjusted, is ready for observation. Then, if the sights be directed successively to any two objects, the degrees and minutes contained between the two posi- tions of the nonius, on the limb of the horizontal circle, will show the horizontal angle of the quadrant. And likewise, if the sights be directed to any object, by moving the horizontal circle and semicircle of altitude, the degree and minute marked by the nonius on the last-mentioned semicircle will be the angle of altitude, if on the quadrant or part nearest the eye ; or of depression, if on the re- moter quadrant. Remark. It is proper in this place to describe the na- ture and use of the admirable contrivance, commonly called a vernier or nonius. It depends on the simple cir r TO OBSERVE BY THE EQUATORIAL. 185 cumstance, that if any line be divided into equal parts, the length of each part will be greater, the fewer the di- visions ; and, contrariwise, it will be less in proportion as those divisions are more numerous. Thus it may be observed, that the distance between the two extreme strokes on the nonius, in the equatorial before us, is ex- actly equal to eleven degrees on the limb, but that it is divided into twelve equal parts. Each of these last parts will therefore be shorter than the degree in the propor- tion of 11 to Vl\ that is to say, it will be one-twelfth part, or five minutes shorter. Consequently, if the mid- dle stroke be set precisely opposite to any degree, the rela- tive positions of the nonius and the limb must be altered live minutes of a degree, before either of the two adja- cent strokes next the middle, on the nonius, can be brought to coincide with the nearest stroke of a degree ; and so, likewise, the second strokes on the nonius will require a change often minutes; the third of fifteen, and so on to thirty, when the middle line of the nonius will be seen to be equidistant from two of the strokes on the limb; after which, the lines on the opposite side of the nonius will coincide in succession with ihe strokes on the limb. It is clear from this, that whenever the middle stroke of the nonius does not stand precisely opposite to any de- gree, the odd minutes, or distance between it and the de- gree immediately preceding, may be known by the num- ber of the stroke on the nonius, which coincides with any of the strokes on the limb. It must be observed, how- ever, that as the degrees in the several quadrants are rec- koned in opposite directions, so likewise the nonius has two sets of numbers ; for the use. of which it need only be remembered, that they always begin .from the middle, and go to 30 minutes, and thence from the opposite SO minutes in the same direction to the middle ; and that, they must always be reckoned in the opposite direction to the degrees on the limb. In this instrument they must be read in the opposite direction; but when the nonius-plate has its divisions fewer than the number of parts on the limb to which it \0L. IV. b2 186 OBSERVATIONS ON THE EQUATORIAL. is equal, they coincide successively in the same direction as that of the motion of the index. The angles by this small equatorial are usually shown to 5' of a degree, and the time to 1 minute. Equatorials, when made for accuracy, are of much larger dimensions, and the various circles moved by teeth and pinion. The equatorial circle is commonly made from 6 to 18 inches diameter, and the angles shown to 1 minute of a degree, and the time to 10 seconds. It is an instrument not now so much in repute as for* merly. The several circles are very difficult to be made without some small eccentricity.* The better kind are therefore now made upon a simpler construction ; or, in- stead of them, a vertical and horizontal circle, called a circular instrument , which, having but two principal cen- tres or axes, is found more accurate and permanent. The altitude and azimuth of the objects are only by this ob- tained ; but these are the chief data in practical astrono- my and topography, and from them all the requisite de- ductions and purposes can readily be obtained. The usual diameters of the principal vertical circle of the circular instrument are from 1 2 inches to 2 feet, but there are no fixed dimensions ; the construction of the observatory, or place of observation, is the chief guide to the astronomer in the choice of his instruments. To these instruments, as well as to the equatorial of the better kind, achromatic telescopes are applied, that will enable the observer to see the planets and stars in the day time. * It is now the practice in all the best and large angular instruments to apply two opposite verniers or nonii, by which any small eccentricity is easily observed and allowed for after the observation.-' [ 187 ] LECTURE XLIV. OF THE FIXED STARS. JN O part of astronomy gives such enlarged ideas of the structure and magnificence of the heavens, as the consideration of the number, magnitude, and distance of the fixed stars. We admire, indeed with propriety, the vast bulk of our own globe ; but when we consider how much it is sur- passed by most of the heavenly bodies, what a point it de- generates into, and how little more even the vast orbit in which it revolves would appear, when seen from some of the fixed stars, we begin to conceive more just ideas of the extent of the universe, and the boundaries of crea- tion. The most conspicuous and brightest of the fixed stars of our horizon is Sirius. The earth, in moving round the sun, is 190,OCO,000 miles nearer to this star in one part of its orbit, than in the opposite ; yet the magnitude of the star does not appear to be in the least altered, or its distance affected by it ; so that the distance of the fixed stars is great beyond all computation. The un- bounded space appears filled at proper distances with these stars, each of which is probably a sun, with attend- ant planets rolling round it. In this view, what, and how amazing, is the structure of the universe ! Though the fixed stars are the only marks by which astronomers are enabled to judge of the course of the moveable ones, and we have asserted that their relative positions do not vary ; yet this assertion must be confined within some limits, for many of them are found to under- 188 OF THE FIXED STARS. go particular changes, and perhaps the whole are liable to some peculiar motion, which connects them with the universal system of created nature. Dr. Herschel even goes so far as to suppose, that there is not, in strictness of speaking, one fixed star in the heavens ; but that there is a general motion of all the starry systems, and conse- quently of the solar one among the rest. There are some stars, whose situation and place were heretofore known, and marked with precision, that are no longer to be seen ; new ones have also been disco- vered, that were unknown to the ancients, while num- bers seem gradually to vanish. There are others, which are found to have a periodical increase and decrease of magnitude ; and it is probable, that the instances of these changes would have been more numerous, if the ancients had possessed the same accurate means of examining the heavens, as are used at present. New stars offer to the mind a phenomenon more sur- prising, and less explicable, than almost any other in the science of astronomy ; I shall select a few instances of the more remarkable ones for your instruction : a consi- deration of the changes that take place, at so immense a distance as the stars are known to be from you, may ele- vate your mind to consider the immensity of his power, who regulates and governs all these wide-extended mo- tions ;' ' € who hath measured the waters in the hollow of his hand, and meted out the heavens with a span." Who turns his eye on Nature's midnight fare, But must inquire — What hand behind the scene, What arm almighty, put these whei ling globes In motion, and wound up the vast machine? \> lid l di 111 hape ; in the northern side three very faint start* may be seen, as also one or two in the southern : the verticies of the longer axis seem less bright, and not so well defined as the rest. PLANETARY NEBULA. These are so named from a singulariry of appearance, which renders it difficult to class them. Their light is so uniform and vivid, the diameters so small and well defined, as to make it improbable that they should be N common nebulae : if nebulae, they must be compressed and condensed in the highest degree. Though the words condensation and cluster often occur in the foregoing extract, we are by no means to infer that any of the celestial bodies, in our nebula, are nearer to one another than we are to Sinus, whose distance is supposed to be not less than 18,717,442,526 of miles. The whole extent of the nebulae, being in some places TELESCOPIC APPEARANCE OF THE PLANETS. 199 near 500 times this distance, must be such, that the light of a star placed at its extreme boundary, supposing it to fly with the velocity of 1 2,000,000 miles every mi- nute, must have taken near 3000 years before it could reach us.* These immense spaces, these numerous hosts of sys- tematic universes, are probably connected the one with the other. Like so many immense circuses, by the mu- tual contact of their circumambient spheres, they press each other : these aerial atmospheres being also connect- ed and interwoven together by an infinity of insertions, constitute a celestial sphere, which is again linked with others, till by an infinity of orbs they obtain a form, which is the origin and pattern of all forms, in which all the variegated sidereal revolutions harmoniously concur to one and the same end ; that of mutually strengthening and establishing each other, and forming a celestial union. OF THE TELESCOPIC APPEARANCE OF THE PLANETS. OF THE SUN. " The observations which might with fulness of evi- dence confirm the opinion of planetary worlds, seem to be placed out of our reach, and we can scarce hope to make our optical instruments sufficiently perfect to ren- der the inhabitants thereof visible to us. All, therefore, that we can do, is to examine whether the planets be ac- commodated with those things which we are used to consider as necessary to animal existence. Lands, seas, clouds, vapours, and an atmosphere, or body of air, are objects that we may expect to find on the face of an in- habitable world." By means of the telescope, we are enabled in some measure to ascend into the celestial region, and view the sun, moon, and stars, as they would appear to us if they were brought so many times nearer to us as the telescope * The exact positions of new planetary nebulae, &c. as discovered by Dr. Herschel, air' communicated to me by him, are placed on the new 18 inch British celestial globt E. Edit. 200 THE TELESCOPIC APPEARANCE magnifies ; the light proceeding from the luminary we are looking at being diminished in the same proportion. The telescope is one of those discoveries, of which no idea could have been formed, previous to the period in which the Supreme Being was pleased to unveil to the human mind some of the mysterious powers of glass: the importance of this discovery, and the extent to which it may be carried, still lie hid among the secrets of infinite wisdom. It is by this instrument, more than by any other, that we have been led onward in our advances to- wards a perfect knowledge of the heavenly bodies, and that astronomy has been raised, from little more than a catalogue of stars, into a science. When we look at the sun through a telescope even of moderate power, the eye being defended by a piece of coloured or smoked glass, nay, sometimes even by the naked eye, when guarded in the same manner, we dis- cover on his surface many black, or rather less bright spots, of various sizes and shapes. Sometimes these spots will vanish in a very short time after their first appear- ,ance ; sometimes they travel over his whole disk, or vi- sible surface, from west to east, when they disappear, and in twelve or thirteen days appear again, so as to be known, by their magnitude and figure, to be those that had dis- appeared before. Those, however, which are of the long- est continuance, do not appear to have much. solidity of consistence, for in a little time they also vanish, or become bright like the rest of the surface. The spots are more frequent at some periods than at others; in some years, the sun's disk has for many months been perfectly free from them ; in others, he has for months been more or less obscured !>y spots : the most remark- able phenomena of these spots, as observed by Schenier and Hevetius, are as follow : 3 . Every spot, which has a nucleus, of dark part, hath also an umbra, or fainter shade, surrounding it. 2. The boundary betwixt the nucleus and umbra is always distinct and well defined. 3. The increase of a spot is gradual, the breadth of the nucleus and umbra dilating at the same time. 4. In like manner, the decrease of a spot is gradual, the breadth of the nucleus and umbra diminishing at the same lime. OF THE PLANETS. 201 5. The exterior boundary of the umbra never consists of sharp angles, but is always curvilinear, how irregular soever the outside of the nucleus may be. 6. The nucleus of a spot, whilst on the decrease, often changes its figure, by the umbra incroaching irregularly upon it; insomuch, that in a small space of time new incroachments are dis- cernible, whereby the boundary between the nucleus and umbra is perpetually varying. 7. It often happens, that by these incroachments the nucleus of a spot is divided into two or more nuclei. 8. The nuclei of the spots vanish before the umbra. 9. Small umbras are often seen with- out nuclei. 10. A large umbra is seldom seen without a nucleus in the middle of it. 11. When a spot, which con- sisted of a nucleus and an umbra, is about to disappear, if it be not succeeded by a faecula, or spot, brighter than the resc of the disk, the place it occupied is in a very lit- tle time not to be perceived. In the Philos. Trans, vol. Ixiv. the reader will find se- veral curious observations on these spots by Professor Wilson and the Rev. Mr. Wolaston. The latter gentleman says, he once saw, with a twelve-inch reflector, a - spot burst to pieces, while he was looking at the sun ; the ap- pearance was to him as that of a piece of ice, when dash- ed on a frozen pond, which breaks to pieces, and slides in various directions. The spots are by no means confined to one part of the sun's disk, though we do not know that any have been observed about his polar regions. Though their direction is from east to west, yet the paths they describe in their course over the disk, are exceedingly different, sometimes being in straight lines, sometimes in curves ; at one time descending from the northern to the southern p*rt of the disk, at other times ascending from the southern to the northern part. The larger spots, most of which exceed the whole earth in magnitude,, last a considerable time, sometimes three months before they disappear, at which time they are generally converted into spots exceeding the rest of the sun in brightness. The general opinion concerning their nature is, that they are volcanoes, or burning moun- tains of immense size ; and that when the eruption is VOL. IV. 2 D 202 OF THE PLANETS. nearly ended, and the smoke dissipated, the fierce flames are exposed, and appear as luminous spots. D. Wilson supposes them, on the other hand, to be excavations in the luminous matter, or atmosphere, that environs the body of the sun. The diameter of a spot near the middle of the disk, is measured by comparing the time it takes in passing over a cross-hair in a telescope, with the time wherein the whole disk of the sun passes over the same hair. It may also be measured by a micrometer. Hevelius observed a spot that rose and vanished in 16 or 1 7 hours. None have been observed to continue longer than 70 days.* OF THE MOON. When we look at the moon with the naked eye, we discern a great number of irregular spots on her disk, distinguished by their dark colour from the brighter or more glaring parts ; but when viewed through a tele- scope, their number is prodigiously increased ; and it is perceived, that many of these appearances are occasioned by vast obscure pits or cavities, and elevations or moun- tains. The spots in the moon always keep their places, not being moveable like those of the sun. Sometimes more or less of the northern, and southern, and eastern^ and western part of the disk is seen, which is owing to what is called her libration. These mountains and cavities are known to be such, from the shadow they cast. In the first and second quar- ters, when the light of the sun falls obliquely upon them, the elevated part casts a triangular shadow on the side opposite to the sun ; whereas, with respect to the cavi- * Dr. Hemchel'8 opinion of the spots en the sun has been given in my note in page 15 of this volume. It may be proper to add here, that from the changes in the atmosphere of Jupiter he accounts for the phenomena of his belts; and on a similar principle he illustrates the various appearances of a solar spot which he observed in the sun in 1779. This spot, he says, ex- tended above 50,000 miles, and tiq. think** may be easily jnd satisfactorily explained, if we allow that the real body of the sun itself was seen on this oc- casion, though we rarely see more than its shining atmosphere. This hy- pothesis he also applies to the solution of phenomena exhibited by other spots, as observed by him*.*. E. Edit. t)F THE PLANETS. 203 ties, these have that side which is opposite to the sun illu- minated, and that which is next the sun is dark and ob- scure, the same as would happen to a hollow bason, pla- ced on a table at some distance from a candle, in a room where there was no other light. The shadows shorten as the sun becomes more directly opposed to the anterior face of the moon, and at length disappear at the time of the full. During the third and last quarters, the shadows appear again, but all fall towards the contrary side of the moon, though still with the same distinction, namely, that the mountains are dark and shady on the side farthest from the sun, and the pits are dark on the side next the sun. The full moon is a very pleasing sight through a tele- scope, and has a great variety of lustre and colour; but this is not the phase on which to discover the mountains, these are best seen at the increase or decrease ; for, be- sides the evidence derived from the shadows, we may then see the tops of these mountains catching the rays of the sun before they reach that part of the surface on which their bottoms are placed. On April 19, 1787, Dr. Herschel observed some ap- pearances on the surface of the moon, which, judging by analogy from things perceived here with us, he thought he might term volcanoes. Three of these he observed in different places of the dark part of the moon ; two of them appeared nearly extinct, or going to break out; the third, as an actual eruption of fire, or luminous matter. On the 20th it burnt with greater violence, and might be computed to be about three miles in diameter : the eruption resem- bled a piece of burning charcoal, covered by a thin coat of white ashes ; all the adjacent parts of the volcanic mountain were faintly illuminated by the eruption, and were gradually more obscure as they lay at a greater dis- tance from the crater. Dr. Herschel had, in 1 783, observed an eruption, somewhat similar to that of the foregoing volcano. Indeed an appearance of this kind had been seen before, by Don Ul/oa, in an eclipse of the sun. It was a small bright spot, near the margin of the moon, which he supposed to be a hole with the sun's light shin- ing through it. 204 OF THE PLANETS. That the moon is surrounded by an atmosphere, is rendered probable by many observations of solar eclip- ses, in which the edge or limb of the sun was observ- ed to tremble just before the beginning. The planets are likewise observed to change their figure from round to oval, just before the beginning of an occultation be- hind the moon, which can be attributed to no other cause, than that their light is refracted by being seen through the moon's atmosphere. That we see no clouds, will not appear surprising, if we consider, that the lunar days and nights are thirty times as long as ours ; it will be easy to conceive, that with them the phenomena of vapours may be very different from what they are with us ; perhaps their clouds and rain, if any, may be condensed into visible quantities only during the absence of the sun, and of course when they must be invisible to us.* Mercury being at all times near the sun, we can on- ly distinguish by the telescope, a variation of his figure, which is sometimes that of a half moon, sometimes a little more or less than half. Venus, when in the form of a crescent, and at her brightest times, affords a more pleasing telescopic view than any other of the heavenly bodies ; her surface is diversified with spots, like those of the moon ; by the motion of these, the time she takes up in revolving up- on her axis is discovered. With a powerful telescope, mountains, like those in the moon may be seen.f Mars appears always round and full, except at the time of the quadrature, when its disk appears like that of the moon about three days after the full. By the * Dr. Herschd conceives, that probably all the planets emit light in some degree from their circumambirMt atmospheres, consisting of vanouselastic fluids, some of which exhibit a shirting brilliancy, while others are merely- transparent ; and that from the removal of this fluid die dark body of the planet becomes visible. As a proof of this, he alleclges the observation of a lunar eclipse in 1790, in which there could be no illumination from the rays reflected by our atmos- phere, the focus in which they meet being more than 189,000 miles beyond the moon. — E. Edit. t Dr. Herschcl has observed a faint illumination in the unenlightened part of the planet Venus, which he ascribes to some phosphoric quality of its atmosphere. — E. Edit. OF THE PLANETS. 205 spots which are seen on its surface, its diurnal revolu- tion has been ascertained. From its characteristic rud- diness, and from other phenomena, it has been suppos- ed that its atmosphere is nearly of the same density with ours. Dr. Herschel has observed two white lu- minous circles surrounding the poles of this planet ; they are supposed to arise from the snow lying about those parts. The appearance of Jupiter through a telescope, opens a vast field for speculative inquiry. The surface is not equally bright, but is distinguished by certain bands, or belts, of a duskier colour than the rest of the sur- face, running parallel to each other, and to the plane of its orbit. They are not regular or constant in their ap- pearance ; sometimes only one is seen, at other times eight have been seen ; their breadth is likewise variable; one belt growing narrow while another in its neighbour- hood becomes broader, as if one had flowed into the other ; in this case an oblique belt has been observed to lie between them, as if for the purpose of forming a communication. Sometimes one or more spots are formed between the belts, which increase till the whole are united in one large dusky band. There are also bright spots to be discovered on Jupiter's surface ; these are rather more permanent than the belts, and re-appear af- ter unequal intervals of time. The remarkable spot, by whose motion the rotation of Jupiter on his axis was ascertained, disappeared in 1694, and was not seen again till 1708, when it re-appeared exactly in the same place, and has been occasionally seen ever since. The disappearance and re-appearance of the spots is not so wonderful as the changes that have been observed in the belts ; the elder Cassini saw one evening five belts upon the planet, but while he was viewing them, they underwent the most surprizing change. In an hour from their fullest appearance there remained only three out of five, and one of these scarce perceptible. The most remarkable telescopic appearances of this planet, are the satellites, but these I have particularly described under the head of satellites. 206 OF THE PLANETS. Though the great distance of the planet Saturn, and the tenuity of its light, do not permit us to distinguish the varieties of its surface ; yet some of the first dis- coveries made by. the telescope were on this planet, and the ring is still one of the most curious phenomena we are acquainted with. There is not, indeed, any thing in the whole system of nature more wonderful than this ring, which appears nearly as bright as any part of the surface of the planet : by what means it is suspended, or by what law supported ; whether it be ojily a bright but permanent cloud, or a vast number of satellites dis- posed in the same plane, whose blended light gives it to us the form of one continual body, we can only form crude conjecture. M. Messier has observed on the anses of this ring several luminous white twinkling points, differing in vivacity from each other. Sometimes our eye is in the plane of the ring, and then it becomes invisible : as its plane always keeps pa- rallel to itself, it disappears twice in every revolution of the planet, that is, about once in fifteen year ; and he sometimes appears quite round for nine months together. At other times the distance betwixt the body of the planet and the ring is very perceptible, insomuch that Dr. Clarke's father saw a star through the opening. When Saturn appears round, if our eye be in the plane of the ring, it will appear as a dark line across the mid- dle of the planet's disk ; if the eye be elevated above the plane, a shadowy belt will be visible ; when the plane appears, the ring next the body is the brightest ; when the ring appears of an elliptical form, the parts about the ends of the largest axis are called ansse. These, a little before and after the disappearing of the ring, are of unequal magnitude. It has been supposed, that the ring has a rotation round an axis. With very long telescopes two belts have been disco- vered on Saturn, which appear parallel to that formed by the edge of the ring ; these are said to be perma- nent : Cassiht and Fatio perceived a bright streak upon Saturn which was not permanent, being visible one day, and disappearing the next, when another came into view OF COMETS. 207 near the edge of his disk. Besides these there are its five satellites, mentioned under their proper heads. OF COiMETS. Comets are a kind of stars, appearing at unexpected times in the heavens, and of singular and various figures, descending from far distant parts of the system, with great rapidity, surprizing us with the singular appear- ance of a train, or tail ; and, after a short stay, are car- ried off to distant regions, and disappear. They were imagined in ancient times to be prodigies hung out by the immediate hand of God in the heavens, and intended to alarm the world, Their nature being now better understood, they are no longer terrible. But, as there are still many who think them to be heavenly warnings, portents of future events, it may not be impro- per to inform you, that the Architect of the universe has framed every part according to divine order, and sub- jected all things to laws and regulations ; and that he does not hurl at random stars and worlds, and disorder the system of the whole glorious frame, to produce false apprehensions of distant events, fears without founda- tion, and without use. Religion glories in the test of reason, of knowledge, and of true wisdom ; it is every way connected with, and is always elucidated by them. From philosophy we may learn, that the more the works of the Lord are understood, the more he must be ador- ed ; and that his superintendency over every portion is more clearly evinced, and more fully expressed by their unvaried course, than by ten thousand deviations. The existence of a universal connection betw r een all the parts of nature is now generally allowed. Comets undoubtedly form a part of this great chain ; but of the part they occupy, and of the uses for which they exist, we are equally ignorant. It is a portion of science whose perfection is reserved for some distant day, when these bodies and their vast orbits may, by long and accurate observation, be added to the known parts of the solar system ; when astronomy will appear as a new science, 208 OF COMETS. after all our discoveries, great as we at present imagine them to be. The astronomy of comets is very imperfect ; for but little can be known with certainty, where but little can be seen. Comets afford few observations on which to ground conjecture, and are for the greatest part of their course beyond the reach of human vision ; but that they are not meteors in the air is plain, because they rise and set in the same manner as the moon and stars, they are called comets from their having a long tail somewhat resembling the appearance of hair ; some, however, have appeared without this appendage, as well- defined and round as planets. It is generally supposed, that they are planetary bo- dies, making part of our system, revolving round the sun in extremely long elliptic curves ; that as the orbit of a comet is more or less eccentric, the distance to which they recede from the sun will be greater or less. Very great difference has been found by observation in this respect, even so great, that the sides of the elliptic orbit in some cases degenerate almost into right lines. They are very numerous : 450 are supposed to belong to our solar system. Those comets, which go to the greatest distance from the sun, approach the nearest to him at their return. The motions in the heavens are not all direct, or according to the order of the signs, like those of the planets. The number of those which move in a retro- grade order, is nearly equal to those whose motion is direct. Their orbits of most of them are inclined in very large angles to the plane of the ecliptic. The velocity with which they move is variable in eve- ry part of their orbit ; when they are near the sun, they move with incredible swiftness ; when very remote from him, their motion is inconceivably slow. When they appear, they come in a direct line to- wards the sun, as if they were going to fail into his body : and after having disappeared for some time, in consequence of his extreme brightness, they fly off on GF COMETS. 209 the other side as fast as they came, continually loosing their splendour, till at last they totally disappear. Their apparent magnitude is very different, sometimes seeming not bigger than the fixed stars, at other times equal in diameter to Venus. Hevelius observed one in 1652, which was not inferior to the moon in size, though not so bright ; its light pale and dim, its aspect dismal. A greater number of comets are seen in the hemisphere towards the sun, than in the opposite ; and are generally invisible at a smaller distance than that of Jupiter. Mr. Brydone observed one vx Palmero, in July 1770, which, in 24 hours, described an arc in the heavens upwards of 50 degrees in length ; so that if it was far distant from the sun, it must have moved at the rate of upwards of 60 millions of miles in a day. They differ also in form from the other planets, con- sisting of a large internal body, which shines with the reflected light of the sun, and is encompassed with a very large atmosphere, apparently of a finer matter, much re- sembling that of the Aurora Borealis ; this is called the head of the comet, and the internal part the nucleus. When a comet arrives at a certain distance from the sun, an exhalation arises from it, which is called the tail. The tail is always directed to that part of the heavens which is directly or nearly opposite to the sun, and is greater and brighter, after the comet has passed its peri- helion, than in its approach to it ; being greatest of all when it has just passed the perihelion. The tail of the comet of 1680 was of a prodigious size, extending from the head to a distance scarcely inferior to that of the sun from the earth. No satisfactory knowledge has been acquired concern- ing the cause of that train of light which accompanies the comets. Some philosophers imagine, that it is the rarer atmosphere of the comet impelled by the sun's rays. Others, that it is the atmosphere of the comet, rising in the solar atmosphere by its specific levity ; while others imagine, that it is a phenomenon of the same kind with the Aurora Borealis ; and that this earth vol. iv. e a l 210 OF A PLURALITY OF WORLDS. would appear like a comet to a spectator placed in ano- ther planet. The number of the comets is certainly very great, con- siderably beyond any estimation that might be made from the observations we now possess. There are some,* who do not think the present astro- nomy of comets well established ; and that as so many small ones are frequently seen, they think that nothing can be determined with certainty, till some better marks are discovered for distinguishing one from another, than any at present known ; and that even the accomplish- ment of Dr. Halley's prediction is uncertain: for it is very singular, that out of four years, in which three co- mets appeared, the only one, in which no comet was to be seen, should be that very year in which the greatest astronomers that ever existed had foretold the appearance of one ; and, in accounting for its non-appearance, Mr. Clairault would have been equally supported by cometic evidence,! whether he concluded the comet to have been retarded or accelerated by the action of Jupiter or Saturn : a comet appeared in 1757, as well as in 1755, and had he determined the retardation of the comet to be twice as great as he did, another appeared in 1760 to have veri- fied his calculations. OF A PLURALITY OF WORLDS. The fixed stars are generally supposed to be of the same nature with our sun, each of them attended by pla- nets which are inhabited by rational creatures, like this earth. Instead, therefore, of one sun, and one world, we find, that the region of unbounded space is peopled with suns, and stars, and worlds. This opinion has been held and taught by many of the most celebrated philosophers and astronomers, both in ancient and modern times ; in this view of things, our system resembles a single individual * Encyclopaedia Britannica, vol. ii. p. 765. Second Edition. f There does not indeed seem any evidence to prove the return of the same comet. OF A PLURALITY OF WORLDS. 211 of some one species of being in outward nature, diversi- fied from all its fellow individuals, by differences unes- sential to the kind and species ; but which constitute that beauty, which arises from uniformity amidst variety. That the fixed stars are suns, shining by their own light, is probable, on account of their immense distance from us ; for, as it is impossible that at these distances they could be seen by any reflexion of light from the sun, it is natural to suppose them endowed with a power of emitting light from their own bodies. By comparing the apparent diameter of objects at different distances, it is clear, that our sun would appear like a star, were he removed to the distance at which they are placed ; and that therefore it is truly reasonable to suppose, that the fixed stars are equal, if not superior in magnitude, to that which is the centre of our system; and that they are made for the same purposes with the sun, to bestow light, heat, and vegetation, on a certain number of planets revolving round them.* Of their immense distance from us, and the vastness of the space they occupy, the reader may form some idea, when he is told, that numbers amongst them are at too great a distance to be adequately expressed by figures, and beyond the reach of admeasurement ; and this will be heightened, if he considers, that the smallest of the stars visible to the eye are much more remote than the larger ones, and that the telescope discovers stars which are too distant to be perceptible to the naked eye : that the instrument, like our eyes, has its limits ; but the ex- tent of the heavens has no bounds. The fixed stars being so far removed from, and for the most part invisible to us ; it can scarcely be conceived by the narrowest mind, that they form a part of our system, or were created only to give a faint glimmering light to * Dr. Herschel closes his conjectures on the sun, Sec. wi.h the following general inference. " It seems therefore, on the whole, not impossible, that in many cases stars are united in such clo^e systems, as not to lea\e much room for the orbits of planets or comets, and that consequently, upon this account also, many stats, unless we could make them mere useless brilliant points, may themselves be lucid planets, perhaps unattended by satellites." E. Edit. 212 OF A PLURALITY OF WORLDS. the inhabitants of this globe : for one additional moon would have afforded us more light than the whole host of stars ; such an opinion is unworthy of our reason, in- adequate to our conceptions of the Deity. It would be also absurd to suppose, that the Author of nature had made so many suns without planets, to be enlightened by their light, and vivified by their heat ; but more so, to ima- gine so many habitable worlds enlightened by suns with- out inhabitants ; we may, therefore, safely conclude, that all the planets, of every system, are inhabited. This reasoning is still further strengthened, by consi- dering the immensity of the starry heavens, in which are innumerable hosts of stars, created as the means to some great end. " Every star may be the centre of a magni- ficent system, attended by a retinue of worlds, irradiated by its beams, and revolving round by its active influence." Thus the greatness of God is magnified, and the gran- deur of his empire made manifest. He is not glorified on one earth, or in one world, but in ten thousand times ten thousand. " If we could wing our way to the high- est apparent star, we should there see other skies expand- ed, other suns that distribute their inexhaustible beams of day ; other stars that gild the alternate night, and other, perhaps nobler, systems established in unknown profusion, through the boundless dimensions of space. Nor does the dominion of the Sovereign of all things terminate here ; even at the end of this vast tour we should find ourselves advanced no further than the fron- tiers of creadon, the commencement of the great Jeho- vah's kingdom.* This mode of reasoning applies with greater force to the planets of our own system, and gains additional strength from other considerations. For who would ven- ture to assert, that infinite love and consummate wisdom had formed such immense material masses, some of which exceed our earth in size, convey them in revolutions round the sun, furnish them with moons, grant them the alter- nate changes of night and day, vicissitudes of seasons, and all this only to emit their scantly light on our earth. * Htrvty's Meditations. OF* A PLURALITY OF WORLDS. 213 Or who that has seen any engine, a windmill for in- stance, and who knows the use of it, if he travel into another country, and there see an engine of the same sort, will not reasonably conclude that it is designed for the same purpose ? So when we know that the use of this planet, the earth, is for a habitation of various sorts of animals, and we see other planets at a distance from us, some bigger, some less than the earth, moving periodi- cally round, revolving on their axes, and attended with moons ; is it not highly reasonable to conclude, that they are all designed for the same use as this earth is, and that they are habitable worlds like that we live in ? Who can conceive them .Unpossess'd, By living soul, desert and desolate.. Only to shine, yet scarce to contribute, Each orb a gleam of light ? Or that the Almighty, who has not left with us a drop of water unpeopled, who has in every instance multiplied the bound of life, should leave such immense bodies desti- tute of inhabitants ? It is surely much more rational to suppose them the possession of human beings, beings formed with capacities for knowing, loving, and serving their Almighty Creator ; blest and provided with every object conducive to their happiness, and many of them in a far greater state of purity than the inhabitants of our earth, and therefore in possession of higher degrees of bliss, and placed in situations, furnishing them with scenes of joy, equal to all that poetry can paint, or religion pro- mise : all under the direction, indulgence, and protection of infinite wisdom and goodness.* The more the heavenly bodies excite our astonishment, from their size, their distances, the regularity of their mo- tions, or any peculiarity or perfection we discover in those attractions by which they seem retained in their places, the more clear it is to any reasoning head, that they could not have made themselves : and that close connexion be- * See the Rev. Mr. IVoolaston's Reflexions SJ14 ON PHYSICAL ASTRONOMY^ tween cause and effect, which the farther we search the more clearly we discern, though it has staggered the faith of many a celebrated naturalist, is itself a proof, if he had not stopped short of the conclusion, that all these must have been the contrivance of consummate wisdom, and guided by an unerring hand. Yet, at the same time, he who sees that every little cor- ner of this earth of ours is replete with animal life, though but one race on it seems to be endowed with reasoning faculties, cannot but be led on to a conjecture at least, that all those vast bodies he discovers in the heavens may be peopled with their gradations of inhabitants likewise ; and that each of them, not improbably, contains its ra- tional beings too, to acknowledge and adore the Creator of them all. So far the heathen philosopher may go : though, if he be a modest inquirer after truth, he will not dogmatize, or enter into any particular detail of what is so totally out of his reach. LECTURE XLV. ON PHYSICAL ASTRONOMY.* 1 HE causes of the celestial motions have in all ages been the objects of philosophical curiosity. Men have generally conducted their researches on this subject upon principles of analogy. Some resemblances have been noticed between the motions of the celestial bodies, and other motions nearer at hand, and more familiar to us ; and the same resemblances have been supposed to exist between their causes. * Professor Robinson's Outlines of Mechanical Philosophy, p. 105. OF PHYSICAL ASTRONOMY. 215 I shall notice four of these different resemblances, or analogies. 1. The motions of the heavenly bodies have been thought to resemble the spontaneous motions of intelli- gent beings. Aristotle, Leibnitz, Tucker, Monboddo, and some others, both in ancient and modern times, have taught that the planets were conducted by spiritual in- telligent beings. ^ Though accounts of the celestial phenomena may be given by means of this resemblance, that are chargeable with no false reasoning ; yet as they afford no explana- tion, they answer no purpose in philosophy. 2. The celestial motions have been thought to repre- sent the motions of bodies carried about centres by means of solid connexions. This motion suggested to philosophers the opinion, that the planets were attached to solid orbits, which turn round the axis of revolution : this opinion has been falsely attributed to Aristotle. It is altogether contradictory to our ideas of the etherial matter that occupies celestial space, and not easily reconcileable to the elliptic motion of the planets. 3. The celestial motions have been thought to resem- ble the motions of bodies carried round by a circulating fluid. Many philosophers have supposed the planets to :>e earned round in such vortices. Descartes and Leib- nitz were at great pains to establish this doctrine. More nodern writers* have removed the difficulties, and obvi- ited the objections made to this system. It will there- ore be necessary to lay before you some of the argu- nents urged in its favour; in doing this, I shall be under ne necessity of repeating some of the observations that nave made before. These writers urge, that so long as we keep within the mits of natural and experimental philosophy, we must ccount for the motions in nature, by referring them to orporeal causes ; and where this cannot be performed i£dS^^ Ess *y™ the First Principles of pS^P^ **• -*- »*on'» Observations on the Moving Powers 216 ON PHYSICAL ASTRONOMY. satisfactorily, we must give them up, or wait with patience for some better clue of investigation, or some further light from experience. It is contrary to sound philosophy to amuse ourselves with names and qualities, which contradict the known laws of mecha- nism, and supercede the operation' of the elements. Nothing is intelligible in philosophy but the action of matter upon matter ; the power of impulse is the only sensible and experimental cause of motion ; and there is the strongest presumption from analogy in fa- vour of the universal material mechanism of the opera- tions of nature. All other principles of motion are founded on conjecture, and incapable of proof. If you attempt to soar above this principle in theory, you are always obliged to descend to it in practice. Natural philosopy has been principally advanced by the experi- ments which have been made on the elements ; but these experiments prove, that matter interferes in pro- ducing all the changes and motions that are observed in bodies distant from each other. Look into, and observe the operations in nature : how does the sun act upon the fruits of the earth, but by the mediation of its light? How do the clouds water the earth, but by the mediation of air ? How does the che- mist produce such wonderful changes in natural bodies, but by the mediation of fire ? In a word, every experi- ment, every observation proves, that in all cases where distant bodies are found to affect each other, there is always something to mediate, whether we do or do not perceive it ; and when this mediation can be no further traced, natural philosophy is at an end, and the fictions of imagination begin, which are oi~ equal value, by whatever name they may be called,, or with whatever parade of demonstration they may be introduced. It is very singular, they observe, that inquirers after physical truth should observe and acknowledge mechan- ism in the greater part of nature, and yet not be led thereby to inquire, whether it be not universally ex- tended ; the more so, as matter and motion must have the same invariable properties. If vapours rise mecha- nically, why may not a stone descend by the samelaw? ON PHYSICAL ASTRONOMY. 217 If fluids circulate in organized bodies by continued im- pulse, why may not a planet revolve in the organized system of the universe by the same cause ? All true philosophers agree in considering the uni- verse as a great machine, so created, fitted, and dispo- sed by the power of God, as to perform all the opera- tions, which are carried on throughout the whole. There is a connexion and communication between all the distant parts thereof. No one part can be consi- dered as acting without being acted upon. It is highly unphilosophical to assert, that matter, considered in general, or any part thereof, has essential separate qualities, by which one part acts upon another. It is the essential property of no one wheel in a machine to move its fellow, though, in consequence of its being placed in the station it is fitted for, it acts upon its fel- low, because it is acted upon. If you interrupt the con- tact in a machine, you destroy the motion in all those parts where the communication is destroyed. It is just the same with the whole system of nature, you cannot take up any parcel of matter and say of it, this has essential properties which empower it to be a natural agent. A philosopher ought to consider it as a concrete, with a certain disposition of parts liable to be acted upon by the subtiler parts of the machine, which can by no means be restrained by art therefrom* It might be as justly asserted, that it is the essential pro- perty of animal substances to live, as that it is the es- sential property of the loadstone to attract. The promoters of the opinion now under considera- tion, urge further, that every known operation in na- ture is mechanical ; and that in all experiments, where the explanation is clear and certain, the effects are pro- duced by matter acting upon matter ; and we are able to trace this mechanism in such a variety of instances, that unless the world be governed by opposite and con- tradictory principles, it must obtain throughout the whole. Thus the body of man, which is the highest piece of machinery in nature, is made to see, to hear, and speak, VOL. iv. 2.r 213 ON PHYSICAL ASTRONOMY. upon mechanical principles ; and it dies, unless there be a constant impression of a material force upon it, from the element of air. Again, from the pressure of air, the mercury is made to rise in the tube of the barometer. Hail, snow, and vapour, are formed in the atmosphere by the dif- ference in its temperature ; the clouds are sustained therein, and driven about to water the earth ; plants grow and are nourished thereby. For those effects where the cause is not so obvious, you find fire a more subtile agent, whose reality is proved, and its operations pointed out both by observa- tion and experiment. It may be transferred from one parcel of matter to another. It will enter the pores, and fill the interstitial vacuities of all other substances. It acts with a force and velocity adequate to all the ef- fects we can desire to ascribe thereto. It gives an elas- tic force to air, and occupies every space from which the air is exhausted. In some cases it acts as light, in others as fire ; light, as it illuminates and renders ob- jects visible ; fire, as it burns and consumes what it acts upon. Thus you find the fluid etherial matter of the hea- vens acting by impulse on the solid matter of the earth, being instrumental in every one of its productions, and necessary to every stated phenomenon of nature. We are forced by the evidence of every phenomenon in nature, by every experiment in philosophy, to con- clude, that impulse* is the only material cause of mo- tion. All the properties of matter are such as fit them to act, and to be acted upon in a mechanical way. They are all such as can be adapted to the known principles of mechanism among artists. We are, therefore, bound by every rule of sound reasoning to consider it as the cause of all the motion, and continuance of motion, in the material universe. It is the one certain and only *No rr.er.hnnic.il motion cat) subsist without a plenum ; this must be, wherever such mechanical motion subsists. This is so necessary a con>e- quence of motion being carried on by impulse, that it needs no other de- monstration. ON PHYSICAL ASTRONOMY. 219 universal known cause. Neither the properties of mat- ter, nor experiment, nor observation, afford any other. No independent motion can be discovered. It is therefore wrong to consider the motion of any body ab- stractedly, or as a thing by itself. The system of nature is connected and related ; and, to be understood, must be considered under those relations and connections. Speculations which carry you out of the world, can never inform you how things are carried on in the world. Matter subsisting as a part of the created world has motion, but, if separated from the rest, would have no more motion than a limb divided from the body ; and he who studies the nature of motion by taking matter ab^ ractedly, is studying motion from that which has no motion belonging to it. Another strong argument adduced in favour of this system, is derived from the continuance and permanency of the motions observed in nature. That there is a universal principle of motion throughout the system of things, is self-evident. We know that matter moving can be the cause of motion in matter at rest ; and we know of no other physical cause capable of producing such motion. Here, therefore, we must look for the causes of permanent motions. Of the motion in different bodies, it is observable, that some retain the motion they have acquired, without any diminution, while others are soon reduced to a srate of rest. When a body retains its motion without diminu- tion, it fi is moved by such causes as would renew its mo- tion, if it were stopped. When a cloud is flying before the sun, the same wind that drives it on, would restore its motion if it could be stopped. In the same manner the sails of a wind-mill, after you have brought them to a state of rest, and even confined them, will receive a new motion from the wind, as soon as the obstruction is removed. If you stop the motion of the lungs by an ef- fort of the muscles, you find that the natural causes that act upon the body tend to renew the motion, and cannot be hindered from effecting it, without a consider rable effort. '220 ON PHYSICAL ASTRONOMY. Every lasting motion is such a one, therefore , that will be renewed upon its own principles. This observation is of great importance towards accounting truly for the undecaying motions of the universe, to all which it may undoubtedly be extended : so that if it were possible to stop the planet Jupiter in his orbit, the establised causes that act upon him, would renew his motion without any artificial motion. A body continues to move as long as the natural causes of motion continue to act thereon ; and rest, which is mechanical death, inevitably follows, when the causes of motion are no longer present. There may be subtile cases, in which it may be as hard for us to trace the causes of motion, as to show why life remains for some time in an animal body under water without re- spiration. Still, however, the general assertion must be true, if every effect must have its cause ; for then to every permanent effect there must be a permanent cause. It is therefore not only absurd, but contrary to every analogy in nature, to suppose that any of the durable motions of the celestial bodies depend upon projection in a vacum : because if you were to stop a body moved upon this principle, you have no means of renewing its motion, it must either fall into the sun, and thus come to a point of rest, or be dead and motionless for ever, without some miracle to give it a new motion ; but this being contrary to the conditions of every undecaying motion, which will be renewed on its own principles in the ordinary course of nature, and by means .already established, is not to be admitted into philosophy. They further urge as a reason for rejecting the hypo- thesis of a projectile impulse, that it obliges its sup- porters to make the universe a vacum : because those elements which are ordained to act upon matter, and keep up the life and motion of the world, would stand in the way, hinder the freedom, and disturb the opera- tion of an imaginary principle, projection. They con- sider projection not only as a hypothetical, but as an artificial and unnatural principle, that cannot be proved to obtain any where in nature. If it be received, they say it must be received as an article of faith. ON PHYSICAL ASTRONOMY. 221 Experiments have been made with a central-force ma- chine, to illustrate the doctrine of centripetal and centri- fugal forces.* But they by no means apply to any case in nature, for the moving body is always connected by a line to its centre of motion ; a circumstance that ne- ver can be reconciled to motion in a vacuum, where no connection is supposed ; nay, is even objected to upon principle. But these experiments are still further de- ficient, because the centrifugal force being a consequence of, or derived from the artificial revolution of the whirling body, cannot be used as a cause of the motion : for it is the nature of all causes to be prior to their effects, but here it is posterior ; the body is never dis- posed to fly off in a tangent till it has acquired its revo- lution. This force can, therefore, never be applied to account for any of the celestial motions, because it brings us to this absurdity, that there is nothing to ac- count for the motion, but the motion itself, or the con- sequence of the motion. The same objections apply, and even further, to an- other illustration, namely, casting round a weight sus- pended in a sling ; for the power of the sling restrain- ing the body from flying off in a tangent, bears no ana- logy to a power actually drawing the moving body to- wards its centre of motion. It has been objected to this reasoning, that no body can move in a space filled with matter, commonly called a plenum. But this entirely depends on the condition of the matter and the circumstances of the moving body : if the matter filling the space be a fluid, whose parts can easily slide over one another, they will be able to move in different or contrary directions at the same time, and while the place of the whole mass remains the same, the place of the parts of which it is composed may be continually changing. * The machine here alluded to, is the whirling-table ; the description of which I have given in vol. iii. p. 319, et seq. Ii is a machine, in my opinion, oi a very evident and illustrative nature, shewing by experiments the laws of central, &c. forces whatever may be the real cause of motion. — E. Edit. 222 ON PHYSICAL ASTRONOMY. The fulness of the space is therefore no objection to the free motion of the parts of any fluid among them- selves ; neither is it any objection to the motion of any solid body in such a fluid medium. Though a vessel be filled with water closely stopped, and the fluid so com- pressed, that a very small point made to enter therein would burst the containing vessel, yet any solid will move freely therein from one side to the other, or from the top to the bottom ; because the parts of the fluid which are displaced before, fall into that space behind quitted by the body. So fast as the body proceeds, just so fast do the parts of the fluid recede ; so that there is neither impediment nor vacuity. The same is true in other cases ; there may be motion, provided there be a circu- lation among the parts. When a solid body is moved in a fluid by any artifi- cial force or violence, contrary to the nature of the me- dium in which it moves, the parts of the medium, by endeavoring to recover their natural state, will resist the motion°of the body till the equilibrium be restored, and the body at rest. Such will necessarily be the case of all violent motions ; it is soon destroyed by resist- ance though the time in which it is destroyed may dif- fer from a variety of circumstances. But on the other hand, if the motion of the body arise from the motion of the medium in which it moves, then the resisting nature of the medium is no longer an objection to the motion of the body, neither can it be, for it is the cause of its motion ; and it is absurd to suppose, that the cause of the motion can resist the motion it causes. No inference, therefore, from the resistance of mediums can lead us to the necessity of a vacuum. A vacuum is only necessary when a motion is proposed, which is independent of the action of eve- ry medium ; but nature knows of no such motion. A variety of motions may be exhibited, for whose production the presence of a resisting medium is abso« lutely necessary ; and they show, that so far from a vacuum being necessary to the continuance of motion in any space, the motion is promoted and occasioned by a resisting medium. That hypothetical train of ON PHYSICAL ASTRONOMY. 223 reasoning which leads us to conclude, that if less mat- ter were in the space, the motion would be more free and continue much longer, is as unphilosophical, as it would be if, in order to enable a man to run faster, we should rid him of the incumbrance of his boots and spurs, by cutting off his legs. Air is, you know, a resisting medium, yet, instead of retarding the motion of the lamp-machine, which I be- fore showed you, by its resistance it preserves that mo- tion ; and if the motion be at last discontinued, it does not arise from defect or irregularity of the cause, but from the imperfection of the materials. If the materials which are acted upon would but continue in the same state, the motion would be unretarded as long as air and fire, which are the causes thereof, subsist in the world. In this experiment the causes are not artificial and vio- lent, as in the central-force machine, but such as are supplied by nature itself, in its regular mode of action ; which both begins and continues the motion. What is performed by the agents in nature in the one case, may certainly be done in others. The planets may be car- ried round in their orbits by the same means. The heavens may be filled throughout with an etherial fluid, not infinitely rarefied, unresisting, and impotent, but dense and continuous. in its parts. The writers in favour of the mechanical system urge, that their opponents have no notion or means of resolv- ing their axioms, or relative laws of motion, to mecha- nism, but consider them merely as laws ; another word, as they use it, for ultimate, spiritual, unmechanical pow- er. As the penetration of some among them has car- ried them so far as to suppose an impelling etherial medium for maintaining attraction, gravitation, &c. &c. it is rather surprizing that they could not perceive that the same medium was necessary for supporting their laws of motion, rest, resistance, &c. for the difficul- ty does not lie in accounting for gravitation, or any particular kind of motion, but in finding powers to produce and maintain motion in general. If these be mechanical, it is easy to suppose, that the contriver may have adjusted the mechanism so as to produce the 224 ON PHYSICAL ASTRONOMY. particular tendencies. But if they be unmechanical, you may call them laws, properties, or any other name, either with or without a meaning. How detrimental is it to the increase of knowledge in the powers and agency of nature, to have the most curious productions of these powers reduced to unintelligible laws, charac- terized by words without meaning, and which render their inventors no wiser than the most heedless and unattentive ! Without instrumental, or second causes, there can be no regular course of nature ; and without a regular course, nature could never be understood. The order and course of things, and the experiments we daily make, show that there is a mind that governs and actu- ates this mundane system as the proper real agent and cause ; the inferior and instrumental cause seems to be fire ; with respect to attraction, it cannot produce, and in that sense account for the phenomena, being itself one of the phenomena produced and to be accounted for. What is said of forces residing in bodies, whe- ther attracting or repelling, it can only be considered as a mathematical hypothesis, not as any thing real and existing in nature. The mechanical agency of the elements accords with the descriptions and illusions of the sacred scriptures. The heathens were in some degree acquainted there- with. When this doctrine was in their hands, a princi- ple of intelligence was ascribed to the active elements, and they were taken for the Gods who govern the world. But with those who are taught that the True God is distinct from, and above the world of matter, though virtually present by a providential inspection and superintendance, it serves only to enlarge and exalt their ideas by setting before them the visible evidence of divine wisdom, which with so exquisite a contrivance, and such simplicity of design, hath adopted physical causes to the production of their respective effects. We have now to consider, 4thly, the mathematical principles of philosophy. The celestial motions have been thought to resemble those exhibited to us in the phenomena of magnetism and electricity ; these and KEPLER S LAWS. 22,5 the celestial bodies seem to act upon each other at a distance, without any observed intervening impulse. Accordingly, many philosophers, both ancient and mo- dern, have imagined that the planets are influenced by causes similar to those of more familiar phenome- na. But these philosophers had formed no accurate notions of the agency of the causes of the motions from which they attempted to derive an explanation ; neither had they examined attentively the circumstances of the motions which they attempted to explain. At last, Sir Isaac Newton contented himself with an investigation of the laws observed in the agency of the causes of the celestial motions, discovered that these laws were the same with those observed in the agency of the causes of the motion of common heavy bodies, and from this dis- covery gave a theory of mathematical astronomy. We are indebted, however, to Kepler for the generalization of facts, which form the basis of the mathematical theorw kepler's laws. Kepler s first law is, that the planets , in revolving round the sun, describe equal areas in equal times. Kepler' *s second law is, that the orbits described by the planets are ellipses, having the sun or the primary planets in the focus. Kepler 9 s third law is, that the squares of the periodical times of the planets are as the cubes of their mean dis- tances from the sun. That this, as the square of the time which a planet, A, takes to revolve in its orbit, is to the square of the time which any other planet, B, takes to run through its orbit ; so is the cube of the mean dis- tance of A from the sun, to the cube of the mean distance of B from the sun. OF DEFLECTING FORCES.* In consequence of the inertia of matter, all motion is considered as equable and rectilineal, as being in a * Professor Robinson's Outlines of Mechanical Philosophy, p. 34 to 107. VOL. IV. 2 G 226 OF DEFLECTING FORCES. straight line with the direction of the moving force ; and as preserving this direction until it be hindered or put out of its way by some extrinsic cause. If therefore a body turn in a curve, that curvature must proceed from some external force continually act- ing upon the body ; and whenever that force ceases to act, the body will move forward in a right line, touch- ing the curve in that point where the body is at the in- stant of time when the force ceases to act. When you observe a change in the direction of any motion, you may infer the action of a force, whose di- rection crosses that of the former motion. This may be called a deflecting force. The change of direction is measured by the angle con- tained between the former and the new direction. When the motion of a body is curvilineal, the deflex- ion is continual, and you may infer the continual action of a deflecting force. On the other hand, the continual action of a deflecting force produces a curvilineal mo- tion. In a curvilineal motion the change of direction is mea- sured by the angle contained between the tangents to the curve. A curvilineal motion is therefore always a compound motion; but the great bodies of this system, as the pla- nets, move round the sun in curve lines ; on these prin- ciples, there must therefore necessarily be two powers acting on them ; one impelling them to move in a straight line, the other deflecting or bending them continually to- wards a centre. You may, therefore, consider deflecting forces as al- ways directed to or from a point ; in the first case they are called centripetal forces, in the second case they are called centrifugal forces. In general, they are termed central forces ; and the point, through which their direc- tion always passes, is called the centre of the forces. Among the various curvilinear motions which may arise from the action of central forces, there is a circum- stance in which they all agree, and which enables the mathematician to investigate the forces by which they are produced^ OF DEFLECTING FORCES. 227 If a body move in a curve line, ABCDEF, plate 15, fig. 3, by means of a force always directed to a fixed point S, the curve is all in one plane, and the areas, ASB, ASC, ASD, described by the straight line joining the body with the point S, are proportional to the times of description ; /'. e. equal areas are described in equal times, unequal areas in unequal times. Thus the trian- gular areas ASB, BSC, CSD, &c. described by the straight line joining the body, with the point S, are pro- portional to the times of description. Let the time be divided into equal parts, let the body be acted on by an impulse that would carry it from A to B, in the first given particle of time ; then in the second particle it would go an equal space, and describe the line B c, equal to the line A B. But when the body is arrived at B, let a deflecting cen- tripetal force so act upon it, that while its first impulse would carry it to c, the deflecting force would carry it to V; complete the parallelogram B V C c, and it is evident from the doctrine of compound forces, that the body would in the second particle of time describe the diago- nal B C. Now, as C c is parallel to S V, the triangles SBC, S B c, are between the same parallel lines, and as such, are by geometry proved to be equal ; for the same rea- son the triangles S C D, S E F, are proved to be each equal to S B A. If any number of these triangles be added together, the total sums, as AD S, F C S, will be proportional to the times wherein they are described. If the lines, A B, B C, be continued round a centre, they will form a polygon, and if the sides of the polygon be indefinitely increased in number, and indefinitely de- creased in length, they will form a curve, — a circle or an ellipsis : and the proposition will be true of these curves, that a line drawn from the centre to a body in the cir- cumference of the circle, or from the focus to a body in the circumference of the ellipsis, will sweep equal areas in equal times. The power, therefore, directed towards the given point S, has no effect on the magnitude of the area described 228 OF DEFLECTING FORCES. by the line supposed to be drawn from the body to that point. It may accelerate or retard the motion of the body, but affects not the area or space described by the line. The line will still continue to describe the same spaces in equal times, about the given point, as it would have done if no new force had acted on the body, but it had been permitted to proceed uniformly in the line of projection. As one impulse towards the given point has no effect on the area or space described by the ray or line from the body to that point, so any number of successive im- pulses directed to the same point can have no effect on the area ; and if you suppose the power directed to that point, to act continually, it will bend the way of the body in motion into a curve, and may accelerate or retard its velocity, but can never affect the area described in a given time by a line supposed to be drawn from the body to the given point, which will always be of an invariable quantity, equal to that which would have been described in the same time, if the body had proceeded uniformly in a right line from the beginning of the motion. The converse of the foregoing proposition shows, that if a body, A, describe a curve all in one plane, and if there be a point, S, so situate in this plane, that a line drawn therefrom to the circumference describes propor- tional areas in proportional times, then is the body urged round by a force tending towards that centre. In other words, the equable increase of the areas described by a line drawn from a body to a given point, is an indication that the direction of the power that acts upon the body, and that deflects it into a curve, is directed to that point. By the same propositions we may illustrate and explain the revolutions of the primary planets in elliptical orbits, not much differing from circles, round the sun, which is in one of the foci of each ellipsis. Let the ellipsis ABCDEFGHIKLM, plate 15, fig. 4, represent the orbit of a planet moving therein round the sun S, according to the order of the letters, the sun, S, being in one of the foci of the ellipsis ; let the time of its revolution be divided into any number of equal parts, suppose twelve; in moving from A through BCD, &c. the planet approaches nearer the sun, and the central tei OF DEFLECTING FORCES. 229 dency continually increasing its velocity, it goes through greater arcs in equal times, till it come to G ; from thence its motion continually carries it to a greater dis- tance from the sun, and it describes in equal times smaller and smaller arcs, till it return to A, from whence it pro- ceeds as before. Now, the triangular spaces passed over by a line drawn from the planet to the centre of the sun will be equal, be- cause in the planet's going the first half of the ellipsis from A to G, the arcs which may be considered as the base of the mixed triangles described in equal times, grow longer and longer, as the legs grow shorter, so as to pre- serve the equability of the triangular space : in the other half or the ellipsis, in the planet's going from G to A, the arcs grow shorter ; but this is compensated by the greater length of the legs. The sum of what has been proved is ; 1 . That the areas or spaces revolving round an immoveable centre are pro- portional to the times ; and, 2. That if a body revolving round a centre, describe about it areas proportional to the times, the body is actuated by a force directed to that centre. But, by Kepler's first law, we know, " that the pri- mary planets describe round the sun, and the secondary planets describe, round their respective primary planets, areas proportional to the times." From hence it is infer- red, that the primary planets are retained in their orbits by forces which are always directed to the sun ; and that the secondary planets are retained in their orbits round their primary planets by forces which are always directed to those primary planets; Kepler's second law is, " that the orbits described round the sun, and round the primary planets, are ellipses, hav- ing the sun or the primary planet in the focus." From hence it is inferred, that the accelerating force, by which a planet is retained in the different parts of its elliptical orbit, is inversely proportional to the square of its dis- tance from the sun, or from its primary. Kepler's third law is, " that the square of the periodic times of planets revolving round common centres, are proportional to the cubes of their mean distances." From this it is inferred, that the forces by which the planets are 230 GRAVITATION OF THE MOON retained in their different orbits, are inversely proportion- al to the squares of their distances from the sun. The same reasoning applies to the satellites. Hence it is also inferred, that the forces by which dif- ferent planets are retained in their different orbits, are not forces of different kinds, but the same force operating at different distances. The secondary planets accompany the primary planets by the action of a force always directed to the sun, and inversely proportional to the square of the distance from the sun. That the moon is a heavy body, and gravitates towards the earth in the same manner as terrestrial bodies.*' Sir Isaac Newton, considering that the power of gravity acts equally on all matter that is in or near the surface of . the earth, that it is not sensibly less on the tops of the . highest mountains, that it affects the air and reaches up- ward to the utmost limits of the atmosphere, was indu- ced to think it might be a more general principle, and extend to the heavens, so as to affect the moon at least, which is the nearest to us of all the bodies in the system. He afterwards extended this principle still further, and showed, that the planets consisted of the same gravitat- | ing substance of which the earth is formed. These effects of the power of gravity upon terrestrial bodies mav be reduced to three classes. 1 . "When in con- sequence thereof, a body at rest, supported by the ground, suspended by a string, or by any other means kept from falling, endeavours always to move. In such cases the effect of gravity is measured by the pressure of the qui- escent body upon the obstacle that hinders its motion. 2. When a body descends in a vertical line, its mo- tion is then continually accelerated, in consequence of the incessant action of the power of gravity ; or if it be pro- jected upwards in the same right line, its motion is conti- nually retarded by the same power acting incessantly up- on it in a contrary direction. In such cases the power * Maclaurin's Sir Isaac Newton's Discoveries, p. 214 to 265. TOWARDS THE EARTH. 231 of gravity is measured by the acceleration or retardation of the motion produced in a given time by the power continued uniformly for that time.* 3. When a body is projected in any direction different from the vertical line, the direction of its motion is con- tinually varied, and a curve line is described in conse- quence of the incessant action of gravity ; which in such cases is measured by the flexure or curvature of the line described by it ; for the power must be the greatest that deflects the course of the body most from the tangent or direction in which it was projected. Effects of each kind of the power of gravity continu- ally fall under your observations near the surface of the earth ; for the same power which renders bodies heavy while they are at rest, accelerates their motion when they descend perpendicularly, and bends their motion into a curve line when they are projected in any other direction than that of their gravity. We can judge only of the powers that act on the ce- lestial bodies by the effects of the last kind ; we see bo- dies near the earth falling towards it ; but this is a proof of the moon's gravity, which cannot be obtained unless the present state of things were dissolved. When a body is projected in the air, you do not see it fall in the perpendicular towards the earth, but you see it falling every moment from the tangent to the curve, that is, from the direction into which it would have mov- ed, if its gravity had not acted for that moment. And this proof is obtained of the moon's gravity ; for though you do not see her falling directly towards the earth in a right line, yet you observe her every moment descending towards the earth from the right line, which was the direction of her motion at the beginning of that moment, and this is as evident a proof of her being acted upon by gravity or some power similar to it, as her recti- lineal descent would be, if she were allowed to fall freely to the earth. If we were in possession of engines of a sufficient force, bodies might be projected from them so as not only to be * See Lecture on Mechanics. 232 GRAVITATION OF THE MOON O 3v carried a vast way without falling to the earth, but so a^ to move over a quarter of a great circle thereof; or, ab- stracting from the resistance of the air, to move round the earth without touching it, and after returning to the first place, commence a new revolution with the same force which they first received from the engine, and after that a third, and thus revolve as a moon or satellite round the earth for ever. If this could be effected near the earth's surface, it might be done higher in the air, or even as high as the moon, could the engine or an equivalent power be carried up and made to act there. By increasing the force of the power, a body proportionally larger might be thus pro- jected, and by a power sufficiently great, a body not infe- rior to the moon, might be at first put in motion, and be- ing perpetually restrained by its gravity from going off in a straight line might for ever revolve about the earth. Thus Sir Isaac Newton saw, that the curvilineal motion of the moon in her orbit, and of any projectile at the sur- face of the earth, were phenomena of the same kind, and might be explained upon the same principle extended from the earth, so as to reach the moon ; and that the moon was only a larger projectile that received its motion in the beginning of things from the Almighty Author of the universe. But to make this more evident, it was necessary to show, that the powers which act on the moon, and on projec- tiles near the earth, and which bend their motions in a curve line, were directed to the same centre, and agreed in the quantity of their force as well as in their direction. All we know of force relates either to its direction or quantity, and a constant coincidence or agreement in these two respects is sufficient ground to conclude them to be the same, or similar phenomena, derived from the same or like causes. Now I showed you, in the lecture on mechanics, that the gravity of heavy bodies is directed towards the centre of the earth ; and it appears from Kepler's first law, as I have shown you in this lecture, that the power which acts on the moon, incessantly bending her motion into a curve, is directed towards the same centre; for astronomers find, TOWARDS THE EARTH. 233 that the moon does not describe an exact circle about the earth, but an ellipse, and that she approaches to the earth, and then recedes from it in every revolution, but still so as to have her motion accelerated while she ap- proaches the centre of the earth, and retarded as she recedes from it, describing equal areas in equal times ; an indication, as you have already seen, that she is acted on by a power directed accurately or nearly towards the centre. There is, therefore, a power which deflects the moon from a rectilineal course, and which like gravity, makes her descend towards the centre of the earth ; so that if the projectile force were destroyed, she would fall to the earth in a direct line ; and as this power acts inces- santly, bending every moment her path into a curve, it would make her descend to the earth with an accelerat- ed motion, like that of heavy bodies in their fall. It remains therefore only to show, that the power which acts on the moon, agrees with gravity in the quantity of its force, as well as in other respects. But before we compare them in this particular, I must ob- serve to you, that the power which acts upon the moon, is not the same at all distances, but is always greatest when she is nearest the earth. To be satisfied of this, it is only necessary to observe, as before, that to bend the motion of a body into a curve when it moves with a greater velocity, requires more power than when it describes the same curve with a less velocity. Though what I have just asserted is sufficiently ob- vious, it may appear more fully by considering a dia- gram; imagine therefore a tangent, plaU 1 5,Jig. 5, drawn at the beginning of a small arc described by the body ; and as this is the line which the body would have fol- lowed, if no new power had acted upon it, the effect of that power is estimated by the depression of the other extremity of the arc under that tangent : now it is plain, that in arcs of the same curvature, the greater the arc is, the farther must one extremity of it fall be- low the tangent drawn at the other extremity ; and, con^ VOL. IV. 2 H 234 GRAVITATION OF THE MOON sequently, when a body describes a greater arc, it muse be acted upon by a greater power than when it describes a less arc in the same time. Now, as the moon approaches the earth, her motion is accelerated, being swiftest at her least distance, slowest at her greatest distance ; and the arches she describes at her greatest and least distance have the same curvature ; therefore the force which acts upon her at her least distance, when her motion is swiftest, must be the greatest force. It will not now be difficult to see according to what law this power varies at her least and greatest distance from the earth. To render this easier, let us assume a simple case, and suppose that her least distance is half that of her greatest. If this were true, the moon would move with double velocity in her least distance ; and the space described by a ray from her to the earth might be equal to the space described in the same time at her greatest distance : so that she would describe at her least distance an arc in one minute equal to the arc she would describe in two minutes at her greatest distance, and would fall as much below the tangent at the begin- ning of the arc in one minute in the lower part of her orbit, or the perigee, as in two minutts in the higher part of her orbit, or her apogee. If, therefore, her projectile force were destroyed at her least distance, she would fall towards the earth as much in one minute, as in two minutes if her projec- tile force were destroyed at her greatest distance. But the spaces described by a falling body are as the squares of the times, and such a body descends through a quadruple space in double time ; so that the moon descending freely would necessarily fall four times as far in two minutes as in one minute ; that is, through four times as much space in one minute at her least dis- tance, as at her greatest distance in the same time. But the forces with which heavy bodies descend are in the same proportion as the spaces described, in con- sequence of those forces, in equal small parts of time \ consequently, the force which acts at the. least distance is quadruple that which acts at a greatest distance, when the latter is supposed to be double the former ; or the TOWARDS THE EARTH. 235 forces are as 4 to 1, when the distances are as 1 to 2. The force therefore which • acts upon the moon, and bends her into a curvilinear orbit, increases as the dis- tance from the centre of the earth decreases, so as to be quadruple at half that distance. In the same manner it is shown, that if her least dis- tance were the third part only of her greatest distance, her velocity would be triple at the least distance, to preserve the equality of the areas described by a ray drawn from her to the centre of the earth ; and that she would be acted upon there by a power, which would have the same effect in one minute, as in three minutes at her greatest distance ; so that if she were allow- ed to descend freely from each distance, she would fall nine times as far from the least distance as from the greatest in the same time ; consequently, the power it- s.lf which causes her to descend would be nine times greater at the third part of the distance, or the distances being as 1 to 3, the force would be as 9 to 1, or in- versely as the squares of the distances. In the same manner it appears, that when the great- est and least distances are supposed to be in any pro- portion of a greater to a less number, the velocities of the revolving planet are in the inverse ratio of the same numbers ; and that the powers which deflect or bend its- motion, into a curve, are in the inverse ratio of those numbers. To consider this in general j let T, plate 15, Jig, 5, represent the centre of the earth, ALP the moon's elliptical orbit, A the apogee, P the perigee, A H and P K the tangents at those points, A M and P N any small arcs described by the moon in equal times at those distances, M H, N K, the subtenses of the angles of con- tact, terminated by the tangents in H and K ; then M H and N K will be equal to the spaces that would be described by the moon, if allowed to fall freely from the respective places A and P in equal times ; and will be in the same proportion to each other, as the powers which act upon the moon, and inflect her course at those places. 236 GRAVITATION OF THE MOON ire Let A m be taken equal to P N, and m h parallel AP meet the tangent at A in h ; now, as the curvature of the ellipse is the same at A as at P, m h is equal to K N ; and if the moon were to fall freely frcm the places P and A towards the earth, her gravity would have a greater effect at P than at A, in equal times, in proportion as m h is greater than M H. But m h is the space which the moon would describe freely by her gra- vity at A, in the time which m h would be described by her projectile motion at A, and M H is the space through which she would descend freely by her gravity at A, in the time in which A H would be described by her pro- jectile motion ; and these spaces being as the squares of the times, it follows, that m h is to M H, as the square of A h to the square of A H, or, because of the equa- lity of the areas T A H, T P K ,as the square of T P to the square of T A. Therefore, the gravity at P is to the gravity at A, as the square of T A to the square of T P ; that is, the gravity of the moon towards the earth increases in the same proportion, as the square of the distance from the centre of the earth decreases. Sir Isaac Newton shows the universality of this law, in all her distances, from the direction of the power that acts upon her, and from the nature of the ellipsis, the line which she describes in her revolution ; and it follows from the properties of this curve, that if you take small arcs described by the moon in equal times, the space by which the extremity of any arc descends towards the earth below its tangent at the other extremity, is always greater in proportion as the square of the distance from the focus is less ; from which it follows, that the power which is proportional to this space observes the same proportion. The moon's orbit, according to astronomers, differs not much from a circle of a radius equal to 60 times the semidiameter of the earth ; and the circumference of her orbit is therefore about 60 times the circumfer- ence of a great circle of the earth. From this the circumference of the moon's orbit is easily computed, and as she finishes her revolution in TOWARDS THE EARTH. 237 27 days, 7 hours, and 43 minutes, it is also easy to cal- culate what arcs she describes in one minute. The next thing is to compute how much this arc of one minute is deflected below a tangent drawn at the other end : now geometricians prove, that this space is nearly a third proportional to the diameter of her orbit, and the arc'she describes in a minute ; whence, by an easy calculation, this space is found to be about 16 feet 1 inch. But you have seen, that this space was described in consequence of her gravity, or tendency towards the earth, which is therefore a power, that at the distance of 60 semidiameters of the earth, is able to make her descend in one minute through 1 6 feet 1 inch. Now, as this power increases as she approaches the earth, let us see what its force would be at the sur- face thereof; and for this purpose, let us suppose her to descend so low in her orbit as at her least distance to pass by the surface of the earth ; she would then be 60 times nearer to the centre of the earth, and move with a velocity 60 times greater, that the areas describ- ed by a drawn line from her to that centre in equal times, might still continue equal. The moon, therefore, passing by the earth at her low- est ebb, would describe an arc in one second of time, the 60th part of a minute, equal to that she describes in one minute at her present mean distance, and would fall as much below the tangent at the beginning of the arc in a second, as she falls from the tangent at her mean distance in a minute ; that is, she would, near the surface of the earth, fall ]6 feet 1 inch in 1 second of time. Now this is exactly the same space, through which all heavy bodies are found by experience to descend by their gravity near the surface of the earth. The moon, therefore, would descend at the surface of the earth with the same velocity, and every way in the same manner, as heavy bodies fall towards the earth ; and the power which acts upon the moon, agreeing in di- rection and force with the gravity of heavy bodies, and 238 GRAVITATION OF THE PRIMARY acting incessantly every moment, as their gravity does, must be of the same kind, and proceed from the same cause. Thus Sir Isacc Newton showed, that the power of gra- vity is extended to the moon ; that she is heavy, as all bodies belonging to the earth are found to be ; and that she is retained in her orbit by the same cause which occasions a stone, a bullet, or any other projectile, to describe a curve in the air. If the moon or any part of her were brought down to the earth, and projected in the same line, and with the same velocity as a terrestrial body, it would move in the same curve. On the other hand, if any body were carried from our earth to the distance of the moon, and projected in the same direc- tion, and with the same velocity with which the moon is moved, it would proceed in the same orbit which the moon describes, and with the same velocity. Thus the moon is a projectile, and the motion of every projec- tile gives an image of the motion of a satellite or moon. That the primary planets are heavy bodies ^ and gravitate towards the sun ; and that the secondary planets gravi- tate towards their respective primaries. Observation proves, that each of the primary planets bend their path about the centre of the sun, are acce- lerated as they approach to him, and are retarded as they recede from him, always describing equal areas in equal times ; from whence it follows, that the- power by which they are deflected must be directed to the sun. This power also varies always in the same manner as the gravity of the moon towards the earth. The same reasoning, by which the gravity of the moon towards the earth at her greatest and least dis- tances were compared together, may be applied in com- paring the powers which act on any primary planet at its greatest and least distances from the sun ; and it will appear, that these powers increase as the squares of the distances from the sun decrease. AND SECONDARY PLANETS. 239 But the universality of this law, and this uniformity of nature, still farther appear, by comparing the mo- tions of the different planets. The power which acts on a planet that is nearer to the sun, is manifestly greater than that which acts on a planet more remote, both because it moves with more velocitv, and because it moves in a less orbit, which has more curvature, and of course the body requires more force to be deflected from its rectilinear course. By- comparing the motions of the planets, it is found, that the velocity of a nearer planet is greater than that of one more remote, in proportion as the square-root of the number expressing the greatest distance, to the square- root of the number expressing the lesser distance ; so that if one planet be four times farther from the sun than another, the velocity of the former would be half the velocity of the latter, and the nearer planet would describe an arc in one minute equal to the arc described by the former planet in two minutes ; and the nearer planet would describe, by its gravity, four times as much space as the other would describe in the same time ; by the laws of falling bodies, the gravity of the nearer planet would therefore appear to be quadruple, from the consideration of its greater velocity only. But fur- ther, as the radius of the lesser orbit is supposed to be four times less than the radius of the other, the lesser orbit must be four times more curved, and the extre- mity of a small arc of the same length will be four times farther below the tangent drawn at the other extremity in the lesser orbit than in the greater ; so that though the velocities were equal, the gravity of the nearer pla- net would on this account only be found to be quad- ruple. On both these accounts together, the greater velocity of the nearer planet, and the greater curvature of its or- bit, the deflecting force, or its gravity towards the sun, must be supposed sixteen times greater, though its dis- tance from the sun is only four times less than the other ; that is, when the distances are as 1 to 4, the gravity is reciprocally as the squares of these numbers, or as 2 6 to 1 . By comparing the motions of all the planets it 240 GRAVITATION OF THE PRIMARY is found, that their gravities decrease as the squares of their distances from the sun increase. The same principle that governs the motion of the pla- nets in the great solar system, governs also the motion of the satellites in the lesser systems, of which the greater is composed. There is the same harmony in their motions compared with their distances, as in the great system. Jupiter's sa- tellites are continually bent from the lines that are the direction of their motions, each describing equal areas in equal times, by a ray drawn to the centre, to which their gravity is therefore directed. The nearer satellites move with greater celerity, in the same proportion as the nearer primary planets move more swiftly round the sun ; and their gravity therefore varies according to the same law. The same is to be said of Saturn's satellites. There is, therefore, a power that preserves the sub- stance of these planets in their various motions, acts at their surfaces, and is extended around them, decreasing in the same manner as that which is extended from the earth and sun to all distances. They accompany their primary planets in their motion round the sun, and move about them at the same time, with the same regularity as if their primaries were at rest. It is as in a ship, or in any space carried uniformly for- ward, in which the natural actions of bodies are the same as if the space were at rest, being no way affected by that motion which is common to all the bodies. As every projectile, while it moves in the air, gravi- tates towards the sun, and is carried along with the earth about the sun, while its own motion in its curve is as re- gular as if the earth were at rest ; so the moon, which is only a greater projectile, must gravitate towards the sun, and while it is carried along with the earth about the sun, is not hindered by that motion from performing its monthly revolutions towards the earth. It is the same with respect to the other secondary planets. Thus the motions in the great solar system, and in the lesser particular systems of each planet, are consistent with each other, and are carried on in a regular harmony. AND SECONDARY PLANETS, &C. 241 without any confusion or mutually interfering with one another, except what necessarily arises from small ine- qualities in the gravities of the primary and secondary planets, and the want of exact parallelism in the direction of these gravities. Observation shows, that the deflexion of the moon to the earth, and of the planets to the sun, is accompanied with an equal and opposite deflexion of the earth to the moon, and of the sun to the planets ; from which it is inferred, that the forces which produce these deflexions are mutual, equal, and opposite. As the planets are deflected towards each other, and as these deflexions are inversely proportional to the square of the distance from the planet towards which they are inflected ; it follows, that all the bodies of the solar sys- tem turn towards each other .with forces which are in- versely proportional to the squares of the distances. The curve which a body describes determines the law of its gravitation, or the relation which subsists between the intensity of the gravitating force, and the distance from the point to which it gravitates. If the gravitation of every particle of gravitating matter be supposed to be the same in the same circumstances, then the relation which is observed between the distances and the periodic times, will determine the proportion of gravitating matter in a planet ; and on this supposition it has been concluded from the phenomena, that this pro- portion is the same in all. But as the above supposition is not formed from any direct arguments, all that can be justly inferred from this observed relation is, that the gra- vitation of each planet, taken in cumulo, is proportional to its quantity of gravitating matter. OF THE CENTRE OF THE SOLAR SYSTEM. Sir Isaac Newton having found, that the celestial bo* dies all mutually gravitate towards each other, it follows that no one body in the whole system can be supposed to be entirely void of motion. VOL. IV. 2 I 242 CENTRE OF THE SOLAR SYSTEM. mly The centre of gravity of the whole system is the on point therein, which can be supposed quiescent ; it is the only immoveable point, round which all the bodies in the system move with various motions. On an accurate examination of the tendencies of the planets, it is found, that the centre round which each planet revolves, is not the centre of the sun, but the point which is the common centre of gravity of the sun and pla- net, whose revolution is considered. Thus, the mass of the sun being to that of Jupiter as 1 to T ^ 6T , and the distance of Jupiter from the sun being to the sun's semi- diameter in a ratio somewhat greater, it follows, that the common centre of gravity of Jupiter and the sun is not far distant from the surface of the sun. By the same method of reasoning it is found, that the common centre of gravity of Saturn and the sun falls within the surface of the sun ; and also, that if all the planets were placed on the same side of the sun, the com- mon centre of gravity of the sun and all the planets, would scarce be one of his diameters distant from his centre. It is about this centre of gravity that the planets revolve; and the sun himself oscillates round this centre in pro- portion to the actions of the planets exerted on him. When, therefore, the motion of two bodies, whereof one revolves round the other, is considered rigorously, the central body should not be regarded as fixed, as they both revolve round their common centre of gravity ; but the spaces they describe round this common centre being in the inverse ratio of their masses, the curve described by the body which is the greatest mass, is almost insensi- ble ; for which reason, the curve described by the body, whose revolution is sensible, is only to be considered, and the small motion of the central body, which is regarded as fixed, is neglected. The earth and the moon, therefore, revolve round their common centre of gravity, and this centre of gravity re- volves round the centre of gravity of the earth and sun. The case is the same with Jupiter and his moons, Saturn and his satellites, and with the sun and all the planeis. And the sun, according to the different position of the IRREGULARITIES PRODUCED BY GRAVITY. 243 planets,* moves successively on every side around the com- mon centre of gravity of our planetary system. This centre is tlje point where all the bodies of our pla- netary system would meet, if their projectile forces were destroyed, though the sun is in perpetual agitation ; be- ing, as I have shown you, so near it, he may with pro- priety be considered by astronomers as the centre of the solar system. Gravity produces some small irregularities in the motion of the planets. The regularity of the planetary motions is disturbed by their mutual gravitation, each disturbing the motions of the others, with a force proportional to its quantity of matter directly, and to the square of its distance from them inversely. In order to calculate these disturbances, it was necessary for mathematicians previously to ascer- tain the quantity of matter in the sun and planets. When a fleet of ships is carried away by a current that affects them equally, it has no effect on their particular motion amongst themselves, nor is the motion from the current discovered by them, unless they have some body in sight, that is not affected thereby in the same manner. The regularity in the motions of a planet, A, round the sun, would not be disturbed by the gravitation to a planet B, if the sun, and the planet A, did gravitate to B with equal forces, and in parallel directions ; and the disturb- ance of the motion A, arises from the inequality and obli- quity of the gravitations of the sun, and of A and of B. In consequence of this disturbing force, the motion of the earth in its orbit is retarded from the time that Jupiter is in opposition, till the time that he is in quadrature with the sun. It is then accelerated till he be in conjunction, then retarded till he be in quadrature, and then accelerated till he be again in opposition. The earth's gravitation to the sun is increased while Jupiter is in or near the quadratures, and diminished while he is in or near the conjunction and opposition. The augmentation of the earth's gravitation to the sun is greatest when Jupiter is in quadrature, being then about 244 IRREGULARITIES PRODUCED BY GRAVITY. . •siwff of the whole gravitation to the sun. The diminu- tion of the earth's gravitation to the sun is greatest when Jupiter is in opposition, being then about bW of the whole gravitation. The diminution of the earth's gravitation to the sun when Jupiter is in conjunction, is about 7—000 of the whole gravitation. In consequence of this change in the earth's gravita* tion to the sun, the line of the apsides of the earth's orbit changes its place in the heavens, sometimes advancing, and sometimes retreating ; but, on the whole, advancing, because the earth's gravitation to the sun is more dimi- nished than it is augmented. In like manner, the aphelion of any inferior planet ad- vances in consequence of the gravitation to the superior planets ; but the aphelion of a superior planet retreats in consequence of the gravitation to the inferior planets. For these reasons, and because Jupiter and Saturn are much larger than the inferior planets, the aphelia of all the planets, excepting Saturn, advance, while the aphe- lion of Saturn retreats. The accelerations and retardations of the planets Mer- cury, Venus, the Earth, and Mars, arising from their mutual gravitations, and their gravitations to Jupiter, nearly compensate each other ; and no effects of them are perceived in any long tract of years. But the posi- tion of the aphelia of Jupiter and Saturn is such, that the retardations of Saturn sensibly exceed the accelerations j so that the anomalistic period of Saturn is increasing, at present, about a day in a century. On the contrary, the period of Jupiter is diminishing. The disturbances occasioned by the mutual gravita- tions of the planets and comets are considerable. The comet of 1777 has suffered a remarkable change in its motions by the action of Jupiter. The earth's motion round the sun is remarkably affect- ed by the moon. In consequence of the mutual gravitations of the pla- nets, the nodes of a disturbed planet retreat on the orbit of the disturbing planet. Hence the nodes of all the pla- nets retreat on the ecliptic, except that of Jupiter, which APPROACH AND RECESS OF TH PLANETS. 245 advances by retreating on the orbit of Saturn, from which it suffers the greatest disturbance. OF THE APPROACH AND RECESS OF THE PLANETS TO AND FROM THE SUN IN EVERY REVOLUTION. Having shown you, that the forces which produce the regular motions of the planets vastly exceed those that disturb them, I shall explain more fully, how the motions in their orbits proceed from the actions of those powers; and how the planet is made to ascend and descend by turns, while it revolves about the centre of its gravita- tion. We have nothing similar to this in the motion of heavy bodies at the earth's surface ; but you must remember, that the force, with which heavy bodies are projected from our most powerful engines, is inconsiderable, com- pared with the motions which their gravity could gene- rate in them in a few minutes ; and they move over such small spaces when compared with their distances from the centre of the earth, that their gravity is considered as acting in parallel lines, without any sensible error ; so that the centrifugal force arising from the rotation about that centre, is altogether neglected. But when the motion of a projectile in the larger spa- ces is examined, and traced in its orbit, it is necessary to take in the centrifugal force, arising from its motion of rotation about that centre ; and then it will appear, that there are indeed some laws of gravity, which would make the body approach to the centre continually, till it fall into it, but that there are other laws which make bodies to approach, and Suffer them to recede from it by turns. If a planet at B, plate 15, Jig. 6, gravitate, or is at- tracted towards the sun, so as to fall from B to y in the time that the projectile, force would have carried it from B to X, it will describe the curve, B Y, by the combined action of these two forces, in the same time that the pro- ectile force singly would have carried it from B to X, or :he gravitating power singly have caused it to descend trom B to y ; and these two forces being duly propor- 246 APPROACH AND RECESS OF THE PLANETS, tioned, and perpendicular to each other, the planet obey- ing both, will move in the circle B Y T. But, if whilst the projectile force would carry the pla- net from B to b, the sun's attraction, which constitutes the planets gravitation, should bring it down from B to 1 , the gravitating power would then be too strong for the projectile force, and would cause the planet to describe the curve B C. When the planet comes to C, the gra- vitating power, which always increases as the square of the distance from the sun diminishes, will yet be strong. er for the projectile force ; and by conspiring in some degree therewith, will accelerate the planet's motion all the way from C to K ; causing it to describe the arcs BC, C D, D E, E F, &c. all in equal times. Raving its mo- tion thus accelerated, it thereby gains so much centrifu- gal force, or tendency to fly off, at K in the line K k, as overcomes the sun's attraction ; and the centrifugal force being too great to allow the planet to be brought nearer the sun, or even to move round him in the circle K 1 m n, &c. it goes off, and ascends in the curve K L M N, &c. its motion decreasing as gradually from K to B, as it increased from B to K ; because the sun's attraction now acts against the planet's projectile motion, just as much as it acted with it before. When the planet has got round to B, its projectile force is as much diminished from its mean state about G or N, as it was augmented at K ; and so, the sun's attraction being more than suf- ficient to keep the planet from going off at B, it describes the same orbit over again, by virtue of the same forces or powers. A double projectile force will always balance a quad- ruple power of gravity. Let the planet at B have twice as great an impulse from thence towards X, as it had before ; that is, in the same length of time it was pro- jected from B to b, as in the last example, let it now be projected from B to c ; and it will require four times as much gravity to retain it in its orbit ; that is, it must fall as far as from B to c : otherwise it could not de- scribe B D, as is evident by the figure. But, in as much time as the planet moves from B to C in the higher parts of its orbit, it moves from I to K, or from K to L, TO AND FROM THE SUN. 247 in the lower part thereof ; because, from the joint action of these two forces, it must always describe equal areas in equal times, throughout its annual course. These areas are represented by the triangles B S C, C S D, D S E, E S F, &c. whose contents are equal to one an- other, quite round the figure. As the planets approach nearer the sun, and recede farther from him, in every revolution, there may be some difficulty in conceiving the reason why the power of gra- vity, when it once gets the better of the projectile force, does not bring the planets nearer and nearer the sun in every revolution, till they fall \ipon and unite with him ; or why the projectile force, when it once gets the bet- ter of gravity, does not carry the planets farther and farther from the sun, till it remove them quite out of the sphere of his attraction, and cause them to go on in straight lines for ever afterwards : but, by considering the effects of these powers, this difficulty will be remov- ed. Suppose a planet at B to be carried by the projec- tile force as far as from B to b, in the time that gravity would have brought it down from B to 1 ; by these two forces it will describe the curve C ; when the planet comes down to K, it will be but half as far from the sun, S, as it was at B ; and -therefore, by gravitating four times as strongly towards him, it would fall from K to Y in the same length of time that it would have fallen from B to 1 in the higher part of its orbit, that is, through four times as much space ; but its projectile force is then so much increased at K, as would carry it from K to k in the same time ; being double of what it was at B, and is, therefore, too strong for the gravitating power either to draw the planet to the sun, or cause it to go round him in the circle K 1 m n, &c. which would require its falling from K to W, through a greater space than gravity can draw it, whilst the projectile force is such as would carry it from K to k ; and therefore the planet ascends in its orbit KLMN, decreasing in its velocity for the causes already assigned. [ 248 ] THE MOON'S IRREGULARITIES. There is nothing that shows betterthe excellency of the Newtonian philosophy, or more clearly demonstrates the truth of its principles, than its so easily and clearly accounting for those many irregularities of motion, to which all the secondary planets, and the moon in par- ticular, are subject. Though these are called irregularities, yet they are not to be apprehended as random or fortuitous ones, bat such as are regular under the like circumstances, and subject to numbers and calculation. For it was by observing the period of those lunar ine- qualities, that Dr. Halley was enabled to foretel an eclipse of the sun, with an exactness little inferior to the obser- vation itself. It hath been seen before, that gravitation is a prim ciple belonging to all gravitating matter ; and that bodies, describing orbits about another placed in the centre of their motion, by a centripetal and projectile force, de- scribe equal areas in equal times. As this is the law by which the primary planets regu- late their motions about the sun, so likewise, were there no sun, by the same law would the moon regulate her motion about the earth. This tendency of the moon towards the sun, then, is the cause of those inequalties in her motion, which are called her irregularities. These are commonly reckoned eight, arising from causes now to be mentioned. J . That variation, whereby, if we suppose E the earth, plate 15, fig. 7, and the circle A B C D, the orbit of the moon, while the moon describes the quadrant A B, that is, while she goes from the quadrature to the conjunction, the force tending towards the sun at S, conspires with the force tending towards the earth at E, and therefore accelerates her motion. But while she goes from the conjunction B, to the next quadrature C, the force tend- ing towards the sun will act contrary to the force tend- ing towards the earth, and therefore will retard her mo- tion. the moon's irregularities. 249 In the same manner, while she goes from the quad- rature C, to the next syzygy D, the same force tend- ing towards S, will accelerate her again ; but while she goes from thence to the quadrature at A, it will again retard her. The moon, therefore, in her monthly revolution about the earth, is, by this action of the sun, alternately acce- lerated and retarded. 2. This force tending towards the sun being the dis- turbing force, or that force which prevents the moon from describing about the earth equal areas in equal times, will be greatest at the octants. For, this force being resolved into two others, after Sir Isaac Newton's manner, one of them at the quadratures or syzygies will be found to point from or towards E, the centre of the earth, directly, and therefore will not hinder the moon from describing equal areas in equal times ; the other, likewise, in those places will be found to tend towards the centre of the sun, and, therefore, nei- ther of them will prevent the moon there from describ- ing equal areas in equal times, /'. e. will not at the quad- ratures disturb the moon's motion at all. But, when the moon is in the octants, as at L, plate 15, Jig. 8, this force being resolved into two others, one of them, as L H, will point directly to or from the centre of the earth, and therefore will increase or dimi- nish the moon's tendancy towards the earth, but not hin- der her from describing equal areas in equal times. But the other, as L I, or H G, points neither towards the centre of the earth nor sun, and therefore, in the oc- tants, prevents her describing equal areas in equal times. But this being the mid-way between the quadrature and the syzygy, in both which places this disturbing force doth not prevent the moon from describing equal areas in equal times, it follows, that at the octants, this dis- turbing force will be the greatest of all. And for this reason it hath always been found more difficult to obtain the moon's place in the octants agreeing with observation, than at the syzygies or quad- ratures. VOL. IV. 2 K 250 the moon's irregularities. 3. The moon's orbit is more curved in the quadra- tures than in the syzygies. For, her motion being accelerated during her progress from the quadratures to the syzygies, in the syzygies her motion will be quicker than it ought otherwise to be, and therefore her centripetal force less than it would otherwise be. She will, therefore, at the syzygies de- scribe the portion of a larger curve, which, consequently, will be less curved than a smaller. That is, instead of describing the curve A B, plaie \5,fig+ 9, or CD, she will describe the curve E F, or G K. On the other hand, while the moon goes from the syzygies to the quadratures, her motion is continually retarded, and therefore, at the quadratures, her motion will be slower than it would otherwise be. At the quad- ratures, therefore, the moon will describe the portion of a lesser curve, which consequently will be more curved than a larger one. That is, instead of describing the curve A B, or C D, she will describe the curve E F, or GK, plate 15, fig 10. Therefore, at the quadratures, the moon's orbit is more curved than at the syzygies. 4. Since these irregularities in the moon's motion pro- ceed, as was said, from the action of the sun, it will follow, that where the action of the sun is greatest, the irregularities arising from k will be greatest also. But the nearer the earth is to the sun, the greater will be the action of the sun upon the moon ; and the more she tends towards the sun, the less will she tend towards the earth. When, therefore, the earth is at the perihelion P, plate 15, fig. 11, and consequently at its least distance from the sun, the action of the sun upon the moon will be greatest, and destroy more of its tendency towards the earth than at any other distance, as S D, S C, S B, &c. Therefore, when the earth is at the perihelion P, the moon will describe a greater orbit about the earth, than when the earth is at any other distance from the sun, and consequently her periodical time will then be th? longest. % the moon's irregularities. 251 But the earth is at its perihelion in the winter, and ■consequently, the moon will then describe the outermost circle about the earth, and her periodical rime will be the longest. And this agrees with observation. For the same reason, when the earth is at its apheli- on A, the tendency of the moon towards the earth will be the greatest, and consequently her periodical time the least. And in this case, which will be in the summer, she will describe the innermost circle about the earth. 5. Since the moon, from what has been said, appears to describe an elliptical orbit about the earth E, plate lo, Jig. 12, in the focus of it; and since her centripetal force towards the earth, by means of the action of the sun, is continually increasing or decreasing, but not equably, that is, sometimes less, and sometimes more, than in the inverse duplicate ratio of the distance of the moon from the earth ; therefore, in this case, the line of the moon's apsides, A B, will be continually going back- wards or forwards. That is, the axis, AB, will not al- ways lie in that situation, but go backwards into the si- tuation K L, or forwards into the situation F G. Since, however, taking one whole revolution of the moon about the earth, the action of the sun more di- minishes the tendency of the moon towards the earth, than it augments it, therefore the motion of the apsides forwards exceeds their motion backwards. Upon the whole, therefore, the apsides of the moon's orbit go for- wards, or according to the order of the signs. 6. Because the moon describes an eccentrical orbit about the earth at E, plate 15, Jig. 13, the action of the sun upon her sometimes increases her tendency to- wards the earth, and sometime diminishes it, i. e. makes her gravity towards the earth increase or decrease too fast. If, while the moon ascends from her lower apside, A, her gravity towards the earth decrease too fast, instead of describing her semi-ellipsis ABC, and coming to the higher apside at C, as she would otherwise do, she will run out in the curve B F D, and come to the higher apside at F. But the curve, A B F D, is more eccentric than the curve A B C D. 252 the moon's irregularities. . Therefore, when the gravity of the moon towards the earth decreases too fast, the eccentricity of her orbit will increase. On the other hand, when the moon is going from her higher apside C, plate 15, fig. 14, to her lower, A, and her gravity towards the earth increases too fast, instead of describing the same ellipsis C D A, and so coming to the lower apside at A, she will approach nearer to the earth, and describe the curve D F B, and so come to the lower apside at F. But the curve, C D F B, will be less eccentric than the curve DABC. Therefore, when the gravity of the moon towards the earth increases too fast, the eccentricity of her orbit will decrease, and the orbit itself will approach nearer to a circle. Therefore, the eccentricity of the moon's orbit will be continually varying. 7. In considering the irregularities of the moon's mo- tion, w r e have hitherto supposed the plane of her orbit as coinciding with the plane of the ecliptic, because her mo- tion would be affected by the irregularities hitherto spo- ken of, if in reality it did so. But it hath been before observed, that one half of the moon's orbit A C B, plate 15, Jig. 15, is raised above the plane of the ecliptic A E B G, and the other half, A D B, depressed below it ; and the points A, B, where the moon's orbit crosseth the plane of the ecliptic, are called the nodes, and the line A B, joining these points, is called the line of the nodes. But when this line of the nodes, A B, lies in conjunc- tion with the sun, S, it is at rest ; but in all other posi- tions it goes backwards. When this line of the nodes, AB, lies in the quadra- ture, plate 15, fg. 16, with the sun S, it goes back- wards fastest of all. 8. It has been formerly observed, that the orbit of the moon is inclined to the plane of the ecliptic in a certain angle ; but this angle is not constantly the same, but sometimes greater, and sometimes less, according to the position of the line of the moon's nodes with respect to the sun. CONCLUSION. 253 ■ When the line of the nodes, A B, plate- 1 5, fig. 17 % passes through the syzygies, the plane of the moon's orbit produced passes through the centre of the sun S, and, consequently, not being affected by the action of the sun, is then at its greatest state, making an angle with the ecliptic of about 5 degrees 18 minutes. When the line of the moon's nodes, A B, plate 1 5, fig. 18, lies in a quadrature with the sun S, then, suppos- ing the line, A B C D, to represent the plane of the eclip- tic, and A E B F the orbit of the moon, let the moon be supposed to have just now passed the ascending node at A, and going to her conjunction with the sun at E. The moon will then be going farther and farther from the plane of the ecliptic A B C D, and, were there no action of the sun, would come in conjunction with him atE. But, because of the action of the sun, the moon, in going from the quadrature at A, towards her conjunc- tion, will be perpetually drawn down towards the eclip- tic, and therefore will not come to a conjunction of the sun at E, but at G, making an angle with the ecliptic, GAC, less than E AC. But the sun continuing still to act, after the moon has arrived at her conjunction, will go on to draw her down towards the ecliptic ; by which means she will not cross the ecliptic in the point B, her former node, but in some other point nearer to the sun, as K. But wherever the moon crosses the ecliptic, is her node. Therefore, her node in the mean time hath gone back- ward from B to K, plate 15, fig. 19, and the moon hath described a semi-orbit A G K, making a less angle with the ecliptic, than the orbit AE B, which she would have described had there been no action of the sun at all. CONCLUSION. Aristotle concludes his treatise, De Mundo, with ob- serving, that " to treat of the world without saying any thing of its author would be impious," as there is nothing 254 CONCLUSION. we meet with more frequently and constantly in nature, than the traces of an all-governing Deity. The philosopher who neglects these traces, and con- tents himself with the appearances only of the material universe, and the mechanical laws of motion, neglects what is most excellent; and prefers what is imperfect to what is supremely perfect, finitude to infinity, what is nar- row and weak to what is unlimited and almighty, and what is perishing to what endures for ever. Those who do not attend to the manifest indications of supreme wis- dom and goodness perpetually appearing before them, wherever they turn their views or inquiries, too much re- semble those ancient philosophers, who made night, mat- ter, and chaos, the original of all things. The plain argument for the existence of the Deity, ob- vious to all, and carrying with it irresistible conviction, is derived from the evident contrivance and fitness of things for one another, which we meet with throughout all parts of the universe. There is no need of nice or subtile rea- sonings in this matter ; a manifest contrivance immedi- ately suggests a contriver. It strikes us like a sensation ; artrul reasonings against it may puzzle us, but can never shake our belief. No person, for instance, that knows the principles of optics, and the structure of the eye, can believe that it was formed without skill in that science ; or that the ear was formed without the knowledge of sounds ; or that male and female in animals were not formed for each other, and for continuing the species. All our accounts of na- ture are replete with instances of this kind. The admirable and beautiful structure of things for final causes, exalts our idea of the contriver ; the unity of the design shows him to be one ; revelation, that this one is Jesus Christ. The great motions in the system performed with the same facility as the least, suggest his almighty power, which gave motion to the earth and the celestial bodies, with equal ease as to the minutest parti- cles. The subtilety of the motions and actions in the in- ternal parts of bodies, shows that his influence penetrates the inmost recesses of things, and is every where exert- ed. The simplicity of the laws that prevail in the uni- CONCLUSION. 255 verse, the excellent disposition of things, in order to ob- tain the best ends, and the beauty which adorns the works of nature, far superior to any thing in art, suggest his con- summate wisdom. The usefulness of the whole scheme, so well contrived for the intelligent beings that enjoy it, with the internal dispositions and moral structure in those beings themselves, show his unbounded goodness. These are arguments which are sufficiently open to the views and capacities of the unlearned, while at the same time they acquire new strength arid lustre from the disco- veries of the learned. God acting and interposing in the universe, shows that he governs it, as well as that he formed it ; and the depth of his counsels, even in con- ducting the material universe, of which a great part sur- passes our knowledge, keeps up an inward veneration and awe of this great Being, and disposes us to receive what may be otherwise revealed to us concerning him. It has been justly observed, that some of the laws of nature now known to us, must have escaped us if we had wanted the sense of sight. God can bestow upon us other senses of which we have at present no idea, with- out which it may be impossible for us to know all his works, or to have more adequate ideas of his nature. In our present state, we know enough to be convinced of our dependency on him, and of the duty we owe to him as the Lord and Disposer of all things. Though the power of God is manifested in all his works, it is in the heavens that it still seems to beam forth in its greatest lustre. By his power acting there, he di- rects the courses of the planets, determines the circum- stances of their motions, and fixes the times of their re- volutions. As a General at the head of an army, he gives the signal to the heavenly bodies, and immediately they shoot forth, and proceed in their proper orbits. It is in consequence of the laws laid down by him, that the moon goes round the earth in a month. It is he that has combined the two motions of the earth, one by which we obtain the vicissitudes of day and night ; the other, by which the seasons of the year are brought about. He it is, who at the appointed times sends salutary winds, and fruitful rains and dews ; who gathers together the waters 256 CONCLUSION. in their sources, and causes them to flow from thence lr the beds of rivers to their great receptacle, the sea. It is he who makes the buds to open, the fruits to ripen, and animals to be prolific, ordering all things according to their different nature, regulating their birth, their growth, and their dissolution. Though the Author of so many wonders is invisible, you cannot on that account deny his power, or doubt his existence. You cannot see your soul, yet the effects it produces in you and around you, are sensible proofs of its existence. It is the same with many of the operations in nature. In like manner, God also, though invisible in himself, is visible in all his works, and in them appears equally strong in power, admirable in wisdom, eternal in duration, and supreme in perfection. The whole universe conspires to celebrate his praise, from whom it derives all its majesty and beauty. The sun that shines in brightness declares the ineffable splendour of its Almighty Creator. The moon and stars proclaim to an understanding heart the adorable power of the hand that guides them. The earth, so richly stocked with pro- ductions of higher and lower rank, with the various kinds of vegetable and animal life, paint in the strongest terms the riches of the divine nature, from whom issues all that adorns the earth, improves the mind, and delights the senses ; governing all things with infinite wisdom, good- ness unlimited, power uncontrouled. That a Divine Mind presides over and governs the uni- verse, is indeed the natural conclusion drawn by common reason from the evidence of common sense. For, who that sees this universal, frame thus wondrous fair, but must infer the cause of it to be full of wondrous beauty ? Who, that observes ever so slightly that constancy which is in the motions of the planets, and in the risings and set- tings of the fixed stars, &c. can possibly imagine the in- constancy of chance to be the mover ? What man, not disordered in his own mind, can suppose any other thing than mind to be the cause of that everlasting order, which appears in the regular interchanges of the elements, and the circling returns of the successive seasons. ON ELECTRICITY. 257 As far as I have conducted you through various branch- es of natural philosophy ; as far as I have proceeded in giving you a general view of the system of the world, beauty has every where struck your eye, and engaged you to proceed and scrutinize further the operations in nature. The more accurate your scrutiny, the more you will discover of regularity, symmetry, and order, in the •constitution of nature's frame ; the further you penetrate into her deep recesses, dividing and subdividing, opening and unfolding, the minutest part of every visible form, still the more you will find of beauty within beauty, and find every order to contain a variety of other orders. LECTURE XLVL ON ELECTRICITY, MR. STILLINGFLEET has well observed, that if the whole scene of nature were laid open to our view, were we permitted to behold the connexions and dependencies of every thing on every other, and to trace the economy of nature through the smaller, as well as the greater parts of this globe, we should probably find, that the Great Architect had contrived his works in such a manner, that we cannot properly be said to be unconcerned in any one of them ; and therefore, those studies, which seem upon a slight view to be quite useless, may in the end appear of no small importance to mankind. If you look back into the history of arts and sciences, you will be convinced, that men are apt to judge too hastily of things of this nature ; you will there find, that he who gave curiosity to his creature, man, gave it for VOL. IV. 2 L 258 ON ELECTRICITY. good and great purposes ; and that he rewards with useful discoveries what in the first instance are con- demned as trifling or minute researches. But it is true, that these discoveries are not always made by the searcher, or his cotemporaries, or even by the immediate succeeding generation ; but there can be no doubt, but that advantages of one kind or other always accrue to mankind from an investigation of the operations in nature. Some men are born to observe and record what perhaps by itself is perfectly useless, but yet of some importance to another, who follows and goes a step further ; then another succeeds ; and thus by degrees, till at last one of superior genius comes, who, laying all that have been done before his time toge- ther, brings on a new face of things, improves, adorns, and exalts human society. All those speculations concerning lines and numbers so ardently pursued, so exquisitely conducted by the Grecians, what did they aim at ; what did they produce for ages ? A little arithmetic, and the first elements of geometry, were all they had need of. This Plato as- serts ; and though, as being himself an able mathe- matician, and remarkably fond of those sciences, he recommends the study of them, yet he makes use of motives that have no relation to the common purposes of life. When Kepler, from a blind and strong impulse, merely to find analogies in nature, discovered that fa- mous law between the distance of the several planets from the sun, and the periods in which they complete their revolutions, of what importance was it to him or to the world ? Again, when Galileo, pushed on by the"same irresisti- ble curiosity, found out the law by which bodies fall to the earth, did he, or could he foresee, that any good could come from his ingenious theorems ; or was there any immediate use made of them ? Yet, had not the Greeks pushed their abstract specula- tions so far ; had not Kepler and Galileo made the above- mentioned discoveries, we never could have seen the ON ELECTRICITY. 259 greatest work that ever came from the hands of man, Sir banc Newton's Principia. Some obscure person, whose name is not so much as known, diverting himself idly, (as a by stander would have thought,) with trying experiments on a seemingly contemptible piece of stone, found out a guide for mari- ners on the ocean ; and such a guide, as no science, how- ever subtile and sublime its speculations may be, however wonderful its conclusions, could ever have attained. It is the same with electricity. Who could have supposed, on seeing a person amusing himself with the effect of excited amber on light bodies, that this was one of the the first links in a science that should teach men how to disarm the clouds of lightening, divest the storm of its terrors, and give life and power to the animal frame. Other instances might be produced to prove, that bare curiosity, in one age, is the source of the greatest utility in another ; and what has been frequently said of the chemists, may, perhaps, be applied to every other kind of virtuosi. They hunt, perhaps, after chimeras and impossibilities, and find something really valuable by the bye. We are but instruments under the Su- preme Director, and do not know, in many cases, what is of most importance for us to search after ; but we may be sure of one thing, that if we study and follow nature, whatever paths we are led into, we shall at last arrive at something valuable to ourselves and others, but of what kind, we must be content to be ignorant. The nature of aqueous vapours, of fire, and the electrical fluid, will clearly prove to you, that a num- ber of substances may act in nature without being known to us ; and that it is our ignorance of their ex- istence, which envelopes in obscurity so many pheno- mena. If it were not for the visible diminution of water when its surface is exposed, and for the hygroscopical ap- pearances, we should not have known that aqueous va- pours existed in the atmosphere. Notwithstanding all these phenomena, there are still those who do not ad- mit their existence. It is not difficult, however, to show, that the effects produced by this fluid, while in an im« 260 ON ELECTRICITY. perceptible state, are incomparably greater than the im- mediate symptoms of its existence. Again, without heat, an effect which is only produc- ed by fire when it is disengaged or free, we should have been ignorant of the existence of fire : yet how great and various are the effects it produces in the combined or latent state ! Heat is a symptom of the presence of this fluid, and of its degree of density, when free and disengaged ; but if you seek to follow it in the phe- nomena of nature, you find, that when it escapes from observation, it is acting the most important of parts. It is the same with light, the companion of fire. If it were not for the impression it makes upon our eyes, we should be ignorant of the greatest and most imme- diate agent of all terrestrial phenomena. Thus you see, that there are substances of the greatest importance for modifying those which are more grossly perceptible, and of which, in the mean time, we have little or no knowledge, though they are producing the greatest effects. But still further, the motions of light occasioned by the electrical fluid, when its natural equilibrium is dis- turbed, are the only signs which give us notice of its existence. All electrical phenomena concur in proving the existence of a certain fluid, possessed of certain characters, capable of particular modifications, and dis- seminated over the whole surface of the globe ; the why or wherefore is still unknown, we are still ignorant of its functions. But we are at the same time ignorant of the cause of so many phenomena in nature, that we ought not to despair of being able to discover those with which it is connected, and how it influences them by its composition and decomposition. It results from the observations I have now made, that the known expansible fluids have two kinds of pro- perties ; one whereby they manifest themselves to one or more of our senses, the other by which they act imper- ceptibly in a number of phenomena. It is not then ne- cessary, either as a proof of the existence of a sub- stance, or of its being a principal agent in phenomena, that it should be manifested to our senses. But it is ELECTRICAL APPEARANCES. 261 essential in nature, as soon as you consider physical ob- jects, that to every phenomenon there be a cause ; and the only method of assigning a reasonable one, where they are not immediately discoverable, is analogy. When therefore certain phenomena, whose cause is hidden, are analogous to other phenomena that we attribute to the intervention of some substance, we are naturally led to some substance as a cause of the first mentioned phe- nomena ; and nothing will oppose their admission, if they explain what cannot be explained without, and if there be nothing which renders the existence of the substance obscure. Here I cannot refrain from observing, that the French philosophers, after extending the influence of electrici- ty over all nature, now pass it by as if unworthy of no- tice ; its name is not even to be found dans le tableau de la novelle nomenclature. Philosophy owes much to the assistance it has receiv- ed from mathematicians ; but this only happens when they apply themselves to the study of phenomena ; when neglecting these, calculations are made to serve a hypothesis ; the more elegant and beautiful they are, the more detrimental they become to science. It it thus, that Mpinus^ by a mathematical theory of electricity, has closed the door on all our reserches into the nature and operations of this fluid. ELECTRICAL APPEARANCES. From these preliminary observations, which I con- sidered, as necessary to excite your attention to the electric fluid, and to lead you to look out for, and to trace its connexion with other agents in nature ; I shall proceed to point out some of the most striking electri- cal appearances. The knowledge obtained of electrici- ty, like most other articles of science, has arisen from very small beginnings, and by very slow degrees to its present height. It had been known for ages, that amber, jett, and other bodies, would, upon rubbing, attract and repel light bodies, as hairs, feathers, down 3 262 ELECTRICAL APPEARANCES. dust, &c. and as this property was most conspicuous in amber, which in the Greek is called electron, the pe- culiar power of that body was termed electricity. Up- on further enquiry, it was found, that not amber only, but several other substances had the same properties in a high degree ; that glass, resinous substances, silk, dry wood, &c. have the same properties ; and that any of these when dry, and rubbed for a short time, would attract light substances. I rub this stick of sealing wax with soft flannel, and you see that it attracts any light substances, as hairs, feathers, &c. that I bring under it ; rub a dry glass tube with dry silk, and you will find it produce the same effect. Let us now darken the room, rub the glass tube again, and you will see sparks of fire follow your hand ; present your knuckle to the tube, and these sparks will be formed into pencils or brushes of light, attended with a crackling noise like that of a green leaf in the fire. The friction has, in these instances, manifested to the senses the existence of a substance that was before imperceptible. The body that is made by friction to exhibit these appearances, is said to be excited. The appearances are termed signs of electricity. Here is a fine downy feather tied to a silk string ; I electrify it strongly by touching it with the excited glass tube, and it immediately flies from, or is repelled by the glass tube. I now present an excited stick of seal- ing-wax, and the feather immediately flies towards it. Thus you see, that what is attracted by excited wax, is repelled by excited glass. This experiment gave rise to a very important distinction in electricity, implying a contrariety of agency therein, and one power or agent by the glass was denominated vitreous, the other resinov.s electricity.* Further discoveries showed, that glass or wax would, according to the circumstances in which they were situate, produce either power. * Now generally called by electricians, fioaitive and negative electricity. E. Edit/ PRINCIPLES OF ELECTRICITY &C. 26$ Our two next experiments lead us also to another very- important distinction in this branch of science. I sus- pend a brass ball by a wire from the end of the glass tube opposite to my hand, and excite the tube as before ; as soon as the tube is excited, you will find, that the ball has acquired all the electric properties of the tube ; it will, like it, attract light bodies, and give the spark. Let us now suspend the ball by a silk string, and ex- cite the tube as before ; you may now rub as long as you please, but the ball will exhibit no signs of electri- city. Here then we have two substances, through one of which, the wire, the electric properties may be con- veyed ; whereas the other, that is, the silk, prevents their passing to the ball. The wire is therefore called a conductor of electricity. The silk is termed a non- conductor. Or, in more general terms, all those bodies, through which the electrical fluid is transmitted freely, are term- ed conductors. Those bodies, through which it does not pass so freely, are called non-conductors, A body resting entirely upon non-conductors is said to be insulated. Thus, in the last experiment, the ball was insulated, because it was suspended by a silk string, which is a non-conductor. Insulation prevents the dis- sipation of the electrical appearances. THE PRINCIPLES OF ELECTRICITY DEDUCED FROM EXPERIMENTS ON ATTRACTION AND REPULSION. I have already shown you, that a light body electri- fied by excited glass, is repelled thereby, but will be at- tracted by excited wax ; and that, on the other hand, if it be electrified by excited wax, it will be repelled thereby, but will be attracted by excited glass. This observation you must keep in mind, without it you can never understand the operations of electricity. The greater part of the experiments in this science, and the whole of the reasoning thereon, depend on a reference to these facts. 264 PRINCIPLES OF ELECTRICITY For the following experiments, I make use of small light balls formed out of the pith of elder ; they are suspended by fine linen threads from small cylinders of wood, and are insulated upon a common wine-glass, that is dry, free from dust, fibres of down, &c. I electrify two balls, thus suspended by excited glass, and you see they repel each other. I destroy this electri- city by touching them with my hand. I again electrify them, but with excited wax, and they again repel each other. I bring the balls electrified by wax towards those electrified by glass, and they immediately fly towards each other. From these experiments you will infer, 1. That bodies electrified vitreously repel each other. 2. That bodies elec- trified resinously repel each other. 3. That bodies electri- fied with contrary powers attract each other. As those light substances which possess the same elec- tric power repel each other, it will always be easy for you to discover with what power they are electrified. If they be repelled by excited glass, they possess the vitreous elec- tricity ; if they be attracted thereby, they are resinously electrified ; on the contrary, those attracted by excited wax, are vitreously, and those repelled thereby, resi- nously electrified. In ascertaining the nature of the electric power, you must avoid bringing the bodies to be tried near each other suddenly ; or one strongly electri- fied too near one that is weakly ; as it may render the experiment doubtful, for reasons that will soon be appa- rent. Before I go any further, it may be proper to point out to you the leading features of that theory of electricity, Mr. Eeles's,* which I adopt in these lectures. I consider, * See Edes's Philosophical Essays in several letters to the Royal Society, London, 1771. On a comparison of this work with the greater part of the modern writers on electricity, yon will find, that they have been gradually giving up the most essential distinctions oftheFranklinian theory, and adopt- ing those of Mr. lieles. See Wilson, Hcnly, Gray, Milner, Brooke, Peart, Read, Sec. Sec. And you will find him laying down principles, and making experiments, that have within a few years been brought forward as new; some have indeed been rejected at first, because deemed contradictory to a favourite theory, but which have since been fully acknowledged. . DEDUCED FROM EXPERIMENTS. 265 with him, all those electrical operations that are manifest- ed to the senses, as occasioned by two distinct, positive, and active powers, which equally and strongly attract and condense each other ; but when by any circumstance they are rendered unequal to each other, the increased power expands into an atmosphere. These two powers exist together in all bodies ; in their natural state they are always conjoined ; the electric signs, or what we call electricity, are only rendered sensible to us by the separation of these powers. In other words, though the electric matter is acting the most important part among the operations in nature, in its united, and to us, latent and invisible state, yet it becomes no object to our senses, till its powers are separated and rendered un- equal. When the powers are separated and brought into ac- tion, the increased power expands, and forms what may be termed an electrical atmosphere. If any body be im- merged in this atmosphere, the powers thereof are sepa- rated, and that which is of the same kind with the atmos- phere is repelled, while the contrary power is attracted : as long as the body remains immerged therein, the powers remain separated. It is however to be observed, that in exciting electrics, the powers are never entirely separated. The diminished power acts inward to the electric, while the increased power acts outward with an extensive at- mosphere. I hold an excited glass tube over this metallic cylinder, placed on a dry wine-glass, as an insulating stand, plate 1, jig. 1, (Electricity), but at a certain distance from it, which distance will depend upon the power of the glass ; it repels the vitreous electricity of the tube into the balls, which will diverge with vitreous electricity, and will of course recede from excited glass. I remove the excited glass from over the balls, and they close. A temporary separation of the electric matter inherent in the cylinder, is in this instance produced by the influence of the ex- cited glass ; as soon as this influence is removed, the powers unite, and the balls close. I now place two cylinders, with their ends in contact with each other, plate 1, jig. 2, and hold the excited VOL. IV. 2 M 266 PRINCIPLES OF ELECTRICITY. tube over the end A ; each pair of balls diverges. While they are in this state, separate them one from the other, and you will find the balls of A to be electrified vitreously, and these of B resinously ; proving, that while the body remained immerged in the atmosphere, the electric pow- ers thereof were separated, one being at each end. Bring the tubes together again, and the balls immediately close; proving, 1. That the separated powers attract each other. 2. That when united, they condense each other, and that all electric signs are immediately lost: 3. The co-exist- ence of the two powers in the cylinders. Again electrify the balls equally, but with the same powers, then bring-the ends of the cylinders together, and the divergence of the balls will not be altered ; which shows, that equal atmospheres of the same kind do not act on each other. Hold an excited glass tube over the cylinder, plate 1, fig. 4, and at the same time keep one of your fingers in contact with the opposite end of the cylinder, remove the glass tube and finger together, and the balls will diverge with resinous electricity ; for, on trying, you will find them fly towards excited wax, and recede from excited glass. The vitreous power is repelled by the excited tube, and passes into the finger, which, in exchange, communicates resinous electricity to the cylinder. The tendency of an electric atmosphere to produce the contrary electricity, in the bodies contiguous to it, is plea- singly illustrated by the following experiment. In this, there are four cylinders, A, B, C, D, plate 1, Jig. 3; excited glass held over A, repels the vitreous power into B^ and draws the resinous into A ; in the same manner, B repels the vitreous power of C into D, and draws the resinous into C ; separate B and D from A and C, just before the excited glass is removed, and you will find A and C possessing the resinous, B and D the vitreous elec- tricity ; as you will find, by bringing the excited glass to- wards the balls, those at A and C will move towards the glass, those at B and D will recede from it. You saw in a former experiment, where the balls were equally electrified with contrary powers, that on bringing the cylinders together, the powers united, and all electri- OF THE ELECTRICAL MACHINE. 267 cal signs vanished. But if one be electrified more than the other, that which is least so, loses all its electricity after contact, and the two remain electrified, with the excess of the electricity of that which was strongest. From these experiments it appears, that the increased power expands itself, and acts outwards, and that in pro- portion to the subtraction of the other power ; and that this is the sphere of the expanded power, which is called an electric atmosphere. It appears further, that no substance seems to be elec- trified, while the powers are equal in or on that body ; but in proportion as there is a greater quantity of one power, than there is of the other, then the increased power acts outwards from that body, and the body will be elec- trified with that power, and will repel any other body electrified with the same power ; but will attract any sub- stance electrified with the contrary power ; and after con- tact between them, all electrical signs vanish, if they were equally electrified ; but if unequally, both will remain electrified with the excess of the strongest power. These positions will be confirmed by other experiments, in which you will see the contrary directions of the two powers. OF THE ELECTRICAL MACHINE, plate 1, fig. 5, AND ITS MODE OF ACTION. By turning the handle of the machine, and of course the glass cylinder which moves with it, electricity is pro- duced ; and this we shall find, as before, of two kinds, each strongly attractive of the other, though repulsive of a similar kind ; when united, the expansive power they before exerted, is condensed, and all electric signs va- nish. To render these positions clear, I insert a wire, B, in the cushion, and another, C, in the conductor ; each of these is furnished with a brass ball at top, and each of them has a sliding wire with balls on its end, that it may be set at any convenient distance from the other. On turn- ing the cylinder, you observe, 1. That I can obtain an electric spark from the balls of either wire on presenting 268 OF THE ELECTRICAL MACHINE, my knuckle thereto. 2. That a strong spark will pass from one ball to the other. 3. That on holding a cork ball suspended by silk, between the two brass balls, it is alternately attracted and repelled from one to the other. 4. Electrify a pair of insulated pith or cork balls by the cushion, and you will find them to possess the resinous electricity ; electrify them by the conductor, and they will possess the vitreous power. 5. Join the balls toge- ther, and all electrical signs vanish. On the other hand, if you place both wires either on the conductor, or the cushion, you will find, that no spark will pass between them, that the cork ball remains stationary, being neither attracted nor repelled by the balls, and this because they both possess the same kind of electricity. From these experiments we infer, that the conductor and the cushion are electrified with different powers; that pne attracts what the other repels, and that when they are united, they exhibit no signs of electricity : that on the separation of the powers by excitation, one power at- taches itself to the excited electric, the other to the rub- ber. The whole variety of electrical experiments appear to be nothing more than different modes of destroying or restoring an equilibrium. By destroying the equilibrium, two positive powers are at the same time produced. By restoring the equilibrium, all things return to their natu- ral state, and every appearance of electricity ceases. The two powers are so connected, that one can never be exhi- bited without producing the other. It is probable, that in the general operations of nature, this fluid always acts in its united form, that in which it is to our senses latent and invisible. On turning the cylinder and separating it from the silk, the electric powers are separated, the cylinder gives its resinous power to the cushion in exchange for the vi- treous ; the conductor in like manner exchanges its pow- ers with the cylinder ; for, as long as the cushion com- municates with the table by a chain, and you continue turning the cylinder, you will find the conductor strongly electrified with the vitreous power. Take the chain from AND ITS MODE OF ACTION. 269 the cushion, and suspend it from the conductor; on turning the cylinder you will find the cushion strongly electrified with the resinous power. Connect the cushion and conductor by a chain, and the powers re-unite al- most as soon as they are separated, and the electrical signs disappear. We now see, why conducting substances cannot be electrified unless they are insulated. It is because the two powers join instantaneously in the non-conductor, and can therefore exert no sensible action. When I turn the cylinder slowly, only a small quan- tity of the fluid is excited, and it does not fly far in the form of a spark ; but when I turn somewhat faster, and make the black silk adhere to the glass, the quantity of excited electricity is considerably increased. The flash or spark passes through a greater space, and assumes a crooked or zig-zag direction, resembling the flashes of lightning. The brilliancy of the spark depends much on the pressure of the atmosphere ; for the spark which ex- plodes in air is vivid like lightning ; but if the same be tried in an exhausted receiver, instead of a spark and ex- plosion, you have only a silent, faint, diluted stream. Before I proceed to other experiments, I shall explain our machine more fully, and show you how to excite it powerfully. The parts of the machine, which fall more immediately under your attention, are, 1 . The electric, or the glass cylinder which is to be excited. 2. The me- chanical contrivances by which it is put in motion. 3. The cushion and its appendages. 4. The conductor, or con- ductors. The glass cylinder of the machine before you, plate 1 , Jig. 5, is put in motion by a simple winch A. This is less liable to be out of order, than those that are turned with a multiplying wheel and pulley, and will also ena- ble you to excite the machine more powerfully.* The cylinder, F G I K, is supported by two strong perpendi- * There are some instances in which, 1 think, a multiplying wheel and pulley are preferable. They are less awkward and tiresome to beginners than a handle. I think those machines the most complete, which are capa- ble of being used occasionally either way E. Edit. 270 OF THE ELECTRICAL MACHINE, cular pieces D E. The axis of one cap of the cylinder moves in a small hole at the upper part of one of the sup- ports. The opposite axis passes through the upper part of the other support. To this axis the winch or handle is fitted. The cushion is supported and insulated by a glass pillar ; the lower part of this pillar is fitted into a wooden socket, to which a regulating screw, H, is adapted, to increase or diminish the pressure of the cushion against the cylinder. A piece of silk comes from the under edge of the cushion, and lies on the cylinder, passing between it and the cushion, and pro- ceeding till it nearly meets the collecting points of the conductor. The more strongly this silk is made to ad- here to the cylinder, the stronger is the degree of ex- citation. Before the cylinder, or opposite to the cushion, is a metallic tube Y Z, supported by a glass pillar L M, with a set of points in the front, called the collector, to collect the electricity from the cylinder. This tube is sometimes called the prime conductor, often only the con- ductor .* For the more conveniently trying the experi- ments on the two powers, and exhibiting the different states of the cushion and conductor, there are two wires, B, C, to be fixed occasionally, the one to the conductor, the other to the cushion ; on the upper part of these, are balls furnished with sliding wires, that they may be set at various distances from each other. Before the electrical machine is put in motion, exa- mine those parts which are liable to wear either from the friction of one surface against another, or to be in- jured by the dirt, that may insinuate itself between the rubbing surfaces. When any grating or disagreeable noise is heard, the place from whence it proceeds must be discovered, wiped clean, and rubbed over with a small quantity of tallow ; a little sweet oil or tallow should also be occasionally applied to the axis of the cylinder. * The conductor exhibited in the figure is of the T shape, which is a -convenient, but not an essential form; they are more frequently made cy- lindi'iccilj with the collecting points placed on the ' side. ....K. Edit. AND ITS MODE OF ACTION. 271 The screws that belong to the frame should be ex- amined, and if they be loose, they should be tightened. The different working parts of the machine having been looked into, and put in order, the glass cylinder, and the pillars which support the cushion and conduc- tor, should be carefully wiped with a dry old silk hand- kerchief, to free them from the moisture which glass attracts from the air, being particularly attentive to leave no moisture on the ends of the cylinder, as any damp on these parts carries off the electric fluid, and lessens the force of the machine : in damp weather it will be proper to dry the whole machine, by placing it before, but also at some distance from, the fire. Take care that no dust, loose threads, or filaments, adhere to the cylinder, its frame, the conductors, or their insulating pillars ; because these will gradually dissipate the electric fluid, and prevent the machine from acting powerfully. Rub the glass cylinder first with a clean, coarse, dry, warm cloth, or a piece of wash-leather, and then with a piece of dry, warm, soft silk ; do the same to all the glass insulating pillars of the machine and apparatus ; these glass pillars that are varnished must be rubbed more lightly than the cylinder. A hot iron may, in some cases, be placed on the foot of the conductor, to evaporate the moisture, which would otherwise injure the experiments. To excite your machine ', clean the cylinder and wipe the silk. Grease the cylinder by turning it against a greasy leather, till it be uniformly obscured. The tallow of a candle may be used. Turn the cylinder till the silk flap has wiped off sa much of the grease, as to render it semi-transparent. Put some amalgam on a piece of leather, and spread it well, so that it may be uniformly bright ; apply this against the turning cylinder ; the friction will immedi- ately increase, and the leather must not be removed un- till it ceases to become greater. Remove the leather, and the action of the machine will be very strong. 272 MOMENTUM OF THE ELECTRICAL FLUID. The pressure of the cushion cannot be too small, when the excitation is properly made. The amalgam is that of Dr. Higgins, composed of zinc and mercury ; if a little mercury be added to melted zinc, it renders it easily pulverable, and more mercury may be added to the powder, to make a very soft amalgam. It is apt to crystallize by repose, which seems in some measure to be prevented by triturating it with a small proportion of grease : and it is always of advantage to triturate it before using. A very strong excitation may be produced by ap- plying the amalgamed leather to a clean cylinder, with a clean silk ; but it soon goes off, and is not so strong as the foregoing, which lasts several days.* OF THE MOMENTUM OF THE ELECTRICAL FLUID. The great strength and velocity displayed by the electrical fluid in its motions, is an object well worthy your investigation : and if it be granted (and I think I shall be able to prove it to you) that the electric matter is the same with the solar fluid, then will the ultimate cause of its momentum be the power by which the light of the sun is propagated, the pressure of which being equal all round upon all bodies, it can neither move them one way nor another. But if, by means of any other power, this pressure be lessened upon any parti- cular part, the current of matter will set forwards to- wards that place, with a force proportioned to the dimi- nution of the pressure. Thus, in the common experi- ments of the air-pump, when the air is exhausted from the receiver, the pressure of the superincumbent atmos- phere is directed towards every part of the glass ; so that * In summer, a machine and apparatus, kept in a room free from dust, will require merely wiping with a dry cloth. In winter, and if the appa- ratus be damp and foul, carefully wiping it all over, and a general small degree of warming by the fire, will be necessary. Supposing the silk used clean and perfect, a small application of amalgam to the cylinder or rub- ber will occasion the cylinder to be strongly excited. Streams of fire, or a crackling noise at the knuckle, when presented to the cylinder, will be a sure proo£ of the machine being in the best order. — E. Edit, ATTRACTION AND REPULSION. 275 if it be of a flat square shape, and not very strong, it will certainly be broken. Now, there is reason to sup- pose, that after the air is exhausted from the receiver, it is full of another subtile fluid of the same nature with the electric. If this could also be extracted from the receiver, the pressure on its sides would be much greater, because not only the atmosphere, but the whole surrounding ether, would be urged towards that place ; and it is not probable, that this pressure could be resist- ed by any finite force whatever. The momentum, therefore, of the electrical fluid de- pends on two causes ; the pressure of the atmosphere upon the electric matter, and the pressure of one part of this matter upon another, which is extended through- out the immensity of space. The force and velocity of the fluid depend therefore, in a great measure, on that which surrounds us. There is a certain state of this fluid, that we violate by our experiments ; when this violation is small, the powers of nature operate gently in restoring the disorder we have introduced ; but, when any considerable deviation is occasioned, the same powers restore the original constitution with ex- treme violence. EXPERIMENTS ON ELECTRICAL ATTRACTION AND REPULSION. To the top of this wire, three large downy feathers are affixed by three linen threads. I insert the lower end of the wire into the prime conductor ; upon turn- ing the cylinder, the plumage expands every way, the threads also recede as far as possible from each other. If I place my finger near the feathers, all the plumulas bend towards it ; if I move my finger this way or that, they all move after it as if alive ; I put my hand on the conductor, immediately the threads lose their diver- gence, the plumulse collapse, and fall close together ; I take my hand away, the threads diverge, and the fea- thers expand as before. VOL. IV. 2 N 274 EXPERIMENTS ON ELECTRICAL 1 cannot explain to you the mechanism which occa- sions the threads to diverge ; but I can state those facts which must concur to occasion it. We know that those light bodies, which possess the same kind of electricity, separate from, or repel each other ; the finger commu- nicates to them the contrary power ; towards this, there- fere, they are impelled by their nature, in order to re- store an equilibrium which our operations "have destroy- ed. By putting my hand on the conductor, the powers are immediately exchanged and united, and the electri- cal effects cease. I place this cork ball, suspended by a silk, so that it may be even with the conductor, and at about six in- ches from it. I turn the machine, but the cork remains quiet ; touch it with the end of the wire in your hand, and the vitreous power of the ball is driven into \ou, and an equal quantity of the resinous is communicated to the ball, which will then therefore fly with great ra- , pidity towards the conductor ; direct the pointed end of the wire towards the ball, and it will keep it fixed to the conductor, by continually supplying it with the resinous power -j remove the wire, and the ball parting with its resinous power to the conductor, in exchange for the vitreous, of which the conductor has the greatest quan- tity, it becomes electrified therewith, and repelled from the conductor. Analogous to the foregoing experiment is the follow- ing, with a piece of linen thread, which, from the viva- city of its motions is termed the animated thread. For this purpose I present a fine thread towards the electri- fied conductor, and it will fly backwards and forwards in a very pleasing manner, according as it conveys the vitreous power to the hand, or the resinous to the con- ductor, to which it will sometimes be affixed, for the same reason as the ball in the preceding experiment. Let a thread hang from the conductor, and present another towards it, they will attract and join each other : present any non-conducting substance, as a brass ball, near the two threads ; the lower one, or that held by the hand, will fly from the ball, while that affixed to the conductor flies towards it. The vitreous atmosphere ATTRACTION AND REPULSION. 275 of the conductor repels the vitreous power of the ball into the hand, and draws the resinous power into it ; the ball being therefore resinously electrified, attracts the upper thread, but repels the lower one, which is in the same state with itself, as acted on by the same causes. In this experiment, the afflux and efflux of the two pow- ers is, as it were, visible to the senses. You will find, that a contrariety of power must always precede, and is absolutely necessary to all electrical attraction, and, indeed, to every communication of electricity. I suspend a small copper plate from the conductor ; underneath this, and at a small distance from it, is a larger copper plate which rests upon a proper stand, on the lower plate I put a leaf of gold, turn the cylinder, the leaf rises upon the plate, and expands itself into a perfect plane, with one corner opposite the upper, the other corner opposite the under plate, moving quickly upwards and downwards between both ; I lower the under plate by degrees, the motion of the leaf has now ceased, and it remains suspended in the air between the two plates ; darken the room, and you will find the leaf supported, as it were, by pillars of fire ; now, as no substance can be thus supported in equilibrio, but by the joint action of two forces acting in opposite di- rections, we have a clear proof that there must be two forces thus acting in the present instance. Place some small paper figures of men, women, &c. on the lower plate, plate 1, fig, 7 ; turn the cylinder, and you see the images rise up, moving from one plate to the other. They generally move in an erect posi- tion, sometimes leaping one upon another, and moving in such a variety of postures, as to afford much enter- tainment. The dance between them has so droll an ap- pearance, if well-conducted, that there are few who can look upon it without laughing. I have before observed to you, that there are two powers in electricity ; now, the heads of the puppets are electrified with the one power, and the feet with the other; they are therefore repelled at both ends, and never come in contact, un- less the lower part of one touch the higher part of the other, and then they approach and stick together. 276 EXPERIMENTS ON ELECTRICAL The foregoing experiment is not only amusing, but instructive : you will find that a very minute alteration in their figure will make the images dance between the plates, or remain fixed to the upper or under plate ; for this end, the upper part should be always so much larger than the lower part, as to contain a part of the power going in, as much greater than what goes out, as will be equal to the gravity of the paper : with a little prac- tice, you will be able to make one of them dance for some minutes without touching either the top or bottom plate. To further illustrate the affluence and effluence of the two powers, dry the head of one of the images, and the power thrown out from the conductor cannot enter that puppet so freely, as the contrary power from the lower plate enters the feet, which are not so dry ; the image will therefore ascend to the upper plate, and remain there : reverse the experiment, by drying the feet and wetting the head, and the image will remain fixed to the lower plate. These, as well as many other ex- periments, will prove to you, that it is not the mere component parts of the body that are acted on in elec- trical experiments, but that it is the different states of the electrical powers inherent or adhesive to the body which occasions the effects ; and that, strictly speaking, it is the opposite powers only that attract each other, and that no substance is ever attracted, until it have acquired a contrary electricity. If the two powers cannot be put in action, the expe- riment will not succeed ; for, if you place your images on a clean dry pane of glass, and hold this under the up- per platef, first removing the lower plate and its stand, you will find that the images will not be put in motion, notwithstanding you continue to turn the machine. Glass does not transmit the two electricities, and there- fore no contrariety in the electric state of the image can be occasioned, and consequently it will not move back- wards and forwards between the two plates. But, if any means be used to cause an exchange in the powers, as by holding your finger under the glass plate, they will be driven backwards and forwards as before. ATTRACTION AND REPULSION. 277 Here is a small apparatus, consisting of three bells with two clappers between them, plate 1, jig. 6 ; they are suspended from a straight piece of brass, the two outer ones by small brass chains, the middle bell and the clappers are suspended on silk ; from the middle bell there is a chain which goes down to the table ; I hang the bells on the conductor, and turn the machine, and the clappers fly from bell to bell, affording you a pleasing peal by electricity. The power from the con- ductor is conveyed down the chains to the exterior bells ; by means of the chain, the exterior bells repel the same power with which they are electrified from the bell or clapper, which, on the powers being thus se- parated, are driven to the outer bell by the contrary power which sets in from the table, &c. through the middle bell ; the ball becoming electrified with the same power as the middle bell, is driven back, and will continue going from one to the other, as long as the outside bells are kept in an electrified state by the ma- chine. If you take hold of the silk cord which is tied to the lower end of the chain that comes from the middle bell, and thereby raise that chain from the table, the ring, ing will immediately stop ; for, silk being a non-con* ductor, prevents the afflux and efflux of the fluids.* As the apparent attraction and repulsion of all light bodies depend on the afflux and efflux of the separated powers, I shall not, in showing you every experiment, enter into a detail of these circumstances, hoping that what I have already said will render that point suffi- ciently clear. I turn the machine with one hand, and hold the other about three or four inches from the end of the conductor ; drop a small lock of cotton upon the hand near the conductor, and the cotton immedi- ately jumps from my hand to the conductor and back again, stretching itself out both ways into an extended * Eight bells, forming the octave, are sometimes placed on a round board with a clapper and fly, and set in motion also by the electric fluid, which afford a pleasing musical effect. — E, Edit. 278 METHODS OF IMITATING THE form, and moving so quick that you will scarcely be able to perceive its form. Here is a small toy, somewhat resembling a hog ; I have coated it with ermine, in the hairs of which I have inserted a few pieces of cotton pulled out, so as to be of a considerable length ; place this upon the con- ductor, I turn the machine, and the hairs of the ermine diverge, and the pieces of cotton are discharged and driven some feet from the conductor. This apparatus, by thus discharging its quills, may be called with pro- priety the electrical porcupine * Few branches of philosophy afford so much enter- tainment as electricity ; here the useful and the agree- able are intimately blended, and while you are investi- gating science, you are entertained by the variety and beauty of the experiments. It was the strong attractive and repulsive powers exhibited by electricity, that first engaged the attention of natural philosophers ; by these they were led on to pursue the subject, as it were by enchantment, and have been richly rewarded by dis- coveries both interesting and important. A few more of the leading experiments, which have been so advantageous to science, will not be . uninter- esting. METHODS OF IMITATING THE PLANETARY MOTIONS. Rackstrow's orrery consists of small glass balls blown very thin ; they are placed on a wooden board, and environed with circles of brass wire insulated with seal- ing-wax, or glass, of such a height that the centre of the balls may be nearly parallel to the wire circles. One of these circles may represent the orbit of Saturn, another that of Jupiter, &c. the circles being connect- ed with the conductor of the machine by a wire, and a glass sphere placed between each, the spheres will per- form their revolutions round their orbits, and at the same time acquire a rotation round their axes. * Communicated by Mr. IVisset* PLANETARY MOTIONS. 279 When the machine is set in motion, the balls will be first attracted to the brass circles, by which means the point that touches the brass circle will become electrified, and be immediately repelled ; other parts will in the same manner be attracted and repelled, by which means the glass ball acquires a kind of spinning motion on its axis, at the same time it must have a progressive motion round the circle. Provide a ball of cork about three quarters of an inch in diameter, hollowed out in the internal part by cutting it in two hemispheres, scooping out the insides, and then joining them together with paste. Having attached this to a silk thread, between three and four feet in length, suspend it in such a manner that it may just touch the knob of an electric jar, the outside of which communi- cates with the ground. On the first contact, it will be repelled to a considerable distance, and after making se- veral vibrations, will remain stationary ; but if a candle be placed at some distance behind it, so that the ball may be between it and the bottle, the ball will instantly begin to move, and will turn round the knob of the jar, mov- ing in a kind of ellipsis, as long as there is any electricity in the bottle. This experiment is very striking though the motions are far from being regular ; but it is remark- able, that they always affect the elliptical rather than the circular form. Cut a piece of India paper in the shape of an isosceles triangle, which may be about two inches long and two tenths of an inch in breadth ; then erect a brass ball of two or three inches diameter on a brass wire one-sixth of an inch in thickness, and two feet six inches long, on the prime conductor ; electrify the conductor, and then bring the obtuse end of the piece of paper within the atmosphere of the ball ; let it go, and it will revolve round the ball, turning often on its own axis at the same time. TUMBLER AND BALLS. Put a pointed wire into one of the holes which are at the end of the conductor, YZ, plate 1, Jig. 1, hold a 28Q ELECTRICAL FLUID dry glass tumbler over the point, then electrify the con- ductor and turn the tumbler round, that the whole inte- rior surface may receive the fluid from the point ; place a few pith balls on a metallic plate or the table, and cover them with this glass tumbler : the balls will immediately begin to have a rapid motion upwards and downwards, as if they were animated, and will continue to move for a long time. ELECTRICAL FLUID UNIVERSALLY DISSEMINATED, AND IN CONTINUAL ACTION. That the electrical fluid is universally disseminated and in continual action, has long been the opinion of those who have paid attention to it. To prove this to others, various instruments have been contrived to detect the smallest variations, and discover the minutest signs of its existence ; these have been generally named electrome- ters ; and among these, that described by the Rev. Mr. Bennett of Wirksworth, may be considered the first, as being by far more sensible than any of the rest. This is one of them, plate 2, fig, 1 . The foot, A, is made of brass, and about three inches high, that you may move the instrument by without touch- ing the glass ; the cylindrical glass, B, in which the gold leaf is suspended, is about five inches high, and one in diameter ; the cap, C, is made of brass, and flat on the top, that the various substances whose electricity is to be examined may be conveniently placed thereon. The dia- meter of the cap is larger than that of the glass, and its rim is about an inch deep, hanging parallel to the glass, in order to keep ir clean and dry ; within this is another circular ring that goes over the glass, and is lined with a soft substance to make it fit close within this rim ; at the centre of the cap a tube is fixed, wherein the peg is placed to which the two slips of gold leaf or silver are fastened. If there were no glass, the gold leaf would be so agi- tated by the least motion of the air, that it would be en- tirely useless. To prevent the gold leaf from being at- tracted and torn by flying to the glass, two pieces of tin UNIVERSALLY DISSEMINATED. 281 foil are fastened, with varnish on the opposite sides of the glass, where it may be expected to strike these slips and carry off the superfluous electricity, and increase the sen- sibility of the instrument. The experiments made with this instrument, not only show that the electrical fluid is universally disseminated, but that the smallest motions in nature disturb its natural equilibrium, and separate the two powers, and thus mani- fest; it to our senses. That this fluid is the etherial medi- um, or element of fire, connected with some material substance, can scarcely now be doubted : if so, all the oscillations in nature put it in action, or, what is more probable, it is the cause of those oscillations. Mr. Ben- net's electrometer will prove to you, that all solution of continuity excites electricity: and I believe there is scarcely any instance where its action is manifested, but what may be traced to this source. In other words, every thing that will increase one power, or lessen the other, produces electric signs. Not to interrupt too much the progress of our lectures, I shall relate to you some of Mr. Bennet's experiments. 1. Powdered chalk was put into a pair of bellows, and blown upon the cap, C, of the electrometer ; the stream of chalk produced vitreous electricity, when the nozzle of the bellows was only six inches distant from the cap ; but the same stream electrified it with the resinous power, when at the distance of three feet. In this experiment, the quality of the electricity seems to be changed by dis- persing or widening the stream, and making it pass through a longer tract of air ; it is also changed by pass- ing the stream through a bunch of fine wires, silks, or feathers, placed in the bellows: it is resinous when blown from a pair of bellows, the iron pipe being taken off to enlarge the stream. This last experiment seems to answer best in damp weather. The vitreous electricity generally remains ; but in the resinous, the gold leaf collapses as soon as the cloud of chalk has passed. 2. A piece of chalk drawn over a brush, or powdered chalk put into a brush, and projected on the cover, gave resinous electricity. The electricity was not permanent. vol. IV. 2 o 282 THE FRANKLIN1AN THEORY. 3. Powdered chalk blown with the mouth, or a pair of bellows, from a plate placed upon the cover, gave a per- manent vitreous electricity. If a brush be placed upon the cover, and a piece of chalk be drawn over it, when the hand is withdrawn, the leaf gold gradually expands with vitreous electricity, as the cloud of chalk disperses.* OF THE FRANKLINIAN THEORY. It was not my intention at first to have particularly no- ticed the defects of this theory ; but, as some late writers have endeavoured to conceal its errors, either by giving up some of the most essential parts, or by endeavouring to bend facts to accommodate them to this theory, it be- came necessary to point out a few of its defects and incon- sistencies. Many parts thereof, I conceive, would never have been accredited, if it had not been necessary for party purposes, to establish the author's reputation as a philosopher.! From hence we may learn, that all new discoveries should be admitted with caution, for they are seldom ac- curate and free from errors ; we are too often apt to be led away by glimmerings of light, or even false views of objects, which are often of worse consequence than a to- tal want of knowledge. I shall enumerate the leading principles of the Frank- linian system, those which have always been considered in that light by the best writers and ablest advocates in favour of this system ; and we may, therefore, justly conclude, that whosoever gives up any of these, so far abandons the principles on which it is founded. 1. That the operations of electricity depend on the action of a simple homogeneous fluid. * A curious and useful machine, called a doubter, has lately been invent- ed by the Rev. Mr. Bennet. It is an instrument by which the smallest quantity of positive or negative electricity, or the vitreous and resinoufj may be continually doubled till perceivable by common electrometers, or bv visible sparks. It has in its mode of action been improved by Mr. Nichol- son* For a description of which, see our Author's Essays on Electricity, 4th Edition, p. 414.....E. Edit. t Oil this head, the anecdotes to be related are numerous and curious. THE FRANKLINIAN THEORY. 283 2. That the electric matter violently repels itself, but attracts all other matter. 3. That glass and all other electrics, though they con- tain a great quantity of electric matter, are nevertheless impermeable thereto. 4. That by the excitation of an electric, the equilibri- um of the contained fluid is broken, and one body be- comes overloaded with electricity, while the other is de- prived of its natural share. 5. Electricity is positive when a body has more than its natural share ; the electricity is negative, when a body has less than its natural share. With respect to the first position, you will find, that its friends can bring no experimental proof to show the homogeneity of this fluid, or its actions; but, on the con- trary, they are forced by experiment to acknowledge a contrariety of state in every operation. The second position is not only destitute of proof, but contradictory to all experiments ; for the electric fluid never attracts matter as such, but only on account of the state of the electric matter therein. It is not repulsive of itself; the appearances, on which this idea is grounded^ are owing to the resistance of the air. Both attraction and repulsion cease where the powers can unite without this resistance. In the course of these lectures, you will see many ex- periments that prove the permeability of glass. The no- tion of its impermeability is altogether hypothetical, for it is not supported by any one determinate experiment, and is contrary to every electrical appearance. You will find the ideas of the Franklinians concerning it quite con- tradictory, some allowing that its influence acts through glass, yet maintaining that it is impermeable thereto; others allowing certain kinds of glass to be permeable. Indeed, you may gather from their writings, that the best, and the worst vitrified glass, that cold and warm glass are all more or less permeable.* • Lyon's Remarks on the leading Proofs of the Franklinian Theory Milner'a Experiments and Observations on Electricity, &c, &c. 284 THE FRANKLINIAN THEORY. The fourth and fifth principles may be considered as one, for they are so intimately connected, as not to be separated : whatever weakens the proofs of the one, di- minishes those of the other. The whole Franklinian hy- pothesis falls to the ground, if the supporters thereof can- not prove, that positive electricity is a superabundant quantity, an accumulation of the electric matter in the body positively electrified, and negative electricity a de- privation of the quantity of this matter natural to a body. Now, in the first place, we have strong reasons to sup- pose, that every electric appearance is occasioned by the fluid being in a divided and weakened state ; but putting this consideration out of the question, let us ask the sup- porters of the Franklinian system for a proof of this po- sition, and, strange to tell, you will find it destitute there- of. You will find them only reasoning in a circle, prov- ing the thing itself; a method from which no conclusion can be drawn. Thus, for instance, a body that is posi- tively electrified, attracts one that is negatively electrifi- ed, because the first has too much, and the other too little electricity. Demand how they prove one has too much, and the other too little of this fluid, and they an- swer, because they attract each other ! According to their principles, the electrical fluid is as active when redundant, as when deficient ; and yet when it is in an intermediate state, it is inactive. " Two light bodies suspended in contact, in their natural state show no signs of electricity ; take away part of their electric fluid, and they repel each other ; take away still more, and the power of repulsion increases ; so that the more a body is deprived of its electric fluid, the more active and extensive is its electric action."* Another mode by which they endeavour to support their system is, by showing that the electric fluid always moves in one direction, that is, from the positive to thfi negative. Now if it can be proved, as I think it has al- ready been in a great degree, that there are two powers acting in contrary directions ; the negative electricity will * Peart on Electric Atmospheres. OF THE ELECTRIC SPARK, &C. 285 turn out to be a positive active power, and the Frankli- nian hypothesis will fall to the ground, being destitute of any proof. I shall hereafter show you, that in the dis- charge of the Leyden phial, there is not only a power acting from the inside to the outside, but also at the same instant a power acting from the outside to the inside. Whosoever allows two currents acting in opposite direc- tions, whatever may be his pretences, gives up the Frank- linian theory, and confesses himself unable to maintain it on the original principles laid down by the author, and vindicated by Canton, Le Roy, Priestly, Becket, Henly, Beccaria, Cavallo, &c. &c. Further proofs of the inconsistency and weakness of this theory will be shown in the course of these lectures ; but they are so numerous, that to expose them all, would occupy too much of our time ; one or two more I shall mention here ; thus you will find Dr. Gray, in the Philo- sophical Transactions, proving Dr. Franklin* s account of the charge and discharge of the Leyden jar to be errone- ous ; yet endeavouring to support the weakest part there- of. Mr. Brooke, in his Miscellaneous Experiments, has shown, what Mr. Eeles had shown years before, and that by reasoning, a priori from his theory, contrary to the ideas of the best judges and friends of Franklin's theory, that during the time of charging a Leyden jar, both inside and outside have the same kind of electricity. Mr. Read has demonstrably proved, by a method previously point- ed out by Mr. Eeles, that in the discharge of the Leyden phial, a vacuum forming a part of the circuit, the electric matter moves in contrary directions; yet such is the force of philosophic authority, that both Mr. Brooke and Mr. Read endeavour to bend these facts to support a theory, with which they are utterly irreconcileable. OF THE ELECTRIC SPARK, AND OF THE INFLUENCE OF POINTS. I bring the knuckle of my hand near the conductor, and a spark with the appearance of fire passes between the conductor and my hand, and I feel a sensation somewhat resembling a stroke from the end of a small wire. I re- ^ 286 OF THE ELECTRIC SPARK, AND move my knuckle further from the conductor, and the spark is longer, forming several curves in its passage, having the exact appearance of a flash of lightning. In this experiment, as much of one power passes from the finger to the conductor, as of the other from the conductor to the finger. No spark will pass unless there can be this interchange of power ; and the spark is always from those parts where the exchange can be most readily effected. Where the two powers can be easily changed, which is the case with pointed metallic bodies, the equilibrium is restored silently, and the conductor is of course gra- dually divested of its electric appearances : but where the surface is large, and a contrary state is not so easily produced, the electricities are as it were compressed, and do not escape till they have acquired a power to overcome the intervening space of air, when it explodes, and the spark is vivid like lightning. As soon as I present a needle, or any other fine pointed substance, to an electrified body, the electric fluid is urged there with great velocity, and the electri- city is said to be drawn off. This drawing off, however, does not extend to any* great distance, not even all around the electrified body, if you keep turning the machine at the same time that you present the point. To prove this, place the wire, to the end of which a number of fine threads are fastened, in one of the holes on the top of the conductor ; turn the machine, the threads on the wire diverge, and spread out like rays proceeding from a centre -, now present a point towards one side of the conductor, but at some distance from it, and you see the threads on one side loose their diver- gence and hang down, while those on the other side con- tinue to diverge. Indeed a point never acts beyond the electric at- mospheres, nor does it act upon them any further than it is immerged therein, and then only so far as it can draw the resinous power through them, and part with so much of the vitreous to them. Suspend a piece of down, or a small ball, by silk, so that it may hang against the side of the conductor *, when you turn the machine, it will INFLUENCE OF POINTS. 287 be electrified, and fly to the extreme part of the con- ductor's atmosphere ; now stop turning, and bring a point towards the outside of the down, and instead of the down being driven in towards the conductor, it will fly to the point, till it has exchanged powers with the point ; then it will fly to the conductor, and be electri- fied, and again repelled ; when it comes to a certain dis- tance from the point, it will fly towards it, and be elec- trified thereby, and so on, as long as the conductor re- mains electrified. When the down is on the verge of the electric atmos- phere, immerge your point in the atmosphere, and you will see the down approach the conductor in pro- portion to the immersion of the point, and this is as of- ten as you move the point forward to the conductor, but no further ; so that the point acts only while in contact with the electric atmosphere. While the machine is turning, and the point immerg- ed in the electric atmosphere, there is a strong stream of the resinous power flowing in from the point to the conductor, and that in proportion to the vitreous pow- er carried off by the point. If this stream meet an elec- trified cork ball, or piece of down, it will change their powers, and electrify them with the resinous power, by which means they are attracted to the conductor, and will be fixed there by the continual stream of the re- sinous power ; draw back your hand to lessen the resi- nous stream, and you will see the down move from the conductor by degrees, and remain between the two powers, without being forced to the conductor, or being able to fly far therefrom. The foregoing experiments are most decisive with a weak electricity. That the spark or passage of the electrical fluid, from the prime conductor to any conducting substance, depends upon the greater or less degree of difficulty in producing the contrary current, is further evinced by placing a point at the end of a piece of sealing-wax, and at a small distance from that part of the metal in contact with the sealing-wax, paste a small round piece of tin-foil, at a little distance from this another piece &c. put your finger upon one of the pieces of tin foil 288 OF THE ELECTRIC SPARK, AND that is farthest from the metallic point, and present the point towards the conductor, and you will find that it does not act near so powerfully, nor at so great a dis- tance as in the former case ; and if you approach it suf- ficiently near the conductor, a spark will pass between it and the conductor. Connect your fingers immedi- ately with the metal, and you will not be able to obtain a spark, and the electric appearances of the conductor will be sooner destroyed by the quicker interchange of the contrary powers. As the spark, which explodes, and is bright in the air, becomes silent, faint, and diluted in vacuo ; so, on the other hand, the electricity, that would pass imper- ceptibly in air, may be made to explode, and become bright, by passing it through mediums more resisting than air. I place a metallic vessel nearly filled with common oil on the conductor ; I shall immerge therein a point, from which, in the open air, I can scarcely obtain any visible appearance, and you see that, under these cir- cumstances, strong sparks pass between the point and the bottom of the vessel, and the oil is thrown into a vio- lent ebulition, by the afflux and efflux lof the two elec- tricities. Here is a pointed wire suspended vertically from the conductor, the point being downwards, from which I can obtain no spark, though the machine is acting pow- erfully. I immerge it in a small bottle of oil, and put my thumb opposite the point ; the spark is loud, the oil is curiously agitated, and, if you examine the bottle, you will find it perforated. Round this glass tube, plate 2, Jig. 2, at small but equal distances from each other, pieces of tin-foil are pasted in a spiral form from end to end, hence it is called the spiral tube ; this tube is inclosed in a larger one, fitted with brass caps at each end, which are connected with the tin-foil of the inner tube. Hold. one end in the hand, and apply the other near enough to the elec- trified prime conductor to take sparks from it, a beauti- ful and lucid spot will then be seen at each separation of the tin-foil y these multiply, as it were, the spark taken INFLUENCE OF POINTS. 289 from the conductor ; for if there were no break in the tin-foil, the electric fire would pass off unperceived.* Here are several spiral tubes, plate 2, Jig. 4, placed round a board, a glass pillar is fixed to the centre of the board,, on the top of this pillar is a brass cap, carrying a fine steel point, to support a wire furnished at each, end with a brass ball, and nicely balanced. I place this under a ball proceeding from the conductor, so that a continued spark from this ball to the centre of the sus- pended wire, gives this wire a rotatory motion, and the- balls in their revolution will give a spark to each spiral tube, which, in its passage from one spot to the other, forms a most beautiful species of illumination. Take this piece of silvered leather, and put it round your head, and then stand upon the stool with glass feet, connecting yourself with the conductor by a chain. If, while I turn the machine, any one pass their knuc- kles near the hoop of leather moving them round it, the leather will be beautifully illuminated, and brisk flashes of electric lightning will pass between the knuckles and conductor. This experiment has been termed the diadem of beatification^ Spirits of wine may be easily fired by the electric spark ; to insure success in making the experiment, it is best either to heat the metallic ladle into which the spirits are to be placed, or else just to fire the spirits, and blow them put, a few seconds before they are elec- trified. This experiment may be performed two ways : 1. By placing the ladle with the spirits on the con- ductor, and then taking a spark through the spirits, which will set them on fire. Or, 2. If a person stand on the insulated stool, plate 2, Jig. 6, and hold in one hand a chain or wire from the electrified conductor, * These sort of experiments must be exhibited in a perfect darkened room, and then the effect is incomparably brilliant and pleasing. Pieces of tin-foil placed on slips of glass with their intervals, forming names and va- rious figures, will have very pleasing effects when exhibited by the electric spark.—E. Edit. t For a further and greater variety of experiments on these principles, see my Essay on Electricity, last edition....E. Edit. VOL. IV. 2 P 290 OF MOTIONS PRODUCED BY and in the other a spoon with the spirits of wine, and another person on the floor bring his knuckle, or a brass ball, quickly to the surface of the spirits, they will be instantly in a flame. You may vary this experiment thus : 3. Let the electrified person on the stool hold the Spirits as before, while another person, standing also on an insulated stool, holds in his hand an iron poker, one end of which is made red-hot ; he may then apply the hot end to the spirits, and even immerge it in them, without firing them ; but, if uninsulated, he may set the spirits on fire, with either the hot or cold end. The spirits could not be kindled while the person was insulat- ed, because the electrical powers could not in that case be separated ; and hot iron, immersed in spirits, will very seldom or never set them on fire. You must have already observed, from what you have seen, that when the quantity of electricity is small, it is incapable of striking at a considerable distance, and the direction of the spark appears straight ; but when it is strong, and capable of striking at a greater distance, it assumes a crooked zig-zag direction. In every electrified conductor, the electricity always es- capes from that part of the surface, where the powers are most separated. The spark is of a different colour according to the density ; when it is rare, it appears of a bluish colour ; when more dense it is purple ; when highly condensed, it is clear and white like the light of the sun. The middle part of an electric spark, where the two powers meet, often appears diluted, and of a red or violet colour, the ends being more vivid and white ; when very strong, it will branch out and di- vide into many parts. OF MOTIONS PRODUCED BY THE ELECTRIC STREAM. Whenever there is an efflux of one power of elec- tricity there is also an afflux of the other power, if any conducting substance be placed so near and in such cir- cumstances, as that it can be drawn therefrom. THE ELECTRIC STREAM. 291 Here is a brass cross, plate 2, fig. 5, supported on a point like a compass- needle, with each of its points bent the same way ; place this upon the conductor, and as soon as I turn the machine, it turns with great rapidity, but always from the points, because the electric fire, flying off from the points, acts forcibly on the air, and is consequently re-acted upon, which occasions the mo- tion. Take the fly and its point, and hold it in your hand under the conductor, and it will turn in the same manner, by a stream of electricity of a contrary power to that thrown off from the conductor, which is drawn in from you, and delivered from the points of the fly to the conductor. Now insulate the fly, and place it at the same distance from the conductor, and it will not move, because no electricity can be drawn through it ; but hold a pin near it, and the fly will immediately begin to turn, as it draws a sufficient quantity of elec- tricity from you through the pin. On this principle, those who are desirous of blend- ing agreeable entertainment with philosophy, may con- trive a variety of curious machines, whose motions may be produced by the electrified stream, which will afford much entertainment to those who can relish domestic in- nocent amusement; and by these, science will be be- nefited ; for, to render any science familiar, is to render it prevalent, and the more it prevails in practice, the more likely it is to produce useful discoveries. If small boats, or little swans, &c. be made of cork or light wood, they may be attracted, and made to swim in any direction, by applying a finger towards them ; a fine needle stuck into the end of the boats, in the man- ner of a bowsprit, will cause them to be repelled from the hand held over it, and they may be steered by it, stern-foremost, to what point of the compass you please. The boats may have the addition of sails to them, and may then be made to move briskly before an electrical gale, from the point of a wire held in the hand. The operator in these tricks would certainly be looked upon as a magician, if thee lectrical machine were kept out of sight. But a more striking sight would be a number of these boats, with each of them a twirling 292 SUBDIVISION OP FLUIDS. fly, about an inch in length, fixed to the top of the mast ; the hand held over them would set them all in motion ; in the dark they would appear as so many rings of fire, moving in various courses, and following the hand in any direction.* OF THE DIFFUSION AND SUBDIVISION OF FLUIDS BY ELECTRICITY. From experiments made by Abbe Nollet, it appears, that electricity augments the natural evaporation of most fluids, particularly of those which have the greatest ten- dency to evaporate ; that, in this respect, it acts most powerfully upon the fluids when they are contained in metal vessels ; but it never makes any fluids evaporate through the pores either of metal or glass. When fluids, that are passing through capillary tubes, are electrified, the stream is subdivided ; and if the tube be less than one-tenth of an inch in diameter, their motion is general- ly accelerated. I suspend this metal pail, to the bottom of which a capillary tube is adapted, to the conductor ; before I turn the cylinder, the tube carries off the water only by inter- rupted drops ; but on turning the cylinder, and electri- fying the water, the dropping from the tube is changed into a continued stream. On applying my finger to the conductor, the electricity is interrupted, and the water again only descends in drops : my finger taken away, the water runs in a diverging stream : darken the room, and you perceive a fiery stream descend from the tube. This experiment has been termed the electrical jet de feu. Insulate two pails with capillary tubes ; connect one with the cushion, the other with the conductor ; turn the machine, and the water, which is dispersed into very minute particles, when they are near enough, is brought together by the effort of the two powers to join each * Becket on Electricity. OF THE LEDEN PHIAL. 293 other ; the drops coalesce and come down like a heavy shower of rain. I place a drop of water on the conductor, and turn the machine. On presenting my knuckle towards this drop, long zig-zag sparks are obtained from the drop of wa- ter ; the drop takes a conical figure ; my knuckle is wetted. The spark was considerably longer than could be obtained from the conductor without the water. Fasten a piece cf good sealing-wax to the ball on the end of the conductor, but place it in such a manner that it may be easily set on fire by a taper ; set it on fire while I turn the machine ; the wax becomes pointed, and shoots out an almost invisible thread to a considerable distance. If you receive the filaments on a sheet of paper, the pa- per will be covered in a very curious manner by the elec- trified wax threads ; the wax flying to those places where it can unite with the contrary power. LECTURE XLVII. OF THE LEYDEN PHIAL. jL)R. Preistley has well observed, that electricity has one advantage over most other branches of natural philosophy : it furnishes matter of entertainment for all persons promiscuously, while it is also a subject of impor- tant speculation for the most philosophic minds. Neither the air-pump, nor the orrery, nor any experiments in hydrostatics, optics, magnetism, &c. ever brought toge- ther so many, or so great concourses of people, as those of electricity have singly done. 294 OF THE LEYDEN PHIAL. If you only consider what it is in objects that makes them capable of exciting that pleasing astonishment, which has such charms for all mankind, you will not wonder at the eagerness with which persons of both sexes, and of every age and condition, run to see electrical experi- ments. For here you see the course of nature overturned to all appearance, and by causes seemingly inconsider- able. For it exhibits to you bodies rising and falling, moving this way and that, and suspended by others contrary to the principles of gravitation, and this by powers which have been put in action only by a very slight friction. Here you may see a piece of cold metal, or even water or ice, emitting strong sparks of fire, so as to be able to kin- dle many inflammable substances. Nor will you find any thing more astonishing than what I am going to exhibit to you. You will find a common glass jar, after a little preparation, capable of giving a person such a violent sensation, as nothing else in nature can give ; and that the discharge of the bottle is attended with an explosion like thunder, and a flash like lightning. Before I enter into the theory of charged glass, I shall show you in what manner it is charged and discharged. This jar is coated on the outside and lined on the inside with tin-foil, to about two inches short of the top, which is stopped with a piece of wood, see plate 1, Jig. 12. A wire passes through the wooden top, and is connected underneath with two other wires, which are bent so as to touch the inside coating of the jar; a smooth ball is fixed on the top of the wire. To discharge the jar without receiving what is called the shock. For this purpose two instruments have been con- trived, one called the common discharging rod, plate 1, Jig. 8, which is nothing more than a semicircular brass wire, furnished with two brass balls, one at the end of each wire. The other, which is of very extensive use in electrical experiments, is called the jointed discharging rod, Jig. 9; it is furnished with a glass handle; the wires are moveable, and may be set to any given distance by means of the joint ; the ends, to which the balls are screw- ed, are pointed. OF THE LEYDEN PHIAL. 295 Place the jar on the table, so that the ball on the top of its wire may be about one-eighth of an inch from the ball and wire placed in the prime conductor Y Z, plate 1 9 fig. 5. Turn the machine, and sparks will fly from the ball of the conductor to the ball of the jar : continue turning as long as you perceive the fire pass between the conductor and ball of the jar ; when it ceases, you may leave off turning, and consider the jar as charged. This done, take hold of the discharger by the middle, and ap- ply one knob first to the outside coating near the bottom, and keeping it there, put the other to the ball of the jar, and it will be discharged of its fire with a loud snap, but the person who holds the discharger feels nothing from the discharge.* Now charge the jar, and touch the outside coating with one hand, and then bring the other to the ball of the jar, you will then act the part of the wire discharger, and re- ceive a shock ; it has affected you through your arms and breast, and the phial is discharged. You may easily con- trive, by way of recreation, to render the surprize occa- sioned by this experiment more entertaining, by connect- ing a chain with the outside coating, and concealing it under a carpet, at the same time connecting another with the top,f placing it in such a manner, that a person may put his hand upon it without suspicion, at the same time that his feet are upon the other wire ; but great care should be taken, that these shocks be not too strong, and that they be not given to all persons indiscrimi- nately. When a single person receives a shock, the company is diverted at his sole expense ; but all contribute their share to the entertainment, and all partake of it alike, when the whole company form a circle by joining hands, the person at one extremity of the circle touching the outside-coating, while he, who is at the other extremity, • With young beginners this should be particularly attended to, as they will thus avoid any disagreeable effect of a shock. No shock can be receiv- ed but from a charged jar, or from a considerable portion of a glass sur- face covered with tin-foil E. Edit. t This may be conveniently done by what is called a medical electrome- ter, either fitted to the ball and wire of the jar, or to the conductor. 296 OF THE THEORY OF touches the ball of the jar. All the persons who form this circle being struck at the same time, and "with the same degree of force, it is pleasant to see them all start at the same moment, to hear them compare their sensa- tions, and observe the very different accounts they give. It is often convenient, sometimes necessary, to know the state of a jar with respect to the charge ; Mr. Henly's quadrant-electrometer is the best instrument yet known for this purpose. It consists, plate 1, Jig, 17, of a per- pendicular stem formed at top like a ball, and furnished at its lower end with a brass ferril and pin, by which it may be fixed in one of the holes of the conductor, or at the top of a Leyden bottle. To the upper part of the stem, a graduated ivory semicircle is fixed, about the middle of. which is a brass arm or cock, to support the axis of the index. The index consists of a very slender stick, which reaches from the centre of the graduated arc to the brass ferril ; and to its lower extremity is fastened a small pith ball nicely turned in the lathe. When this electrometer is in a perpendicular position, and not electrified, the in- dex hangs parallel to the pillar ; but when it is electrified, the index recedes more or less according to the quantity of electricity. OF THE THEORY OF THE LEYDEN BOTTLE. I shall now endeavour to explain to you the theory of this mysterious bottle ; and you will here see, that the electric powers, when in equilibrio, do really condense each other ; and that one power always expands in pro- portion as the action of the other is withdrawn, or in pro- portion to the increase of one power, and the diminution of the other ; and that when the bottle is charged, it is equally electrified on both sides, but with different powers of electricity ; and when a communication is made by a conductor, the increased power on the outside flies in, and the increased power within flies out, to make the powers equal within and without. Place a Leyden bottle upon the insulated stand, form a communication between it and the conductor, give the machine a few turns, and both sides of the bottle will be THE LEYDEN BOTTLE. 297 electrified with the vitreous power, as you may easily prove, by touching them with down or a small ball sus- pended by silk ; for, when this is electrified by touching the outside, it will be also repelled by the ball which com- municates with the inside. Place an insulated bottle so that the ball may commu- nicate with the conductor ; let a wire also be connected with the coating, so as to form a communication with the table. Now turn the machine, and, 1, On applying a cork ball, you will not find any signs of electricity in the coating, but you will find the ball, or inside, electrified with the vitreous power. 2. Remove the wire commu- nicating with the table, and you will find the coating also electrified with the vitreous power ; and this as often as you remove the wire, till the bottle be fully charged. 3. When the bottle is fully charged, remove its commu- nication both with the conductor and table, touch the coating, and the cork ball will remain suspended by it without any sign of being electrified ; then touch the knob of the bottle with your hand, the cork ball will be strongly repelled from the coating, and be electrified with the resinous power. 4. Take another cork ball suspended by silk, and touch the knob of the bottle therewith, and the cork ball will be electrified with the vitreous power and repelled. 5. Now touch the coating with your finger, and the cork ball will be repelled much farther by the ball ; but that which is repelled from the coating, now flies towards it, and remains at rest, till you touch the knob of the bottle with your finger ; it will then be elec- trified as at first, and be violently repelled ; the ball which was electrified by the knob of the bottle will now fly to- wards it. This change in the extent of the atmosphere of the different powers, takes place almost instantaneously as often as you touch the ball or coating. Or you may connect the knob of the bottle wdth the conductor by a wire, and suspend a cork ball to touch the conductor ; then touch the coating, and the ball will be repelfed from the conductor, while that next the coat- ing is attracted ; touch the knob of the bottle, and the ball will be repelled from the coating and attracted by the VOL. IV. 2 Q 298 THEORY OF conductor, and so on, as often as you touch the knob or coating. From hence it seems plainly to appear, 1. That the bottle is electrified with the vitreous power on the inside, and the resinous on the outside. 2. That when the equili- brium of these powers is destroyed by lessening the quan- tity of one, the extreme part of the other expands itself into an extensive atmosphere; but the atmosphere of the lessened power is condensed, as appears by the cork balls falling close to the conductor and coating. 3. It remains to be shown, how these powers came to be thus situate on the inside and outside of the bottle, or why they do not mix through the glass where they seem to have the great- est tendency to unite. Here it will be necessary to con- sider the separation of these powers between the globe and the cushion, for all the other phenomena are only con- sequences of the separation that takes place between these. Now, the cylinder parts with its resinous power to the cushion, in exchange for the vitreous ; the conductor, in like manner to the globe, and the inside of the bottle to the conductor ; and so the exchange would go on with the next conducting substance, but that the bottle gives Some obstruction to the passage of the electrical powers ; by which means, the vitreous power, which passes through the glass to the conckicting substance upon the outside of the bottle, is carried off, together with the vitreous power of the coating, along the wire which communicates with the table, in exchange for an equal quantity of the resi- nous power brought back by the wire to the coating of the bottle ; till at length, the resinous power on the out- side is able to counterbalance the vitreous power on the inside, and thus affords an opportunity for drawing off the resinous power on the inside of the bottle to the con- ductor ; so that the bottle remains a partition between the two powers, and they cannot change place through the peculiarly constructed pores of the glass, while their surfaces are opposed in such quantities. For, when the junction is made in the open air, or when their surfaces are opposed in any quantity, it is not done without violence, occasioning a loud noise and a flash of fire, while bursting through to meet each other; THE LEYDEN BOTTLE. 299 for, wherever the different powers unite in any quantity, they are much condensed. The violent convulsion felt through the body by com- pleting a circle with the hands, is only occasioned by the different powers passing in opposition through the same nerves. For, if one person touch the coating, and an- other the top of the bottle, the bottle will be discharged without giving either of them the shock. Now, it is very clear, that as much fire passed through either of them, as if each had singly discharged the bottle. But in this case, the fire is diffused through all parts of the body, and the fire brought in is drawn from all parts of the body ; and, consequently, the nerve cannot be so much shocked as in the former case, when all the fire passes in opposition through the same nerves. EXPERIMENTS ILLUSTRATING THE THEORY OF THE LEYDEN PHIAL. Charge an insulated bottle, remove it from the con- ductor, and let a cork ball suspended by silktiang against the outside of the bottle ; touch the outside or coating with your finger, the ball will not be affected ; but, touch the knob of the bottle, and the ball immediately flies off, strongly electrified with the resinous power ; and thus you may go on for a considerable time, altering the ba- lance of the powers within and withoutside the bottle, by alternately touching the top and the bottom of the bottle. The defenders of Franklin's system will hardly say, it is the return of the positive electricity which elec- trifies the ball negatively. The fact is, that when you touch the top, you take a spark of the vitreous power from the inside, and, in exchange, give as much of the resinous power thereto ; by these means, the force of the vitreous power within the bottle is lessened, which leaves the resinous power on the outside in greater quantity than the vitreous withinside, and consequently at liberty to exchange with any non-electric in contact with it ; and thus the ball becomes electrified with the resinous power. Charge a bottle fully, and remove the wire from the table, and make the coating communicate with the con- 300 EXPERIMENTS ILLUSTRATING THE ductor instead of the knob, and then turn the machine, and the resinous power with which the coating is electri- fied becomes covered with the vitreous power, and you may take as many sparks from it as you please, without making any change in the charge of the bottle ; for, when you stop turning, and remove the communication with the conductor, and touch the outside of the coating with the finger, all signs of the vitreous power disappear ; and when the circle is completed, the bottle is discharged with as loud a report as it would have made before you appli- ed the conductor to the coating ; for, the vitreous power within the bottle being undisturbed, kept an equal quan- tity of the resinous power firmly fixed to the outside of the bottle. But the case is different when you give the vitreous power from the inside an opportunity to escape. Thus, when the bottle is fully charged as before, remove the wire that communicates with the table, and bring the coat- ing in connexion with the conductor ; after a turn or two of the cylinder, take a spark from the ball of the bottle, and you will find that it will fly to a considerable dis- tance, often double the distance at which you can draw a spark from the conductor, because the vitreous power covering the resinous power on the coating, lessens the action on the vitreous power within the bottle, and there- fore leaves that power greater freedom to fly off; but as you go on taking sparks, they gradually lessen, because after a few, the vitreous power in the bottle is lessened, and the resinous power within increased by the quantity received in exchange on every spark ; and thus by a few sparks the bottle is discharged ; but if you go on to take more sparks, the bottle will be re-charged with the resi- nous power withinside, instead of the vitreous, with which it was before charged. Again, suppose fifty turns of the cylinder will charge your bottle, make only twenty-five, and then remove the communication between the coating and table ; and as you turn on. whether you continue the communication from the conductor to the top of the bottle, or shift it to the coating, you will find the bottle electrified on both sides with the vitreous power ; remove the bottle from THEORY OF THE LEYDEN PHIAL. 301 the conductor, and then discharge it with an insulated discharger, and you will find the bottle still electrified, both within and without, with the vitreous power ; but this electricity will disappear, by touching either the ball or coating w r ith your ringer. To illustrate further the reciprocal exchange of the electric powers, here is an insulated bottle with a wire proceeding from the bottle, at right angles to which is a wire for receiving a needle with reversed points ; make the top of the bottle communicate with the conductor, and all the time the bottle is charging the needle will turn ; but when the bottle is chatged, the needle stops. Then touch the top of the bottle with your finger, or any conductor, and the needle will turn till the bottle is dis- charged. Now, while the bottle is charging, if you touch the needle with a piece of bog-down, or a cork ball, sus- pended by silk, you will find it electrified by the vitreous power, w : hich flies off in exchange for the resinous power drawn in from the air to the outside of the bottle ; and while the bottle is discharging, if you apply the dow r n or ball in the same manner to the needle, you will find them electrified with the resinous power, which flies off from the outside of the bottle in exchange for the vitreous power drawn in through the points from the air ; while the vitreous power from the inside of the bottle makes the same exchange for the resinous power through your finger, to make these different powers equal to each other, withinside and withoutside the bottle. Place two Levden bottles on an electric stand, with their coatings in contact ; and while you charge one from the conductor, let a person on the floor touch the top of the other bottle with his finger ; you wall find the first bot- tle charged with the vitreous power inside, and the second with the resinous power inside. Now, the exchange here is evident ; for, while the resinous power from the inside of the first bottle changes place with the vitreous thrown in from the conductor, the vitreous from the coating changes place for so much of the resinous from the coat- ing of the second bottle ; and the vitreous in that bottle changes place for so much of the resinous power drawn in through the man on the floor. 302 EXPERIMENTS ILLUSTRATING THE I charge a Leyden phial, and set it aside to be in readi- ness to ascertain the state of another. I now take the bot- tle with the projecting wires, plate 1, Jig. 10, unscrewing the ball from the wire at the coating, and suspending a pair of pith balls therefrom This done, I bring the knob of the bottle to the conductor ; I work the machine, and the phial will charge slowly, and the balls will repel each other ; while I am turning and the bottle charging, bring the knob of the first bottle towards the balls, and they will be repelled thereby. This plainly proves, that the outside of the bottle is electrified vitreously while it is charging, that is, with the same electricity as the inside. Let us discharge the bottle with the projecting wires, and charge it again as before, and you will still find, that whilst it is charging, the balls will fly from the knob of the first bottle ; I cease turning, and the balls cease to re- pel each other ; they now touch each other, and again recede, but with a contrary electricity, for they are now attracted by the knob of the first bottle. This shows that the difference between the two sides cannot appear, while they are charging, or while vitreous electricity is forced through the jar. Let us now discharge both bottles, in order to try an- other experiment, to determine the state of the outside du- ring the charge. I first put the ball on the end of the wire of the bottle with the projecting wires, bring the knob thereof to the conductor, holding the knob of the first bottle against the coating of that with the projecting wires ; by working the machine, both will be charged. As soon as they are pretty well charged, and while the machine is working, remove the first bottle from the other ; after this is removed, cease working the machine as soon as possible. I now connect by a wire the two out- side coatings, and bring the balls to each other. If, while the bottles were charging, the outside of that with pro- jecting wires had been resinously electrified, the inside of the second would have been so also ; and on their being thus brought together, both bottles would be discharged : but this is not the case, for the insides of both are charg- ed with the vitreous electricity, the coating having ex- changed powers with the bottle charged thereby. This THEORY OF THE LEYDEN PHIAL. 303 Experiment shows, that to consider one side of a phial to be positive, and the other negative, at the time they are charging, is erroneous. The criterion of the resinous and vitreous electricity, as determined by the light on metallic points, gives full evidence in favour of Mr. Eeles's theory, while it is di- rectly opposed to that of Dr. Franklin. For you will here find, that during the time that the bottle is charging, the outside exhibits the sign of vitreous electricity. To prove this, I place a pointed wire at the end of the conductor, and place this apparatus with the sliding wire, plate 1 , Jig. 11, on one of the insulated stands, first removing the bottle therefrom ; I then unscrew the balls from the projecting wires of the remaining insulated bottle, and also from the sliding wire, which leaves the points that were under the bottle exposed and ready for our opera- tions. Things being thus prepared, I place the insulated bot- tle so that the point, from the inside, may be about half an inch distance from that in the conductor, and let one of the points of the sliding wire be at the same distance from, and opposite to, the point projecting from the out- side of the insulated bottle. I now turn the machine, and as soon as the charge begins, the signs of the electrici- ties are visible, illuminating the points of the interrupted circuit. The point on the prime conductor gives the brush or sign of vitreous electricity; the sign on the point opposed to it on the knob of the bottle is resinous. The light from the wire, that projects from the coating of the bottle, is the brush, or vitreous ramified light ; but that of the point opposed thereto is the star, or sign of resi- nous electricity, as they ought to be, according to Mr. Eeles's theory, not " contrary to the kind or source of electricity from whence they proceed," which is the case on the principles of the Franklinian theory.* * Rcid's Summary View of Spontaneous Electricity, p. 81, 82 [ 304 ^ EXPERIMENTS SHOWING THAT TN THE DISCHARGE THE LEYDEN JAR, THE TWO ELECTRICITIES RU! INTO UNION FROM OPPOSITE DIRECTIONS. The three first experiments I shall mention to you, were made by Mr. Atwood, of Cambridge, and were de- scribed by him in the Analysis of a Course of Lectures, which he read at Cambridge. He slightly charged the surfaces of an electric insulated plate, and discharged it through an interrupted circuit, formed of needles placed in a groove of wax, the distance between the needles being very small ; the two powers were visible, on the discharge illuminating the points of the interrupted circuit, each power extending farther from the surface contiguous thereto, in proportion to the strength of the charge ; but when this was sufficiently strong to make the illuminations proceeding from each side meet, there was an explosion of the whole charge. The length of the interrupted circuit made by Mr. Au wood was twelve feet. Mr. Atwood charged a cylindrical plate of air, under the receiver of an air-pump, and found that the more the air was exhausted from between the surfaces, the more readily and easily the powers united. He made an exhausted receiver part of the electric cir- cuit, and on using such charges as were not sufficient to form an explosion, he found the electric light proceeding in opposite directions from the parts communicating with the vitreous and resinous surfaces. When a Leyden jar is charged but slightly, if you touch the coating with a finger of one hand, and at the same time bring a finger of the other to the knob of the jar, you will receive a smart blow upon the tip of each finger, but the sensation reaches no higher. Charge the jar a degree higher, and you will feel a stronger blow, reach- ing to the wrists, but no further. When it is charged somewhat higher, a severe blow will be received, but which will not reach beyond the elbows. Lastly, when the jar is strongly charged 5 the shock will be perceived FROM OPPOSITE DIRECTIONS. 305 at the wrists and elbows, but the principal blow is felt at the breast, as if a blow from each side met there. This plain and simple experiment of Mr. Symmer obviously suggests the existence of two currents proceeding in contrary directions, accords with those of Atwood and Volta* and is in direct contradiction to that assertion of the Franklinians, " that the same quantity of electric matter, which is thrown upon one of the surfaces of glass in charging, is driven from the other, and that in the discharge this accumulated quantity is restored to the deficient surface." When a jar is charged very high, the electricities will often, in their endeavours to unite, force a hole through the jar, and push out the coating on both sides, sometimes melting it ; the burr of tin-foil pro- truded from the middle of the glass strongly indicates, that the two electricities meet at the middle of the glass ;f there also the greatest effort is exerted. Mr. Read says, that when the charge for melting of fine wire is of a proper intensity to melt it into fine globules, he has observed the wire to be of a paler red heat in the middle than at the extremities, and the melting to begin at the middle, leaving a portion un- melted at each end. At other times, though less fre- quent, the wire was observed to be of a more glowing heat in two parts, and these were generally near the middle. These effects clearly show, that the vitreous and resinous electricities of the charged jar meet in great force near the middle of the wire, which is di- rectly contrary to the leading notions of Franklin's theory. The remarkable tendency of the divided fluids to unite, is often perceived in a full-charged Leyden bot- tle, at the upper edge of the outside coating, and at the edge of the cork on the neck of the bottle ; rays of light darting from each, and soliciting as it were a union, and sometimes forming an actual circuit. * See my Essay on Electricity, t Read on Spontaneous Electricity, &c. p. 44„ VOL, IV, 3 R [ 306 ] THE SAME PRINCIPLES CONFIRMED BY THE APPEAR- ANCES OF THE ELECTRIC SPARK. The electric spark appears of different colours ac- cording to its density ; when it is rare, it appears of a bluish colour ; when more dense, it is purple ; when highly condensed, it is clear and white, like the light of the sun. The middle part of an electric spark often appears diluted, and of a red or violet colour, while the ends are vivid and white ; this appearance cannot be account- ed for by the theory of a single fluid moving in one di- rection, but is a proof of two currents moving in op- posite directions ; the electric signs growing weaker where the two powers unite. Mr. Read* has well, and I believe first observed, that the place of re-union is much less luminous, and in some cases quite dark ; and that this is the natural effect of the union of the two electricities ; at that point the distinctions of the vitreous and resinous cease, and there the electric light vanishes. These appearances are best observed, by viewing in the dark a strong electric spark passing be- tween two bodies electrified with contrary electricities. Though the appearances of the electric light on a point and ball, as well as of the electric spark, are sub- ject to many variations, yet there are certain signs ge- nerally peculiar to each kind of electricity. For in- stance, if the resinous part of a spark be small, or what has been usually termed the luminous globule, then the middle part is generally of a purplish colour. When ramified rays issue from the vitreous part, then the resinous is more extended, stretching out towards the vitreous. When the vitreous and resinous elec- tricities strike into each other in dense light, in va- rious parts of the intermediate space, then their exact place of union is generally observable by a dark spot. Mr. Read with propriety considers the loss of light in * Read's Summary View of Spontaneous Electricity, p. 47. 48, and 4P» ELECTRIC LIGHT IN VACUO. 307 any part of an electric spark, whether total or partial, as the immediate effect and constant sign of the re- union of the two electricities. Mr. Read observes, that whether the resinous light •assumes the figure of an oblong flame, or of a luminous globule, in either case the vitreous light is seen to ap- proach, and unite with it in all possible directions. The effect of a vitreous surface appears to extend farther than that of a resinous surface. THE OPPOSITE DIRECTIONS OF THE TWO ELECTRI- CITIES PROVED BY THE APPEARANCES OF THE ELECTRIC LIGHT IN VACUO. Though I have already pointed out to you some ex- periments in vacuo that illustrate this point, yet those of Mr. Read are so decisive, that not to mention them, would be to deprive you of essential information on this subject. For these experiments, Mr. Read used a glass tube, three feet seven inches long, furnished at each end with a brass cap, one of the caps being fitted to the plate of the air-pump ; from each cap a brass wire, on which was a brass ball, projected within the tube ; when this tube is sufficiently rarefied, the charge of a Leyden phial will readily pass through the rarefied air. In making these experiments, you must only slightly charge your Leyden jar ; for, if the charge be strong enough to force the whole contents swiftly through the rarefied air, the motion of the fluid is too rapid, and the light to resplendent to permit an exact observation of its appearance. On making the discharge in the dark, you will per- ceive, the moment the circuit is formed for that purpose, a light within the tube, but chiefly at each end. These lights are of the contrary kinds of electricity, and ac- cord with the side of the bottle to which they are con- nected. You may sometimes perceive the two lights to have a manifest tendency to meet near the middle of the resisting medium. Mr. Read has observed the light: S08 OPPOSITE DIRECTIONS OF THE within the tube to be considerably diminished in splen- dour where the two powers unite ; and so it ought to be, for when the two electricities unite and regain their natural state, they loose their light, for it is only in a divided state that the electrical matter is luminous ; the same appearances are produced in the tube by the sim- ple spark, that is, the contrary electricities are observed at each end*. But this is still further confirmed by a new observa- tion and decisive experiment of Mr. Read's. He sus- pended his exhausted tube in a horizontal direction, by silk lines from the ceiling ;f one end was placed so as to receive an electric spark from the conductor of his machine ; at half an inch from the other end, there was a metallic communication with the earth. On turning the machine, the tube is filled with elec- tric light, and continues so long as the action of the machine is continued. Mr. Read first observed, that the instant the supply ceases, the light divides near the middle of the tube, and flies back to the ends ; ful- ly evincing the truth of Mr. Eeles's theory, by showing that the light within the tube is not all of one kind of electricity ; the tube includes both electricities in one appearance of light. The moment the action of the machine is discontinued, the afflux and efflux cease, and each electricity returns to its own place, where the separation first commenced. To ascertain beyond dispute, that the light within this kind of exhausted tube consisted of vitreous and resi- nous light, he made the following experiment. The glass tube was suspended as before, and two Leyden phials in a horizontal position, but lying on glass stands, were placed one at each end of the tube, with their me- tallic knobs nearly in contact with the metallic caps of the glass tube. In this disposition of the apparatus, the coating of one bottle is to receive a spark from the * Read) p. 51, 52, 53. t It is more convenient to insulate the glass tube or luminous conductor by glass pillars, as \>late 1, Jig. 13. TWO ELECTRICITIES PROVED. 309 prime conductor, and the coating of the other a spark from the metallic communication with the earth. On turning the cylinder, sparks were perceived to pass in the four intervals of air, and at the same time a luminous appearance within the glass tube. On re- moving the bottles, and examining their charges, they were found to correspond with the lights within the tube, to which they were opposed. One bottle was vitreous- ly, the other resinously electrified.* These experiments clearly prove, that there is at the same time one power acting from within, towards the outside of a charged Leyden phial, and another power acting from the outside towards the inside of the phial ; and thus concur with others in showing, that electrici- ty consists of two distinct positive powers acting in con- trary directions, and towards each other. Here is a glass coated flask from which the air has been exhausted, that you will find, on trial, to illustrate pleasingly the theory of electricity, plate l,Jig. 14. From the experiments on the theory of the Leyden bottle, I shall now proceed to some entertaining ones with the same instrument. No electrical experiments answer so well the joint purposes of pleasure and sur- prize, as those that are made with the Leyden phial. And philosophers are so far from laughing at the asto- nishment of the ignorant at these experiments, that they cannot help viewing them with equal, if not greater astonishment themselves. There are indeed, as Dr. Priestley has observed, many electricians still living, who can well remember the times when, with respect to these things, they themselves would have ranked among the same ignorant and staring vulgar. What would the ancient philosophers have said, what would Newton himself have said, to see the present race of electricians imitating, in minature, all the known effects of lightning ; nay, essaying to disarm the thun- Mr. Relet from his theory, pointed out, in 1758, the mode of making mis experiment, ind foretold what would be the result. This is only one among many i: s ances, where, in reasoning a/morf, he has pointed out results, that the Frankliniansof the day denied. 310 OPPOSITE DIRECTIONS OF THE der of its power of doing mischief, and without any appre- hension of danger to themselves, drawing lightning from the clouds into a private room, and amusing themselves at their leisure, by performing with it all the experiments that are exhibited by electrical machines ? • One cannot indeed consider the present improved state of philosophy, without indulging, with the Rev. Mr. Jones, a wish to exhibit to the wise men and heroes of ancient times some of those wonderful improvements which are now so familiar to us, but were totally unknown to them. . I would give, says he, to Aristotle the electrical shock: I would carry Alexander to see the experiments upon "Woolwich warren, and exhibit to him all the evolutions and firings of a modern battalion : I would show to Ju- lius Ctesar, the invader of Britain, an English man oi war; to Archimedes a steam engine, and a reflecting telescope.* Entertaining electrical experiments are not withou! their use, for they give even to philosophic minds an op. portunity of examining things under different points o: view, and often arest the attention to objects which hac before escaped their notice. To strike a hole through a card. Having charged youi jar, hold a card with one hand close to the coating of th< jar'near the bottom, then apply one knob of the discharg ing rod to the card, and the other to the ball of the bot tie, and the discharge will pass through the card, anc will make a hole through it with a burr on each side, o which I shall take more notice hereafter ; it will have i strong sulphureous smell. If die experiment be made with two cards instead o one, the cards must be placed but at a very small distano from each other ; each of the cards, after the explosion will be found pierced with one or more holes, and eacl hole will have burrs on both surfaces of the card. To stain paper, you must lay a chain upon a sheet o white paper, and pass a shock through it ; the paper Wil Jones's Physiological Disquisitions. TWO ELECTRICITIES PROVED. 311 e found to be stained with a blackish tinge at every junc- ure of the links. If you make this experiment in the .ark, a spark with a kind of radiation will be seen at ach juncture ; and the chain will appear illuminated like . line of fire ; an iron chain answers best the purpose. You may also, by the discharge, stain glass with go/d- eaf; for this end, take two slips of common window- dass, each about an inch broad, and three or four inches ong; then take a narrow slip of gold or silver leaf, and ;>ut it between the glasses lengthwise, letting the ends of he leaf hang half an inch without the glasses at each end ; )lace the glasses in the small wooden press, and fix them here by a gentle pressure, and then lay them down on he table, so that one end of the metal leaf may be in :ontact with the coating at the bottom of the jar ; and vhen the jar is charged, put one end of the discharging od upon that part of the leaf that lies without the glass, vhich is farthest from the jar, and apply the other end )f the discharger to the top of the jar, and the fluid will >ass through the metal leaf; and when the glasses are aken asunder, you will find, that the leaf has been actu- .lly melted by the electric lightning, and driven into the ery substance of the glass.* A pane of glass, coated on each side, the coating being very where about two inches from the edge, with a pic- ure pasted on the upper side, and put into a frame, is ailed the magic picture ; one line of tin-foil, that goes rom the coating of the under side, is made to communi- ate with the bottom of the frame ; the back edge of the )ottom rail and the frame is covered with tin-foil. Set he face of the picture against the ball of the conductor, md turn the machine. Then take it away, and holding t in a horizontal position by the top of the frame, drop t small piece of money upon the head. You may then lesire any person to take hold of the lower rail of the * The wooden press is generally adapted to the universal discharger* date 1, Jig. 15, the jointed balls and wires of which may be readily and •onveniently applied to the extremities of the gold-leaf above-mentioned. V chain or wire from the outside of the jar is to be connected with the ring; >f one wire, and the chain of the discharging red with the other, before the Uncharge is made...„E,EDiT. 312 OPPOSITE DIRECTIONS OF THE frame with one hand, and to take off the piece of money with the other ; in attempting to do this, he will fail of his design, for the moment he touches the money he will receive a strong shock. You must continue to hold the frame all the while, and will have nothing to fear, be- cause none of the electric virtue, with which the picture is charged, can come to you, as you are not in the cir- cuit. This bottle is called the spotted bottle, plate 1, Jig. 18, because it is only coated with small pieces of tin-foil, placed at a little distance from each other ; charge this bottle in the usual manner, in a darkened room, and you will see strong sparks of electricity fly from one spot of tin-foil to the other, making the passage of the fluid on the outside very visible. Discharge this bottle, by bring- ing a pointed wire gradually near the knob, and the un- coated part of the glass between the spots will be pleas- ingly illuminated, and the noise will resemble that of small fired crackers. If the jar be discharged suddenly, the outside surface appears illuminated. To produce these appearances, the glass must be very dry. Hold a phial in the hand which has no coating on tk outside, and present its knob towards an electrified con- ductor ; the fire, while it is charging, will pass from the outside to the hand, in a pleasing manner ; on the dis- charge, beautiful ramifications will be seen upon the un- coated part of the jar. By setting fire to some tow in a tin house, you have a representation of that awful appearance, a house inflames. To make this experiment succeed, take a piece of soft tow, dry it well, and then rub, or fill it pretty well with rosin, and place it between the balls in the inside of the house ; the balls should not be far asunder, nor the charge too high ; connect the hook at the bottom of tht house with the bottom of the jar ; let the top of the jar be connected with the conductor, and when it is charged 5 put one ball of the jointed discharger on the conductor, and bring the other down upon the ball above the house ; the explosion will set the tow on fire, whose flames will pass through the windows^ and make the house appear like one on fire. TWO ELECTRICITIES PROVED. 313 You may pleasingly illustrate the nature of the Leyden phial, by suspending two sets of bells therefrom ; one set connected with the inside, the other with the outside, see plate 1, y%. 16. Hook up the chain from the bells communicating with the inside, that they may have no connexion with the table ; charge \hk bottle in the usual manner ; during the charge, the set suspended from the outside will continue to ring. After the bottle is charged, unhook the wire of the bells suspended from the inside. Touch now the wire A, and the bells will cease ringing, but the other set will begin to act ; take the finger from A, and apply it to B, and the bells at B will be quiet, while those at A will be set in motion, and so on alter- nately, till the bottle be discharged. EXPERIMENTS WITH THE ELECTRICAL BATTERY. The most formidable part of the electrical apparatus is the electrical battery, that is, a number of jars connected together in a box ; the bottom of the box is covered with tin-foil ; from these a hook projects on the outside of the box, by which any substance may be connected with the outside of the jars ; their insides are all connected by wires. With a battery you may perform a great number of very surprising and interesting experiments ; and though, if very large, it is a formidable appendage to an electri- cal machine, and ought always to be used with caution, yet it cannot be said, that the apparatus of an electrician is complete without it ; its effects in rending various bo- dies, in firing gun-powder, in melting wires, and in imi- tating all the effects of lightning, never fail to be viewed with astonishment. There is some caution necessary in the use and man- agement of a battery, and you should be careful never to make part of the circuit, and to prevent those that are viewing the experiments from touching the battery, or ap- proaching too near any part of the apparatus ; the quad- rant electrometer, plate 1, Jig. 17, should be always used vol. iv. 2 s 5314 EXPERIMENTS WITH THE with it ; it is best to place it upon the ball, which unites the internal wires, but it should always be elevated two or three feet above the ball. A battery cannot be charged so high in proportion, as a single jar ; the quadrant elec- trometer, therefore, never rises so high as 90 degrees, seldom higher than to 60 or 70 degrees, more or less. in proportion to the size of the battery, and the force of the machine. I must observe to you here, that if one jar in your battery be broken, you must remove the bro- ken jar before the rest can be charged. Mr. Atwood made, with his battery, a very curious ex- periment on the perforation of paper by the electric fluid; combined with those that I shall afterwards relate to you, you will find it to prove, with great clearness, the existence and action of the two electric powers. He suspended a quire of paper by a line, in the man- ner of a pendulum, from a convenient altitude, while quiescent in a horizontal direction perpendicular to the plane, the rods of communication not touching the pa- per ; the phenomena were, first, the aperture mentioned in the leaves, being protruded both ways from the mid- dle :* second, not the smallest motion was communicated to the paper from the force of the discharge. A quire of the thickest and strongest paper was made use of for this experiment, the height from which it was suspended sixteen feet. It is an extraordinary appearance on the hypothesis of a single electric fluid, that a force suf- ficient to penetrate a solid substance of great tenacity and cohesive force, should not communicate the smallest mo- tion to the paper, when a breath of air would cause some sensible vibration in it. But the other phenomenon, /. e> the opposite direction in which the leaves are protruded, tends very much to strengthen the opinion of two oppo- site currents ; indeed, when the two facts are taken to- gether, it is scarcely possible to reconcile the hypothesis of a single power with matter of fact. * The burr of the paper pointed one way on one side, and the opposite way on the other side, as if the hole had been made in the quire, by draw- ing two threads through it, in a contrary direction. ELECTRICAL BATTERY. 315 Mr. Symmer placed in the middle of a paper book, of the thickness of a quire, a slip of tin-foil ; in another of the same thickness he put two slips of tin-foil, including the two middle leaves between them ; upon passing the electric stroke through them, he found the following ef- fects. In the first, the leaves on the side of the foil were pierced, while the foil itself remained unpierced ; but at the same time he could perceive, that an impression had been made on each of its surfaces, at a small distance from each other ; such impressions were still more visi- ble on the paper, and might be traced as pointing differ- ent ways. In the second, all the leaves of the book were pierced, excepting the two holes that were between the slips of foil, and in these two, instead of holes, the two impressions in contrary directions were visible. When a quire of paper, without any thing between the leaves, is pierced by the electrical stroke, the two pow- ers keep in the same track, and make but one hole in their passage through the paper ; not but that the power from above, or that from below, sometimes darts into the paper at two or more different points, making so many- holes ; but these generally unite before they go through the paper. They seem to pass each other about the mid- dle of the quire, for there the edges are most visibly bent different ways ; whereas, on the leaves near the outside, the holes very often carry more the appearance of a power issuing out, than of one darting into the paper. When any thin metallic substance, such as gilt leaf, or tin-foil, is put between the leaves of the quire, and the whole is struck ; the counteracting powers deviate from the direct track, and make their way in different lines to the metallic body, and strike it in two different points dis- tant from one another, about one-fourth of an inch, more or less ; the distance appearing to be generally less when the power is greatest ; and whether they pierce or only make impressions upon it, they leave evident marks of motion from two different parts, and in two contrary di- rections. When two slips of tin-foil are put into the middle of the quire, including two or more leaves between them, if the electricity be but weak, the counteracting powers only 316 EXPERIMENTS, &C. strike against the slips, but leave an impression ; if the shock be stronger, one of the slips is pierced, but seldom both ; and it appeared in general to Mr. Symmer, that the power which issued from the outside, acts with greater force, than that which proceeded from within. To break thick pieces of glass. Place a thick piece of glass on the ivory plate of the universal discharger, plate 1, Jig. 15, and a thick piece of ivory on the glass, on which a weight from one to seven pounds is to be placed; take off the balls a, b, bring the points of the wire against the edge of the glass, and pass the discharge through the wires, by connecting one of the wires with the hook of the battery, and forming a communication, when the battery is charged, from the other wire to the ball. By this operation the glass will be broken, and some part of it shivered to an impalpable powder. When the piece of glass is strong enough to resist the shock, the glass is often marked by the explosion with the most lively and beautiful colours. Place a piece of very dry white wood between the balls of the universal discharger, the fibres of the wood to be in the same direction with the wire, pass the shock through them, and the wood will be torn to pieces ; or run the points into the wood, and then pass the shock through them. To melt wires by the electrical fluid, you ought to have a battery containing at least thirty square feet of coated surface ; you may then connect the outside coating with a wire of about one-fiftieth of an inch in diameter, and from twelve to twenty-four inches in length ; fasten the other end of the wire to one of the balls of the discharg- ing rod ; on making the discharge the wire will become red-hot, then melt and fall upon the floor or table in glow- ing globules. Sometimes the sparks are thrown to a con- siderable distance ; if the force of the battery be very great, they will be entirely dispersed by the explosion.* * For a further variety of experiments, see our Author's Essay on Elec- tricity. I am preparing a new edition oi this work for the press, which will be published in the beginning of the next year, 17^9, with many correc- tions and augmentations.... E. Edit. C 317 ] LECTURE XLVIIL OF LIGHTNING, AND THE USEFULNESS OF METALLIC CONDUCTORS TO DEFEND BUILDINGS FROM ITS EF- FECTS, N OTHING can be more natural than to pass from the electrical battery to lightning itself, for the former seems to be more than an imitation ; it is nature invested in her own attire. The light and sound accompanying these phenomena, when exhibited on the great scale of nature, are indeed so awfully sublime, that we can scarce with propriety reflect on the weakness of those, who, in ages less informed, supposed it to be the immediate minis- ter of vengeance from an angry Deity. They are now more rationally considered, as the natural means of re- storing a necessary equilibrium ; the rough discords of nature productive of general harmony. The phenomena of lightning are always surprizing, and sometimes terrible ; there is no appearance in which there is more diversity, no two flashes being observed ex- actly similar to each other. On a summer's evening, it may often be perceived to play among the clouds ; this kind is quite inoffensive, and is not accompanied with thunder. When the lightning is accompanied with thunder, it is well-defined, and has generally a zig-zag form ; some- times it only makes one angle like the letter V, sometimes it appears like the arc of a circle. But the most formida- ble and destructive form which lightning is ever known to assume, is that of balls of fire. The motion of these is very often easily perceptible to the eye, but wherever they fall, much mischief is the result of their explosion* 318 OF LIGHTNING* The next to this, in its destructive effects, is the zig- zag kind; for that species, whose flashes are indistinct, and whose form cannot be easily observed, is seldom known to do much hurt. You may consider the co- lour of lightning as an indication of its power to do mis- chief, the palest and brightest flashes being the most destructive. There seems to be a kind of omnipresent property in the zig-zag kind of lightning when near. If two persons be standing in a room, looking different ways, when a loud clap of thunder happens, accompanied with the zig-zag lightning, they will both distinctly see the flash, not only by that indistinct kind of illumina- tion of the atmosphere, which is occasioned by fire of any kind, but the very form of the lightning itself, and every angle it makes in its course will be as distinctly perceptible, as though they had looked directly at the cloud from whence it proceeded. If a person were at that time io be looking on a book, or other object which he held in his hand, he would distinctly see the form of the lightning between him and the object. This pro- perty seems peculiar to lightning. The effects of lightning are generally confined with- in a small space : and are seldom similar to those which accompany explosions of gun-powder, or of inflam- mable air in mines. Instances of this kind, however, have occured ; the following is one of the most remark- able of which we have any distinct account : " Au- gust 2, 1 763, about six in the evening, there arose at Anderlight, about a-league from Brussels, a conflict of several winds borne upon a thick fog. This conflict lasted four or five minutes, and was attended with a frightful hissing noise, which could be compared to no- thing but the yellings of an infinite number of wild beasts. The cloud then opening, discovered a kind of very bright lightning, and in an instant the roofs of one side of the houses were carried off and dispersed at a distance; above 1000 large trees were broken off, some near the ground, others near the top, some torn up by the roots ; and many both of the branches and tops carried to the distance of 60, 100, or 120 paces ; OF LIGHTNING. 319 whole coppices were laid on one side, as corn is by or- dinary winds. The glass of the windows, which were most exposed, was shivered to pieces. A tent in a gen- tleman's garden was carried to the distance of 4000 paces ; and a branch torn from a large tree, struck a o-irl in the forehead as she was coming into town, at the distance of 40 paces from the trunk of the tree, and killed her on the spot." Thunder-storms will sometimes produce most violent whirlwinds, such as are by some philosophers attributed to electricity ; nay, even occasion an agitation of the waters of the ocean itself; and all this too after the thunder and lightning has ceased. Of this we have the following instances : " Great Malvern, October 16, 1761. On Wednes- day last we had the most violent thunder ever known in the memory of man. At a quarter past four in the afternoon, we were surprised with a most shocking and dismal noise; 100 forges all at work at once, could scarce equal it. Upon the side of the hill, about 400 yards to the south-west, there appeared a prodigious smoke, attended with the same violent noise, as if a volcano had burst out of the hill ; it soon descended, and passed on within about one hundred yards of the south end of the house ; it seemed to rise again in the meadow just below it, and continued its progress to the east, rising in the same manner for four different times, attended with the same dismal noise as at first ; the air being filled with a nauseous and sulphureous smell ; it gradu- ally decreased till it was quite extinguished in a turnip field, about a quarter of a mile below the house ; the turnip leaves, with leaves of trees, dirt, sticks, &c. fill- ed the air, and flew higher than any of these hills. The thunder ceased before this happened, and the air soon after became calm and serene." Lightning is in the hands of nature, what electricity is in ours ; the wonders we now exhibit at pleasure are little imitations of those great effects which frighten and alarm us, they seem to depend on the same mechanism ; the same properties, the zig-zag sparks, their similar action on conducting substances, the power of rending,, 320 OF LIGHTNING. inflaming, and dispersing in every direction the sub. stances on which it acts with power, the giving polari- ty to feruginous matter, &c- all concur to show their identity. But independent of these similarities, the thing is proved by the plainest and clearest evidence ; when the atmosphere is charged with thunder clouds, we can by an electrical kite draw from it the matter of lightning, and with this matter perform every known electrical experiment. You have seen, that the electric powers never become sensible to us, except when they are separated, and then chiefly in their passage from one body to' another in opposite directions ; and that an equal quantity of a dif- ferent power must be conducted from the earth to the cloud to produce lightning. There must be the same reciprocal exchange of powers to occasion lightning from one cloud to another. When two clouds, which are highly electrified with the different powers, come near together, they approach with an increasing force till they flash in exchanging powers. But as clouds are formed of distinct particles, and every particle has its share of both electric powers, according to the equality or inequality of quantity of each power in each particle, it is more or less electrified; and on the various combinations of these powers, will arise the mode in which the clouds approach each other, and in which they exchange their different powers. When the electrified particles are made so to approach each other, that their atmospheres are pressed off toge- ther to a great distance from the cloud, they then act nearly the same as if the cloud was one continuous bo- dy ; but after the flash, those particles which have exchanged powers, and in which the two electricities are united, being no longer buoyed up by these agents, fall dow in rain, hail, &c. That these atmospheres are extended to a great dis- tance from the cloud, appears from all experiments made both here and abroad ; for in them it is plain, that an atmosphere goes up from the earth, of the pow- er which is contrary to that of the cloud, which would OF LIGHTNING. 321 not take place if the atmosphere of the cloud did not reach the earth. When one of these highly electrified clouds ap- proaches so near to the earth as to exchange powers with it, then is the damage done to those things through which the exchange is made, which are generally those bodies that rise nearest the cloud. Many are the observations which show, that the at- mosphere of the clouds are condensed at the time of their junction by a flash, and that the contrary electrici- ty is then, as it were, drawn up from the earth. Thus, in Mr. Ludolfs account, Phil. Trans, vol. xlvii. at eve- ry clap of thunder the electricity seemed extinct, and did not return till after the space of about 30 seconds ; the threads which by their divergence indicated the elec- tricity, approached each other suddenly, as if they had been pushed together with force. The Abbe NoIIet 9 and many others, have observed similar appearances. In an observation of Abbe Nollet, the clap of thunder put a stop for some time to the force of the electrici- ty ; all this may be easily illustrated by our electrical apparatus. Bring two cork balls suspended by linen threads from the end of a wire, within the atmosphere of an electrified conductor, and they will be electrified with a power contrary to that which electrifies the con- ductor, receding from each other, but flying towards the conductor ; take a spark from the conductor, and they immediately collapse, the electricity drawn into them from your body returning thereto. It often happens, as before observed, that clouds electrified with the contrary powers are driven together, and the particles coming into contact, the powers is ex- changed without that violent flash which usually accom- panies a thunder storm. In this case, the particles ge- nerally descend in heavy showers of rain ; but the ex- change of powers is most complete in the middle of the united clouds, and the heaviest part of the shower is generally from the middle of the cloud. In confirmation of this, I shall only mention one ob- servation, though many might be produced j it was VOL, IV. 3 T 322 OF LIGHTNING. made by Mr. Eeles'in October 1760 ; the clouds were very distinct, and the showers heavy. In three differ- ent clouds he found the showers from the beginning electrified with the vitreous power ; the showers from the middle of each cloud showed no sign of electrici- ty, and the end of each cloud was resinously electrified, the wind N. W. There was no appearance of electri- city in the middle of the showers, because the electric powers were there united to each other in every drop ; their atmospheres and actions were therefore insensible. Rain, hail, and snow, often exhibit signs of being electrified, for the clouds are seldom so equally electri- fied with the different powers of electricity, as upon meeting to render them equal in each descending drop. In large flakes of snow, the electricity is often very evident ; for when they come near a non-electric body, they are driven towards, and cling about it like an elec- trified feather. It is not easy to form any idea of what some writers mean by a negative cloud or negative stroke. Is it a mere inanity which knocks down steeples, rends trees, tears up the earth, and kills men and cattle, &c. Can that, which is not, act ? You saw, by an experiment I lately exhibited to you, that if two electric plates, or two jars, be charged, and a communication be made from the vitreous side of one, to the resinous side of the other, no discharge will fol- low, unless a communication be formed between the other two surfaces at the same time. The natural electricity in the atmosphere is frequent- ly discharged in this manner : two clouds being elec- trified with opposite powers, the surfaces of the earth immediately under them are likewise electrified with powers contrary to those in the clouds above them ; and the moisture of the earth forming a communication between the two contiguous charged surfaces, when- ever the two clouds meet, there will fellow a discharge both of the clouds and surfaces on the earth opposite to them. If the earth should be dry, and consequently afford a resistance to the union of the two electricities accumulated on or under its surface, there will follow OF LIGHTNING, 323 an explosion in the earth as well as in the atmosphere, which will produce concussions and other phenomena which have Frequently been observed to happen in dry seasons, particularly in those climates which are the most liable to storms of thunder and lightning. The various cases of lightning are too numerous to be here considered, and too imperfectly known to be ac- curately explained. What I have said will, I hope, give some general notions of the method in which it operates, and lead you to a further investigation of the subject. You may from thence also readily account for its seemingly capricious nature ; sometimes it will strike trees, high nouses, &c. without touching cottages, men, or animals in the neighbourhood ; while in other in- stances, low houses and cattle have been struck, while , high tre&s, steeples, &c. near them have escaped. All this is very easily accounted for, upon Mr. Eeles's theory of a double current, and the efforts in nature to restore the electrical fluid to a latent state, whenever by any means the powers thereof have been separated. Thus, in great thunder storms, there is a portion of the earth under the cloud which is electrified thereby with the contrary electricity ; those objects therefore, which form the most perfect conductors between the clouds and that portion of the earth, will most probably be struck, as being the readiest way by which the two opposite powers can unite, and restore the electrical equilibrium both in the cloud and the earth, one part of the flashes ascending from the earth, the other de- scending from the cloud. Let us suppose a cloud, vitreously electrified, to be formed over a certain part of the earth's surface ; the electric power of the cloud first separates that of the at- mosphere, and while it is thus operating, the atmosphere is resinously electrified ; in a little time the air becomes vitreously electrified, and then both it and the cloud act as one body. The surface of the earth then begins to be electrified, and the powers therein to be separated, and a continual effort is made by the contrary electri- cities to unite between the earth and the cloud. If those causes which first produced the electricity still act, the 324 OF LIGHTNING. c in power becomes inconceivably great, and the flashes in uniting will tear every thing to pieces that resists their passage. Mr. Read justly observes, that a portion of the earth may be highly electrified, and yet we may be insensible thereof, because we are involved therein ; for where all things are equally involved in an electrical atmosphere, there can be no visible signs of the presence of the elec- tric matter. Thus, if two or more persons be electri- fied, while standing on the same insulation, they show no signs to each other of being electrified.* Whatever be a person's situation, whether in the house or open field, he is liable to be involved in an electric charge, whether it be stationary, or moving with the clouds. Mr. Read found himself so involved once in Hyde Park ; the at- mosphere had a menacing appearance with a heavy black cloud at no great distance ; on taking his pocket elec- trometer out of its case, and holding it in his hand, it instantly diverged near one inch. It is not probable, that the restoration of the equilibrium, or returning stroke, as it is often called, will hurt any one, unless he be in the direct path of the flash. I have already observed, that it is probable that the operations of the electrical matter are most universal and important in its latent and united state ; and that, whenever by separation it becomes visible, there is then a general stress throughout the greater part of our sys- tem to restore the equilibrium ; and that this stress is greater in proportion to the quantity separated ; that this separation in many instances is spontaneous ; , and that as this fluid is universally disseminated, there is no occasion to consider the appearances of electricity in va- pour, &c. as the means whereby this fluid is conveyed to the clouds. From M. de Luc's observations, it would hence ap- pear, that lightning often arises from the sudden pro- duction of a great quantity of the electrical fluid, that which is then manifested, not being apparent as elec- * Ready p. 61 and 68. OF LIGHTKING. 325 tricity, but just before we perceive its effects. This is further confirmed by his observations when on moun- tains, where he had often opportunities of viewing these phenomena. Thus, in a storm on the Buet, one of the Alps, while the air was perfectly transparent and dry (the last circumstance being determined by the hygrometer), clouds began to form in different parts ; these, when thickened and united, embraced the sum- mit of the Buet, and supported themselves against Mount Blanc, and the summits of the neighbouring mountains. M. de Luc and his companions were over- whelmed with rain ; there was also a vast deal of light- ning, which was often violent, and lasted for a conside- rable time. M. de Saussure has also given instances where the clouds formed a conducting communication with the ground, and yet the lightning continued with- out interruption. From these phenomena, air perfectly transparent and dry, containing neither the vapours of which the cloud is formed, nor the electric fluid, but only the in- gredients proper to give them birth, he infers, that by some unknown cause, clouds of a certain kind are formed spontaneously, and during the progress of their formation, the electricity is produced in great abun- dance exploding every time it is thus formed ; and that before this, the electric fluid no more existed in that state, than the aerial fluids which are disengaged from gunpowder, existed as such before the gunpowder was exploded. I need scarce observe to you, how much Mr. Eeles's theory is confirmed by this account of M. de Luc. You may gain some idea of the prodigious quantity of the electric fluid, that is sometimes manifested, and passing between the clouds and the earth, by an instance or two with which we are furnished by M. de Luc. Thus, a cloud was observed at the top of the mountains of Turin : it was formed of a mass, whose obscurity rendered it terrific, producing in those places, over which it was situate, night at noon day ; this mass was ploughed as it were by lightning, which was soon after followed by a grumbling kind of thunder ; there fell 326 POINTED AND KNOBBED CONDUCTORS. so prodigious a quantity of water and ice from the cloud, that the country was ravaged by the torrents, the hedges were bearen down, and the ditches half filled with hail. Erfurt, a small city in Germany, was struck in one night in forty-two different places ; seven per- sons were killed, three houses were set on fire, but quenched by the rain, which came down in torrents. Now where shall we find, on the vapour theory, known humidity in any strata of transparent air, sufficient to explain the formation of such clouds, and the torrents of rain which were discharged from them ? OF CONDUCTING RODS. We are now prepared to consider the advantages of conducting rods. You know that the electrical fluid is always impelled to those places where an exchange of powers can be most easily made, or where the union of the two powers is least resisted. If then there should happen, in any of the preceding instances, to be a house furnished with a conducting rod, directly between that part of the cloud and that part of the earth, where there is the greatest effort for restoring the equilibrium, the con- ductor will be struck, and will probably prevent the building from receiving any injury. If there be no con- ductor, the lightning will for the foregoing reasons pass at the same place, but the building will probably be damaged, because the materials resist the passage of the electrical powers. OF POINTED AND KNOBBED CONDUCTORS. A great dispute has been carried on among electri- cians concerning the termination of conducting rods, for preserving buildings from lightning ; some warmly contending, that they should be terminated by knobs or balls ; others as strenuously contending, that they should be pointed. Ever since the identity of electricity and lightning has been proved, conductors of some kind have beer POINTED AND KNOBBED CONDUCTORS. 327 generally allowed to be necessary for the safety of build- ings in thunder storms, as they afford a ready passage for the union of the contrary electricities. Electricians seems to have forgotten, that neither lightning nor elec- tricity ever strikes a body, merely for the sake of the body, but because that body is a means of restoring the disturbed equilibrium. When a quantity of electricity is excited by means of an electric machine,, a body communicating with the earth, will receive a strong spark from the prime con- ductor 5 it receives this spark, not because it is capable of containing all the electricity of the cylinder and con- ductor, but because, the natural situation of the fluid be- ing disturbed by the motion of the machine, the natural powers make an effort to restore the equilibrium. No sooner, then, is a conducting body, communicating with the earth, presented to the prime conductor, than the whole effort of the electricity is directed against that body ; not merely because it is a conductor, but because it affords a place, by which the natural powers can more readily unite, and which they would do by other means, though that body were not to be presented. That this is the case, we may easily see, by presenting the same conducting substance in an insulated state to the prime conductor of the machine, when we shall find only a small spark will be produced. In like manner, when lightning strikes a tree, a house, or a conducting rod, it is not because these objects are high, but because they are situate in that place, where, from a variety of causes, the impetus of the two powers can be lessened by uniting with each other. From hence you will perceive the fallacity of that ! kind of reasoning, which is generally employed con- cerning the use of thunder rods. Because a point presented to an electrified body, in our experiments, draws off the electricity in a silent manner, Dr. Franklin and his followers have conclud- ed, that a pointed conductor will do the same thing to a thunder cloud, and thus prevent any kind of danger from a stroke of lightning* 328 POINTED AND KNOBBED CONDUCTORS. , But, for this very reason, Mr. Wilson and his part have determined, that the use of pointed conductors is utterly unsafe ; they justly consider the Franklinian idea of exhausting the clouds of their electricity, to be not less absurd, than it would be to clear away an inundation with a shovel, or exhaust the atmosphere with an air- pump. They bring many instances, where a point will receive a full stroke, and assert that it solicits a discharge ; and that, being often unable to conduct the whole elec- tricity of the atmosphere, it is impossible for us to know whether the discharge it solicits may not be too great for the conductor to bear; and, consequently, all the mis- chiefs arising from thunder storms may be expected, with this mortifying circumstance, that this very con- ductor hath probably solicited the fatal stroke. I must also further observe to you, that the Franklini- ans, granting them all they ask, still make their pointed conductors of too much consequence ; for it is now well known, that points have no influence at all, unless they be immerged in the electrified atmosphere. If a point- ed body do not communicate with the earth, but the communication be interrupted by a short interval, it will receive a full spark. It will also receive a full spark, if it be suddenly brought sufficiently near a strongly electri- fied body : this case applies strongly against pointed con- ducting rods for shipping. It will also receive a full spark at a considerable distance, if surrounded with non- conducting substances. The circumstances on which an explosion depends are too many to be here enumerat- ed ; in general it may be said that, with respect to a point, it will depend on the suddenness of the discharge, on the proximity of the cloud, on the velocity in its mo- tion, on the quantity of electricity contained in it, and on the contrary electricity opposed to it. If a small cloud hang suspended under a large cloud loaded with electric matter, pointed conductors on a building under- neath will receive the discharge by explosion, in prefer- ence to those terminated by balls ; the small cloud will form an interruption, which allows only an instant of time for the discharge. If a single electric cK u J be dri- ven with considerable velocity near to a pointed con- rOINTED AND &NOBBED CONDUCTORS. 329 ductor, the charge may be caused to explode upon it by the motion of the charged body. A pointed conductor has not even the power of at- tracting the lightning a few feet out of the direction it would choose itself: of this we have a most decisive instance in what happened to the magazine at Purfleet, in Essex. That house was furnished with a conductor, raised above the highest part of the building ; never- theless, a flash of lightning struck an iron cramp in the corner of the wall of the building, considerably lower than the top of the conductor, and only forty-six feet in a sloping line distant from the point. The conductor, with all its power of drawing off the electric matter, was neither able to prevent the flash, nor to turn it forty-six feet out of its way. The matter of fact is, the lightning was determined to enter the earth at the place where the board-house stands, or near it ; the conductor fixed on the house offered the easiest com- munication, but forty-six feet of air intervening between the point of the conductor and the place of the explosion, the resistance was less through the blunt cramp of iron and a few bricks moistened with the rain to the side of the metalline conductor, than through the forty-six feet of air to its point ; for the former was the way in which the lightning actually passed. An objection to the use of conductors of either kind may be also drawn from the accident which hap- pened to the poor-house at Heckingham, Norfolk, which was struck by lightning, though furnished with eight pointed conductors, and which, I am well assured from good authority, were uninterrupted, continuous, and at the time of the stroke perfectly connected with the com- mon stock. Hence it is evident, that the effect of con- ductors, in general is too inconsiderable either to lessen fear or animate hope. The thunder house , plate 2, fig. 3, as it is usually call- ed, is the apparatus principally used to illustrate the Franklinian method of preserving houses from damage by lightning. It consists of a mahogany board, shaped like the gable end of a house. It is fixed upright on a VOL. iv. u 330 POINTED AND KNOBBED CONDUCTORS. horizontal board as a stand ; a square hole is made in the gable board, into which is fitted, so as to go in and out easily, a square piece of wood ; a wire is fixed in the one diagonal of this board, and wires are also fixed in the gable board, one from the upper part, the lower end of which comes to one corner of the square hole; the upper end of the other wire coincides with the oppo- site corner, and goes down to the bottom of the gable board. The upper wire has a brass ball on the top ; this may be occasionally taken off, which leaves a point ex- posed ; at the bottom of the lower wire there is a hook : connect the hook at the bottom with the outer coat- ing of a jar, place the square piece in the hole, so that the metallic wire shall not coincide with the other two ; when the jar is charged, bring the discharging rod from the knob thereof to the ball of the house ; an ex- plosion will ensue, and the square piece be driven out to a good distance from the gable board. Put the square piece into the hole in such a manner, that the ends of the diagonal may not coincide with the ends of the wire of the gable board, then make the dis- charge as before, and the metallic circuit being now com- plete, the square board will remain jn its place.* Take off che ball, and the point will prevent an explo- sion, and its accumulating therein in any sufficient quan- tity to do any damage. The prime conductor is supposed to represent a thun- der cloud discharging its contents on some metal pro- jection on the top of a building ; and -this is considered as receiving no damage when the conductor is perfect; but when the connexion is imperfect, the fluid in pass- ing from one part to the other, damages the building. * If two square pieces be made and placed in die two different direc- tions, the property is then shown atone discharge of the jar. In respect to the best form for conductors; from a great variety of ex periments that 1 have made, and also been a witness to, pointed conduc- tors have all the properties of blunt ones under all circumstances, and, moreover, some advantageous properties peculiar to their pointed termina- tions. Upon the whole, I recommend, and give the preference to con tors with very fine and pure metallic points....E. Edit, I 331 ] LECTURE XLIX. ON THE NATURE OF ELECTRICITY; OF ANIMAL ELECTRICITY, &C. AFTER pointing out to you the principal phenome- na of electricity, and exhibitijig to you many of the most interesting and entertaining experiments in this branch of natural philosophy ; I shall now endeavour to trace out its connexion with the great agents in the operations of nature, and thus lead you to form some idea of what electricity is, and of its use in the great system of things. Whatever it may be, it is certain, and that without any exaggeration, that whether you look to the heaven's above, or the earth beneath, you can scarce perceive any thing that is not acted upon, and in a manner perfectly subject- ed to the operations of this wonderful fluid. That electricity is real matter, and not a mere proper- ty, is evident from a variety of circumstances. When it passes between bodies, it divides the air, and puts it into those undulations which give us the idea of sound. It emits the rays of light in every direction, and those rays are variously refrangible and colorific, as other light is: and, if light is acknowledged to be matter, it is contra- ry to reason and experience to suppose that the thing which emits it should not likewise be material ; neither are the other senses unaffected at its presence : its smell is strongly posphoreal or sulphureous. The sense of feeling is a witness of its presence, not only from the sparks which, when received from the conductor of a powerful machine, are pungent, and will pass through two or three persons standing on the ground, but also 332 ON THE NATURE OF ELECTRICITY. from the shock. A stream of electric matter has also evidently a subacid taste. In contemplating the system of nature, you perceive three kinds of fluids of extreme subtilty, and very much resembling one another : these three are, fire, light, and electricity. Their resemblance is so great, that it is not surprizing to find it the general conception of all unin- formed minds, that they are ultimately the same ; on ex- amining the evidence of their identity, you will find it exceedingly strong. If it is true, that natural effects are not to be ascribed to many different means or agents, where one will suf- fice, these three should be considered as different modi- fications or states of the same fluid. Light or solar fire will burn in fuel, and act in solid matter with greater effect than the most violent fire of a furnace. Common fire, like that of the sun, will promote vegetation and ripen fruits. The electric fire will light a candle and fire gunpowder, like the common fire -, will afford a spectrum of the seven primordial colours, in common with light j and will throw metals into fusion with a violent scorching heat.* Let us leave generals, and descend more into particulars. These three fluids all agree in one property, that of exciting heat in certain circumstances, and not doing so in others. Fire, in the common acceptation of the word, always excites heat ; but in its latent state, it lays aside this pro- perty, and in vapour, for instance, is cold to the touch. Light, when collected into a focus by a burning glass, /. e. when its rays converge to a centre, and diverge, or attempt to diverge from one, produces heat. Electricity, when its force is concentrated and converg- ed, produces heat, as I shall soon show you by its effect on a thermometer. This does away the objection for- merly made to those who asserted, that electricity was that elementary fire which pervaded all substances : the objection was, that though the electric matter emitted * J^ieSs Ph}siol;gic al Disquisitions. ON THE NATURE OF ELECTRICITY. 333 light, and had the appearance of fire, it wanted ics essen- tial characteristic of burning ; and where great quanti- ties of the fluid were forced through substances, they in- sinuated, that it might be occasioned by the internal com- motion excited among their small particles. There is no occasion to dwell upon the weakness and fallacy of the objection, as it is completely removed by many facts. 1 . By the effect of electricity upon the ther- mometer. 2. By the experiment that was made at the Pantheon by Mr. Wilson, with the immense apparatus that was constructed for making experiments on the preferable utility of pointed or knobbed conductors, for preserving buildings from lightning. The electric aura from this machine fired gunpowder in the most unfa- vourable circumstances that can be imagined, namely, when it was drawn off by a sharp point, in which case it has generally the least force. Upon a staff of baked wood a stem of brass was fixed, which terminated at the top in a wooden point; this point was put into the end of a small tube of Indian paper, made somewhat in form of a car- tridge, about an inch and a quarter long, and T ths of an inch in diameter. When the cartridge was filled with common gunpowder unbruised, a wire communicating with the earth was fastened to the bottom of the brass stem. The charge in the large conductor being kept up by the motion of the cylinder, the top of the cartridge was brought near to the conductor, so as even frequently to touch the tin-foil with which it was covered. In this situation, a small faint luminous stream was frequently observed between the top of the cartridge and the metal. Sometimes this stream would set fire to the gunpowder the moment it was applied ; at others, it would require half a minute or more, before it would take effect. The difference in time was supposed to arise from some mois- ture in the powder. Tinder was fired much more rea- dily. It now appears clearly, that the electric fluid moving through bodies, either in small quantities, or with rapidi- ty, or in very great quantities, will produce heat, and set them on fire : it seems therefore scarce disputable, that this fluid is the same with the element of fire. 334 ON THE NATURE OF ELECTRICITY. These are far from being the only instances of their identity ; for fire is brought into action by friction, as well as electricity. Fire dilates all bodies : the electric fluid has also a dilating power, which is evident from its action on a thermometer, though, in general, the force with which bodies cohere together is greater than the dilating power of electricity. Fire promotes and accelerates vegetation as well as germination. Electricity does the same. Electricity, as well as fire, accelerates evaporation. The experiments made by Mr. Achard on the eggs of a hen, and by others on the eggs of moths, prove that electricity, as well as heat, favours the developement of those animals. The electric fluid, in common with fire, will throw metals into fusion. If substances with equal degrees of heat touch each other, the heat is diffused uniformly between them. In the same manner, if two bodies with unequal degrees, or different kinds of electricity, touch each other,- an equili- brium will be established. If bodies of different kinds, and of equal degrees of heat, are placed in a medium of a different temperature, they will all acquire, at the end of a certain time, the same degree of heat. There is a considerable difference, however, in the space of time in which they acquire the temperature of the medium : ex. gr. metals take less time than glass to acquire or lose an equal degree of heat. On an attentive examination of the bodies which re- ceive and lose their heat soonest, when they are placed in mediums of different temperatures, they will be found to be the same which receive and lose the electric signs soonest. Metals, which become warm or grow cool the quickest, are the substances in which the electric powers unite most readily. Wood, which requires more time to be heated or cooled, receives and loses electricity slower than metals. Lastly, glass and resinous substances, which receive and lose slowly the electric fluid, acquire with difficulty the temperature of the medium which surrounds them. If one extremity of an iron rod be heated red-hot, the other extremity, though the bar be several feet long, will ON THE NATURE OF ELECTRICITY. 535 become so warm in a little time, that the hand cannot hold it, because the iron conducts fire readily ; but a tube of glass, only a few inches long, may be held in the hand, even while the other end is melting. The electric fluid, in the same manner, passes with great velocity from one end of a rod of iron to the other ; but it is a considera- ble time before a tube of glass, at one end of which an excited electric is held, will give electric signs at the other. These observations prove, that several bodies that re- ceive and lose with difficulty their actual degree of heat y receive and lose also with difficulty their electricity. The electric powers may be put in action by heat and cold. Mr. Canton procured some thin glass balls, of about an inch and a half in diameter, with stems or tubes about eight or nine inches in length, and electrified them, some vitreously on the inside, others resinously, and then seal- ed them hermetically ; soon after he applied the naked balls to his electrometer, and could not observe the least sign of their being electrical ; but holding them at the fire, at the distance of five or six inches, they became strongly electrical in a short time, and more so when they were cooling. These balls would, every time they were heated, give the electric power to, or take it from other bodies, according to the vitreous or resinous state of it within them. Heating them frequently diminished their power, but keeping one of them under water a week did not in the least impair it. The balls retained their virtue above six years. The tourmalin, and many other precious stones, are also known to acquire electricity by heat. The tourmalin has always at the same time a vitreous and resinous elec- tricity ; one side of it being in one state, the other in the opposite. Sometimes one side will at the same time pos- sess both electricities. These powers may be excited by friction and by heat ; nay, even by plunging it in boiling water. Many instances prove, that electricity is produced by liquifaction. Thus, where chocolate is manufactured in large quantities, a vivid light is frequently seen flashing upon its surface after melting, and it will also attract light 336 ACTION OF ELECTRICITY. substances, separate pith balls, &rc. When it had lost this property, Mr. Henly found it might be restored by melting it together with a small quantity of olive-oil. If sulphur be melted in a glass vessel, and taken out when cool, both it and the glass will be found strongly electri- fied. I have already shown you, that electricity is produced by the evaporation of water ; I shall now relate Mr. Read's* mode of performing this experiment. He insu- lates a large hollow tin cone, containing about four sheets of tin plates, with many yards of small wires coiled up within it ; one end of the wire is extended from the cone to a very sensible electrometer. The cone and wire col- lect and condense the ascending electrified vapour, as it quits the insulated vessel containing the fluid. The elec- trometer connected with the cone is vitreously electrified ; that connected with the vessel from whence the vapour arose, is in a resinous state. Mr. Read has also, by burning different substances in insulated vessels under his tin cone, shown that bodies, in passing from a solid to a fluid state, produce the two electricities ; the quantity observed is in general very small, on account of the intimate affinity between flame and electricity. ACTION OF ELECTRICITY ON A THERMOMETER.' Insulate a sensible mercurial thermometer, and place the bulb between two balls of wood, one affixed to the conductor, the other communicating with the ground ; and the electric fluid, in passing between the two balls, will raise the mercury in the thermometer considerably. With a cylinder of about seven inches and a half in dia- merer, the fluid passing from a ball of lignum vitse to a ball of beech, and thence to the ground, elevated the quicksilver in the thermometer from 68° to 1 10°, repeat- edly to 105°. The thermometer was raised from 68° to 85°, by the fluid passing from a point of box to a point of Read's Summary Mew of Spontanecus Electricity, &c. ON A THERMOMETER. §37 lignum vitae; from 67° to 100° from a point of box to a ball of box ; from 66° to 100° from a ball of box to a brass point; from 69° to 100° from ball to ball; the bulb of the thermometer being covered with flannel. < c If then these fluids, fire, light, and electricity, which thus mutually and in all respects assume each others pro- perties, be not the same ; experiment is a thing not to be depended upon, and the most obvious rules of philo- sophizing, adopted and approved by all parties, are no better than specious deceptions. " More, indeed, need not be said to any observer of na- ture ; but it is necessary to accumulate proof, in order to lessen the prejudices of modern philosophers, who have altogether neglected to study and trace the great agents of nature. For these, it may be necessary to point out other links, in which they may see the connexion between fire, light, and electricity. Thus, as heat is diminished, or bodies are cooled, elec- tricity succeeds in its place. All electric bodies by heat are rendered conductors, and can no longer be excited ; but, as soon as the heat is removed, their electric pro- perty returns. Water is a conducting substance ; by being frozen, its conducting powers are lessened ; when cooled down to twenty degrees below of Fahrenheit's scale, it be- comes an electric, and will emit sparks by friction, like glass. The atmosphere is a natural electric ; but, by a certain degree of heat, it loses in a degree this property, and becomes a conductor, nor is there any doubt that its electric properties are increased, in proportion to the de- gree of cold imparted to it. Mr. JEpinus mentions some facts in a letter to Dr. Guthrie^ which will illustrate this subject ; they relate to phenomena that are known to take place in Russia, when a great cold has continued for several weeks. Mr. JEpi- nus was sent for,, to see an uncommon phenomenon. On going into the apartment of Prince Or/off, he found him at his toilet, and that at every time his valet drew the comb through his hair, a pretty strong crackling noise was heard ; and on darkening the room, the sparks were seen following the comb in great abundance, while the VOL. IV. 2 X 338 ACTION OF ELECTRICITY Prince was so completely electrified, that strong sparks could be drawn from his hands and face ; nay, he was even electrified, when he was only powdered with a puff. A few days after, Mr. JEpinus was witness to a more striking effect of the electric state of a Russian atmos- phere. The Great Duke of Russia sent for him one evening in the twilight, and told him, that having briskly drawn a flannel cover off a green damask chair in his bed-chamber, he was astonished at the appearance of a strong bright flame that followed ; but, considering it as an electrical appearance, he had tried to produce a simi- lar illumination on different pieces of furniture, and could then show him a beautiful and surprizing experiment. His Highness threw himself on his bed, which was co- vered with a damask quilt, laced with gold, and rubbing it with his hands in all directions, the young Prince, who had then reached his twelfth year, appeared to be swim- ming in fire, as at every stroke flames arose all around him, darted to the gold-lace border, ran along it, and up to that of the bed, and even to the very top. While his Highness was showing this experiment, Prince Orloff came into the room with a sable muff in his hand, and showed us, that by only whirling it five or six times round his head in the air, he could electrify himself so strongly, as to send out sparks from all the uncovered parts of his body. The inlaid floors had be- come so dry, as to form a complete insulation. In the winter time, therefore, we must consider the frozen surface of the earth, the water, and atmosphere, as forming one electrical machine of enormous magni- tude ; for, the natural cold of those countries is often so great as to cool water to more than 20 degrees be- low 0, and thus render it an electric. That something of this kind is real, appears from the excessive bright au- rora borealis, and other electric appearances, far exceed- ing any thing in this country. In the summer time these appearances are not remarkable, but an excessive heat prevails from the long continuance of the sun above the horizon. The quantity of heat in summer being succeed- ed by a proportionable quantity of electricity in winter, one can scarce avoid concluding, that the heat in sum- ON A THERMOMETER. 339 mer, or disengaged fire, becomes electric fluid in winter, wMch going off through the celestial expanse, returns a i ro the grand source of light and heat ; thus mak- iu room for the succeeding quantities which are to en- li : the earth during the following season. If the identity of light, fire, and electricity, be admit- ted, the source from whence the electric fluid is derived into the earth and atmosphere is very evident ; it can be no other than the sun or source of light. The vast quantity of light continually proceeding from the sun to the earth, must in a great measure be absorded there- by ; but, from the other operations in nature, it is pre- vented from remaining there: it is therefore in continu- al circulation, to make room for new quantities continu- ally coming from the sun. It must however be observed, that as this fluid is variously combined, it cannot appear in its natural form of fire or light, till it be disengaged, and capable of receiving a motion similar to what it had when proceeding from the sun. This change of matter into a different form, with the subsequent regeneration of it into its primative form, is, says Mr. Jones, one of the greatest secrets of nature, whereby the world is kept from decaying, either with respect to its matter or its motion. By means of a cir- culation in matter, the lasting motions of nature are maintained, and its stores unexhausted.* The experiments that I shall now lay before you, do in the strongest manner prove the identity of the electric fluid and light, and that both are transmitted through electric as well as other substances ; and that it is on the motion of this fluid that transparency de- pends ; that, when this medium is at rest, the body is opake j when set in motion, it becomes transparent. LUMINOUS EXPERIMENTS. To render an ivory ball luminious. Take a strong spark through the centre of the ball, and it will be il- luminated throughout. * See Jonse's Physiological Disquisitions, p, 51. J40 LUMINOUS EXPERIMENTS. ™k a i To obtain a crimson coloured spark. Take a spark through a ball of box wood, and it will appear of a beautiful crimson, or rather a fine scarlet colour ; or the shock may be passed through pieces of wood o different thicknesses and density ; which will afford very ample field for observation and experiment. To make a bottle of water luminous* Connect one end of a chain with the outside of a charged jar, let the other end lie on the table, place the end of another piece o chain at about one quarter of an inch distance from th former ; then set a decanter of water on these separat ed ends, and, on making the discharge through th chain, the water will appear perfectly and beautifull luminous. There is scarce any substance, fluid or solid, bu what may be rendered luminous, by passing the elec trie fluid through it, and thereby separating the elec trie powers inherent in the body. In water, spirit, oil animal fluids of all kinds, the discharge of a Leyde phial of almost any size will appear very splendid provided you take care to place them in the circuit, that the fluid may not pass through too great a quant ty of them. To perform this, place the fluid, on which the exp riment is to be made, in a tube three quarters of inch in diameter and four inches long ; stop up th orifices of the tube with two corks, through whic push two pointed wires, so that the points may approac within one-eighth of an inch of each other; the fluid, in passing through the interval which separates the wires is always luminous, if a force be used sufficiently strong ; the glass tube, if not very thick, always breaks when this experiment succeeds.* To make the passage of the fluid luminous in the acids, they must be placed in capillary tubes, and two wires introduced, as in the preceding experiment, whose points shall be very near each other. It is a well-known fact, that the discharge of a small Leyden phial, in pass- See Mr. Morgan's paper, Phil. Trans. LUMINOUS EXPERIMENTS. S41 ig over a strip of gold, silver, or Dutch metal leaf, ill appear very luminous. By conveying the contents f a jar, measuring two gallons, over a strip of gold- :af one-eighth of an inch in diameter and a yard long, will frequently give the whole a dazzling brightness, ou may give this experiment a curious diversity, by tying the gold or silver leaf on a piece of glass, and len placing the glass in water; for, the whole gold leaf ill appear most brilliantly luminous in the water, by xposing it thus circumstanced to the explosion of a attery. The difficulty of making any quantity of the electri- al fluid luminous in any body, increases as the con- ucting power of that body increases ; because the two owers unite sooner in proportion to the conducting ower, and consequenrly all electric signs vanish. In order to make the contents of a jar luminous in •oiling water, a much higher charge is necessary, than rould be sufficient to make it luminous in cold water, ?hich is universally allowed to be the worse conductor. There are various reasons for believing the acids to >e very good conductors ; if, therefore, into a tube illed with water and circumstanced as has been already lescribed, a few drops of either of the mineral acids )e poured, it will be almost impossible to make the luid luminous in its passage through the tube, as the wo powers unite immediately. The ease with which the electrical fluid is rendered uminous in any particular body, is increased by increas- ng the rarity of the body. The appearance of a spark, )r of the discharge of a Leyden phial, in rarefied air s well known. But we need not rest the truth of the ^receding observation on the several varieties of this act ; similar phenomena attend the rarefaction of ether, rf spirits of wine, and of water. Spark in rarefied water, spirit of wine, ether, and uids. Into the orifice of a tube forty-eight inches long md two-thirds of an inch in diameter, Mr. Morgan zemented an iron ball, so as to bear the weight which presses upon it when the tube is filled with quicksilver, leaving only an interval at the open end, which con- 342 LUMINOUS EXPERIMENTS, tains a few drops of water. Having inverted the tube, and plunged the open end of it into a bason of mercu- ry, the mercury in the tube stood nearly half an inch lower than it did in a barometer at the same instant, ow- ing to the vapour which was formed by the water. But through this rarefied water, the electrical spark passed as luminously as it. does through air equally rarefied. If, instead of water, a few drops of spirits of wine be placed on the surface of the mercury, phenomena, similar to those of the preceding experiment, will be dis- covered, with this difference only, that as the vapour in this case is more dense, the electrical spark, in its pas- sage through it, is not quite so luminous as it is in the vapour of water. Good ether substituted in the room of the spirits of wine, will press the mercury down so low as the height of 16 or 17 inches. The electrical fluid, in passing through this vapour, unless the force be very great indeed, is scarcely luminous ; but if the pressure on the surface of the mercury in the bason, be gradually lessened by the aid of an air-pump, the vapour will become more and more rare, and the electric spark, in passing through it, more and more luminous. The brilliancy and splendour of the electric spark is always increased when it is compressed into a smaller compass. That is, a spark, or the discharge of a bat- tery, which we might suppose equal to a sphere one quarter of an inch in diameter, will appear much more brilliant, if the same quantity of fluid be compressed into a sphere one-eighth of an inch in diameter. This obser- vation is the obvious consequence of many known facts; if the machine be large enough to afford a spark, whose length is nine or ten inches, this spark may be seen some- times forming itself into a brush, in which state it oc- cupies more room, but appears very faintly luminous ; at other times, the same spark is seen dividing itself into a variety of ramifications which shoot into the surrounding air. In this case, likewise, the fluid is diffused over a large surface, and in proportion to the diffusion, so is the faintness of the appearance. A spark, which in the open air cannot exceed one-quarter of an inch in diameter, LUMINOUS EXPERIMENTS. 343 will appear to fill the whole of an exhausted receiver, four inches wide and eight inches long : but in the for- mer case it is brilliant, and in the latter it grows fainter and fainter, as the size of the receiver increases. This observation is further illustrated by the following ex- periments. Introduce two pointed wires into the vacuum, so that the fluid may easily pass from the point of the one to the point of the other ; when the distance between them is not more than the one-tenth of an inch, in this case we shall find a brilliancy as great as in the open air. Into a Torricellian vacuum, thirty-six inches long, con- vey as much air as will fill two inches only of the ex- hausted tube if it were inverted in water ; this quanti- ty of air will afford resistance enough to condense the fluid, as it passes through the tube in a spark thirty- eight inches in length. The brilliancy of the spark in condensed air, in water, and in all substances through which it passes with difficulty, depends on principles si- milar to those which account for the preceding facts. In the appearances of electricity, as well as in those of burning bodies, there are cases in which all the rays of light do not escape; and the most refrangible rays are those which escape first or most easily. The electrical brush is always of a purple or bluish hue. If you con- vey a spark through a Torricellian vacuum, made with- out boiling the mercury in the tube, the brush will dis- play the indigo rays. To an insulated metallic ball, four inches in diameter, fix a wire a foot and a half long ; this wire should ter- minate in four ramifications, each of which must be fix- sd to a metallic ball half an inch in diameter, and placed at an equal distance from a metallic plate, which must communicate by metallic conductors with the ground. A powerful spark, after falling on the large ball at :>ne extremity of the wire, will be divided in its passage from the four small balls to the metallic plate. When you examine the division of the fluid in a darkened room, you will discover some little ramifications, which will yield the indigo rays only : indeed, at the edges >f all weak sparks, the same purple appearance may be 844 LUMINOUS EXPERIMENTS. discovered. You may likewise observe, that the near 6r you approach the centre of the spark, the greater h the brilliancy of its colour. The influence of different media on electrical light i analogous to their influence on solar light, and will heir us to account for some very singular appearances. Let a pointed wire, having a metallic ball fixed tc one of its extremities, be forced obliquely into ; piece of wood, so as to make a small angle with tfec surface of the wood, and to make the point lie abou one-eighth of an inch below the surface. Let anothei pointed wire, which communicates with the ground, bi forced in the same manner into the same wood, so tha its point likewise may lie about one-eighth of an inct below the surface, and about two inches distant fron the point of the first wire. Let the wood be insulated and a strong spark, which strikes on the metallic ball will force its passage through the interval of wood whicl lies between the points, and appear as red as blood. Tc prove that this appearance depends on the wood's absorp tion of ail the rays but the red ; when these points wen the deepest below the surface, the red only came tc the eye through a prism ; when they were raised a little nearer the surface, the red and orange appeared ; whei nearer still, the yellow ; and so on, till, by making th< spark pass through the wood very near its surface, ai" the rays were at length able to reach the eye. " Previous to the discoveries that have been made ii modern times, relative to the chemical effects of light somcmathematical philosophers disputed its existeno as a particular fluid, and even that of fire itself; the; crudely imagined, that the phenomena of light aru heat were only particular modifications of the sub stances, in which they appeared ; a kind of vibratioi of their particles, transmitted by means of a medium as in the case of sounds." , " They applied the mathematics to this hypothesis in order to explain some particular phenomena ; and a every thing that appears to be deduced from mathema tical theorems easily seduces those who do not apph themselves to examine data, this theory, which effect u LUMINOUS EXPERIMENTS. 345 ally barred the road to the most important physical re- searches, met with many partizans : but chemistry and meteorology have now come in to terminate the contro- versy ; and there are at present very few philosophers who do not agree, that lucidity and heat are the effects of two fluids, namely, light and fire, which produce those particular phenomena whenever they are at liberty ; but which, at the same time, may be so combined with other substances as to lie hidden in them without vproducing these effects, till again set at liberty. By an attention to these great agents, the stud of nature has proceeded with rapidity, and the present asra will probably on this account be as much celebrated in the history of science, as those in which Pascal demonstrated the pressure of the air on bodies, and in which Newton discovered the principle of gravity. " Our progress in the knowledge of the origin of bodies, has been much advanced in this age, since che- mists and philosophers have begun to examine their vo- latile products, in other words, the elastic fluids ; but this would have been doing but little, had not the ad- vances in other branches of natural knowledge led them to discover, that the phenomenon of heat proceeded from a particular substance susceptible of chemical affinities, namely, fire, the immediate cause of heat. Here is then a substance of the highest importance in the composi- tion of bodies, which nevertheless escaped the attention of philosophers, while they only estimated and express- ed the amount of their products by their weights. Is it possible for any one to suppose, that we have hereby discovered all the imponderable substances that enter into the composition of natural bodies ?" Ought we to neglect the phenomena of lucidity, while every thing announces to us, that light is a chemical substance ? This neglect is scarcely now to be appre- hended, as philosophers are aware, that great chemical effects may be produced by imponderable substances. The phosphoric phenomena of certain mineral substan- ces indicate clearly, that light enters as an ingredient into their composition. Wilson and Beccaria have shown, vol. iv, 2 Y MS LUMINOUS EXPERIMENTS. that every substance in nature is more or less phospho- rical ; and you have just seen, that there is scarce any substance but what you may render luminous by sepa- rating its electric powers. The relation* of these two imponderable substances, whose existence is now established beyond a doubt, is such as in many other instances is found to subsist be- tween such substances as enter into the composition the one of the other. Light frequently does not sensibly act otherwise than as the cause of lucidity, or of luminous phenomena ; and fire in the same manner, only as the ciuse of heat : but at other times fire, in producing heat, produces also in the end its luminous effects ; and in some circumstances light, in making visible the objects, by its reflexion contributes to produce heat. These phenomena clearly indicate, that one of these substances contains the other, but that under certain circumstances it may be so decomposed, as to permit either of them to exercise its own peculiar properties. The'most excellent Boerhaave, in bis analysis of fire, has so clearly established the universality and importance of this element, and so stripped it of the mystic dress, in which it was enveloped before his time, that one would imagine it scarce possible for philosophers to have resolv- ed so many of its subtile effects into occult or fanciful properties ; yet, that such has been the case is evident from the slightest inspection of modern theories. Again, though the most obvious phenomena in nature, and nu- merous experiments, tend to ascertain beyond all doubt, that the matter of common light or fire pervades all na- ture, and fills all things ; yet, as I have before observed, the whole has been overlooked as an accidental filtration that implied no consequences, nor interfered with the various unintelligible properties of bodies, notwithstand- ing its access to their innermost penetralia. It is evident, that the natural omnipotence of light de- pends on the sun ; by him in a natural sense the matter of fire, as his issue, is omnipresent and all-sufficient. If the life of all things depends on the activity he commu- nicates to them, is it not probable, that it is the influence of the solar fluid that generates and maintains that life in LUMINOUS EXPERIMENTS. 347 all its specific characters, in every being according to its kind ? And that life, whether it be vegetable or animal, is such as it is according to the state of the fire in it ; and that every dead £ hing is only so, because its fire is quench- ed ? The ancient philosophers affirmed, that the light of the sun, which gave life and motion to all things, must be in all things ; they therefore conceived all things to be replete with this fluid. Is it not highly probable then,* that this terraqueous globe is only an accumulation of materials introduced in the boundless ocean of the solar fluid as a theatre, on which, under the direction and guidance of the Almighty, it may display its inexhaustible energy and powers ; the terres- trial mass being so disposed and arranged by its Divine Author, as to become a seminal bed of materials, where light and fire may pierce, animate, and display an endless variety and succession of beings ? This fluid extricates all the forms, and generates all the powers of nature, out of the* materials provided for it to possess. It is impossible to form any clear or distinct idea of the agency of the solar fluid in the air, in animals, in vege- tables, &c. without first considering it more in general ; nor can you properly have a view of the universal agency of the element productive of fire, light, and electricity, and its importance to the animal frame, unless you take an enlarged prospect of its action. Besides, knowledge often makes more rapid advances by reasoning upon known facts than by discovering new ones, which, by their novelty, too often lead to hasty undigested theories. In the disquisition upon these fluids, I have always an eye upon the doctrine of electricity; and the preceding as well as following experiments all concur in showing the analogy that runs through nature ; and you will find that electricity, though not in name, has been the doc- trine of all ages. I shall therefore continue to treat of these wonderful fluids. Of all that are known in the uni- verse; the mobility of the matter of light is the greatest. There is not the smallest speck of colour in the beams of See vol. ii. Lecture xxi. 348 LUMINOUS EXPERIMENTS. the sun, that does not obediently receive perpetual im- pressions from him in all lineal directions, by night as well as by day. The sun, as the fountain of motion, is also continually agitating this fluid either radially or ob- liquely, by the lateral shocks and friction of the radii upon those parts of the fluid that lie out of the line of the sun's irradiation ; these, together with the constant vicissitudes of day and night, preserve a constant motion in ail its internal parts. But even this is inadequate to convey to you a just idea of the constant, positive, intense energy, from the activity of the matter of light. Of this you will form a better idea, by examining the mode of its action in the interior parts of the most rigid and solid bodies. For in the most secret recesses of the most solid and passive substances, the matter of light is so far from existing in an indolent quiescent state, that it is impossible to form an adequate idea of its incessant and active energy under these circumstances. Yet this state of bodies is but little thought of by philosophers in their researches into its properties, either common or special ; which I shall illus- trate by considering the cases of sonorous bodies, and the phenomenon of hammering cold iron red-hot. If this fluid resided within bodies in an indolent and passive state, it could exert no reluctation on any mecha- nical force, disturbing its passive occupation within bo- dies ; whereas, in fact, its natural state is never disturb- ed without an active irritation being excited in the fluid, to recover and repossess its organical and interstitial in- herency, greater than that by which it was expelled ; it returns with a force not barely sufficient to recover the dimensions it occupied within bodies, but with a violence capable of expanding them as much beyond their natu- ral size as the external blow or concussion tended to compress them within it : hence a vibratory colluctation takes place between that action which preserves bodies in their natural crasis, and the' 'rapid returns of the fluid to its natural state; these vibrations continue for a time, and die away imperceptibly. This intense agitation, excited by the collision of bo- dies, is not confined to their points of contact, but per- LUMINOUS EXPERIMENTS. 349 vades their whole substance, and oscillates in every part. This is demonstrated to the eye and ear, when a musical chord is struck. You have specimens also of it in all elastic sonorous bodies. When a bell is struck, the sound conti- nues labouring in the ear for a considerable time after- wards ; nor is the tumult subsided when our sense of it fails ; it passes through a gradual decay below the stand- ard of sense. Suspend an iron poker from the head, by the teeth, and the iron discovers no great degree of any sonorous quality ; yet if it be struck, you will have a very striking sensation of the vibratory motion its whole substance re- ceives from the stroke, by the teeth's transmisson of their feeling to the ear. Physicians talk of the irritability of our nervous system, as a very mysterious and wonderful phenomenon ; but there are more striking examples of this irritability in the most rigid dead substances. Substances, such as glass, bell-metal, &c. which are so rigid that few instruments will make an impression on them, yet are capable of be- ing agitated through every atom of their substance ; nay, in some cases, to be burst in pieces by the impression of certain sounds. A wine-glass will burst in pieces by the action excited through its substance by certain tones of voice ; columns of marble or porphyry are tremulous to thunder explosions, and to certain tones of an organ. This excessive mobility of parts throughout the whole substance of the most rigid bodies, clearly implies a great turgency of their substance with some very active fluid, so that a small increase of its action is ready to burst them in pieces. A slight resistance to the internal agitation of a bell will cause it to crack. Now it is impossible to conceive, that such a tremulous motion should be produced through the whole continuity of such hard bodies, unless they contained in themselves some inconceivably active element, exerting a constant nisus to force their parts to as great a distance from each other as possible, and barely counteracted by the power that maintains their cohesion. ■ The symptoms of this restless activity within solid bo- dies are not confined to such as are commonly called elas- 350 LUMINOUS EXPERIMENTS. tic. Thus iron yields more striking proofs of this latent active principle, than any substance of greater elasticity than itself, and thus discloses to our sensible conviction precisely what that principle or restless element is, that exerts its energy so powerfully within all terrestrial bo- dies. For the power within bodies, that sustains and preserves their form, is not a passive power. It is positive re-action to the approach of the parts of the body. The law of re- action, being equal to action, resides ultimately in the con- stitution of this powerful fluid medium. Whenever the spaces it occupies within the surfaces of bodies are press- ed nearer one another by any sudden shock or collision, and consequently this medium, for an instant, driven out, the next instant it returns with violence, not enough to re- gain its place in the body, but equal to that with which it was ejected ; and therefore, in returning, it dilates its spa- ces as much beyond their sizes, as they were compressed below their natural standard by their collision ; by which means, a temporary oscillation is excited between the ef- forts of that power, which circumscribes bodies, and binds them to their natural sizes, and the internal medium, which was irritated by the stroke, to act with a force equal thereto. If the strokes, which dispossess this fluid of the spaces it naturally obtains within bodies, be quickly and succes- sively renewed, before the coliuctations raised by former ones have subsided, the internal agitation may thereby soon be raised to such a height, as to break forth and manifest itself in the form of actual fire. Every material being through all the forms of nature, is a composition of this celestial fluid and terrestrial mat- ter ; you will find the distribution of material substances into these two classes to be the real key to all natural knowledge ; it not only distinguishes this globe from the celestial fluid in which it swims, but it is to be applied to every individual terrestrial substance; which must be con- sidered, if you would comprehend the phenomena of na- ture, as an intimate composition of these two elements ; the latter being the organ or case to the energy of the former, and the modifier to its incessant activities, while the former is the medium used by mind to impress those LUMINOUS EXPERIMENTS. 351 characters on the latter, which are known as the distin- guishing properties of different bodies. This fluid, according to the variety of the phenomena by which its energy has been discovered to us, has been called under different circumstances, light, fire, electri- city, materia subtilis, materia media, &c. At other times it has been divested of its materiality, and has been con- sidered merely as a principle annexed to or inherent in matter, under the terms of occult quality, nisus, attrac- tion, electric attraction, elasticity, irritability, stimulus, sym- pathy, vital principle, life, &c. &c. This invisible fire is ever ready to exert and show it- self in its effects, cherishing, heating, fermenting, dis- solving, shining, and operating in the various manners, according to the subjects which employ and determine its force. It is present in all parts of the earth and fir- mament, though in most cases latent and unobserved, till some occasion produces it in act, and renders its ef- fects visible ; it exists in our constitution, and indeed in every form in nature in two modes, interstitially and or- ganically. If the pores of gold, which is one of the dens- est known substances, exceed its solid or earthly parts, how much greater must the proportion of solar fluid be in our frame than in that of gold 1 To illustrate this I shall refer to the element of water. Now water, by its transparency, certifies to your senses, that light has free access into and through its substance ; and that it probably fills up its interstices, as water does a spunge when soaked in it. But we know further, by the fluidity and the volatilization of water, that the matter or light of fire has not only access to its interstices, but penetrates and occupies its similar elementary particles ; for these particles could not be rendered volatile, but by internal dilation, nor could they be dilated, but by some- thing that reached their internal parts. These particles then are the organical parts of water, which have their individuality as separable elementary parts, as well as their similarity of character, preserved by that etherial principle that possesses them. ^ These points being cleared, you will now have an ob- vious solution of the difficulties which have attended the 352 LUMINOUS EXPERIMENTS. question, What is the principle of natural life ? Modern physiology has indeed bewildered the conception of its pupils, by not distinguishing between the term life, used metaphysically for our system of consciousness, or as the result of our whole composition explicable only by the Creator, and the same term life, used physically to denote the natural power that presides in reciprocally regulating, and being regulated by the mechanism and disposition of the whole, and every part and particle of our corporeal frame. It is by the unremitting reciprocal corruscations of this vital principle in the fluids and solids, according to the different qualities and consistencies they assume in dif- ferent parts of our constitution, that the whole system of life is displayed and maintained in every individual. Light is not more instantaneously dispatched by reflexion from a mirror, or by that power which every point of the air has of reflecting lightning, than that with which the same fluid, under the character and modification of the vital principle, acts from place to place in the human frame. For the moment of willing, and moving any member, is undistinguishably the same ; so likewise the moment of being touched, and the touch being felt. But these instantaneous transmissions in our frame are not confined to such as we have a conscious perception of; they are incessantly transacting ; the remotest vibrating artery cor- responding with the heart, does not more immediately and constantly feel its power, than the material princi- ple of vitality through its whole form in our structure feels the permanent influence of its own power concen- tered in and irradiating from the brain, the nerves being the directors of the various intended energy of the pow- ers of natural life. This vivifying plenum, occupying and organizing every particle and interstice in our composi- tion, can discharge its whole nisus according to the inti- mation and direction of any nerve or nerves, as instantly as electricity does through the substance of the body that receives the shock. When you consider the rarefying and expansive force of this element, which is capable in an instant of time to LUMINOUS EXPERIMENTS. 353 produce the greatest and most stupendous effects, you have a full proof not only of the power of fire, but al- so of the wisdom with which it is managed, and with- held from bursting forth to the utter ravage and de- struction of all things ; and it is very remarkable, that this same element, so fierce and destructive, should yet be so variously tempered, and applied by Divine Provi- dence, as to be the genial and cherishing flame of all natural life. So bright and lively are the signatures of a Divine Mind operating and displaying itself in fire and light throughout the world, that, as Aristotle observes, " all things seem full of divinities, whose apparitions on all sides strike and dazzle our eyes." And indeed the wisest men of antiquity, how much soever they attri- buted to second causes, and the force of fire, yet sup- pose it always to be governed by a mind or intellect ac- tive and provident, restraining its force, and directing its operations. The order and course of things, together with what we daily experience, fully proves that there is a Mind that governs and actuates this mundane system, as the proper real agent and cause, and that the inferior instru- mental cause is pure ether, fire, or the substance of light, which is applied and determined by an Infinite Mind in the macrocosm or universe with unlimited power, and according to stated rules, as it is in the mi- crocosm, with limited power and skill by the human mind. There is no proof from reason, or experiment, of any other agent or efficient cause than mind or spirit. When I speak therefore of corporeal agents, or corpo- real causes, you understand them as used in a different, subordinate, and improper sense. The principles whereof a thing is compounded, the instrument used in its production, and the end for which it was designed, are all in vulgar use termed causes, though none of them be, strictly speaking, agent or efficient. Therefore when I speak of the element of fire as acting, it is to be understood only as a mean or instrument, which is indeed the case of all mechanical VOL. iv. 2 z 354 OF ANIMAL ELECTRICITY. causes whatsoever. They are nevertheless sometimes termed agents, or causes, although by no means active in a strict and proper signification : when therefore force, power, virtue, or action, are mentioned as sub- sisting in an extended, corporeal, or mechanical being, these terms are not to be taken in a true, genuine, real, but only in a gross and popular sense, which sticks in appearances, and does not analyze things to their first principles. In compliance with established language, and the use of the world, we must employ the current phrases ; but for the sake of truth, we should distin- guish their meaning.* What I have here, as well as in my former lectures, laid before you, concur in proving (nay, all nature gives testimony thereto), " that the fluid etherial mat- ter of the heavens acts by impulse on the solid matter of the earth ; is instrumental in every one of its pro- ductions, and necessary to all the stated phenomena of nature. The elements may then be divided into active and passive ; not that they are such by any inherent or essential difference, but that according to the order es- tablished by the Divine Architect, they are observed to subsist under such relations."! OF ANIMAL ELECTRICITY. I shall here introduce you to the reasons and expe- riments, which induced Dr. Shebbeare to adopt electri- city, as the principle of vital heat and motion, in 1755; and then show how far his opinion has been confirmed by subsequent information. A muscle put in motion by the will, may yet be more actuated by a farther extension of volition, as from walking to running ; by this operation of the mind, there is more of the vital fire determined to the muscles employed in those actions ; muscles are also brought into action by the fire from the electric machine, and * Siris, No. 154, 155. t Jones's Essay on the the First Principles of Philosophy, p. 8. OF ANIMAL ELECTRICITY. S55 palsied limbs have been rendered plump by the same machine, and a power of motion and action restored to those whose palsies have not been of long standing, and which do not take their source from the spinal mar- row. This offers a convincing proof, that vital fire is the cause of muscular motion, and that the vital fire is of the same kind with that produced by our electical machines. After so many experiments on the electrical fluid, and after the discovery of so many phenomena, which are no ways to be distinguished from those of fire, it will scarce be any longer disputed, that they are the same in their own nature. Nor will any one, I presume, af- ter the fire put in action in electrical experiments has been perceived by all our senses, suppose that there can be no less reality in it, than in earth, air, water, or fire, whose reality with respect to mankind depends on the evidence of those very senses. Electricity communi- cates ideas to every sense ; it is light to the eye, odour to the nose, stroke to the touch, subacid to the taste. If you apply heat, either by means of water, or any other method, to the heart of a viper or of an eel taken from the body of those animals, it will again begin to vibrate. Now heat is fire in action, and thus you see the same effect is produced as was effected in the pal- sied limb. The reason why the hearts of vipers and eels, and such like animals, are put into motion by a power of the same nature, though in a less degree than that which moves the heart of larger animals, is, because they are extremely cold by nature, and therefore a less degree of fire actuates on their heart than on those of larger animals. It is not improbable that the same degree of heat, which is necessary to keep a fowl alive, would de- stroy a frog or viper, and burst the cells of the tunica cellularis. After the heart of a viper has discontinued to beat with the application of any certain degree of heat, it will vibrate again on the application of a supe- rior degree. The heart, which in the open air had ceased to move with a certain degree of heat, will vibrate again in vacuo with the same degree ; for the pressure of the 256 OF ANIMAL ELECTRICITY. atmosphere being removed, a less power is required to distend the fibres. Dr. Shebbeare took the heart of an eel, which had been some time dead, and placing it on a card, put it on the conductor ; the first motion that was communi- cated to it was its swelling, or the diastole of the ven- tricles, which not being immediately followed by the contraction or systole, he took the electrical spark therefrom, on which it contracted ; it then dilated again, and upon the application of his finger again contracted; and thus having repeated it several times, the heat con- tinued to perform its dyastole and systole, without be- ing touched ; and when it was removed it ceased, but began again upon being placed on the bar. Lord Bacon has given us a very remarkable instance of the effect of fire upon the human heart. He says, " that upon the embowelling of a criminal, he had seen the heart of a man, after it was thrown into the fire, leap up for several times together, at first to the height of a foot and a half, and then gradually lower, to die of his memory, for the space of seven or eight mi- nutes. Trace vital heat and motion from their source, and you will find these phenomena still more clearly illus- trated. An egg, though it includes all the parts neces- sary for the formation of an animal, will never produce a chicken, unles it be kept in a certain degree of heat for a certain time ; which heat, regularly conducted, is all that is necessary to the production of an animal similar to the parent. That there is nothing more necessary to the produ- cing this animal from an egg, than common fire, has been long known and practised in Egypt, and demonstrated by Mr. Reaumur. There is no other vital principle transfused from the hen to the embryo, than from a common fire. Thus is fire plainly proved to be the first mover in the animal machine, and is the only active, material, or natural principle during its exist- ence ; and it is a principle absolutely necessary for the preservation of health, and generating wholesome fluids. Shall fire be allowed to have the power of beginning OF ANIMAL ELECTRICITY. 357 the vital motion in the womb, or egg, and shall it be refused the power of continuing it after the birth ? Now, for many reasons, which will be seen as we pro- ceed, it appears that the fluid of fire passes by the nerves to the brain and spinal marrow, and from thence to the heart for supplying trie cause of involuntary motion, and that a sufficient quantity is always detained there to go to the muscles at particular times for the performing voluntary motion. This fire (the reality of whose existence is proved by all the demonstrations which can attend the proof of any existence, and whose general properties are now well known) is lodged in the brain, medulla spinalis, ganglions, and nerves, and thence operates on aU-the different parts of the body. The diminution and waste of this fire is continually supplied from the earth. The nerves, which are destined to the sense of feel- ing, are the conductors of this fire to the brain ; while those which are destined to motion, are the conductors by which it is conveyed to the muscles. For a parti- cular explanation of the manner in which it acts, I must refer you to Dr. Shebbeare's masterly performance. It is not the fluid of fire alone that constitutes and preserves the vital heat and vital motion ; but it must for this purpose be brought into a certain state or de- gree of action, which, in a healthy man, amounts to 98° of Fahrenheit* s thermometer ; and according to the de- grees of heat originally destined to each animal, and the excess or decrease of it, will be the state of its activity and health. Nor is it confined to animals ; something of the same kind seems to take place in vegetables. The heat which produces an apple to perfection would never bring forth a pine-apple ; and the firs, which thrive and look green on the bleak and snowy hills of Norway, would perish in the burning sands of Barca ; whilst the spicy vegetables of the east, which breathe incessant sweets amid the glowing soil of Arabia, would languish md expire in that cold clime which breeds the lofty i>ak. 35S OF ANIMAL ELECTRICITY. The heat which hatches the chicken from an egg would destroy the whole race of fishes, if it affected their spawn ; and thus the very same element, which makes an animal complete in one degree, and in one species destroys its existence in another species with the same degree. The degree of heat, which would injure the life of a frog, would not be sufficient to keep the heart of a sheep in action. Health depends on a degree of heat which is natural to each animal, and which was first imparted to it by that Divine Intelligence, who is alone able to actuate and inform, and who has furnished us with powers to keep up this degree, and counteraf. and throw off a greater. In this account of vital heat and motion, there is no- thing new supposed ; no new property assigned either to fire or electricity ; no new formation given to any part of the human body. We require no more of the nerve than that it exists, and that it be a conductor of the electric fluid ; which experiment proves, vital heat and vital motion are here as they are in nature, beginning together, and conti- nuing so through life. Solar fire and the electric fluid are one and the same vivifying principle, actuating all the different orders of material beings : they are so radically the same, that in various instances you find that what was one becomes the other ; and thus facts and philosophy are united ; and the cause of natural life and motion is discovered by reason and experience to be the same with what our senses inform us to be in- tuitively the true one. And permit me to tell you, that in general, whenever the account given to explain the cause of any phenomena in nature, is contradictory to the obvious apprehension of the senses of a plain un- derstanding, there is reason to suspect its truth. That to the agency of fire all animal motion and animal heat are owing is obvious to the meanest capacity ; and if this element cease to act, or if it be disunited from the body, death is the certain consequence. Every part of nature affords facts to support this opinion. Contem- plate the great luminary which enlightens the universe, OF ANIMAL ELECTRICITY. 359 and you will find every ray to be fraught with fire, which it is ready to manifest on meeting a proper recipient. Without the genial warmth they communicate, both animal and vegetable life must cease, and all nature be- come one lifeless, torpid, dismal ruin. All nature bears testimony to the existence of this ethe- rial fluid, and to its incessant active energy. To us, indeed, it often remains latent ; and peculiar circumstances are ne- cessary to excite those signs which render its effects most visible to our senses. The ancients, viewing nature as she is, often attained more accurate notions of her operations than modern philosophers. These, by multiplying expe- riments, without first attaining a correct idea of the facts continually presented for observation in the great labora- tory of nature, have often wasted their time and talents; and, in the end, have bewildered themselves in an inex- plicable labyrinth, or at best, have only placed one spe- cies of ignorance in the deserted room of another. The Platonists and Pythagoreans maintained, that fire was the great instrumental cause in the universe, subor- dinate to the Infinite Creative Mind ; and that it actuated the macrocosm and animated the microcosm. The old naturalists have universally maintained, that fire was in all bodies ; and, however indistinctly they were able to write of it, what they wrote was true. The- opbrastus has spoken of fire in terms that bespeak a con- siderable knowledge thereof. Far from supposing mo- tion to be the cause, much further from supposing it to be the essence of fire, he asserts, that fire is a very dis- tinct thing from the matter in which we see it lodged, and from the motions which we see excite it ; and that it is, in its pure natural state, fine, etherial, imperceptible, and at perfect rest. He hints, that this fire was the breath which the Creator diffused in all matter, which, passing over the waters, made out of them metals, stones, and earth ; and asserts, that it is the instrument which he employs to give all things life and motion. They in general considered earth and water, air and fire, as the component elements of all visible and known cor- poreal beings, and that life was conveyed to them through the elements of air and fire ; that this fire was continu- 360 OF ANIMAL ELECTRICITY. ally operating to apply and adjoin to these bodies the newly arrived matter, converting this matter into a substance of the same nature or form with that part to which ii was applied, and thus fitting it for the growth or increase, as well as the aliment of the part. But then they also con- sidered natural life as only possessed of these powers, be- cause it was the immediate agent of mind : for mind is evidently the cause of form to all things formed by man ; and the cause of union or conjunction to all things united or conjoined by art. It is hardly possible not to agree in many respects with these ancient sages: for, when you look round with a phi- losophic eye, and contemplate the universe with sedulous attention, you will find that there is no effect either beau- tiful, great, marvellous, or terrible, but what proceeds from fire. It can, therefore, be no matter of surprize, that after the discovery of electricity, it was considered as the phy- sical cause of motion, irritability, &c. but it is surely a subject of regret, that medical men have shown such re- ; luctance to the investigation of this subject, and that too many have in every possible way endeavoured to discoun- tenance its application in medicine ; though the agency of this fluid, and its existence in animated nature, has been so fully proved by a variety of experiments, that there can be very little doubt that it is essentially con- nected with, and continually exerting its influence on the human frame. I shall here lay before you some further instances to corroborate what has been already advanced. By means of a small condensing plate, Mr. Cavallo ob- tained very sensible signs of electricity from various parts of his own body, and the head of almost any other per- son. The strong electricity obtained in frosty weather from silk stockings, &c. on being pulled off, as well as that obtained by combing the hair, have been long known. Among others, Mr. Brydone mentions a lady, who, on combing her hair in frosty weather in the dark, had ob* served sparks of fire to issue therefrom. This made him think of trying to collect the electrical fire from human hair alone. To this end, he desired a young lady to stand on wax, and comb her sister's hair, who was sitting in a OF ANIMAL ELECTRICITY. 361 chair before her ; soon after she^had begun to comb, the young lady on the wax was surprized to find her whole body electrified, and darting out sparks of fire against every object that approached her. Her hair was strongly electrical, and affected an electrometer at a considerable distance. He charged a metallic conductor from it, and in the space of a few minutes collected a sufficient quan- tity of fire to kindle common spirits ; and, by means of a small jar, gave many smart strokes to all the com- pany. When the discoveries in this science, says Mr. Brydone, are further advanced, we may find, that what we call sen- sibility of nerves, and many other diseases, which are known only by name, are owing to the bodies being pos- sessed of too large or too small a quantity of this subtile fluid, which is, perhaps, the vehicle of all our feelings. It is known, that in damp and hazy weather, when this fire is blunted and absorbed by the humidity, its activity is lessened, and what is collected is soon dissipated ; then our spirits are more languid, and our sensibility is less acute. And, in the fierce wind at Naples, when the air seems totally deprived of it, the whole system is unstrung, and the nerves seem to lose both their tension and elasti- city, till the north-west wind awakens the activity of the animating power, which soon restores the tone, and enli- vens all nature, which seemed to droop and languish in its absence. Nor can this appear surprizing, if it be from the different state of this fire in the human body that the strictum and laxum proceed, and not from any alteration in the fibres themselves, or their being more or less braced up (among which bracers cold has been reckoned one,) though the muscular parts of an animal are more braced when they are hot, and relaxed when they are cold. From the perpetual electricity of the atmosphere, which is no longer a problem, as its existence and agency in that mass of air which surrounds our globe, has been ascer- tained by numerous, clear, and decisive experiments, it seems but just to infer, that it must exert a certain influ- ence on all the beings contained therein, and principally on organized bodies, among which the human frame claims the pre-eminence. VOL. IV. 3 A 362 OF THE LATER EXPERIMENTS But there is no necessity for deductions from a general view of nature, for we are nowi:. possession of facts, which prove that it is a principal agent in promoting the func- tions of animated beings ; as in the gymnotus electricus torpedo, and silurus electricus. For the similitude esta- blished between the electrical fluid of these animals, and that of nature at large, is such that in a physical sense it may be considered the same. OF THE LATER EXPERIMENTS ON ANIMAL ELEC- TRICITY. When Mr. Walsh first attributed the sensations pro- duced by the torpedo, &c. to electricity, his opinions, and the inferences deduced from his experiments, were vehe- mently opposed by most of the best electricians of the day : the conceptions of these men being limited to the minutiae of experiments, they were incapable of grasping a more extensive subject, or one that was not in all re- spects conformable to the appearances they were used to. Whereas a just view of things should have prepared them to expect various anomalies, while they were investigating the nature of an invisible and subtile agent, subject to a variety of modifications from the substance through which it passes, or with which it may be combined. Hence, in the pursuit of animal electricity, you must not expect to meet with every electric sign ; as, from the very nature of its connexion with animated beings, it will certainly ac- quire properties that are not to be found when it is disen- gaged therefrom. Before I relate any of the experiments of Val/i, &c. I shall lay before you those principles, which I conceive will throw great light on the subject of animal electricity, and by which they may be reconciled to the general agency of nature. You have seen, by a great variety of experi- ments, that electricity is first rendered sensible by a solu- tion of continuity ; you have also every reason to suppose, that the electric matter is carrying on its most important functions when we are unable to perceive any signs of elec- tricity ; you have seen that the electric matter, and what ON ANIMAL ELECTRICITY. 363 we term electricity, are not inseparable beings, that the one may subsist when the other ceases to appear. As the air may occupy a space without producing sound, so the electric matter may reside in a body without ex- hibiting any electric signs. We know also by universal observation, as well as partial experiments, that there is a principle in all bodies which is continually endea- vouring to extend their form, but whose energies are continually counteracted by an exterior force. Now it must be evident, that every solution of continuity will give an opportunity for this expansive dilating substance to escape, when it puts on new and unexpected appear- ances. Hence, as we know this expanding substance to be fire, and have a proof that on its escape it exhibits elec- tric signs, we have a further confirmation of the iden- tity of these elements. I think this view of the subject is in itself a sufficient refutation of Dr. Munro's attempt to prove that the ner- vous fluid or energy is not the same with the electrical;* though many other arguments may be adduced to an- swer the same purpose. His difficulty in conceiving how the electrical fluid can be accumulated within our nervous system, is not greater than that of conceiving how it is accumulated amidst a conducting fluid in the torpedo, &c. nor indeed than of its being accumulated in the Leyden phial, as glass is now known to be permeable thereto. But the difficulty with respect to animals vanishes, when we con- sider that electrical appearances are occasioned by a state of the fluid altogether different from that under which it exists in the animal frame ; when it is in the latter, its powers are united, and its operations imperceptible ; when it appears as electricity, its powers are divided and some of their effects rendered sensible. So far as mechanical stimuli have any relation to fire, so far they will be in some degree similar to the electri- cal fluid, and act in the same manner ; for, stimulants act only as they are the vehicles of fire. The second * Mimro's Experiments on the Nervous System, 364 OF THE LATER EXPERIMENTS objection, therefore, of the professor falls to the ground The same reasoning applies to his sixth objection. His fourth reason, so far from proving tl^at the ner- vous and electrical fluids are not the same, may be con- sidered as a clear proof of their identity, for the two elec- trical powers always act in opposite directions. On the same principle, the nervous energy (the elec- trical fluid in its united state) cannot pass readily up or down a nerve that has been tied or cut, for the tying or cutting of the nerve changes the state of the fluid. Before I proceed to give you an account of the ex- periments relating to animal electricity, I shall lay be- fore you some remarks of the Rev. Mr. William Jones,* from whom we have already profited so much in the course of these lectures, and which are intimately con- nected with our subject. " As the force of the electri- cal fluid, says he, is principally exerted on the nerves and tendons of the body, there is reason to believe that this fluid is the same with that something, which many physicians have discoursed upon under the name of ani- mal spirits. The nerves do not appear as if they were designed to admit any animal fluid or liquor, unless it be an indolent lymph necessary to keep them moist : but their pellucidity indicates that they are properly adapted to give a direct passage to the fluid light ; for they are transparent, and that not transversely, but longitudinally, or in the direction of their fibres. This Mr. Jones observed accidentally, as some eyes of sheep and oxen, which he had procured for dissection, lay on the table ; one of these eyes shone in the day time much in the same manner as the eyes of some animals do in the dark; on examining into this circumstance, he found, that if his hand were interposed between the nearest win- dow and the extremity of the optic nerve, a part of which nearly an inch in length remained with the eye, and was accidentally pointed towards the window, the light im- mediately disappeared. " * Jnnen\s Essay on the First Principles of Natural Philosophy, p ON ANIMAL ELECTRICITY. 365 From this he was led to consider, whether the light that appears in the eyes of some animals in the night time, is really a reflection of light from the eye, as is commonly supposed ; or whether it does not rather pass into the eye, through the optic nerve, from the body of the animal ? It is not easy to conceive how this shining can be occasioned by a reflexion of light from the choroides in the bottom of the eye, when the light to be reflected (as in a dark night) is not visible before its entrance into the eye. If a candle be held before the eyes of a dog, and you place yourself in the line of re- flection, the light will be visibly reflected from his eyes, because the illumination is sufficiently strong; but when there is no visible illumination at all, how should it account for the like effect ? Whence it is more reason- able, that this appearance should be owing to a light from within the body of the animal, which being weaker than the light of the day, but stronger than the light of the night, is visible in the night but not in the day. The light of other bodies which shine in the dark is inhe rent in those bodies, as in putrifying veal, fish, rotten wood, phosphorus, the glow-worm, &c. concerning the last of these, the eminent anatomist and philosopher, T. Bartholine, has the following observation. If a glow-worm be examined, it will appear to have a lucid liquor in the hinder part of its body, where the heart is placed, by which the heart is moved and illuminated ; and this fluid retains its light so long as the heart of the insect re- tains its life and motion. Dr. Priestley , in his Heads of Lectures on a Course of Experimental Philosophy, has given so excellent and compendious a view of the principal experiments that have been made by Valli and others, to determine the electricity of animals, that I cannot do better than lay it before you ; which I the more readily do, as it will save us from the disgusting detail of a variety of cruel expe- riments ; experiments that I hope you will never be in- duced to repeat. One alone will suffice to give you an idea of the nature of these operations. Mr. Valli opened the abdomen of a frog, in order to lay bare the spine of the back, and discover the crural 366 OF THE LATER EXPERIMENTS . nerves which issue from it ; a few lines above this poi he cut the animal in two, and passing his scissars imme- diately under the origin of these nerves, removed the remaining portion of the vertebral column, so as only to leave the vertebral which united the bundle of nerves ; this portion of the vertebras was enveloped with a piece of sheet lead ; the coated part was touched with one end of a metallic conductor, and with the other, the sur- face of the thighs which were previously stripped of their skins. The movements produced thereby were violent and continued for a long time. Having thus explained to you the manner in which the animal is prepared for these experiments, I shall proceed to point out the principal results," as furnished by Dr. Priestley. The nerve of the limb of an animal being laid bare, and surrounded with a piece of sheet-lead, or tin-foil, if a communication be formed between the nerve thus armed, and any of the neighbouring muscles by means of a piece of zinc, strong contractions will be produced in the limb. If a portion of the nerve which has been laid bare be armed as above, contractions will be produced as powerfully, by forming the communication between the armed and bare part of the nerve, as between the armed part and muscle. A similar effect is produced by arming a nerve, and simply touching the armed part of the nerve with the metallic conductor. Contractions will take place if a muscle be armed, and a communication be formed by means of the conductor between it and a neighbouring nerve ; the same effect will be produced, if the communication be formed be- tween the armed muscle and another muscle which is contiguous to it Contractions may be produced in the limb of an ani- mal, by bringing the pieces of metal into contact with each other at some distance from the limb, provided the latter make part of a line of communication between the two metallic conductors. ON ANIMAL ELECTRICITY. 367 The experiment which proves this is made in the fol- lowing manner. The amputated limb of an animal be- ing placed upon a table, let the operator hold with one hand the principal nerve, previously laid bare, and in the other let him hold a piece of zinc ; let a small plate of lead or silver be then laid upon the table at some distance from the limb, and a communication be form- ed by means of water between the limb and the part of the table where the metal is lying. If the operator touch the piece of silver with the zinc, contractions will be produced in the limb the moment that the me- tals come into contact with each other. The same ef- fect will be produced, if the two pieces of metals be previously placed in contact, and the operator touch one of them with his finger. This fact was discovered by Mr. William Cruikshank. Contractions can be produced in the amputated leg of a frog, by putting it into water, and bringing the two metals into contact with each other, at a small dis- tance from the limb. The influence which has passed through, and excited contractions in one limb, may be made to pass through, and excite contractions in another limb. In perform- ing this experiment, it is necessary to attend to the following circumstances ; let two amputated limbs of a frog be taken, let one of them be laid upon a table, and its foot be folded in a piece of silver ; let a person lift up the nerve of this limb with a silver probe, and an- other person hold in his hand a piece of zinc, with which he is to touch the silver including the foot ; let the person holding the zinc in one hand, catch with the other the nerve of the second limb, and he who touches the nerve of the first limb is to hold in his other hand the foot of the second ; let the zinc now be ap- plied to the silver including the foot of the first limb, and contractions will immediately be excited in both limbs. The heart is the only involuntary muscle, in which contractions can be excited by these experiments ; con- tractions are produced more strongly, the farther the coating is placed from the origin of the nerve. 368 OF THE LATER EXPERIMENTS Animals which were almost dead, have been found to be considerably revived by exciting this influence. When these experiments are repeated upon an animal that has been killed by opium, or by the electric shock, very slight contractions are produced ; and no contrac- tions whatever will take place in an animal that has been killed by corrosive sublimate, or that has been starv- ed to death. Zinc appears to be the best exciter when applied to gold, silver, molybdena, steel, or copper ; the latter metals, however, excite but feeble contractions when applied to each other ; next to zinc, in contact with these metals, tin and lead appear most powerful exciters. It has been found, that if a plate of zinc be applied to the upper part of the point of the tongue, and a plate of silver to its under part, on bringing the two metals into contact with each other, a pungent disagreeable feel- ing, which it is difficult to describe, is produced in the point of the tongue. And if a plate of zinc be placed between the upper lip and the gums, and a plate of gold applied to the upper or under part of the tongue, on bringing these two metals into contact with each other, the person imagines that he sees a flash of light- ning, which however a by-stander in a darkened room does not perceive ; and the person performing the ex- periment perceives the flash though he be hood-winked.* After performing this experiment repeatedly, Dr. Munro constantly felt a pain in his upper jaw, at the place to which the zinc had keen applied, which conti- nued for an hour or more ; and in one experiment, af- ter he had applied a blunt probe of zinc to the septum narium, and repeatedly touched with a crown piece of silver applied to the tongue, and thereby produced the appearance of a flash, several drops of blood fell from that nostril ; and Dr. Fowler , after making such an ex- periment on his ears, observed a similar effect. Munro's Experiments on the Nervous System, p. 25. [ 369 ] RESEMBLANCE OF THE FLUID PUT IN MOTION BY THE FOREGOING EXPERIMENT, TO THE ELECTRI- CAL FLUID.* The fluid set in motion by the application of the me- tals to each other, and to animal bodies, or to water, agrees with or resembles the electrical fluid in the fol- lowing respects : Like the electrical fluid, it communicates the sense of pungency to the tongue. Like the electrical fluid, it is conveyed readily by water, blood, the bodies of animals, the metals ; and is arrest- ed in its course by glass, sealing-wax, &c. It passes with similar rapidity through the bodies of animals. Like the electrical fluid, it excites the activity of the vessels of a living animal ; as the pain it gives and hemorrhagy it produces seem to prove. Hence, perhaps, it might be employed with advantage in amenorrhcea. It excites convulsions of the muscles, in the same man- ner, and with the same effects as electricity. When the metals and animal are kept steadily in con- tact with each other, the convulsions cease, or an equi- librium seems to be produced, as after discharging the Leyden phial. GENERAL OBSERVATIONS. A view of the great agents in nature naturally leads us to consider the opinions of those who wish to set religion and reason in opposition to each other, and to suppose that philosophy and revelation can never agree. But, in opposition to such insidious attempts, attempts which never were designed to enlarge the mind or to improve the heart, it may easily be made to appear that, take philosophy in its most improved state, enriched * Munro's Experiments on the Nervous System, p. 25. VOL. IV. a B 370 GENERAL OBSERVATIONS. by the discoveries of ages, examined by the test of the closest reasoning, elevated above the fallacies of the senses and of appearances ; and yet, in this improved state, it shall be found perfectly to correspond with the philosophy of scripture, rightly understood. The word of God is as perfect as his works. Both proceed from the one fountain of truth, who cannot contradict him- self. His word and his works mutually illustrate each other : the one is not to be understood without the other ; for both are the offsprings of divine love, manifested in wisdom, and exercised in power. Creation may be considered as the grand chain of causes and effects, intimately connected together. It is the work of Omnipotence, guided by infinite wisdom, and excited to work by communicative goodness. But, do we not obtain wrong ideas on this important subject, if we imagine that any part of this grand system stands unconnected ? or, as if the Great Master Builder was obliged to collect discordant materials from different parts, and, overcoming the repugnance of their natures, to form one whole out of these heterogeneous sub- stances ? Whereas, the truth appears to be, that in his divine hand, the one naturally and orderly produces the other ; that, which was the effect of a prior princi- ple, becomes the cause of that which follows it immedi- ately ; and again, chis effect becomes an instrumental cause in its turn ; and is thus extended in a long series, until all are completed in outward nature. Let us examine how this will agree with the Mosaical account of the creation ; for, although we may readily allow, that that book contains more interesting and im- portant subjects than the detail of the mere creation and formation of this material system ; yet the natural ac- count, when rightly understood, may be found to be most accurate, philosophical, and just. The great and spiritual truths, conveyed under that form, may yet be delivered down to us in a vehicle of the most accurate philosophical truth ; the stricter the truth, the greater and more perfect the analogy and correspondence ; but it seems to have been the peculiar fate of these sublime and ancient writings of the Hebrew Sage, that they have GENERAL OBSERVATIONS. 371 been supposed to contain what they did not, whilst their real and most important contents have been greatly over- looked. The ideas of the Divine Mind disclosed, the energies of his almighty will exerted, produced motion in different degrees, as the instrumental cause for future productions. Hence the motion of spirits, of minds, of life, of thought, of light, of the heavenly bodies, of blood, and of the sap. Hence this motion, depend- ent and continued from one source of life and motion, may be considered as the key of natural knowledge, which opens the temple of physical truth. Motion is the visible discovery of the divine hand ; motion is the grand connecting link between the spiritual and natural worlds ; by this the energies of the one are impressed on the other. This motion, proceeding from a pure and superior system, was at first most perfect and full, unencumber- ed by matter, unimpeded by obstructions. Now, in the first day (or in the first state of creating things, for as yet there was no sun and earth, and there- fore no measure of day and night), in this first state of things, the Scripture says, light was formed, or rather the matter of light, by the means of pure original mo- don. Now the matter of light is elementary fire. This is evident from the most intimate relation between fire and light ; light being only an effect, an outward visible manifestation of latent fire. This pure elementary fire, the matter or substance of light, produces that rapid motion of light from the sun or stars to the earth, travelling with such amazing ve- locity. Fire and light combined, produced air, or the first and purest etherial particles ; and therefore, in the Mosaical account, the firmament, the expanse, or the atmosphere of the air, was the second day's work, or the second state of things in their progress to perfection and fulness. This elementary principle is not so subtile and active as its parents, fire and light ; yet it is more subtile and active than vapour or water ; therefore it holds the intermediate rank between these, and is a con- necting link in the great chain, as it is produced by fire and light; so again, when partly deprived of these. 372 GENERAL OBSERVATIONS. it is the instrumental cause to form the vapours and Water. That fire and light produce air may be illustrated by Various experiments ; the respiration of plants, and the purity of the air, which they produce, when exposed to the agency of light ; and the great quantities of dif- ferent airs produced in various chemical experiments by the activity of fire. Air condensed, exposed to obstructions, and thus de- prived of the greatest portion of its etherial fire, be- comes first vapour ; and as the fire dissipates, and the motion ceases, it becomes water in the various forms of mist, dew, rain, &c. In this state, it is almost entirely deprived of its original motion ; is less subtile, and more gross ; is become an object of the outward senses, and is subject to the laws of gravitation. Water is the great support of animal and vegetable substances, which at length are reduced to earth in their various changes, from the first principles of active na- ture, down to the lowest, grossest material form ; from the* fountain of life, from the architypal ideas of the Divine Mind, through spirits to fire, light, ether, air, water, earth, down to sluggish inert matter. Fire, light, air, and water, may then be considered as the grand agents in nature. The earth is, as it were, a basis for them to rest and to work upon. In these, the circulation of motion in its descent and degrees is preserved, and the earth is a nidus where they rest, and where their effects are manifested. Thus there was a regular and beautiful descent from the spiritual to the natural world, from motion to resr. The wonderous fabric of the earth was not built of discordant materi- als, of jarring elements, forcibly restrained by the di- vine hand continually checking them ; but the homoge- neous substance arose in a wise and orderly series. Each part being preparatory for that which was to succeed ; ^every thing being a link in the great chain of order and usefulness ; and instrumental cause to produce the suc- ceeding effect, until all was finished and complete, na- ture stood perfect in outward matter : creation was no longer all fire, light, air, or water, but each retained its GENERAL OBSERVATIONS. 373 respective rank; and the gross material world was produced, able to sustain minerals, plants, animals, and man. Thus did the Divine Architect accomplish this great and stupendous work by the most simple means ; by a regular descent from the spiritual to the natural world -> a continued series proceeding from the highest to the lowest, from the purest motion to inactivity, from the highest principles of intelligent mind down to the low- est, grossest heaviest matter. Thus were all things ordered in infinite wisdom : causes were employed most simple and prolific, to accomplish the end designed. Creation was accomplished ; the earth stood complete ; the work of divine power resulting from divine wisdom and mercy. It was made the theatre of his goodness, on which he might display it, and communicate it to his various creatures, who thus might rejoice in their ex- istence ; and manifest his praise, by enjoying happiness, and rising in perfection through endless ages. Thus was the earth designed to be the repository of the human race, the seminary of men ; until, full of years and wisdom, they were ripe for a happier change ; were prepared to quit the perishing body, and to be trans- planted into a paradise of endless delights. The whole material system was also a volume of di- vine instruction opened to man, in which he might read and understand and live for ever ; in which he might discover immense benevolence, design, and order, and thus be led to understand and adore him, who is the source of all things. C 374 ] LECTURE L. ON MAGNETISM.* 1 HOUGH the phenomena of the magnet have for many ages engaged the attention of natural philoso- phers, both from their singularity and importance ; we are not yet in possession of any hypothesis that will sa- tisfactorily account for the various properties of the magnet, or point out those links of the chain that con- nect it with the other phenomena of the universe It is known by the works of Plato and Aristotle, that the ancients were acquainted with the attractive and re- pulsive powers of the magnet ; but it does not appear, that they knew of its pointing to the pole, or the use of the compass. That property of the magnet, whereby, when properly suspended, it turns towards the north, renders it of the utmost service to mankind in general, but more particularly to an Englishman; the riches and power of whose country depend on navigation. The powers of the magnet excited the wonders of the ancients ; they were to them inexplicable, and still remain so. Posterity, instead of being able to remove the difficulties, have only by their researches found out new wonders equally inexplicable. All, therefore, that I shall be able to do, will be to relate to you the prin- cipal qualities of this curious phenomenon. " The magnet is a proof, that nature has many secrets, and that philosophy, if contented with present knowledge, fore- goes most valuable and interesting discoveries, towards See my Essay on Magnetism. OF MAGNETISM. 375 which, perhaps, the previous steps are already trod- den." From its action on the compass in all parts of the world it is plain, that its influence is universal. From our knowledge of this we are naturally led to suppose, that there may be other invisible agents exerting their in- fluence on us, and on our globe. Let the modern philosopher,* who denies the existence of a God, because he cannot perceive him with his cor- poreal eyes, tell you what magnetism is, and how it exists. Let him, who will understand every thing that exists, be- fore he allows of its existence, first employ himself here; and when he has given the world a proof of his powers, let him attempt a higher subject. The loadstone, leading-stone, or natural magnet, is an iron ore or ferruginous stone, found in the bowels of the earth, generally in iron mines, of all forms and sizes, and of various colours. It is endowed with the property of attracting iron ; and of both pointing itself, and also ena- bling a needle, touched upon it, and duly poised, to point towards the poles of the world. Loadstones are in general very hard and brittle, and for the most part more vigorous in proportion to their degree of hardness. Considerable portions of iron may be extracted from them. Newman says, that they are al- most totally soluble in spirit of nitre, and partially in the vitriolic and marine acids. Mr. Kirwan says, that the magnet seems to contain a small quantity of sulphur, it is often contaminated with a mixture of quartz and argil ; it is possible it may contain nickel, for this, when purified to a certain degree, ac- quires the properties of a magnet ; but its constitution has not as yet been properly examined f Artificial magnets, which are made of steel, are now generally used in preference to the natural magnet ; not only as they may be procured with greater ease, but be- * Condorcet, and many of his school, have laughed at mankind for be- lieving in an invisible Being. t Kirwan's Elements of Mineralogy. In the second edition, 1796, p. 158, of thisexcellent work, are giveu further and more correct particulars of this curious mineral E. Edit* 376 OF MAGNETISM. cause they are far superior to the natural magnet in strength, and communicate the magnetic virtue more powerfully, and may be varied in their form more easily, so that the natural magnet is now very little esteemed, except as a curiosity. The power of attracting iron, &c. possessed by the loadstone, which is communicable to iron and steel, is called magnetism* It has been supposed, that iron and the loadstone were the only two bodies which could be ren- dered magneticai ; but it now appears, that nickel, when purified from iron, becomes more instead of less magne- tic, and acquires, what iron does not, the properties of a magnet. 5 * A rod or bar of iron or steel, to which a permanent polarity has been communicated, is called a magnet. The points in a magnet which seem to possess the greatest power, or in which the virtue seems to be con- centrated, are termed the poles of the magnet. The magneticai meridian is a vertical circle in the hea« vens, which intersects the horizon in the points to which the magneticai needle, when at rest, is directed. The axis of a magnet is a right line, which passes from one pole to the other. The equator of a magnet is a line perpendicular to the axis of the magnet, and equally distant from the two poles. < The distinguishing and characteristic properties of a magnet, are. 1. Its attractive and repulsive powers. 2. The force by which it places itself, when suspend- ed freely, in a certain direction towards the poles of the earth. 3. Its dip or inclination towards a point below the hori- zon. 4. The property which it possesses of communicating the foregoing powers to iron or steel. Kirwan's Elements of Mineralogy, second edition, 1796, p. 2ft J. [ 377 ] OF THE TENDENCY OF IRON AND A MAGNET TO AP- PROACH EACH OTHER. This curious property of the magnet was that by which it was first discovered, and by which it engaged the at- tention of the curious. Every substance that contains iron is more or less at- tracted by the magnet. And so universally is this metal disseminated, that there are very few substances that are not in some degree capable of being attracted by the mag- net. You will find it in animals, vegetables, minerals, and even in the air.* Iron is attracted with different degrees of force, ac- cording to the different states of its existence ; but it ne- ver becomes quite insensible to the magnetic power. Even the purest calx, or the completest solution ever made of the metal, when accurately examined, is found to be in some degree obedient to the magnet. TO ASCERTAIN WHETHER A OR IS CAPABLE OF BEING ATTRACTED BY THE MAGNET. If the given body contain evidently a large quantity of iron, on bringing a magnet in contact therewith, you will find it adhere so strongly as to require a certain degree of force to separate them. If the body be not sen- sibly attracted by the magnet in this way, then you may Boat it by a piece of wood or cork on water; in this situa- :ion it is more easily acted on, and consequently small quantities of iron are readily discovered. The magnet should be presented sidewise to the body, and when it is it rest, it is sometimes necessary to bring the magnet within one-tenth of an inch distance from the swimming x>dy in order to perceive the attraction. * Cavaii'o on Magnetisrr, p. 66, VOL. IV, 3 C S78 TO ASCERTAIN WHETHER A still smaller degree of attraction may be discovered by placing the given body upon quicksilver, and then pre- senting a magnet to it. The vessel, in which the quick- silver is contained, should be at least six inches in dia- meter, otherwise the curvature of the fluid will be per- petually carrying the body towards the sides of rhe ves- sel. The quicksilver should be pure, and occasionally cleared by passing it through a funnel of clean writing- paper ; the smaller the aperture of the funnel, the better wiil it answer the purpose. The air should be agitated as little as possible. Attending to these precautions, you will seldom fail to discover whether a body contains any ferruginous particles.* I place a piece of iron on a cork, and put the cork into a bason of water. I present a magnet to it, and it is at- tracted thereby, and follows the magnet, so that I can move it without touching, wherever I please. On this principle, many ingenious and entertaining pieces of me- chanism have been contrived. The tendency between the magnet and the iron is reci- procal ; for, if the magnet be put on the cork, it will fol- low the iron in the same manner as this followed the mag net. And this attraction takes place, although a piece of paper, glass, brass, &c. be interposed between the mag- net and the iron. The reciprocal tendency of iron to a magnet, and of a magnet to iron, is pleasingly illustrated by suspending a magnet under the scale of a balance, and counterpoising it by weights in the other scale ; when thus counterpois- ed, bring a piece of iron towards it, and the magnet will immediately descend. Reverse the experiment by sus- pending the iron from the scale, and the iron will now de- scend and follow the magnet. I place a magnet upon a stand, to raise it some distance irom the table ; I shall bring a small sewing-needle to- wards it, keeping the thread which is in the needle in my hand, to prevent the needle from fixing itself to the mag- * A fine magnetic needle, about four or five inches long, suspended on a pointed stand, as shown at plate 2, Jig. 16, 1 think thj m jst sensible and convenient apparatus for this purpose E. Edit. A BODY HAS ANY IRON. S79 net ; and the needle endeavouring on one hand to fly to the magnet, and being withheld on the other by the thread, remains pleasingly suspended in the air. Mathematicians have endeavoured to compute the force with which the magnetic attraction acts at different distances, but hitherto without success. No law has been ascertained, upon which any dependence can be placed. Though many experiments have been made to disco- ver, whether the force by which two magnets are repel- led or attracted, acts only to a certain distance ; whether the degrees of its action within, and at this distance, is uniform or variable, and in what proportion, to the distances it increases or diminishes ; yet we can only in- fer from them, that the magnetic power extends further at some times than it does at others, and that the sphere of its action is variable. The smaller the loadstone or the magnet is, the great- er is its force, ceteris paribus, in proportion to its size. When the axis of a magnet is short, and of course its poles very near, their action on each other weakens the magnetic force. A variety of other causes will also oc- casion great irregularity in the attraction of magnetism. The attraction is, as I shall show you, always strongest at the poles of the magnet ; and most so when the body is near the magnet, but diminishes as either recedes from the other. It appears also from experiment, that a mag- net attracts another magnet with less force than it does a piece of iron. OF THE POLES OF A MAGNET. It has been already observed to you, that there are certain points of a magnet called the poles, which are pos- sessed of the greatest magnetic force, and in which its virtues seem as it were to be concentrated. This I shall prove by an easy experiment : here are several small iron balls ; I shall try what number of these the magnetic bar will, sustain at different places, and you find that it sup- ports the greatest number near the ends \ this will an* 380 THE ACTION OF THE MAGNETIC swer our purpose in the first instance ; you will find this further confirmed by the subsequent experiments, de- signed to point out with accuracy the situation of the poles of a magnet. I have covered a pane of glass with writing paper, that the difference in colour may enable us to discern more distinctly what effect a magnet has on steel filings strewed over the paper ; I place this pane over a magnet, and sift some fine steel filings thereon ; these you see arrange themselves in a very curious manner ; those points from which the curves seem to rise, and over which the filings stand in an erect position, are the poles of the magnet. Here is a small needle inclosed in a glass ball ; move this over a magnetic bar, and the needle will be perpen- dicular to the bar, when it is over either of the poles. The poles of a magnet may be ascertained with great accuracy by means of a small dipping needle, plate 2, Jig. 7, (Electricity ) ; place this on a magnet, and move it backwards and forwards till the needle is perpendicu- lar to the magnet, it will then point directly to one of the poles. When it is between the north and south poles, so that their mutual actions balance each other, the centre of the needle will stand over what is called the equator of the magnet, and the needle will be ex- actly parallel to the bar ; between this situation and the poles, it inclines to the bar in different angles, accord- ing to its distance from the poles. OF THE ACTION OF THE MAGNETIC POLES ON EACH OTHER. in the action of the magnetic virtue at the poles, there is a strong similarity with that of electricity ; thus the contrary, or north and south poles of two magnets attract each other, but poles of the same name, as two north or two south poles, repel each other. Suspend on a point a touched needle, then present towards its north pole the south pole of a magnet, d it will be attracted by, and fly towards ir ; present l other pole of the magnet, and the needle will fly from POLES ON EACH OTHER. 381 Strew a few steel filings upon a pane of glass, put either the north or south pole of one of the bars under the pane ; the filings will rise upon the glass as the magnet approaches. Bring the same pole of the other bar directly over that under the glass, and when it is at a proper distance, the steel filings will drop flat on the pane. Fix two needles horizontally in two pieces of cork, and put them in water ; if the poles of the same name be placed together, they will mutually repel each other ; if the poles of a contrary denomination be turned to- wards each other, they will be attracted and join. Dip the north or south ends of two magnets in steel filings, which will hang in clusters from the end of the bars ; bring the ends of the bars towards each other, and the steel filings on one bar will recede from those on the other. Dip the south pole of one magnet, and the north pole of the other, into steel filings, and bring the ends near to each other, and the tufts of filings will unite, forming small circular arches. THE ACTION OF THE MAGNETIC POLES RENDERED VISIBLE BY STEEL FILINGS. I place the glass pane covered with paper over a mag- netical bar, and strew it over with steel filings ; on striking the glass gently, the filings dispose themselves in such a manner, as to represent with exactness the course of the magnetic matter. The curves, by which it seems to go from pole to pole, are pleasingly indicated by the arrangement of the filings ; the larger curves rise from one polar surface and extend to the other ; they are larger in proportion as they rise nearer the axis or centre of the polar surface ; the interior curves are smal- ler and smaller in proportion to their distance from the end ; see plate $., fig. 8. The greater the distance be- tween the poles of a magnet, the larger are the curves which arise from the polar surface. Let two magnets be placed in a straight line at a small distance from each other, the south pole of the one op* 582 ACTION OF THE MAGNETIC POLES posed to the north of the other ; lay a pane of glass over them, sprinkle it with steel filings, and then strike the pane gently with a key, and the filings will arrange themselves in the direction of the magnetic virtue ; those that lie between the two polar surfaces, and near the common axis, are disposed in straight lines, going from the north pole of one to the south pole of the other, as if uniting and joining together; plate 2, Jig. 9. Place two north or two south poles under a pane of glass, on which iron filings have been strewed, and the tilings will be disposed into curves, which seem to turn back and avoid each other; plate 2, Jig. 10. In magnetism, as well as in electricity, it is not the mere matter that is attracted, but the state of the mag- netic fluid therein, so that the body always becomes magnetic before it is attracted ; and hence there is no magnetic attraction but between the contrary poles of two magnets. When a piece of iron, or any other substance that contains iron, is brought within a certain distance of a magnet, the powers thereof are separated, and it becomes itself a magnet, having poles, attractive power, and every property of a real magnet. That part which is nearest the magnet has a contrary polarity. The magnetism that soft iron acquires, when placed within the influence of a magnet only lasts while it con- tinues in that situation, but disappears as soon as it is re- moved. But with hard iron, and particularly with steel, the case is quite different. For the harder the iron, or the steel, the more permanent is the magnetism it ac- quires ; but it is also more difficult to render it mag- netic. Thus if two pieces, one of soft iron, the other of hard steel, but both of the same shape and size, be brought within the influence of a magnet, and at the same dis- tance, you will find the iron appear more magnetical than the steel ; but when the magnet is removed, the soft iron instantly loses its magnetism, whereas the steel will preserve it for a long time. A magnet will therefore attract soft iron more forcibly than hard iron, because it can render it more strongly magnetical. RENDERED VISIBLE BY STEEL FILINGS. 383 In the foregoing experiments, the steel filings became so many little magnets, with contrary poles. On the same principles, a large key, or any other untouched piece of iron, will attract and support a small piece of iron, while it is near the pole of a magnet, but will let it fall when removed therefrom. A ball. of soft iron, in contact with a magnet, will at- tract a second ball, and that a third, till the influence becomes too weak to suppost a greater weight. Here is a small spinner, plate 2, fig. 1 1 , with an iron axis ; I spin the spinner, and then take it up by a mag- net, and you will not only find that it will continue spin- ning longer than if it were left to whirl on the table, but a second and a third whirligig may be suspended one under another, and yet continue in motion. The number suspended depends on the strength of the mag- net. OF MAGNETIC CENTRES. There is a point between the two poles, where the magnet has no attraction nor repulsion ; this point is called the magnetic centre^ though it is not always exact- ly between the two poles. Pass the dipping needle, plate 2, fig. 7, over a mag- netic bar, and you will find a place between the two poles, where the needle will be parallel to the bar ; but if you move it ever so little from thence, it immediately inclines towards the poles, and when over either pole, is perpendicular to the bar. This effect is also pleasingly exhibited by surrounding a magnet with small compass needles. I place the nee- dles on these brass stands, so that they may be nearly in the same place with the bar, and you see those near the ends incline towards the pole, but that the two needles near the middle of the bar are parallel thereto, not in- clining to either pole ; see plate 2, fig. 1 2. You may also observe, that the north pole of the magnet attracts the south poles of all the needles, and the south, the north of the needles. 384 OF MAGNETIC CENTRES. Lay a number of magnetic bars in a straight line with the north and south poles together, pass the dipping needle over them, and you will find a magnetic centre at each place of contact, the union of the two powers destroying their action ; separate them, and you have the north and south poles, as at first. Upon the same principles, if a magnetic bar be broken into any two parts, each part becomes a magnet, having two poles ; the ends of which next to where it was bro- ken acquiring a polarity contrary to the other end. Place a magnetic needle upon one of the stands, and when the needle is steady, place an iron bar about eight inches long and between a quarter of an inch and one inch in thick- ness, upon the stand, so that one end of it may be on one side of the north pole of the needle, and so near it as to draw it a little way out of its natural direction. In this situation, approach gradually the north pole of a magnet, to the other extremity of the bar, and you will see that the needle's north end will recede from the bar more and more, in proportion as the magnet is brought nearer to the bar. If the experiment be repeated, with only this difference, viz. that the south pole of the mag- net be directed towards the iron bar, then the north end of the needle will advance nearer and nearer to the bar, in proportion as the south extremity of the magnet is brought nearer to the iron. The reason of this phenomenon is, that, by the ap- proach of the north pole of the magnet, in the first case, the extremity of the iron bar which lies next to it ac- quires a south polarity, and, of course, the opposite ex- tremity acquires the north polarity ; in consequence of which the needle is repelled, because magnetic poles of the same name repel each other ; but in the second case, when the south pole of the magnet is brought near the bar, the end of the bar which is next to it acquires the north polarity, and the opposite end acquiring the south polarity, attracts the north end of the needle. If, whilst the pole of the magnet stands contiguous to one end of the bar, a small magnetic needle be presented within a certain distance to various parts of the surface of the latter, it will be observed, by the attraction and TO RENDER IRON AND STEEL MAGNETIC. 385 repulsion of the needle, that that half of the bar which is next to the magnet possesses the contrary polarity, and the other half the same polarity with the pole of the magnet that is applied to the iron. The magnetic centre, however, or the limit between the polarities, is not always in the middle of the bar ; it is generally nearer that end which is presented to the magnet. This difference is greater as the magnet is weaker, and the length of the bar increases ; but when the bar exceeds a certain length, which depends on the strength of the magnet, then the bar acquires several successive poles, viz. when the north pole of the mag- net is contiguous to one of its extremities, that extre- mity becomes a south pole ; a few inches farther on you will have a north polari-y, then a south polarity, and so on. In this case, the first magnetic centre comes very near that end of the bar which stands next to the mag- net, and other magnetic centres are formed between every pair of successive poles. Those successive poles become weaker and weaker in power according as they recede from that end of the bar which is contiguous to the magnet ; so that in a pretty extended bar, they quite vanish long before they come to the farther end of it ; hence, if one pole of a magnet be applied to the end of a long bar, the other end of the bar will not thereby acquire any magnetism. This will happen, when a magnet, capable of lifting about two pounds weight of iron, is applied to one extremity of an iron bar about one inch square and about five feet long. On removing the magnet, the bar, if of soft iron, will immediately lose all its magnetism ; otherwise it will re- tain it a longer or shorter time, in proportion to its hard- ness. TO RENDER IRON AND STEEL MAGNETIC. The communication of the magnetic power to iron and steel bars, is termed by artists, touching a needle, a bar 9 &c. To give a detail of the various processes used by ditferent artists for communicating magnetism to iron, would take up too much of our time •> I shalL, therefore, vol. IV, 3 d 386 TO RENDER IRON AND only mention two methods, which you will find ade- quate to every common purpose. I first place two magnets,* A, B, plate 2, Jig, 13, in a straight line, the north end of one opposed to the south end of the other, but at such a distance that the bar to be touched may rest upon them, taking care that the end I designed for the south be laid upon the north end of one bar, and the north end on the south pole of the other bar. I now take two other bars, D and E, and apply the north end of Dt and the south end of E to the middle of the untouched bar C, elevating their other ends so as to form an acute angle with the said bar. I now se- parate D and E, drawing them different ways along the surface of the bar C, but preserving the same elevation all the way ; I remove D and E to the distance of a foot or more from the untouched bar 0, and bringing the north and south ends in contact, I apply them again to the middle of the bar C, and shall repeat the process three or four times ; after which I shall touch the other three surfaces in the same manner, and the bar will thereby have acquired a strong and permanent magnetism. This was one of the methods used by Dr. Knight, who first taught us the great advantage that might be obtained from the use of magnetic bars, giv- ing by their means a magnetism to compass needles double in force to that which the strongest natural loadstone could communicate. He was the first also who found the way of working on the natural magnet, so as to increase its power in a great degree, and of inverting its poles at pleasure. You may readily communicate the virtue to un- touched bars by a horse-shoe magnet, shown at plate 2, Jig. 15, either single or compound; the bar to be touched should be laid on two other magnets, as in the * The longer and stronger these are, the better will they answt r the purpose. f The north ends of magnetic bars are generally marked by a line cut across them, as are also the north ends of horse-shoe or other shaped mag- nets, E. Edit. STEEL MAGNETIC. 387- preceding case ; the horse-shoe magnet must be placed on the middle of the untouched bar, with the north end towards that you design to be the south ; you are then to draw it backwards and forwards over the bar five or six times, but be careful when you remove it that it be at that time over the middle of the bar. The same operation is to be used with the other surfaces of the bar. A small compass needle may be touched by being put between the opposite poles of two magnetic bars ; while it is receiving the magnetism, it will be violently agitated, moving backwards and forwards as if it were animated : when it has received as much magnetism as it can acquire in this way, it becomes quiescent.* TO TOUCH A HORSE-SHOE MAGNET. Place a pair of magnetic bars against the ends of the horse-shoe magnet, with the south end of the bar against that end of the horse-shoe which is intended to be the north, and the north end of the other bar to that which is to be the south : the contact or lifter of soft iron to be placed at the other end of the bars. In this situation the magnetic fluid, which circulates through the bars, will endeavour to force a passage through the horse-shoe magnet, and thus facilitate the further communication of the magnetic virtue to the horse-shoe magnet : to this end, rub the surfaces of the horse-shoe with a pair of bars placed in the form of * Magnetism is best and most conveniently communicated to compass needles by the two following methods : 1. By a pair of magnetic bars not less than six inches in length. Fasten the needle down on a board, and with a magnet in each hand draw them from the centre upon the needle outwards ; then raise the bars to a considerable distance from the needle, and bring them perpendicularly dowu upon the centre, and draw them over again. This repeated about twenty times will magnetize the needle, and its ends will point to the poles contrary to those that touched them. 2. Over one end of a combined horse-shoe magnet, of at least two in number and six inches in length, draw from its centre that half of the nee- dle which is to have the contrary pole ; from a considerable distance draw the needle over it again. This repeated about twenty times at least, and the same for the other half, will sufficiently communicate the power. E. Edit.. $SS TO MAKE A MAGNETICAL BAR a compass, or with another horse-shoe magnet, turn- ing the poles properly towards the poles of the horse- shoe magnet, being careful that these bars never touch the ends of the straight bars, as this would disturb the Current of the magnetic fluid, and injure the operation. If the bars be separated suddenly from the horse-shoe magnet, its force will be considerably diminished ; to prevent this, slip on the lifter or support to the end of the horse-shoe magnet, but in such a manner, however, that it may not touch the bars ; the bars may then be taken away, the support slid to its place, and left there to strengthen the circulation of the fluid. TO MAKE A MAGN2TICAL BAR WITH SEVERAL POLES. Place magnets at those parts where the poles are in- tended to be, the poles to be of a contrary name to those required ; and if a south pole be fixed on one part, the two next places must have north poles set against them ; consider each piece between the suppor- ters as a separate magnet, and touch it accordingly. The difference in the nature of steel with respect td its receiving magnetism, is exceedingly great, as is ea- sily proved by touching in the same manner and with the same bars two pieces of steel of equal size, but of different kinds. With some sorts of steel a few strokes are sufficient to impart to them all the power they arc capable of retaining ; other sorts require a longer ope- ration ; sometimes it is impossible to give them more than just a sensible degree of magnetism. Steel that is hardened receives a more perfect mag- netism than soft steel, though it does not appear that they differ from each other in any thing but the ar- rangement of the parts ; perhaps the soft steel contains phlogiston in its largest pores, while hardened steel contains it in the smaller. Iron and steel have very lit- tle air incorporated in their pores ; when they are se- parated from the ore, they are exposed to a most in- tense degree of heat ; and most of the changes to which they are afterwards submitted, are effected in a red-hot state. A piece of spring-tempered steel will WITH SEVERAL POLES. 389 not retain as much magnetism as hard steel, soft steel still less, and iron scarce retains any. From some ex- periments of Mr. Musschenbroek, it appears that when iron is united with an acid, it will nut become magne- tical ; but, if the acid be separated, and the phlogiston restored, it will become as magnetical as ever. In communicating magnetism, it is best to use weak magnets first, and those that are stronger afterwards ; but you must be very careful not to use weak magnets after strong. A magnet can never communicate a greater power than itself possesses, or even of an equal degree ; but, as several magnets of nearly an equal degree of magne- tism, by being joined together, have a stronger power than either of them singly ; in order to impart a stron- ger magnetic power to a given body A, by means of a weak magnet B, you must first render several bodies, C, D, E, F, &c. weakly magnetic, and then by pro- perly joining C, D, E, F, together, you may commu- nicate to another body, or several bodies, a stronger magnetism ; and thus by degrees be able to communi- cate to A the desired degree or magnetic power. A magnet loses nothing of its own power by commu- nicating to other substances, but is rather improved thereby. If bars of iron be heated, and then cooled equally in various directions, as parallel, perpendicular, or in- clined to the dipping needle, the polarity will be fixed according to their position, strongest when they are pa- rallel to the dipping needle, and so less by degrees, till they are perpendicular to it, when they will have no fixed polarity ; but if, upon cooling a bar of iron in water, the under end be considerably hotter than the upper, and the upper end be cooled first, it will some- times become the north pole, but not always. If iron or steel undergo a violent attrition in any one particu- lar part, it will acquire a polarity ; if the iron be soft, the magnetism remains very little longer than while the heat continues. Lightning is the strongest power yet known in producing a stream of magnetism ; it will in an instant render hardened steel strongly magnetical, and invert the poles of a magnetic needle. JJ ! 390 TO MAKE A MAGNETICAL BAR Every kind of violent percussion weakens the power of a magnet. A strong magnet has been entirely de- prived of its virtue by receiving several smart strokes of a hammer ; indeed, whatever deranges or disturbs the internal pores of a magnet, will injure its magnetic force, as the bending of touched iron, wires, &c. Fill a small dry glass tube with iron filings, press them in rather close, and then touch the tube as if it were a steel bar, and the tube will attract a light nee- dle, &c. shake the tube so that the situation of the fil- ings may be disturbed, and the magnetic virtue will vanish. But, though a violent percussion will destroy a fixe magnetism, yet it will give polarity to an iron bar which had none before ; for a few smart strokes of a hammer on an iron bar will give it a polarity, and by hitting first one end of the bar, and then the other, while it is held in a vertical situation, the poles may be changed. Twist a long piece of iron wire backwards and forwards several times, then break it off at the twisted part, and the broken end will be magnetical. The pole of a magnet always produces the contrary polarity on a bar to which it is applied : therefore, if two bars fully touched have the poles of the same name joined together, they tend to produce on each other a force of a contrary name to that with which they are endowed ; and this effect will diminish the polar force of each bar ; consequently, the magnetic force of each longitudinal element of an artificial magnet diminishes as its bulk is increased, and the total force of two mag- nets fully touched, and of the same length, but unequal in bulk, will be in a less ratio than that of their mass. If the magnet do not touch the bar, but be held at some distance from it, the phenomena will be the same ; but the bar will acquire less magnetism than when it was in contact with the magnet. Each point of a magnet may be looked upon as the pole of a smaller magnet, tending to produce on the points of the magnet a force contrary to its own. The effect of this tendency will be greater, in proportion to the force of the point, and its nearness to those points WITH SEVERAL POLES. 391 m which it acts ; and the force of a magnet will depend m the reciprocal action of these points on each other. Hence, a narrow bar will in general be more power- ful than a broader one ; and hence also the exterior ?dges and points of a magnet will have more power han the interior ones of the same bar. Hence, also, magnets should never be left with two lorth or two south poles together ; for, when they are ;hus placed, they diminish and destroy each other's nagnetism, Magnetic bars should therefore be always [eft with the opposite poles laid against each other, or by connecting their opposite poles by a bar of iron. The magnetic power is increased in a magnet, by let- ting a piece of iron remain attached to one or both of its poles. A single magnet should therefore be always thus left. OF ARMED MAGNETS. As both magnetic poles together attract a much greater weight than a single one, and as the two poles of a magnet are generally in opposite parts of its sur- face, in which situation it is almost impossible to adapt the same piece of iron to both at the same time ; two soft pieces of iron are applied to the poles of a loadstone, so as to project on one side the magnet ; these pieces being rendered magnetic, another piece of iron can be conveniently adapted to these projections, so as to let both poles act at the same time. The magnet in this case is said to be armed, the pieces of iron are called the armature, the piece of iron that connects the poles is termed the lifter. Plate 2, Jig. 14, represents an arm- ed artificial magnet. In a similar manner the load- stone, or natural magnet, is advantageously armed. To avoid the expense and trouble of the armature, artificial magnets have been made in the shape of a horse-shoe, of which I have already spoken. Gassendi invented a peculiar kind of armour, by piercing a loadstone in the direction of the axis, and placing a cylinder of iron in the hole, which augment- ed considerably the force of the magnet. 392 OF THE MAGNETISM OF THE EARTH, Here is a straight magnetic bar, the north pole of which supports four ounces. I apply another magnet against it, but so that the north pole thereof is about half an inch from the pole of the other, and it will now sustain near seven ounces. OF THE MAGNETISM OF THE EARTH. What has been usually termed the magnetism of the earth, might with more propriety be termed the mag- netism of the atmosphere. Even the experiments usu* ally adduced to prove the magnetism of the earth, are full proofs that it is an aerial influence ; as you will per- ceive by the account I am going to give you of the ex- periments brought in support of the earth's magnetism. Mr. Savery has adduced several instances to show the force and action of the earth's magnetism ; among others, that it will support small pieces of iron. He hung up a bar of iron, about five feet long, by a loop of small cord at the upper end, and then carefully wip- ed the lower end, and the point of a nail, that there might be no dust or moisture to prevent a good con- tact ; then holding the nail under the bar with its point upward, he kept it close to the bar, holding only one finger under its head for the space of thirty or more seconds ; then withdrawing his finger gently down- wards, that the nail might not vibrate; if it fell off, he wiped the point as before, and tried some other part of the plane at the bottom of the bar. If the ends be similar, and the bar have no permanent virtue, it is in- different which end is downwards ; if it have an imper- fect degree of polarity, one end will answer better than the other. The upper end, A, of a long iron rod, which has no fixed polarity, will attract the north end of a mag- netic needle ; the under end, B, repels the north end of the needle ; invert the iron bar, and the end B, which is now the upper one, will attract the north pole of the needle it repelled before. The case is the same, if the bar be placed horizontally in the magnetic meri- dian ; the end towards the south will then be the north pole. THE MAGNETISM OF THE EARTH. 393 The explanation of this curious phenomenon is easily- deduced from the foregoing observations ; for, since in these northern parts, the earth is possessed of a south magnetic polarity, the lowest part of the iron bar, by be- ing nearest to it, must acquire the contrary, namely, the north polarity ; the other extremity of the bar becoming a south pole. It follows, likewise, and it is confirmed by actual ex- periment, that in the southern parts of the earth, the lowest part of the bar acquires the south polarity ; that on the equator, the bar must be kept horizontal, in order to let it acquire any magnetism from the earth; and that, even in these parts of the earth, the most advantageous situation of the bar is not the perpendicular^ but that a little inclined to the horizon. In short, in every part of the world it must be placed in the magnetical line, viz, in the direction of the dipping needle. If the iron bar* instead of being kept in the magnetical line, be^placed in a direction perpendicular to it, then it will acquire no magnetism, because in that situation the actions of both poles of the earth upon each extremity of the bar are equal. If, instead of the above-mentioned two directions, the bar be placed in any other position, then it will ac- quire more or less magnetic power, according as it ap- proaches nearer to the former or to the latter of the said two directions. Iron bars of windows, which have remained long in a vertical position, acquire a fixed polarity. Mr. Lewen- boek mentions an iron cross, which had acquired a very strong polarity. Mr. Canton proposed to make artificial magnets without the assistance of natural ones ; but in this he was mistaken, for his poker and tongs were natu- ral magnets, and had their verticity fixed by being heated and cooled in a vertical position ; and an iron or steel bar, though without a verticity, while it remains in that posi- tion exerts a polarity, and is able to communicate a fixed verticity to the small bar, and is, therefore, for the lime i natural magnet. And further, every iron bar, from .he largest size to a sixpenny nail, will exert this power when treated as above-mentioned. But how this power is raised so soon to a degree greatly exceeding that which VCL. IV. 2 E ,494 DIRECTIVE PROPERTY OF MAGNETS. communicated it, we do no* know ; nor is it more easy to account for the facility with which the magnetic power is withdrawn by a friction contrary to that which gave it. OF THE DIRECTIVE PROPERTY OF MAGNETS. Let an iron rod be exactly balanced and suspended on a point, so as to revolve in a plane parallel to the horizon ; communicate the magnetic virtue to this rod, and one extremity will be always directed towards the north. Here is an untouched magnet, I place it on a point, and you may observe that I can make it rest in any given situation ; I shall communicate the magnetic virtue to it, and you will then find it no longer indifferent as to its situation, but it will fix upon one in preference to any other, one end always pointing to the north. Whenever a magnet can move itself freely, as if it be suspended by a fine thread, or if it be made to float on water by means of a piece of cork, or if it be balanced on a point, provided it be not disturbed by the vicinity of iron; it will always place itself so as to direct its north pole towards the north, and the south pole towards the south. The directive power of a touched needle is of the great- est importance to mankind ; it enables the mariner to tra- verse the ocean, and thus unites the arts, the manufac- tures, and the knowledge of distant countries, together. The surveyor, the miner, and the astronomer, derive many advantages from this wonderful property. The mariner's compass consists of three parts, the box, the card or fly, and the needle. The card is a circle of stiff paper representing the ho- rizon, with the 32 points of the compass marked on it; the magnetical needle is fixed to the under side of this card ; the centre of the needle is perforated, and a cap with a conical agate at its top is fixed in this perforation ; this cap is hung on a steel pin, which is fixed to the bot- tom of the box, so that the card hanging on the pin turns freely round its centre ; one of the points being, from the property of the needle, always directed towards the DIRECTIVE PROPERTY OF MAGNETS. 395 north pole. The box which contains the card and needle, is a circular brass box hung within a square wooden one, by two concentric rings called jimbals, so fixed by cross centres to the two boxes, that the inner one shall retain a horizontal position in all motions of the ship. The top of the inner box has a cover of glass, to prevent the card from being disturbed by the wind.* Before the compass was invented, the navigating of ships was a tedious and precarious operation, and seldom performed out of sight of land ; but this instrument enables the mariner to tra- vel over the seas almost in as direct and true a tract, as the land carrier directs his carriage in a well-beaten road. It has been already observed, that the ancients do not seem to have been acquainted with the directive power of the magnet. The only thing that seems capable of being mistaken for some such knowledge, is what Jamblichm tells us in his life of Pythagoras ', " That Pythagoras took from Abaris, the Hyperborean, his golden dart, without which it was impossible for him to find his road." But the authority of the writer, as well as the obscurity of the passage, prevents any conclusion being drawn from it. Paul, the Venetian, is said to have introduced the use of the compass in 1260 ; but this is said not to have been his own invention, but borrowed from the Chinese. P. Gaubil says, the directive power of the needle was known to the Chinese as early as the year A. D. 223, under the dynasty of Haz. But the Abbe Renaudot, in his Disserta- tion on the Stone, when the Mahomedans went first to China, has adduced strong reasons to prove, that the Chi- nese knew nothing of the mariner's compass till it was in- troduced there by the Europeans. Vertomanus affirms, that A.D. 1500, he saw an East-Indian pilot direct his course by a compass, framed and fastened like those used in Europe ; but this must be received with some caution, as M. Barlow, in 1597, says, that in a personal confer- ence with two East-Indians he was told by them, that _ * This is called simply the steering comfiass; with the addition of sights, divided circles, &x,. for observing azimuths and amplitudes of the heave nlv bodies, it is called the azimuth ro^C5s...,.E.EsrT. 396 DIRECTIVE PROPERTY OF MAGNETS. instead of our compass they made use of a magnetical needle of six inches or longer, set upon a pin in a dish of white China earth filled with water; that in the bot- tom of the dish they had two cross-lines to mark the four principal winds, *and that the rest of the divisions were left to the skill of the pilot. But to return to Eu- rope, Mr. Pcrrault, in his parallel between the ancients and the moderns, has cited some verses of Guyot de Pro- vim, who wrote in 1 1 80, which show distinctly, that the mariner's compass was known in the south of France at that time. By most writers the invention of the compass is as- cribed to Flavio Gain, of Analsi in Campanee, who lived about the year 1300; and he is said to. have been the first that applied it to navigation in the Mediterranean. Mr. de Lalande informs us, that in Le Tresor de Bru- nei, a manuscript in the French king's library, there is a passage which proves that the compass was made use of about the year 1260. Here however it may be observed, that though a mag- net, which has only two poles, will always, when freely suspended, place itself in the magnetic meridian, or in the same plane with other good magnets ; yet when a magnet has more than two poles, these may be so situ- ate that the magnet will not traverse, that is, will have no directive power. Thus, suppose an oblong magnetic needle to have a north polarity equally strong at each end, and a south polarity in the middle ; it is plain, that as each has an equal tendency towards the north, neither of them can be directed towards the north in preference to the other ; consequently, the needle cannot traverse. Though this case very seldom occurs, yet there are many others where a needle, when fixed to a card on which the points of a compass are drawn, may occasion considerable errors; this has been clearly proved by Dr. Knight and Capt. Greaves. Mr. R. Walker, of Jamaica, has also clearly proved, that the only proper shape for magnetic com- pass needles, is that where the line of direction is in the edge of the bar ; each end of the bar should be pointed. [ 397 ] OF THE VARIATION OF THE COMPASS. Though the north pole of the magnet is, in every part of the world, directed nearly towards the north, yet it very seldom points exactly thereto, and consequently the south pole of the magnet seldom points towards the south. In other words, the magnetic meridian seldom coincides with the meridian of the place, but generally varies from it some degrees eastward or westward. This variation is different in different places on land as well as at sea, and is continually varying in the same place. For instance, the variation is not the same in Lon- don as at Paris, or at the Cape of Good Hope ; and the declination at London, or at any other place, is not the same now that it was twenty years ago. This variation is always reckoned from the north ; that is, if the north end of a needle vary to the east of the north, the variation is said to be easterly ; and if it vary to the west, the variation is said to be westerly. The uncertainty of the quantity of this variation in dif- ferent parts of the world is a great impediment to the per- fecting of navigation ; and philosophers have earnestly endeavoured to investigate its cause, and, if possible, to correct the errors it occasions. Though the directive power of the compass was applied to the purposes of navigation in the fourteenth and fif- teenth centuries, it does not appear, that there were any apprehensions during that time of its pointing otherwise than due north and south. The variation of the compass is said to have been first discovered by Columbus, the latter end of the fifteenth century. But the first person who discovered that it was real, and was the same with all needles in the same place, is generally allowed to be Sebastian Cabot. This was about the year 1497. After the variation was discovered by Cabot, it was thought, for a long time, to be invariably the same at the same places in all ages ; but Mr. Gellibrand, about the year 1625, discovered, that it was different at different times in the same place. 398 VARIATION OF THE COMPASS. From successive observations made afterwards it ap- pears, that this deviation was not a constant quantity, but that it gradually diminished, and at last, about 16.57, it was found, that the needle pointed due north at Lon- don, and has ever since been increasing to the westward of the north. So that in any one place the variations have a kind of libratory motion, traversing through the north to unknown limits eastward and westwards. The present variation at London is about two points, or 23 degrees west of the north. Dr. Halley supposed, that the earth has within it a large magnetic globe, not fixed within to the external parts, having four magnetic poles, two fixed and two moveable, and by this he has endeavoured to account for the phenomena of the needle. His application of this theory to facts is in many respects inadequate, in all la- boured and unnatural. Mr. Eider has shown, that he can with two magnetic poles placed on the surface of the earth, account for all the phenomena as well as Dr. Halley with four ; but his theory has also various imperfections. The variation of the needle may be illustrated by placing several touched needles round a magnetic bar; see plate 2, fig, 12. Now, if the earth be a great mag- net, or if it have only a magnetic atmosphere, it is clear from this experiment, that magnetic needles placed on its surface would have different directions in different places, which is conformable to experience ; and the apparent irregularities in the variation of the needle must be occasioned by the situation of the magnetic poles of the earth. If the magnetic poles agreed with those of the earth, there would be no variation, and the magnetic needle would point to the true north and south. If the axis of the magnetic poles passed through the centre of the earth, it would be easy to assign the quantity of the va- riation at every place ; but as this is not the case, to ac- count regularly for the variation, it is necessary to know the exact situation of the magnetic poles of the earth, their number, force, and distance from the real poles ; whether they shift their place, and if they move, the quantity of motion every year. [ 399 ] OF THE DIURNAL VARIATION OF THE NEEDLl. About the years 1722 and 1723, Mr. George Graham made a number of observations on the diurnal varia- tions of the magnetic needle. In the year 1 750, Mr. Wargentin took notice of the regular diurnal variation of the needle ; and also of its being disturbed at the time of an aurora borealis. About the latter end of the year 1756, Mr. Canton began to make observations on the variation, and in 1759, communicated several va- luable experiments to the Royal Society. The observations were made by him for 603 days ; on 574 out of these the diurnal variation was regular. The absolute variation of the needle westward was in- creasing, from about eight or nine o'clock in the morn- ing till about one or two in the afternoon, when the needle became stationary for some time ; after that, the variation westward was decreasing ; and the needle came back again ro its former situation in the night, or by the next morning. The diurnal variation is irregular when the needle moves slowly eastward in the latter part of the morning, or westward in the latter part of the afternoon ; also when it moves much either way after night, or suddenly both ways in a short time. These irregularities seldom happen more than once or twice in a month, and are always accompanied with an aurora borealis. The diur- nal variation in the months of June and July is almost double that in January and December. Mr. Canton supposes, that the diurnal heat of the sun acts upon the magnetic parts of the earth, or rather upon the magnet included in the earth. But Mr. JEfi- mis has shown, that this supposition is inadmissible, be- cause agreeably to the hypothesis the magnetic nucleus must be very profound, and it is well known, that the solar heat does not penetrate to very great depths; there are caves at no great distance from the surface of the earth, in which a thermometer remains always at the same height. The diurnal heat does not penetrate even these, there is therefore no probability of its ef- fects extending to still greater depths. [ 400 ] OF THE DIP OF THE NEEDLE, If a needle, which is accurately balanced and sus- pended, so as to turn freely in a vertical plane, be ren- dered magnetical, the north pole will be depressed, and the south pole elevated above the horizon : this pro- perty is called the inclination or dip of the needle. As it is very difficult to balance a needle accurately, the poles are generally reversed by a magnet, so that its two ends may dip alternately, and the mean of the two is taken.* This property was discovered by Robert Norman, about the year 1576. I shall give the account of the discovery in his own words : " Having, says he, made many and divers compasses, and using always to finish and end them before I touch- ed the needle, I found continually that after I had touch- ed the yrons with the stone, that presently the north point thereof would bend or decline downwards under the horizon in some quantity ; insomuch, that to the ilie of the compass, which before was made equal, I was still constrained to put some small piece of wax in the south part thereof, to counterpoise this declining, and to make it equal again. " Which effect having many times passed my hands without any great regard thereunto, as ignorant of any such property in the stone, and not before having heard nor read of any such matter; it chanced at length that there came to my hands an instrument to be made, with a needle of six inches long, which needle after I had polished, cut off at just length, and made to stand level upon the pin, so that nothing rested but only the touch- ing of it with the stone : when I had touched the same, presently the north part thereof declined down in such * The dipping needle represented at plate 2, Jig. 7, is one of the com- monest and smallest kind. The mo>t complete and accurate dipping or rather universal magnetic nadle, showing at the same time the horizontal and vertical directions of the magnet, was contrived by the late ii.genious Dv Larimer, a philosopher, who, among the moderns, has perhaps n the greatest variety of experiments and discoveries in the science. See Ilia Concise Essay on Magnetism, 4to. 1795. — E. Edit. INFLUENCE OF THE AURORA BORCJALIS, &C. 401 sort, that being constrained to cut away some of that part to make it equal again, in the end I cut it too short, and so spoiled the needle wherein I had taken so much pains. " Hereby being stroken into some cholar, I applied myself to seek further into this effect, and making cer- tain learned and expert men (my friends) acquainted in this matter, they advised me to frame some instru- ment, to make some exact trial, how much the needle touched with the stone would decline, or what great- est angle it would make with the plane of the horizon/ 1 Thus far Mr. Norman. The dip is said to be subject to a variation. At this time in London it is about 72 degrees ; from some late obvervations it appears to diminish about fifteen minutes in four years. The nature of this phenomenon is plea- singly illustrated by carrying a small dipping needle from one end of a magnetic bar to the other ; when it stands over the south pole, the north end of the needle will be directed perpendicularly to it ; as the needle is moved, the dip will grow less, and when it comes to the magnetic centre it will be parallel to the bar ; after- wards the south end will dip, and the needle will stand perpendicular to the bar, when it is directly over the north pole.* OF THE INFLUENCE OF THE AURORA B0REALIS ON THE MAGNETIC NEEDLE. Messrs. Wilcke and Van Swinden have clearly prov- ed, that there is a connexion between the aurora bo- * Plate 2, Jig. 17, represents an instrument called a magnetometer* or an instrument to ascertain the comparative strength of magnetical bars. A is a brass quadrant divided into 90° ; B, a magnetic needle vertically suspended and balanced ; C, a brass base divided into inches and tenths. The bars to be examined are laid on the base, and their respective powers are shown by the distance of the ends of the bars on the base from on the ire, and the number of degrees on the arc, up to which the needle, B, is repelled. Collections of the several magnetical articles, part of which are shown in plate 2, to illustrate the general principles of magnetism, are selected and packed by us in cases, and which form either to the lecturer or student d new and useful collection of curious instruments. E, Edit, VOL. IV. 3 F 402 OF THE THEORY OF MAGNETISM. realis and the magnetic needle ; they have shown it to be so evident, so general, and so constant, that no one, who examined the affections of the one and the other with attention, could have any doubts on the subject. It remained, however, for Mr. Dalton* to give a com- plete and satisfactory account of this connexion, and it is with great pleasure I take this opportunity of recom- mending his work to your attentive perusal. From various observations he has demonstrated, 1. When the aurora appears to rise only about 5° 10', or \5° above the horizon, the needle is very little dis- turbed, and often insensible. 2. When it rises up to the zenith, and passes it, there never fails to be a con- siderable disturbance. 3. This disturbance consists in a regular oscillation of the horizontal needle, sometimes to the eastward, then to the westward of the mean daily position, in such sort, that the greatest excursions on each side are nearly equal, and amount at Manchester to about half a degree on each side. 4. When the aurora ceases, or soon after, the needle returns to its former station. From these facts alone, says Mr. Dalton, indepen- dent of other observations, we cannot avoid inferring, that there is something magnetic constantly in the high- er regions of the atmosphere, that has a share at least in guiding the needle ; and that the fluctuations of the needle, during the aurora, are occasioned by some mutations that then take place in this magnetic matter in the incumbent atmosphere. OF THE SIMILARITY BETWEEN ELECTRICITY AND MAGNETISM. The powers of magnetism, like those of electricity, are excited and separated by friction. This effect is wonderful in both, but more so in magnetism, where two powers, naturally attracting each other, remain separated in the steel bar for many years, and yet they * Meteorological Observations and Essays, by John Dalfcrn, 1~93. HYPOTHESIS. 403 may be reduced to their natural state by the friction of two other magnets, acting in contrary order to that by which the poles were originally separated. Magnetism and electricity act powerfully at corners, edges, and points. Magnetism may be communicated to a small steel needle, by passing the discharge of a large electrical battery through it. The discharge of an electrical battery through a small magnetic needle will sometimes destroy the mag- netism, and sometimes invert the poles of the magnet. Similar effects have been produced by lightning. OF THE THEORY OF MAGNETISM. Here, as in other parts of natural philosophy, we must content ourselves with mere conjecture. Of the various hypotheses that have been formed to account mechanically for the phenomena of magnetism, that of Mr. Prevost* is undoubtedly the best ; but as it de- pends on a knowledge of Mr. le Sage's mechanical sys- tem of the universe, it will be impossible for me to lay it before you in a satisfactory manner ; you must there- fore be contented with a very imperfect sketch thereof. HYPOTHESIS. There exists in and about our globe a very subtile fluid, possessing the following properties : 1 . It is expansive, and consequently discrete. 2. The molecules of this fluid are formed by the union of two kinds of elements, -A, B, united by affinity. 3. The elements of the different kinds have a great- er tendency to each other than those of the same kind. 4. That excepting the preceding property, these at- tractions follow the same laws as universal gravitation. * Prevent de L'Origine lies Forces Magnetiques a Geneve, 17&8. 404 HYPOTHESIS. 5. This fluid has an affinity with the particles of iron, and which probably acts only at contact, or when near- ly in contact. This fluid is decomposed by iron, and seldom by any thing else. The foregoing properties of the magnetic fluid may be all mechanically explained on the principles of Mr. le Sage. To explain the magnetic phenomena of the earth, it is sufficient to suppose that one aliment of the magne- tic fluid is furnished by nature in a greater abundance in one hemisphere than the other ; or that a small por- tion thereof is decomposed by some of the causes per- petually acting in nature, by which means the terres- trial globe is maintained in a charged state, having a greater abundance of one element in one hemisphere than in the other. This accumulation may principally exist in the atmosphere. On considering that the aurora borealis, the zodia- cal light, electricity, and heat, all in some measure affect the magnetic needle, there seems ground for supposing that one or other of the elements of the magnetic fluid is furnished by the solar rays. When we consider the extent occupied by the mag- netic fluid, we are naturally led to enquire whether its effluvia course incessantly over land and sea, only to turn here and there a mariner's compass ? Being assured that God governs by a long subordination of second causes ; that he not only employs a concurrence of causes to produce one effect, but likewise produces va- rious effects from one and the same cause ; we may safely answer, that there are other uses of the magne- tic effluvia, besides those we discern. Here again, as in every other part of philosophy, we have a further confirmation of the littleness of human knowledge, and see how much pains God has taken, so to speak, to hide pride from man. [ 405 ] LECTURE LI. ON METEOROLOGY. I HERE is scarce any subject in which mankind feel themselves more interested, than in the state of the weather, that is, in the temperature of the air, the influences of wind, rain, &c. It forms a principal topic of common conversation. By the weather, the traveller endeavours to regulate his journies, and the farmer his operations ; by it plenty and famine are dis- pensed, and millions are furnished with the necessaries of life. It is intimately connected with the health of the human body, and with every part of natural history, and more particularly with agriculture. You will therefore find this branch of philosophy peculiarly in- teresting ; the more so as it will lead you to consider the great operations in nature. " Here you may see and admire the changes in the elements, which present us with all that is great and wonderful in nature, and which, with a variety little less than infinite, work together for the good of man, and the preservation of the world." I have long since observed to you, how improperly the science of natural philosophy has been treated by its most zealous advocates and ablest professors ; it is high time for them, after so much labour in vain, to return to the point from whence they should have set out, and now begin to consider the great agency of the ele- ments. It is by this agency, that all the phenomena we perceive are performed ; by it the growth of plants, the life of individuals, are supported and preserved ; by it the planets are maintained in their respective si- tuations, and made to revolve in their orbits. 406 ON METEOROLOGY, " There is no hope in the present mode of philoso- phizing, but of seeing experiments varied, and facts multiplied ; and they may be thus multiplied and varied to eternity without advancing us one step towards a knowledge of the causes operating in nature. The in- defatigable experimentalist may proceed for ever, and flounder like the mole in the dust he raises about him- self ; but by continually heaping up of facts, or mak- ing experiments, he will never be able to trace either the nature or design of the operations carried on in this system of things. " For the universe is a system, in which all the parts are connected and related, and mat- ter, as a part of the created world, has motion ; but he who would understand the nature of motion, by considering motion abstractedly, as is the case with many modern philosophers, is studying motion from that which has no motion belonging to it. There are, as I have before observed to you, no insulated facts in nature ; they are all systematic, or mechanical, having a double reference ; as effects to their causes, and as causes to their effects. The material world is an im- mense body, composed, like our own, of an infinite number of parts, so interwoven together, as to unite in one common centre. It is the business of philoso- phers to point out these connexions, and to explain when they appear to us as separated, and thus lead us to that principle of unity which harmonizes and con- nects all the works of creation. But alas, you find the philosopher continually losing sight of the true construction of nature, and endea- vouring to build systems upon matter independently considered, " upon which he can only raise such a world as never did nor can exist, being as empty and absurd, as it is arbitrary." " You find physicians treating of the nature and causes of diseases, of ble- mishes, of preternatural appearances in the body ; but wholly indifferent, and altogether inattentive to the proceedings of the healthy economy : you will find an hundred dissertations on fevers, for one upon life. The action of stimuli,, and the irritability of the living fibre, have been the subjects of many ingenious discussions : ON METEOROLOGY. 407 the regular and uniform action of the fibre, but of few. It is the same with philosophy ; we have treatises on light, as separated and divided by the prism ; on heat, as mea- sured by the thermometer ; but none on that ocean of the solar fluid, in which all bodies are as it were immers- ed -, none upon the various influences of the sun, upon which the natural life, and the activity of all things in the natural world depend.* If we look into artificial nature, we shall every where find a want of known agents. Hence the variety and changes of opinion with respect to a great number of phenomena that are observed in our laboratories; al- though we can there multiply and vary the processes, and thus subject our conjectures to experiment ; but the phe- nomena being all on a small scale, we are often but very little struck with circumstances, that may in themselves be very important, and which are daily perceived. We are diffident of the exactness of our measures and weights; we suspect some foreign influence from the vessels used, from the disparity in substances of the same kind and name, or from some unknown action of the air and va- pour ; and yet, unless we have learned not to be satisfied with vague conjecture, we seldom attend to the notices which result from the imperfections and inaccuracies of our theories. But in the laboratory of the atmosphere, all the phenomena are carried on upon a scale propor- tioned to their importance among the operations of na- ture, which can be disturbed by nothing foreign to these operations, without producing some characteristic pheno- mena ; every thing has a reference to the vessel itself, u e. to the surface of our globe, whose distinct parts, as mi- nerals, vegetables, and animals, offer masses perpetually changing ; here, therefore, the disagreement of theory with facts must give us great and important lessons.f * Young's Essay on the Powers and Mechanism of Nature. Jones's Physiological Disquisitions. Adajns's Dissertation on the Barometer, &x. t Ue Luc, ldees sur la Meteorologie, a work that should be fully consi- dered by all who mean to understand the subject, and to which I am indebt- ed for a great part of the Lectures on Meteorology. 408 ON METEOROLOGY. If you, however, compare attentively meteorological phenomena with our physical measures, the barometer, the thermometer, hygrometer, &c. you will find your- self unable to reduce them to any law, that can be ex- pressed by the range of these instruments : which shows more evidently than any thing that can be seen in our laboratories, the necessity of admitting other combina- tions than those that are known, and even perhaps other ingredients. The meteorological phenomena, whose causes we have yet to explore, are those that are most common and the most important to terraqueous physics. They are chan- ges of heat independent of seasons and latitude, those of winds, and the variations in the heights of a local baro- meter ; the vicissitudes of rain and fair weather ; aerial electricity and magnetism ; the relations of the state of the air to our sensations ; the small connexion we find between vegetation (as well in general, as for certain par- ticular products), and the different remarkable charac- ters of the seasons. All these grand lines in the opera- tions of nature on our globe are to us, with respect to the producting causes, as a sealed book. These observations on the chemistry of nature are by no means designed to lessen your esteem for that of our laboratories, which is certainly one of the principal sour- ces of all the true science we possess. They are designed to show you, that the smallest meteorological phenomena should be studied with as scrupulous an attention, as that we pay to the modifications of a little air in our close ves- sels ; for it is these phenomena which should guide us in our researches into the nature of the latter. In the atmosphere every cause produces its effects : the subtile fluids are there distributed according to their na- tural tendencies ; and can form or destroy themselves differently, in different times, different soils, different climates : the winds, which have seldom been consider- ed, but as more or less violent, warm, or humid, may, with respect to these fluids, answer more important pur- poses than what have yet been attributed to them. In a word, great general causes act in the atmosphere from which our confined air is secluded. It is therefore only ON METEOROLOGY. 409 by letting meteorology and chemistry go hand in hand, that we can hope to be secure from error in either pur- suit, or be enabled to make advances towards real know- ledge. Our researches into the nature of these interesting ob- jects will be ever vague, and of course attended with but little success, till we have some certain theory on the na- ture of expansible fluids, and in general of all physical agents. Here it may be proper to observe, that we com- monly join to physical laws the idea of their being an ex- planation of the phenomena ; but this notion is errone- ous, for all physical laws, not excepting even those of gravity, are only generalisations. We are now acquaint- ed with many phenomena, where the real agents mani- fest themselves, and whose laws flow from the nature of these agents : it is to such as these only that the idea of a physical explanation can be attached. Thus, to assign to phenomena a real and determined agent, acting in a certain manner, and from which certain effects are the natural consequences; and to show that the laws of these phenomena correspond thereto, is really to explain the phenomena. Now we know phenomena enough, thus connected with real causes, to enable us by analogy to ex- tend very far the real links that connect effects one with another in nature ; remembering always to consider true agents as being more and more general, the further they are removed from us ; or subordinate, as they approach us. These are the proper objects to occupy the attention of a rational philosopher, as they alone can form the foundation of rational physics, one that will give us effi- cacious aid in our experimental researches. What w r ould become of practical mechanics, if in elementary mecha- nics there were no system of its agents, no laws previously determined, which they follow when they operate ? And yet, though all is action in nature, philosophers scarce think of looking for the true agents. Among the means of advancing our knowledge in me- teorology, are the instruments that have been contrived to ascertain the variations in the weight of the atmos- phere ; its changes with respect to temperature ; the de- koL. TV, 3G 410 ON METEOROLOGY. grees of humidity, &c. If every one who is in posses- sion of meteorological instruments would keep a diurnal register of their state, and of the corresponding pheno- mena of the atmosphere, and transmit the result of his observations to the public, he would contribute more to the advancement of this branch of science than he might at first imagine. While he was amusing himself, and gratifying curiosity, he would be promoting knowledge, and probably procuring benefits for posterity. Let no one suffer the apparent improbability of success to discourage him from the attempt. Remember that science advances by slow and gradual steps, that its progress depends on the cultivation of the mind, the acquisition of facts, the removal of obstacles, and the exertion of individuals : the present is ever pregnant with the future, though the con- nexion between them can only be found by long atten- tion and diligent observation. A register for preserving what no memory can retain, becomes an authentic do- cument, a reference from which facts may be combined and compared, and thus one of the purest sources of practical knowledge. To indulge ends so rational to as great an extent as the human powers can reach, and with as much enjoyment as the human mind can bear, Divine Providence hath appointed the means whereby each man's small stock of knowledge and truth may be communi- cated to others without loss to himself; and further, how it may be placed in a common treasury for even" man to draw from thence whatever his occasions or in- clinations may require. These ends are known to be ac- complished, the one by speech,' the other by writing and publishing what is written. It may be observed in general, that meteorological phe- nomena, like all the durable motions of the universe, de- pend upon a circulation of matter. Here it is principally carried on by changing of water into a new form, and a regeneration of it again into its primitive form. It goes off from the surface of the earth in the form of a rare invi- sible expanded vapour ; in the atmosphere, its state is changed, from that of vapour to an aeriform fluid ; by some unknown cause it is again changed into mists and clouds, it is then gathered into drops when it fails, OF THE BAROMETER. 41 i and in this form it returns to the place from whence it came, to take its turn once more in the common course of evaporation, and be again and again circulated to the great promptuary of the world. The principal objects therefore of inquiry are, 1. In what manner the atmosphere is supplied with humidity ? 2. What causes, and what prevents, invisible humidity being formed into clouds ? And, 3. What occasions, and what prevents, visible clouds being precipitated into rain ? That is, to know the various balancings of the clouds, and learn how such ponderous materials are suspended in the air ; and how the waters are bound up in the thick clouds. As the principal means of answering these inquiries are the instruments used to discover the changes in the atmosphere, I shall first describe these, and then proceed to give you some account of meteorological phenomena. The instruments in general use are, 1 . A barometer ; 2. A thermometer ; 3. A hygrometer -, 4. Arain-gage ; .5. An electrometer. OF THE BAROMETER. In treating of the barometer, I shall have only to en- large on some circumstances that were but slightly noti- ced, when I explained this instrument to you in my third lecture. The barometer, as I there showed you, consists of a straight glass tube, about thirty-two inches long, open at one end and close at the other ; this tube is first filled with mercury, and then inverted in a bason of the same fluid, by applying a finger to the open end, so as to prevent any air from entering the tube ; when the open end of the tube is immerged beneath the surface of the mercury in the bason, the finger is withdrawn ; the tube being now set upright, the mercury descends, leaving the top of the tube, and subsiding till it has attained a certain distance from the surface of that in the bason; this is more or less, according to the state of the air at the time of mak- ing the experiment. The tube is then fixed to a frame 412 OF THE BAROMETER. with a scale annexed, to show at all times the height of the mercurial column. From the method of filling the tube, the air is excluded from the top of the column, or that part of the tube above the surface of the mercury in the tube. The external air presses upon the quicksilver in the cistern, and sustains, by its pressure in a contrary direction, a column equal in weight to itself. For it is evident, that the mercury endea- vours to descend with a force equal to that by which its descent is prevented. In other words, the pressure of the atmosphere on a given surface is equal to the weight of a column of mercury, whose base is the given surface, and height equal to that of the atmosphere. The height of the mercury is therefore an adequate measure of the weight or pressure of the air upon a surface equal to the base of the tube containing the mercury. The truth of this reasoning 1 have confirmed by seve- ral experiments, exhibited in my third lecture; you there saw, when a baromete/ was placed under a receiver, that in proportion as the air was exhausted from the surface of the mercury in the bason, the column in the tube descend- ed till at last they were nearly on the same level. The ba- rometer-gage exhibited the reverse of this experiment, for you there saw the mercury rise in the tube in proportion as the air was exhausted therefrom, the open air pressing at the same time upon the surface of the mercury in the baj.on. What is thus exhibited by our instruments is con- firmed by nature, for the higher we ascend in the atmos- phere the shorter is the column of mercury. The barometer with a straight tube, as originally con- trived by Torricellius, is preferable to all the subsequent variations in its form. When it was found that the differ- ent heights of the mercury in the barometer were in some measure connected with the state of the weather, the phi- losopher and artist endeavoured to vary the form of the instrument ; hence a variety of constructions more or less accurate, according to the views of the inventors, and the distance to which they removed the barometer from its original simplicity. They thought, that by augmenting the scale of the ba- rometer, and thus rendering the variations of the mercury OF THE BAROMETER. 413 more sensible, they should sooner discover the minute changes of the atmosphere, and the causes which occa- sioned them : unfortunately, by altering the construction of this instrument, they only multiplied errors, and ren- dered it less capable of answering the purposes for which it was designed. Some of these forms may appear more elegant than the plain barometer, but none of them can be depended upon for keeping an accurate register of the weather, or for observing the extent of the variation thereof in any given situation, or comparing the different changes at one place with corresponding ones at another. Hence it is necessary to point out to you what is requisite to con- stitute a good barometer. SOME OF THE PRINCIPAL REQUISITES OF A GOOD BAROMETER.* 1. It is requisite that the height of the column of mer- cury be altered by no other causes but the changes that arise from the pressure of the air ; and that these changes be truly indicated. 2. That the variations in the height of the column be ascertained by a known measure. 3. That the column of mercury be susceptible of the smallest alterations in the weight of the air. In order that the column of mercury in the tube may be affected by no other cause than the pressure of the lir, it is absolutely requisite that the upper part of the :ube, and the mercury itself, be entirely free from air ; For, if there be any air between the upper surface of the column of mercury and the sealed end of the tube, it will be the source of many errors and much irregularity. Ihe included air will act as a counterpoise against the weight of the atmosphere, and to a certain degree coun- :eract its pressure ; and, therefore, render the indication ^f the instrument uncertain and erroneous. This included lir, being also often combined with humidity, expanded 1 ^ee my Dissertation on the Earometfp and Thermometer, ir96* 414 TO BOIL THE QUICKSILVER by heat or contracted by cold, acts differently at differen times : the only method of preventing these errors, aru perfectly excluding the air from the barometer, is by boil ing the mercury in the tube ; an operation which is care fully performed in all the best instruments. TO BOIL THE QUICKSILVER IN A BAROMETER TUBE.* Choose a tube not less than three feet long with a bon about three or four twentieths of an inch in diameter, bu not more ; and that it may be sufficiently strong, it shoulc be nearly as thick on all sides as the diameter of the bore Let this tube be nicely sealed or closed at one end, anc as clean as possible ; fill the tube with pure mercury tc within two inches of the top, and then hold it with the sealed end lowest in an inclined position over a chafing- dish of burning charcoal, placed near the edge of a table, in order that all parts of the tube may be exposed suc- cessively to the action of the fire, by moving it obliquel) over the chafing-dish. The sealed end is to be first gra- dually presented to the fire ; as soon as the mercury be- comes hot, the internal surface of the tube will be studded with an infinite number of air-bubbles, giving the mer- cury a kind of grey colour ; these increase in size by rui ning into one another, and ascend towards the high( parts of the tube, where meeting with a cooler part the fluid, they are condensed, and nearly disappear. In consequence, however, of successive emigrations towards the upper parts of the tube which are successively heated, they finally acquire a bulk which enables them in their united form entirely to escape. When the first part ol the tube is sufficiently boiled, move it onward by little and little through the whole length of the tube. When the mercury boils, its parts strike against each other, and against the sides of the tube with such violence, that a person unacquainted with the operation naturally appre- hends the destruction of his tube. * Dc Lu:'s Rec'ierches sur les Modifications de FAtmcsphere. IN A BAROMETER TUBE. 415 The great advantages that result from this operation ippear to be these : the whole body of the mercury, and he interior surface of the tube are hereby freed from all he minute and imperceptible particles of dust and mois- ure which they generally contain, and of the little at- nospheres that are seen to surround them ; which, du- ing the tumultuous motions of the mercury, are visibly Iriven up towards its surface, and expelled. The tube and he mercury are deprived likewise of all the air that can >e expelled from them, and particularly from the surface >f the former, by the violent heat and agitation of boiling [uicksilver. As that heat too is a determinate or fixed [uantity, its effects in expelling the air from different tubes vill be nearly equal ; so that, though some small portion )f air may still be left in them, there can be no difference n the quantity of it remaining in different tubes thus uni- brmly treated. Accordingly, the barometers thus pre- )ared not only stand higher than those which have not mdergone this process ; but at the same time they pretty iccurately correspond with each other. M. de Luc observes, that the greatest part of the air vhich is expelled during the process, proceeds from the nternal surface of the tube, where it seems to have form- ed a thin stratum or lining of air, which cannot be dis- odged from thence by the mercury introduced into the ube in the common manner, but requires the violent heat >nd agitation of boiling quicksilver to detach it. But it is 'ery remarkable, that, after this aerial coating has been >nce effectually separated and expelled, if the tube be mptied and some other, even cold mercury, be intro- lucedinto it, the barometer thus extemporaneously made, vill be nearly as perfect or as free from air as before. It vill stand nearly as high as it did when it contained the tiercury that had been boiled in it ; if the same process be iow repeated, it will not be studded with bubbles of air, s in the former operation ; when the mercury has been ompletely boiled, the tubes may be cut off to their proper ?ngth by a file. When the tubes are well boiled, the mercury generally emains suspended at top, mi will not descend to its pro- er level without shaking the tube to bring it down. 416 TO BOIL THE QUICKSILVER That the changes in the height of the mercurial column may be truly ascertained, it is necessary to know at all times the exact distance of the surface of the mercury in the tube from the surface in the reservoir or cistern. The first point of the measure must commence from the surface of the mercury in the cistern ; but this sur- face is variable ; for, when the mercury descends, a quan- tity of it falls into the bason, and raises the surface there- of, and on the contrary, when it rises, a quantity is taken out of the cistern, and the surface thereof is lowered. The scale of inches to the barometer is fixed ; but the surface of the mercury in the cistern from which it originates is continually varying. To remedy this evil, it is necessary that the lower surface should be always kept at the same height from divisions on the scale affixed to the insrru ment. This is effected by means of a floating gage, which was first applied to the barometer by my Father, though others have, since his time, assumed the merit to them- selves. By means of the floating gage, the same screw- that renders the barometer portable, regulates the sur- face of the mercury in the cistern, so that it is always at the place from whence the divisions on the scale com- mence. This gage is never applied to the common port- able barometers, but only to those of the best kind. Another circumstance necessary to be attended to in very accurate observations, is the effect of heat and cold on the barometer, as by these the mercurial column is either dilated or contracted ; for, as all bodies expand and occupy larger spaces when their temperature is increased, the mercury in the barometer will, when heated, be spe- cifically lighter, and will consequently ascend from cause, though the pressure of the air should remain un- changed ; and therefore, in order to know accurate])' tfi< effect of the air's pressure on the barometer, it is ne sary to correct the height by the addition or subtraction of a quantity equal to the influence of the temperatui the air thereon. In some cases, a scale of correction i^ applied to the thermometer accompanying the barometer, and which is indeed a necessary companion to it. 2d. Condition. That the scale should be of some knowr measure. It would have been totally unnecessary to hav; OP THE NONIUS. 417 mentioned this condition, had it not been to prevent you from being imposed upon by venders of imperfect instruments. Some of these instruments have no de- terminate scale affixed to them ; and those which have a scale, have one that is in general ill-graduated ind erroneously placed, so that no comparative observa- tions can be made with them ; and often, indeed, no observation at all ; as, from the small bore of the tube, they act as a thermometer, as well as a barometer. I have already observed, that by enlarging the scale, er- ror is multiplied, and uncertainty produced. 3. Condition. That the smallest changes in the height of the column of mercury may be discerned. To measure the smallest changes, a nonius division moves with the index, by which each inch is subdivided into 100 parts, and the height of the mercury is accu- rately obtained without any danger from parallax, by the peculiar construction of the index. OF THE NONIUS. The scale of inches is affixed to the right side of the tube, the zero or beginning of the scale being at the surface of the mercury in the cistern, the index and its nonius plate slide up and down in a groove, which is parallel to the line of inches, that the index may be set at any time to the upper surface of a column of mercury. Each inch, or line of inches, is divided into ten parts, which are again subdivided into t^n, by means of the nonius scale ; the whole inch being thereby di- vided into 100 equal parts. TO READ OFF, OR ESTIMATE THE DIVISIONS OF THE NONIUS SCALE. I., If that edge of the nonius scale, which is in a line with the index, coincide exactly with any division on vol. IV. s a 418 THE COMMON PORTABLE BAROMETER, the line of inches, that division expresses the height of the index from the surface of the mercury in the cis- tern in inches and tenths of inches. But, 2dly, When the foregoing edge does not coincide with any division, you must look for that division of the nonius, which coincides with a division in the line of inches, and the number on the nonius shows how many tenth parts of the ten hundredth part the index or edge has passed the last decimal division. Thus for example, suppose the edge of the nonius was to point somewhere between 29 inches 8 tenths, and 29 inches 9 tenths ; then if by looking at the nonius, you observe the coincidence at J, it shows the altitude to be 29 inches 8 tenths, and /> parts of another tenth, or 29.85. OF THE COMMON PORTABLE BAROMETER. This instrument, when made with care, will answer for general and domestic observation, but is not suffi- ciently accurate for philosophical purposes. It consists of a tube of a proper length accurately filled w 7 ith mer- cury ; the lower end of the tube is glued to a wooden reservoir, the bottom of which is formed of leather; into this reservoir the superfluous mercury descends, and the air, by pressing upon the flexible leather at the bottom of the reservoir, keeps the mercury suspended at its proper height. This reservoir is concealed from the eye by a neat mahogany cover or box. This tube and reservoir are placed in a frame, on the upper part of which is a silvered brass plate ; on the right hand side of this plate is a scale of inches, reckoned from the surface of the mercury in the cistern ; each inch is divided into ten parts. Close to the line of inches there is a slit or groove for conveniently sliding the no- nius scale and index up and down. The upper edge of the index and nonius scale are in a line. It is the upper edge of the index that is to be set to the uppei surface of the mercury. On the left hand side of the plate the words fair, changeable, rain, are en- graved. At the bottom of the frame there is a screw THE BEST PORTABLE BAROMETER. 419 passing through the mahogany box which covers the reservoir : a flat round plate is placed upon the end of the screw within the box ; this end is designed to press upon the leather bag, and force the mercury up to the top of the tube, and thus prevent it from shak- ing, or violently striking against the top of the tube when transported from one place to another. TO USE THE PORTABLE BAROMETER. \ 1. Suspend it against a wall or wainscot, so that the tube may be perpendicular to the horizon. 2. Unscrew the screw at the bottom of the frame as low as it will go, and the mercury will fall to its proper height, and be obedient to the changes in the weight of the air. 3. Set the upper edge of the index so as to coincide with the surface of the mercury in the tube, and the nonius scale will point out the height of the column. 4. Before every observation, strike the frame gently with the knuckles to disengage the quicksilver from the tube. 5. When the barometer is to be moved from one place to another, turn the screw till the mercury is pressed by it against the top of the tube. DEFECTS OF THE COMMON PORTABLE BAROMETER. It is necessary here just to mention some of the de- fects of this kind of barometer, in order to render the advantages of the better kind more conspicuous. 1. It cannot be so adjusted, as ro be sure that the divisions on the scale are at that height from the mer- cury in the cistern, which is expressed by the numbers affixed to them. As when the mercury falls in the tube, it rises in the reservoir ; and when it rises in the tube, it falls in the reservoir ; its distance is perpetual- ly varying from the divisions of the scale. 2. The tension of the leather forms a considerable resistance to the pressure of the atmosphere. [ 42 ° J OF THE BEST PORTABLE BAROMETER. I This barometer, like the preceding, consists of a glass tube properly filled with mercury, having the low- er end fixed to a wooden cistern with a leather bottom, and this tube and cistern placed in a mahogany frame On the upper part of the frame a brass plate is plac- ed ; on the right hand side of the tube a scale of inches is graduated on the plate, the beginning of the scale being at the surface of the mercury in the cistern : each inch is divided into ten parts, which are again subdi- vided into tenths by the nonius scale. The nonius plate carries two indexes exactly similar to ach other, one placed before the tube, the other behind it. The indexes may be raised or depressed by turning the key, which fits into a small hole in the frame, directly under the groove of the nonius plate. On the left hand of the tube a small thermometer is placed, with Fahrenheit's scale ; there is an index to the thermometer, which may be set by the same key as the barometer, only putting it into the small hole un- der the thermometer, and turning it round till the in- dex points to the mercury in the thermometer. A scale for correcting the expansion of the mercury in the barometer is often graduated close to the scale of the thermometer. The upper part of the barometer is covered with a glass piate, to prevent the silvering of the plate from being injured b\ dirt, or being corroded by the action of the air. OF THE LOWER PART OF THE BAROMETER. The lower end of the tube is immersed in th< tern which contains the mercury ; the cistern is cover- ed with a mahogany box ; at the bottom of the frame is a screw, to raise or lower the surface of the mercun ; at the top of the cistern is a hole, which is fitted with an ivory screw, to be placed there occasionally for the conveniency of transporting the instrument safely from one place to another. . OF THE SCALE OF CORRFCTION. 421 The gag° consists of a small stem of ivory, arising from a float of the same substance ; a circular division is cut round this stem ; the stem passes through a short cylinder of ivory, which is cut open in front ; on this front two small divisions are cut : at the bottom of this c) linder is a male screw, to fit the female screw of the cistern ; the upper part of the gage is protected by a tube of glass perforated at top. TO USE THIS BAROMETER. 1 . The barometer being fixed in a perpendicular posi- tion, unscrew the screw at bottom as far as it will go without forcing it. 2. Take out the ivory screw at the top of the cistern, and place it between the scroles on the upper part of the frame. 3. Screw the gage into the place from whence the ivory screw was taken. 4. Screw up that screw which is at the bottom of the frame, until the line on the float exactly coincide with the two lines on the front of the ivory cylinder. 5. btrike the barometer gently with the knuckles, and then so set the lower edge of the front index to the convex surface of the mercury, that it may be at the same time in a line with the edge of the index behind the tube ; and the nonius will then give the true height of the mercurial column, from the surface of the mer- cury in the cistern. 6. The preceding rule for setting the gage must be complied with previous to every observation. 7. When the barometer is to be transported from one place to another, the gage must be removed, and the solid ivory screw inserted in its place ; after w 7 hich, the mercury in the tube may be forced gently up to the top thereof, by the screw at the bottom of the frame. OF THE SCALE OF CORRECTION. 1 his scale is placed close to that of the thermometer ; but on the right-hand side, the zero, or O degree of 422 OF THE THERMOMETER. this scale, corresponds to the 55th degree of the ther- mometer. 1. If the barometer be at 30 inches, and the ther- mometer at 55 degrees, no correction is necessary. 2. But if the thermometer be under 55^ and the ba- rometer at 30 inches, you must add to the height of the barometer as many of the lOOths of inches as are on the scale of correction opposite to the degree of the thermometer. 3. If the thermometer be above 55 9 and the barome- ter at 30 inches, you must subtract as many lOOths as are indicated by the given degree of the thermometer on the scale of correction. 4. The scale applied to the thermometer answers for the general range of meteorological observations ; but if the height of the barometer be very far distant from 30 inches, it will be necessary to make use of the rule of three, in order to obtain the true correction : for in- stance, let the barometer be at 28 inches, which we call P, c the correction indicated by the thermometer, x the true correction' : then as 30 : P : : c : x ; or — rz x, which is to be added to the height of the barometer whenever the thermometer is under 55 degrees, but to be subtracted when it is above 55. OF THE THERMOMETER. No instrument is of more importance for making discoveries in meteorology than the thermometer, as it points out the temperature or degree of heat of the air and other bodies. Heat and cold are perceptions, the ideas of which we acquire by our senses. Our sensa- tions are, however, inadequate measures of heat and cold, for they depend not only on the substances which excite them, but on the actual state of our bodies at the time : we cannot, therefore infer the exact iden- tity or similarity of the cause, from the sameness of the sensations, unless we can be assured that our bo- dies are in the same state ; if they be not, the same objects will produce very different sensations. Thus, if the hand be plunged into lukewarm water, this water OF THE THERMOMETER. 423 will appear cold, if the hand be warm ; but, if t he- hand be cold, the water will appear to be warm ; though in both cases it possesses the same temperature. Our senses are, therefore, both imperfect and de- ceitful measures of heat ; and we cannot ascertain, by their means only, the state of the surrounding bodies, with respect to heat and cold. This occasioned philo- sophers to seek for some method, by which they might determine the temperature of bodies with more certain- ty. This they found in the property of fire to dilate or expand all bodies, whether solid or fluid ; and of cold to contract or condense them. This expansion and contraction is considered as a measure infinitely more certain of the degrees of heat and cold than the senses. It would appear from this expansion, that fire, when it is agitated by that motion which we call heat, always acted as if it wanted more room ; and this in such a wonderful manner, as if every particle of the space in which it exists were a radiant point or centre, from whence it spreads forcibly outwards in every direction ; and, consequently, when fire thus acting is admitted in- to the pores of bodies, their parts must be stretched out, and their dimensions every way increased, accord- ing to the degree of fire by which they are acted on. Some idea of the force of this expansion may be gained, by considering how vast a weight may be suspended from a bar of iron or brass in a vertical position, with- out separating the parts of the metal, or overcoming the force with which they cohere. Now, this, fire easily executes, so far relaxing the texture of brass and iron, that their parts will fall asunder with nothing but the force of gravity. Thermometers are instruments which measure the degree of heat by the expansion of bodies. Fluids are those generally used, because they are dilated more rea- dily than solids ; and quicksilver is preferred to other fluids, because its expansibility is not affected by the different circumstances in which it is placed ; it does not soil the tube like many other fluids, and at the sam^ time affords an extensive scale of divisions. 42* OF THE THERMOMETER. A thermometer is a tube of glass, the end of which is blown into a ball or cylinder ; the ball and part of the tube is filled with mercury. The fluid in the ball dilates by the heat, and contracts by the cold, which occasions the fluid in the tube to rise and fall ; and the smaller the bore of the tube is in proportion to the bail, the more visible will be the rise of the fluid by a small expansion. We may, therefore, consider this instru- ment as a convenient measure of the changes of heal and cold, which is shown by the scale to which the tube is affixed. But it is not sufficient to have found a measure of heat ; .it must be universal, always speaking the same language, and awaking the same ideas in the mind, in all places, and at all times. To this end it is necessary, 1. That this measure should begin from a known and determinate point. That another point, equally certain as the first, but some distance from it, be fixed upon. And, 3. That the space between them be divided into a certain num. ber of parts, which in all instruments will have a con- stant proportion. It has been fully proved, that the temperature of freezing water, or melting ice, is constantly the same in all places, and at all times. The same may be said of boiling water, under a given pressure of the atmos- phere. If, therefore, the ball of a thermometer be plunged into melting ice, and afterwards into boiling water, and left in each till it acquire their temperature, and marks be made at the respective heights at which the mercury stands in each, two fixed points will be obtained. To be more particular : When ice is at the melting temperature, whatever be the heat you apply to it, it does not become hotter; a thermometer in the middle of the mass continually stands at the thawing point as long as any of the ice re- mains about it, so that the same cause, w r hich in other circumstances would produce heat, here only produces liquifaction. Hence it is, that melting ice, or freezing water, is so well adapted for giving one of the fixed points of a thermometer. The quantity of fire absorb- OF THE THERMOMETER. 42$ ed by ice in melting, is such as would increase the tem- perature of the water about 140 degrees: conversely, water may be cooled 1 8 degrees below the freezing point, without freezing : congelation cannot take place till a certain portion of the combined or latent fire be disengaged ; when any part does congeal, the fire let loose, raises the thermometer to the freezing point, and it continues there till the water be frozen ; after which, as the water in the first case, so the ice m the latter, obeys the external temperature. Continual accession of fire arrives at water when boiling, without increasing the heat thereof; for ebul- lition, under any given pressure, cannot take place, till the vapour produced in the liquid has obtained a de- gree of expansive force sufficient to raise the liquor into bubbles ; under that pressure, and so long as the va- pour retains this heat, it must continue capable of re- sisting the same pressure ; as the heat abates, a decom- position takes place, which occasions the opake steam over boiling water. These principles explain the fixity of the boiling point, for vapours cannot be formed within the mass, unless they have sufficient expansive force to displace or raise it into bubbles : they cannot acquire this force till the heat has arrived at a certain point, and as soon as they have acquired it, they escape in virtue of that ex- pansion : further accession of fire passes off in the same manner, and only accelerates the evaporation. Though boiling water under the same pressure has always the same heat, it may be made, before it does boil, to receive a greater heat than it can retain when it does boil. In a vessel with a very narrow orifice, filled with water, well purged of air, though the water sus- tains no other pressure than that of the atmosphere, yet its particles meet with so much resistance to their sepa- ration, that M. de Luc found it would receive, without boiling, a heat of 22 degrees above the boiling point j as soon as vapours could form themselves, their expan- sive force was so great, that they pushed a large quan- tity of water out of the vessel, in the way of explosion, vol. IV. 3 1 426 OF THE THERMOMETER. but the remainder was immediately reduced to boiling heat. The vapours of boiling water arise from within the mass, but water may yield from its surface vapours of an equal expansive force, provided they be confined in a place of equal temperature with themselves. Thus, if wa- ter be introduced above the mercury in a barometer, the vapours it produces in a temperate warmth will press down the mercury nearly half an inch. In the heat of boiling water, they will depress it to the level of the mer- cury in the bason ; being then become equivalent to the pressure of the atmosphere in a greater heat, they will depress it below the level, and escape at the bottom of the tube, the water giving no signs of ebullition to the last. In making thermometers, care should be taken that tl tubes used for that purpose be very clean, and very dry; the next thing is to examine whether the bore of the tut be equal and cylindrical throughout ; this is easily pt formed, by immerging one end of the tube in mercui and taking it out, previously stopping the other end wit the finger ; by this means a small portion of the mercui will enter the tube, more or less in proportion to the deptl the tube is immerged ; measure the length of this portioi of mercury, and then slide it backwards and forwards ii the tube. If the length thereof be the same in all parts, the tube is a regular cylinder ; but if otherwise, the dia- meter varies, and the tube cannot be used to form a gooc thermometer, unless the divisions on the scale be pro- portioned to the different lengths of this mercurial cylin der. The tube being chosen, the bulb is to be blown ; i the tube be very regular, you may now begin to fiil it : if not you must find the proportions of the inequalities tc adapt the divisions thereto £ to this end tie a paper funne over the end of the tube, and pour a small quantity o: mercury therein ; then hold the bulb over the flame of < candle or lamp, and let some of the air pass out of thu bulb through the mercury ; take it now from the lamp and as the ball cools the mercury will begin to enter th( f ube ; admit about half an inch, take the exact measun OF THE THERMOMETER. 427 :hereof, measure the length of this portion in different parts of the tube, and you will thereby obtain data for proportioning the divisions to its inequalities. If you have reason to suspect that there is moisture in your tube, it would be proper before the preceding ope- ration to clean it ; this may be done by laying the tube on a plate of iron, or over a chafing-dish in which there is only a small fire mixed with cinders ; it should be con- tinued there till it be so hot that you must use a glove or a small pair of pincers to hold it, taking care not to warm the bulb. This process dilates the included air, consumes small particles of dirt imperceptible to the eye, and eva- porates moisture. While things are in this state, sud- denly heat the bulb, and the air being thereby dilated, drives before it all these impurities, and leaves the tube as clean as you can desire. To fill the bulb and tube, tie on a paper funnel as be- fore, and put somewhat more mercury therein than you think will fill the thermometer ; hold the bulb over the flame of a lamp or a small candle newly snuffed, this will expand and force part of the air from the bulb ; when you think a sufficient quantity is expanded, withdraw the tube from the candle ; in proportion as the bulb cools, the remaining air will be condensed, and the space it oc- cupies will be occupied by the mercury ; by thus alter- nately cooling and heating the bulb, it is at last completely filled. When the bulb is nearly filled, you must boil the mer- cury therein, by applying it over the flame of a lamp, or that of a snuffed candle. The air included in the mercury, and that which lines the tube, dilates itself, is collected in small bubbles, and expelled by the first ebullition ; when the mercury boils violently, a great part of the contents will rush up the tube into the paper reservoir. Remove the bulb from the flame, and repeat the operation, till the diminished noise and agitation show that it is deprived of its air and moisture. After the boiling is completed and the tube cool, plunge it in melting ice or snow, which gives the temperature of 32°. Take off the funnel, and hold the bulb in the hand, and afterwards in the mouth ; the heat thereof will cause 42B OF THE THERMOMETER. some of the mercury to drop out of the tube ; cool it again to 32°, and mark where the mercury stands. The distance between this mark and the top of the tube mea- sures the interval between freezing and blood heat, or 32 and 95, that is 63°, and will consequently point out whe- ther the degrees will be large or small, and what extent your scale is capable of.* When the number of degrees to which the length of the tube will extend is thus known, you may settle where- abouts you will have the freezing point, which may be ftearer or further from the bulb, according as your instru- ment is designed to measure great or small degrees of heat or cold. Now prepare the upper part of the tube for sealing, by drawing it out to a fine capillary tube ; then heat the bulb in the candle till a few particles of mercury have fallen off the top of the tube, and afterwards try if the freezing point be sufficiently near the bulb ; if it be not, you must repeat the operation, being careful how- ever not to throw out too much mercury at a time. Have two candles now ready, one to heat the ball, the other to close the tube. The blow-pipe being in readiness, the up* per part of the tube near the flame of one candle, and the bulb near the flame of the other, the mercury will rise, and at last begin to form a globule at the point of the ca- pillary tube ; at this instant the bulb must be withdrawn from the flame of the lower handle, and the flame of the upper one be directed by the blow-pipe upon the point of the tube. This will be immediately ignited, and will close by the melting of its parts before the mercury has perceptibly subsided. When the mercury has fallen, the sealing may be rendered more secure by fusing the whole point of the tube till it becomes sound. To settle the freezing point, you have only to immerge the thermometer so deep in melting snow or ice, that the mercury may be scarcely visible above the surface, and then carefully mark the place at which it stands. For the boiling point, the Royal Society advise a vessel to be pro- vided somewhat longer than the thermometer, with a cover * JVic/idson's First Principles of Chemistry, p. 26, 27, 28. OF THE THERMOMETER. 429 and two holes therein ; one about an inch in diameter for the steam to escape, the other smaller to hold the ther- mometer tube. When this is used, the thermometer must be fastened in the cover, so that the estimated place of the boiling point may be just above the hole; water then must be put into the vessel, but so as not to touch the bulb of the thermometer when the cover is placed on. The cover being* put on, and a thin plate of metal laid on the steam- hole, you are to make the water boil bv heat applied to the bottom only ; the thermometer will thus be surround- ed by steam which will raise its temperature to the boiling point, and this must be carefully marked on the tube. Fahrenheit* s scale is that which is used in England : the freezing point is called 32, the boiling water point 212; so that there are 180 degrees or divisions between them, which may be extended upwards and downwards, as far as is necessary. Foreigners generally use Reaumur's, or rather de Luc's scale, where the freezing point is marked O, and the boil- ing water point 80. Two thermometers are necessary for accurate observa- tion ; one to be suspended within doors, near the baro- meter ; the other out of doors. That without doors should be placed at the north side of the house, or where it will be sheltered from the rays of the sun. I have already shown you, that the increments of ex- pansion in a mercurial thermometer are nearly as the in- crements of heat ; or in other words, that the dilatations and contractions of the fluid are nearly proportional to the quantities of fire, which are communicated to, or sepa- rated from the same homogeneous body as long as they remain in the same state. Thus the quantity of fire re- quired to raise a body four cfegrees in temperature by the mercurial thermometer, is nearly double what is required to raise it two degrees, and four times what is necessary to raise it one. This is proved by putting a thermometer first in cold water, and then into water heated to any de- gree, noting the altitudes ; then putting equal quantities of these two waters together, which will give a mean heat, and the mercury in the thermometer will stand at the mean altitude between the two before observed ; this 430 OF THE RAIN-GAGE. is found to be true, whatever be the temperature of the two parts of water. Though in the sense here explained the thermometer is an accurate measure of heat, yet I have also shown you, that it can only indicate the proportions of that action of fire by which bodies are expanded, but is by no means a measure of the whole quantity of fire disengaged, dis- placed, or absorbed; properly speaking, it is therefore only a scale of expansion indicating certain translations and transfusions of the igneous fluid. It may be proper to observe to you, that glass is dilated and contracted by heat and cold together with the fluid, and consequently the apparent variations in the dimen- sions of the fluid are the difference between the real co- temporary expansions, or sum of the cotemporary con- tractions of the glass and the fluid. The changes arising from these causes are too inconsiderable to be worthy of notice in the general use of this instrument ; the change of dimensions in the glass is prior to that in the fluid ; hence the fluid is found to sink before its rise upon an increase of temperature ; and if the bulb be large, some time may elapse before the fluid acquires the same tem- perature with the glass. The pressure of the atmosphere on the outside of the bulb not being counteracted by the air within, will affect its magnitude, diminishing it as the pressure is increased. The variation on the scale occa- sioned by this cause is, like the preceding one, very small, being never above one-tenth of a degree. OF THE RAIN-GAGE. It is necessary towards forming a systematic idea of the weather and its various changes, to measure the quantity of rain which falls upon the earth ; and this is done by what is called a rain-gage. The rain-gage is a very simple instrument, consisting of a square tin funnel of twelve inches diameter, commu- nicating with a tube or cylinder of tin, into which the rain is conveyed by the funnel. The depth of the water is measured by a rule fixed to a float \ this rule passes OF THE HYGROMETER. 431 through the centre of the funnel. The divisions on the rule show the number of cubic inches of water that have fallen in a given time on a surface of one square foot. To use the rain-gage, so much water should be first poured in as will raise the float, so that the zero on the rule may exactly coincide with the aperture of the funnel. The funnel is so contrived, as to prevent the water from evaporating. This gage should be fixed down firmly in a place where, whatever winds blow, the fall of the rain may not be in- tercepted by the house, or any other impediment. OF THE HYGROMETER. The hygrometer is an instrument intended to discover the moisture contained in the atmosphere. As the substances that are affected by moisture are very numerous, so are also the contrivances that have been executed to indicate the degrees of moisture, and render sensible the smallest variations in the substances influenced thereby. Thus, wood expands by moisture and contracts by dryness ; on the contrary, cord, cat- gut, &c. contract by moisture and lengthen by dryness ; consequently, the contraction and expansion of these substances indicate different states of the air with re- spect to moisture. The twisted beard of a wild oat, with a small index fixed to it, moveable against a scale, makes a very good hygrometer ; for the twisting, being affected by the variations of moisture, moves the index. Mr. de Luc> who has laboured more on this subject, and with more success than any other man, after ma- king an immense number of experiments to find out a substance, whose expansion increases most, nearly in proportion to the quantity of moisture imbibed, found that whalebone and box, cut across their fibres, in- creased very nearly in proportion to the quantity of moisture, and more so than any other substance which he tried. He however preferred the whalebone; 1st. On account of its steadiness, in always coming to the same point at extreme moisture j 2dly, On account of 432 PRINCIPLES OF its greater expansion, increasing in length above one- eighth of its length, from extreme dryness to extreme moisture ; lastly, Because it is more easily made thin and narrow. As the whole atmospheric economy, as far at least as relates to the weather, depends upon, or is connected with the state of vapour it contains, it is rather surpriz- ing, that we find so few hygrometrical observations among ihe many meteorological diaries that have been published. From time immemorial, the effects of moist- ure have been considered as prognostic of the weather, as is evident by the confidence the housewife places in her salt-box, the carter in his whip leather thong, and the sailor in his shrouds. But whether the hygrome- ter be a prognostic or not of the weather, it is certainly of the utmost importance to the natural philosopher, and would probably prove a valuable oracle to the far- mer, which is fully evinced by the following observa- tion of Mr. Marshall, in his minutes of agriculture. " Yesterday morning," says he, " while the hygrome- ter stood at two degrees moist, the peas were by no means fit for carrying ; the balm was green, and the peas soft. About ten o'clock the hygrometer fell to one degree dry ; before one, the peas were in good or- der ; I went into the field, merely on the word of the hygrometer, and found the peas fit to be carried." It is plain therefore, that on a scattered farm, in hay-time and harvest, a hygrometer must be peculiarly useful. Before I proceed to take further notice of hygrome- ters, it will be necessary to remind you of the principles I have already laid down concerning evaporation and vapour ; for, unless these be properly attended to, you will never be able to attain any fixed and certain no- tions of meteorology. Atmospherical fluids are divisi- ble into two classes vapours and aeriform, the distinct- ive characters of which are these : vapours are decom- posed by pressure, but aeriform fluids bear the strong- est compression without decomposition : vapours are decomposed in vessels hermetically sealed by the spon- taneous escape of fire ; but aeriform fluids can only be decomposed by some substance, to which their gravi- LVAPORATION AND VAPOUR. 43cJ fating matter has more affinity than to the fluid which maintains them in an aeriform state. In vapours the proportions of the component parts are very variable, according to the subsisting circumstances ; but aeriform fluids, when once formed, continue in the same state, and can only be changed by chemical causes : the dif- ference arises from the weakness of the union of water in vapour with fire, so that it can separate itself there- from by the mutual tendency of its own particles, when they are brought within a certain distance one of the other, and because fire can so easily quit them, to re- store certain equilibria with respect to itself. By watery vapour, 1 do not here mean visible opake steam or vapour, because that is vapour in a state of de- composition ; I mean the invisible and transparent ex- halations, which constitute a peculiar and distinct fluid, expansible and compressible, and thus, far from pos- sessing the mechanical properties of aeriform fluids, and exercising these properties whether mixed with them or alone* The specific gravity of these vapours is above one half less than that of common air ; that is* when they exercise a certain expansive force, whether alone or mixed with air, their mass is above one half less than a like volume of air, which would exercise the same expansive force. In the course of our lectures on fire, I showed you, that vapour consists of particles of fire, united with those of water, and that there was no foundation for the hypothesis which considered it as a chemical solu- tion of water by air. This is, however, a hypothesis that has been adopted by so many writers, though con- trary to every circumstance duly examined, and of such consequence in meteorology, that 1 shall again make a few remarks thereon. I shall first notice the phenomena of air contained in water, and show you* that these have no relation to the common notions of solution. If water be placed in a receiver, and a va- cuum made, a number of air-bubbles are formed in the midst of the water, which increase in size, and then VOL. IV. 3 K 434 PRINCIPLES OF escape. Now, there is no principle in the theory of dissolution which can explain, why a menstruum, be- cause it is less pressed, should let go the substance that it had dissolved ; whereas it should hold it stronger if the menstruum be thereby dilated. When the water ceases to produce air by this operation, if you agitate it strongly, more air is disengaged ; this also is contra- ry to the theory of dissolution, for this is promoted by the agitation of the menstruum. When both these me- thods cease to be efficacious, more air may be disengag- ed by heat ; here the hypothesis is contradicted at its very foundation ; the sole plausibility on which it rest- ed was derived from the idea, that the air could contain more water when its heat was greatest, which, of course, must also take place with respect to air contain- ed in water, to which you see this fact is diametrically opposed. I have shown you in the lecture on fire, that the phenomena of aqueous vapour are the same in vacuo as in open air, that it may be produced in vacuo without any concurrence of the air. The density of the vapour is the same every where at any temperature, pro- vided the particles thereof keep at a certain distance from each other. This density in every space, and at every temperature, is determined by a certain minimum dis- tance among the particles of the vapour. It is sufficient for their conservation as vapour, either in vacuo or in air, that they be not forced to approach within this distance. The product of evaporation is always of this nature, namely, an expansible fluid, which either alone or in air affects the manometer by pressure, and the hygrometer by moisture, without any difference aris- ing from the presence or absence of air. I may again, therefore, repeat after M. de Luc, that every phenomenon proves, that the hypothesis of the solution of water by air is vague, without any solid foundation, unnecessary for the explanation of evaporation, while it involves every branch of philosophy in obscurity.* * See M. dc Luc's Letters, dans le Journal de Physique, hisldees sur la Metidbrologie. See also further proofs of the errors of the chemical idea of water being Solved by air in Vol. II. Lecture xiv. EVAPORATION AND VAPOUR. 435 Evaporation is a dissolution of water by fire. A most decisive reason in support of this opinion is, that every liquid cools when it evaporates ; the portion of the liquid that disappears, being carried away by a quantity of fire proceeding from the liquid itself. Mr. Watt has shown, that in the common evaporation of water in open air, the quantity of heat lost by the mass, bears to the quantity of water carried away, a propor- tion still greater than that which is found in the steam produced by boiling water. As vapour consists of fire and water united, zrA forming a new compound, the specific properties of each of the component parts are in certain respects sup- pressed, as in other chemical operations, the water loses its faculty of moistening, and the fire that of producing heat ; hence the loss of heat in the evaporation of li- quids, and the augmentation of heat in the decompo- sition of vapour. The particles both of fire and water still, however, retain the faculty of maintaining their respective equilibrium between the medium and sur- rounding bodies. Thus the particles of water still re- tain the tendency of uniting together, and this union takes place whenever they are so near each other, that this tendency can surmount the effort of the fire which keeps them disseminated. Of course, the less the quantity of free fire, or the cause of heat, in a given space, the greater is the dis- tance at which the particles of water can exert their faculty of uniting together, and of abandoning their la- tent fire. The precipitation of water or final union, therefore, takes place when the density of vapour has exceeded certain limits, which limits depend on the temperature ; for the*greater the quantity of free fire in "any given space, the nearer the particles of vapour may approach each other without being decomposed, that is, without the watery particles, in consequence of their natural tendency, re-uniting together, and quit- ting the fire with which they were associated. Thus there is necessarily a minimum of distance of the aqueous particles, beyond which the vapour cannot be compressed without being decomposed j and this is 436 HYGROMETRY. different in different degrees of heat, but constant in the same. When vapours are mixed with air, thev can sustain a much greater pressure than they can bv themselves, because the air supports the pressure, and prevents the particles from being forced within their minimum distance ; and it is thus that vapours subsist in the atmosphere without being decomposed by its pressure. Vapours are decomposed not only by the mutual ap- proach of the particles, but also in virtue of the affinity of water to those substances that are called hygroscopic, of which fire may be reckoned one. The principal law of this affinity is, that the water distributes itself to all the substances of the class that are within its reach, to every one alike, proportional to its capacity of reten- tion. If new fire be introduced into a given space, where there is no superabundant water, it will take away some of the water from all the hygroscopic sub- stances, and diminish their humidity. If some of the fire be taken away, the water that was united thereto will be divided among all the rest ; and if any other hygroscopic substances be introduced, containing a greater or less quantity of humidity than those already there, the surplus of humidity will be divided among them. It is by fire that this disri ibution is effected ; the particles of this element being always in motion, take up the water from one that has more than its share, acid give it out to another that has less. Thus hygros- copic substances have their humidity always propor- tional to the places they are in. Hygroscopic substances are of three distinct classes : 1. Those that seize on the water of vapour by a che- mical affinity with that liquid ; among these are acids, salts, and calces. 2. Those that imbibe the water, by the tendency it has to propagace itself in capillary pores, but from their nature receive no sensible increase oi bulk by its introduction ; such are porous stonesi 3. Those that, imbibing a certain quantity of water, are thereby expanded ; and these are most of the solids of the vegetable and animal kingdoms. M. de Luc, by a long series of experiments, to which I must refer you. HYGROMETFY. 437 hows, that the substances of the last class are the only m s proper for hygrometers, and that even in this iass, to avoid fallacy in respect to the most important >henomena, we must use those that cease to lengthen, mly when they can not be penetrated with more water. ilc-re, however, it will be necessary to define in what ense we use the words moisture and humidity , for in he manner they are commonly used, they sometimes tnply a cause, sometimes an effect ; this ambiguity is lot peculiar to these words, you will find many others ised in philosophy as ambiguous, particularly when hey have oeen applied to certain phenomena, the causes )f which are not determined. Moisture, in a general sense, may be considered as in- visible water, producing observable phenomena. i nus in hygroscopic bodies, the quantity of water vhich expands them, and increases their weight, is :oi coaled within their pores ; and, in the ambient me- lium, that water which affects hygroscopic bodies, be- rig th-.--.re under the form of vapour, is as invisible as he air itself. Bur in respect to hygrometry, where moisture is con- id ored as having correspondent degrees in the medium, he word requires a more particular determination, VIoisture may be either totally absent or absolutely ex- reine, both in the hygroscopic bodies and in the ambi- :nt medium ; hence, both in the whole and in corres- )ondent parts, moisture assumes in the medium the cha- racter or a cause, and in hygroscopic bodies, that of an ffect. These two circumstances furnish us also with a ixed module for determining correspondent degrees. Moisture is totally absent, first, in the medium when t contains no vapour ; and then as a consequence in ngroscopic bodies, because they contain no more wa- er jhat can evaporate, without a decomposition of their '.omponent parts. The case here supposed that is, vhen, by some adequate cause, no sensible quantity A vapour is permitted to remain in the medium, as in he lime vessel used by M. de Luc to obtain the point )f extreme dryness. Moisture is extreme, first, in the medium, whether ir or a space free from air, when no more vapour can 438 HYGROMETRY. be introduced therein, without a part being decompos ed ; and then, as a consequence in hygroscopic bodies because no more water can be admitted in their pores. Here it is to be observed, that from the nature of the last of these maxima the quantity of water which pro duces it, i. e. extreme moisture, in a given body is fix ed, because it is determined by the actual capacity o; its pores ; but the quantity of water which produce: extreme moisture in a medium of a given extent, is a: variable as the temperature. The equilibrium, therefore, between the mediun and hygroscopic bodies in different stages of moisture which equilibrium is the object of hygrometry as a sci ence, does not depend on certain quantities of wate: contained in the medium of which bodies may receivi their share ; it depends on different aptitudes of th< vapour contained in the medium, to communicate wate to those bodies ; which aptitudes vary not only witl the different densities of that fluid, but also in vapou; of the same density according to the temperature.* From the hygrometer we have learned, that in th> phenomenon of dew, the grass often begins to be we when the air a little above it is still in a middle state o moisture ; and that extreme moisture is only certain ii that air, when every solid exposed thereto is wet. I has taught us, that the maximum of evaporation in ; close space is far from being identical with the maxi mum of moisture ; this depending considerably, thougi with the constant existence of the other, on the tempe rature common to the space and the water that evapo rates. It has shown, that the case of extreme moistur existing in the open transparent air in the day, eve: when it rains, is extremely rare ; M. de Luc has onl found it once iu this state, the temperature being 39' Messrs. de Saussure and de Luc have proved by th hygrometer, that the air is dryer and dryer as we a? cend in the atmosphere ; so that in the upper attainabl * See M. de Luc's -paper on Evaporation, from which the remarks < Hygrometers, Sec. u an extract, "Phil. Trans, for 1<"91, part 9. HYGROMETRY. 439 •egions, it is constantly very dry, except in the clouds. VI, de Saussure has shown, that if the whole atmos- )here passed from extreme dryness to extreme incis- ure fhe quantity of water thus evaporated would not •aise the barometer half an inch. Lastly, in chemical merations on the air, the greatest quantity of evaporat- ed water that may be supposed in them at the common emperature of the atmosphere, even if they were at extreme moisture, is not so much as the one hundredth Dart of their mass. The two last very important pro- positions have been demonstrated by M. de Saussure* LECTURE LIL OF RAIN. IN a science so very difficult as that of the weather 5 it is not to be supposed that any thing like a certain and established theory can be laid down : our utmost know- ledge in this respect goes no further as yet than the es- tablishment of a few facts ; and in reasoning upon these, we are involved every moment in questions which seem scarcely within the compass of human wisdom to resolve. To treat it in a satisfactory manner, we ought to have an intimate acquaintance with the constitution of the atmosphere, and the nature of those powerful agents, fire, light, and electricity, by which it seems * See M. de Luc's second paper on Hygrometry, Phil. Trans. 440 OF RAIN. to be principally influenced ; with their peculiar i^fl u . ences upon one another and upon the atmosphere, a d this in every possible variety of circumsranees. Many of the qualities of air, earth, water, and fire, have been indeed discovered and estimated ; but when thji>e come to be united by nature, they often produce a re- sult which no artificial combinations can imitate. Every cloud that mov ri s, — 3very shower that falls, serves to mortify the philosopher, and to show him hidden qualities in air and water that he is unable to explain. The greater part of the received notions on meteoro- logy are vague and incorrect, not only those which re- late to the nature of the causes, but those also which concern the laws of their effects. The same may be said of our notions of the elasticity of the air, of heat when applied to this fluid, of both igneous and aqueous meteors, of sudden and partial winds ; they are all so many enigmas to the philosopher. Indeed, till we were in possession of a good hy- grometer, it was impossible to form any certain conclu- sions concerning the moisture of the air : this difficulty is removed ; M. de Luc has by numerous experim and observations furnished us with a comparative hy- grometer, by which, together with a thermometer, the air can neither lose nor acquire moisture without our being advertised thereof. By the use of this hygrometer we have obtained clear and certain ideas of the causes, by which water, simply evaporated in air, may be precipitated there- from. These causes are the same with those, which in air, where the quantity of evaporated water remains the same, always produce an increase of moisture, the ne- cessary forerunner of the precipitation of water ; and these are two, viz. the compression of the air, and its being cooled : no other causes are indicated by experi- ment. Some philosophers have thought that the air, when rarefied, quitted a portion of the water which, according to them, it held in solution ; but I have shown you that this idea is erroneous, and that rarefac- tion occasioned dryness instead of moisture. OF RAIN. 441 The great question, therefore, in the inquiry con- cerning the immediate cause of clouds, &c. is, What becomes of the water that rises as vapour into the at- mosphere ? What is the state in which it subsists there, between the time of its evaporation and the time of its falling down again in rain ? If it continue in a state of watery vapour, or such as is the immediate product of evaporation, it must possess the distinctive characters essential to that fluid. It must make the hygrometer move towards humidity in proportion as the vapour is more or less abundant in the air. On a diminution of heat, the moisture, as shown by the hygrometer, would increase ; but, on an increase of heat, the humidity would decrease. Again, on this supposition, if hygroscopic substances dryer than the air be introduced therein, they must have the same effect as an augmentation of heat : for, Such are always the properties of aqueous vapour on every hypo- thesis of evaporation. If, therefore, water exists in the atmosphere without these properties, it is no longer vapour, it must have changed its nature. M. de Luc has shown, that the water which forms rain, does not possess these properties ; it must, therefore, have passed into another state. Repeated observations have shown, that the upper regions of the atmosphere, notwithstanding the conti- nual ascent of vapours there, are dryer than the infe- rior regions ; on the submits of high mountains a de- gree of dryness prevails unknown on the plains. If rain be the immediate product of evaporation, it ought always to be preceded and accompanied by a diminution of heat, in that stratum of air where it ori- ginated ; and this diminution, to produce its effect, should be greater in proportion as the moisture was further removed from its extreme term in this stratum \ but, in a great storm on the mountain^ of Sixt, M. de Luc found that the heat had increased instead of di- minished ; this cause could not operate here, and it was therefore impossible that the quantity of water which was then precipitated from the air could have VOL. IV. 3L 442 OF RAIN. been contained there in the form of the immediate pro- duct of evaporation. On every hypothesis of the formation of rain from vapour, it is heat that produces the evaporation, and a diminution of heat that occasions the return of vapour into water, and therefore rain should happen only in the night, or at the coldest time of the day ; whereas experience shows, that it has no connexion with heat or cold. We have rain as often in the day time, when, according to the natural course of things, the heat of the atmosphere should be the greatest, as at night, when the heat ought to diminish ; besides, the heat often diminishes in the day, without producing rain. Whatever be the degree of heat, the air can only part with so much of its water, as it is unable to retain in that degree of heat ; no rain should therefore be form- ed, unless the air were saturated, or at extreme mois- ture, but this also is contrary to fact. Thus, when M. de Luc and his brother were on the Sixt, though the hygrometer was 66\ degrees from ex- treme humidity, thick clouds formed around them, which obliged them to think of retreating ; in a little time thi summit of the mountain was surrounded by them, they spread and covered the whole horizon, a premature night surprized them in a very dangerous road, and a most violent storm of wind, rain, hail, and thunder, lasted the greater part of the night ; it extended over all the neighbouring mountains and plains : after the storm ceased, the rain continued with very few intermissions till the next day at noon. The hygrometer being exa- mined in one of these intervals, only showed 1 A more moisture than before ; and even this increase was no other than what the difference of heat was sufficient for producing ; nevertheless, new clouds were formed, and the rain began again, accompanying our travellers by fits to the bottom of the mountain ; when arrived there, the clouds entirely dispersed, the hygrometer was again observed in the open air, and though the earth was drenched with water, and the heat much less, the hygro- meter was ItV dryer than it had been two days before, after a course of fine weather. Now, where was all this OF RAIN. 443 water, and all the ingredients of the storm, while the hvgrometer showed such a degree of dryness in the stra- tum where it was formed ? The reasoning of M. de Luc is confirmed by the phe- nomena of fair weather ; continued evaporation from the inexhaustible source of vapour, the ocean, and from the earth after it has been soaked with rain, would, if vapour did not change its nature in the atmosphere, ren- der it more and more humid, and bring it at last to a maximum of humidity, as it does under a glass receiver. But experience shows, that though the evaporation con- tinues for several months together on vast extents both of seas and continents, the air does not become moister, but, on the contrary, more and more dry. The diminu- tion of heat in the night produces dew ; but this symp- tom of humidity diminishes from day to day, and some- times ceases altogether. Many attribute the ordinary occurrences of rain to changes in the winds. When it rains with a south wind, it is supposed that these winds are warm, because they come from the south, and that they are more humid be- cause the greater heats in those climates from which they proceed, ought to produce a greater degree of eva- poration ; and that, consequently, when this air meets with a colder part of the atmosphere, the water it con- tained would be precipitated. When it rains by a north wind, it is imagined, that this wind being colder than our air, produces the same effect that this did upon the south wind. There are, however, various reasons which prove, that these winds are not the immediate cause of the phe- nomena. To place this hypothesis in the most favourable light, we will suppose that one stratum of air is at rest and the other in motion, and that both are saturated with the im- mediate product of evaporation. But the quantity of eva- porated water, which constitutes saturation or a maximum of humidity in the air, varies with the temperature, aug- menting or diminishing with the heat. The colder air will, therefore, contain proportionally less evaporated water than the other. When these two airs meet, the one will be cooled, which should produce a precipitation 444 OF RAIN. of water ; but the other will at the same time be as much heated, and therefore capable of receiving the superflu- ous water : at first a mist may be formed, but this will not be durable ; for, as it is in contact with the air that is growing warmer, it is soon dissipated. It sometimes rains with a south wind, which seems to embrace the whole height of the atmosphere ; here it has been gratuitously supposed that this air proceeded from the torrid zone, saturated with water thrbughout its whole height. Granting this supposition, it will not account for the phenomena of rain ; I shall not consider here the dif- ference in the seasons, which ought necessarily to influ- ence these phenomena, which however is not perceived ; for we have often durable rains with this wind in sum- mer, when the change of climate will occasion little or no variation in its temperature. Whatever change the heat of this air undergoes, it will gradually take place on account of the vicissitudes of day and night; for, as soon as the rays of the sun cease to act upon our horizon, the heat in the air decreases in as great a degree as that in which it existed at the same hour of the day. If the air be thus cooled beyond a certain point, the excess is preci- pitated in dew : besides, moisture in the atmosphere is daily destroyed by some cause of which we are ignorant, and re-appears as suddenly in vast abundance in some strata, by causes of which we are equally ignorant. If you consider attentively the consequences of all these facts, you will see that there is very little probability that air, which travels night and day to come to us, and which must necessarily conform, and that successively, in all the intermediate latitudes, to the various causes that de- termine their mean degrees of humidity and heat, can ever occasion the phenomena attributed thereto. The remarks I have just made on the effect supposed to be deducible from different winds, are formed from the notions we gain by observations made in plains. They are strengthened and confirmed when connected with observations made on mountains, for there these winds are found without those deceiving appearances which favour the hypothesis we are combating ; they are found to convey cold there, while they are communicating heat OF RAIN. 445 o the plains below. Now, if the south wind derives its leat from the climate whence it proceeded, why is it not ,varm on the tops of mouncains as well as in the plains? [f it be said, that it is cold also on the tops of high moun- ains in the torrid zone, we reply, that if so, this in it- ;elf is a great mystery ; and further, that no one can sup- Dose that the superior air of this zone preserves its posi- ion and degree during its whole passage, and arrives in he same state at the tops of the northern mountains ; ind we may conclude, that though the air which pro- :eeds from donates warmer than our's be then hotter han the air surrounding us, yet, the greatest heat we ind therein does not in general proceed from this cause, )ut from some difference in its nature, whereby the solar -ays are rendered more powerful and more capable of Droducing heat near the surface of the earth. From observations that have been made on mountains, we may draw the same conclusions with respect to the hu- midity that generally accompanies south winds near the earth. For from these we find, that they do not produce the same effects in the higher region of the atmosphere, but accord with the usual dryness of these superior stra- ta ; they are not, therefore, in themselves the immediate causes of these differences ; for, if this were the case, the higher regions would be as much affected as the lower, or they could not be considered as an assemblage of the same fluids. But in this assemblage there may be un- known fluids, on which the solar rays may in the lower regions have a different influence, arising from circum- stances of which we are ignorant, perhaps from a greater or less density in the mass, or from a difference in their distances from the soil of the earth. Every circumstance seems to indicate, that chemical operations are the gene- ral cause of the phenomena, though in a manner un- known ; among the agents concerned, the solar rays hold the first rank. The necessity of the solar rays for the fructification of vegetables has been long established ; Messrs. Priestley, Ingenbouz, and Se?inebier, have proved that this operation is accompanied by great modifications in the air, modifi- cations which are essentially altered by the presence or 446 OF THE NATURE OF CLOUDS. absence of the solar rays. By these operations we see neu solids rising before us ; arid yet, if we be scrupulous ir the connexion of causes with effects, we must confess oui inability of tracing here the combinations of this firsi substance, which evidently puts in action every substanct on our globe. We know in this instance, that some oj the substances belong to the atmosphere, some to the earth, and that both are modified by the solar rays : fire also participates, but light is a constituent part of fire : water has its share, but water contains fire and light ; some ingredients of air are joined thereto ; but of these ingredients, those that are united depend on the quantity of light. Thus, however, new compounds are formed, possessed of different colours, consistency, odour, flavour, and che- mical properties. All these wonderful operations are pro- duced by the medium of the solar rays from the atmos- phere and from the earth : and these modifications taking place on the earth and on the waters, over the whole sur- face of the globe, must be considered as a class of causes which have considerable influence in meteorology. OF THE NATURE OF CLOUDS. From considering the causes of rain, I proceed to in- vestigate the nature of clouds. As it is from these that rain proceeds, we must acknowledge the blessings we re- ceive through them, though we are not able to account for their various phenomena. They are continually tra- velling over our globe, and by a proper distribution of moisture, rendering the spacious pastures of the wealthy fruitful, and gladdening the little spot of the cottager. " They satisfy the desolate and waste ground, and cause the bud of the tender herb to spring forth j" that the na- tives of the lonely desert, the herds which know no mas- ter's stalls, may nevertheless experience the care of an All-supporting Parent. Clouds are composed of a mass of vesicles like soap- bubbles, which vesicles are easily perceived in proper situ- ations, particularly on high mountains: these vesicles float OF THE NATURE OF CLOUDS. 447 n the air, rising or falling, till they are in equilibrium vith the air, remaining suspended there as long as they reserve the same state. By the nature of the suspen- ion of these aqueous vesicles, they do not alter the pres- ure exercised by the strata in which they are inclosed, leither on itself nor on the inferior strata. When the particles of vapour, properly so called, ap- >roach within a certain distance of each other, which is letermined by the actual quantity of free fire, the parti- :les of water, of which they are composed, tend to unite, md the fire which quits them joins itself to the remaining articles of vapour. From the observation of M. de Luc t appears, that vesicles of liquid water may be formed, md exist when the temperature of the air is at freezing, have already shown you, that there is something else >esides cold, necessary to the formation of ice. It is hence ve see mists, fogs, and clouds, when the thermometer is mder 32°. There is never, however, any great cold in bgs or in clouds ; for the cause, whatever it may be, that >rings vapour beyond its maximum, disseminates also leat. Aqueous vesicles never freeze without changing heir state ; but if the bubbles be broken when the air s at or under 32°, the water thereof freezes : when this lappens in the midst of clouds, snow is the consequence, vhose duration, like that of rain, depends on the quantity )f vesicles that are brought within a certain distance of :ach other ; the destroyed vesicles then group themselves nto flakes of snow by a crystallization, somewhat similar o what are termed by chemists sublimations, u e. the pre- stations of substances dissolved by fire. If the quan- ity of aqueous vesicles be too small to unite and be de- troyed by being brought near together, they may be de- troyed and frozen by causes similar to those which fix ublimates to the sides of the furnace and other receivers, >r which determine congelation in water sufficiently cool- id. In this case a hoar frost is formed. Clouds are always composed of bubbles, formed of iquid water, and they are generally at a temperature r ery little above the freezing point ; the existence of hese vesicles or bubbles is but of short duration, they ise and are destroyed successively, like the brilliant 448 OF THE DURATION OF CLOUDS. sparks we often perceive rising from a chafing-dish, when the coals are animated by a pair of bellows. OF THE DURATION OF CLOUDS. swn Whenever we see a mist or a fog formed by a known cause, we are always certain that the vapours, from which it proceeds, are passing rapidly beyond their maximum ; and the mist ceases when no fresh vapour arrives for its support. The principal known causes of mists and fogs are either the ebullition of water in open air at all tem- peratures, the transpiration and respiration of animals in winter, the evaporation from hot springs in the sam seasons, and the fogs, properly so called, that happen i autumn. In all these cases we know, that the vapour i produced in too great abundance for the temperature o the neighbouring air ; hence a rapid destruction of a part of those which arrive in that space which is occupied b the fog. Meanwhile, the fog only occupies a certai space, which is nearly fixed as long as the circumstance remain the same ; in a word, we always find fogs an mists to cease, as soon as the cause producing the va- pours ceases to furnish them beyond the maximum suit- able to the temperature of the air ; the vesicles are form- ed by a rapid decomposition of superfluous vapours ; as soon as this ceases, the vesicles are dissipated. From a review of known facts you will find, that the following conclusions are well founded. 1st. That vesi- cles are only formed in those cases where vapours get beyond their maximum. 2d. That these vesicles are con- crete water, subject to evaporation like any other water, and which always evaporate when the surrounding air is not at the extreme point of humidity. 3d. It is this last- mentioned circumstance which determines the extent of space occupied by a cloud or fog; for these vesicles only exist in that part, where the source of vapours, whatever it may be, having produced extreme humidity, dissemi- nates superfluous vapours ; so that beyond this space the vesicles evaporate. 4thly and lastly. This evaporation is prevented in whole or in part, either by obstacles that OF THE DURATION OF CLOUDS. 440 oppose the expansion of the mist or fog, or because the source of vapours furnishes them so rapidly, that the vesicles approach near enough to unite, even in the midst of the fog, which occasions them to unite, and the result is a distillation of water. From hence we may conclude, that when a cloud is formed in air, whatever be the cause, it can only subsist there while aqueous vapours continue to be produced in the same place. Thus, the extent occupied by a cloud is an indication of the cause which produces vapours, or of its intensity in some part of this space : extreme humidity exists but very little beyond the extent of the cloud, and, as soon as the cause which furnishes the vapour ceases, the cloud dissipates. We have been accustomed to see clouds from our earliest infancy ; they therefore neither excite attention, nor awaken admiration ; and yet, of ail the objects which surround us, there is none more truly wonder- ful, or more worthy of attention. Those also, who have but little studied the laws of hygrology, are very little astonished at these appearances, because they either suffer themselves to be amused by words, or rest satisfied with a few seducing glimpses : those who have considered the laws of hygrology, and weighed all the circumstances, find they are only carried to the boundaries of known causes ; but they also know how to stop and wait there, till fresh light enables them to proceed further. The traveller who has frequented high mountains knows, that clouds are a species of fog or mist, much resembling those we perceive on plains ; he has also re- marked, that where clouds are scattered in the air, the strata where they are met with are not at extreme hu- midity. Among other instances, M. de Luc mentions one where he saw his own shadow, and that of the rock on which he was situate, projected on a cloud beneath him, in a stratum where there were many other similar clouds extending to a considerable distance. The air was very serene, and there was not the least symptom of extreme humidity. How are such clouds preserved ? Whence do they increase to the eye ? Why, as they VOL. IV, 3 M 450 OF THE DURATION OF CLOUDS. are continually evaporating, are they not immediately dissipated ? The evaporation of clouds, even while they are in- creasing in size, is a circumstance of which you may easily be satisfied, by considering attentively the brok- en edge of a cloud, which has an azure ground behind it. These edges present to the imagination a thousand grotesque forms, which, by their striking changes, will assist you in your researches. Often you may perceive the part you are looking at dissipated in the place where it was first observed ; often it stretches it- self out, the cloud remaining stationary, and vanishes while it is thus extending itself. Sometimes, while one festoon vanishes others are formed, by which the cloud is enlarged ; at other times it diminishes, the festoons successively evaporating, till the whole disappears. It is impossible to consider these various metamorphoses of the same cloud, without supposing that there is in the air a source of vapours, which are produced in the place where the cloud is formed, and that it is by the continued production of fresh vapour that the cloud subsists and increases, though continually evaporating. When they wholly disappear, it is because the evapo- ration is not repaired by the formation of fresh vapour. These phenomena are independent of heat and cold, for clouds are sometimes formed suddenly in the midst of a hot day, and after they have poured down their water, all is clear again. Sometimes they evaporate after sun-set, gradually vanishing in the calmest wea- ther, without change of place. The appearances, on the whole, are such as would be produced by a large mass of water in violent ebullition, suspended invisibly in the atmosphere ; and the similarity in the effect naturally points out an analogy in the cause, that is, a source of vapour in the atmosphere. When it rains, the source which furnishes vapours produces them in such abundance, that the vesicles which are formed are driven against each other even in the be- som of the cloud ; and not having time either to dis- perse or evaporate, they are united ; and the water fall- ing to the lowest part, as in soap-bubbles, they are soon OF THE DURATION OF CLOUDS. 451 burst, and fall as rain. It is to these surcharged vesicles we must attribute the pendent fringes which are some- times seen under the clouds towards the horizon. Ex- perience has shown, that it rains under those clouds ; not that these fringes are rain itself, but the vesicles which fall by the augmentation of their weight. As drops of rain are formed their vesicles are destroyed. Lasting rains proceed from strata of clouds which co- ver the whole heavens ; and it is these that have the greatest connexion with the fall of the mercury in the barometer. The source of vapours comprehending a stratum of considerable extent, the barometer, after it has announced these rains, generally rises, and continues to rise, as long as they last. This is a fact observed, but to us inexplicable. It is no doubt connected in some way with the primitive cause of rain, but with that cause we are unacquainted. The relation of rain with the barometer is a subject as obscure as the cause of rain itself. In the midst sometimes of the finest days, and while ordinary symptoms indicate that the air is dry, and this as well in the vallies as on the mountains, bright and heavy clouds appear on azure ground, announcing sud- den rains. Sometimes these clouds increase enormously and descend ; other clouds form about, and unite to them ; the air is darkened, as if a curtain was drawn be- tween heaven and earth. From the tops of high moun- tains, these clouds may be often seen to accumulate ra- pidly over the plains ; while from these the azure ground of the heavens disappears ; the wind often rises, and blows from different quarters in a kind of whirlwind ; and lastly it pours with rain. As soon as the rain ceases, the curtain is withdrawn, and the calm is restored, the sun re-appears, and no other vestiges remain of this grand phenomenon but the water that is on the ground. When the air is disposed to product this phenome- non, you will often see the clouds rising from the hori- zon ; sometimes from the side where the wind proceeds, sometimes from other quarters. Often theLe heavy showers are partial ; sometimes they re-commence at in- tervals, accompanied with heavy squalls. Sometimes 452 OF THUNDER- these heavy intermitting showers are a prelude of moFe lasting rains ; in which case the clouds unite, and the wind goes down, and you have one or more successive days of rain. OF HAIL, Sudden storms, accompanied with hail and thunder, are among the number of phenomena which show how ignorant we are of the causes of those that we observe in the atmosphere. Hail is a sign of a great degree of cold ; but what is the immediate cause thereof ? Whence a substance, that must require so intense a cold for its formation, in seasons so warm as those in which hail is chiefly formed ? It is supposed in general, that hail- stones are drops of rain, which, falling through a cold- er region of air, are congealed in their passage into a rarefied sort of ice. Dr. Halley gives an account of hailstones that weighed five ounces each, and says, it is very extraordinary that such sort of vapours should continue undispersed in so long a tract as sixty miles to- gether ; and in all the way of its passage occasion so ex- traordinary a coagulation and congelation in the watery clouds, as to increase the hailstones to so vast a bulk in so short a space as that of their fall. OF THUNDER. All the phenomena of stormy clouds are obscure, and I am afraid there is very little probability of explaining them independently of each other. Those that are sa- tisfy d with conjectures may find enough at their ser- vice ; but he who conducts himself by the " scale and chart of truth," will find little to depend upon. It is thus with thunder and lightning : we can neither ac- count for the immense quantities of electricity discharg- ed by the one, nor the rumbling noise of the other. Mr. Volta supp )sed, that water, by being changed in- to vapour, acquired a greater capacity for the electric fluid, and that thus electricity was continually convey- OF THUNDER. 453 ed to the atmosphere by evaporation ; and this he de- duced from an experiment, in which water being eva- porated from a body, left that body negatively electri- fied. This, however, is by no means satisfactory ; for, not to insist on the fallacy of the terms positive and ne- gative ^ as both electricities may be produced by evapo- ration, if the electric fluid passed from the earth to the atmosphere by evaporation, and its return was occasion- ed by the reduction of vapour into water, there would always be more or less lightning when there was vio- lent and sudden rain, for in this case it would be rapid- ly disengaged ; but there is much oftener violent and sudden rain without than with lightning. In this case lightning also should always be preceded by rain, whereas there is often lightning among the clouds with- out any rain. Further, if we are unable to explain rain by the vapours which existed in the air before the formation of the clouds, the source of electricity exist- ing in the clouds ought not to be sought for in vapour. Indeed, on this supposition, as soon as there was a vio- lent rain the lightning would cease, and the fluid would pass cff by the drops, illuminating the air by its passage from drop to drop. There seems to be no other mode of considering lightning, than as an explosion, that is, as a sudden production of a great quantity of the electric fluid ; the fluid which is then manifested not existing as such but just before we perceive its effects ; just as the vapour, of which the clouds are formed, do not exist as vapour in the air until the moment of their appearance : the air, as yet transparent, contained neither the vapour of which the cloud is formed, nor the electric fluids, but the ingredients proper to give birth to both of them. By some cause, of which we are ignorant, clouds of a certain kind are formed. During the progress of their formation, and by fits, the electric fluid is produced in great abundance, and explodes every time it is thus produced. Observations made among mountains where clouds are formed, point out this to be the result of the phenomena. 454? OF THUNDER, In a storm observed by M, de Luc on the Buet, he had an opportunity of observing this phenomenon with all its modifications. The air of the strata where he was situate was perfectly transparent and dry ; the thermometer at 6 of Reaumur. Notwithstanding this, clouds formed here and there : by degrees they aug- mented, then became united, embracing the summit of the Buet, and supporting themselves against Mont Blanc, and the summits of the neighbouring mountains. M. de Luc and his companions were inundated with rain : though the clouds and rain formed a complete conductor, communicating with the ground, yet there was a continuance of thunder for a considerable time, and often very violent. Other instances may be found in the works of M. de Saussure of thunder storms, where the clouds formed a conducting communication with the ground, and yet where the thunder succeeded without interruption. ' The rumbling noise of thunder has been explained by a supposed analogy between the passage of lightning and the electric spark through the air. This explana- tion might have been admitted as plausible, if the rumb- ling noise of thunder had grown weaker and weaker, as being a succession of sounds proceeding successively from points more and more distant ; whereas the sound of thunder often increases, and gives us a distinct per- ception of its proceeding from points which are nearer to us than those from which it set out. It is sometimes intermingled with such terrible claps, as deprive the hy- pothesis of all probability ; or other inconsistencies therein might be pointed out. The rumbling and re- peated echoes, &c. of thunder still remain among the phenomena not yet accounted for. In general, a course of hot weather precedes a thun- der-storm ; and it seldom happens, that very hot wea- ther in the summer terminates without a storm of thun- der. Hence also in the East and West Indies, where the climate is so much hotter, thunder and lightning are not only much more frequent, but much more \ io lent, than in this country. [ 455 ] OF WINDS. Of winds, the observation of our Saviour is still just ; we hear the sound of the wind as it passes by, but we neither know from whence it comes, nor whither it goes ; we cannot determine how it originates, or why it ceases. The great Bacon indeed was of opinion, that by a close and regular history of the wind, continued for a number of ages together, and the particulars of each observation reduced to general maxims, we might at last come to understand the variations of this capri- cious element, and be able to foretel the certainty of a wind with as much ease as we now foretel the return of an eclipse. Indeed his own beginnings in this ardu- ous task seem to speak the pcssibility of its success ; but unfortunately this investigation is the work of ages, and we want a Bacon to direct the process. In the Historia Ventorum, Bacon reckons three sources of winds ; one by descent from the superior regions of the atmosphere, another from the expansion of the low- er air, and a third by expiration from the earth : of which last he proposes it as an object of inquiry, What winds blow out of subterraneous caverns ? Whether they come forth in a large body, or blow insensibly here and there ; and then unite in one stream, like a river formed out of many different springs ? This latter cause has been but little attended to, though this reci- procation between the earth and air is surely a very in- teresting part of natural philosophy. In the language of Holy Writ, God is said " to bring the winds out of his treasures j w as if some hidden storehouse were al- luded to, such as that of the waters and cavities in the bowels of the earth. The annual revolution of the sun is doubtless a gene- ral cause of winds ; but this cause, considered alone, should produce regular winds, whose progress would correspond to, and be connected with the seasons ; the phenomena however observed by no means enable us to perceive this connexion. There is another cause, of which we may form an imperfect idea, by which the winds, 456 OF WINDS. proceeding from the south, may be south west to us, and those which come from the north, north-easr. This cause is the difference in the velocity of the motion of the parts of the earth we inhabit, and that at the equa- tor, or polar circles. If the air was calm at the t quator, that is, moved with the same velocity as the earth, and that in coming from thence to us in the same direction, preserving at the same time a portion of its acquired motion, it would gain upon the earth in this direction, and would thus become south-west. The same cerfect, and the observations we possess too few to form i theory. In the commencement of tbe monsoons, to lse the seamen's phrase, they creep along the shore, they hen spread into the ocean : at first they are feeble, they fterwards become vigorous ; they then gradually dimi- lish, and finally come to a change ; and this twice in a r ear, agreeable to our solar progress. The sun is the undoubted cause of the sea and land >reezes, which are wisely appointed by Divine Providence o make some of the hotter climates habitable. The sea >reezes in the West Indies begin to appear about nine ) , clock in the morning, in a fine black curl upon the vater, approaching the shore ; it increases gradually till loon, and dies away at four or five in the afternoon. Nicholson's Philosophy, vol. ii. p. 61 and 62, 462 OF TRADE-WINDS About six in the evening it changes to a land breeze, which blows from the land to the sea, and lasts till eight in the morning. There is an interval in the morning and evening between the changing of the breezes, when the wind is stationary, like tfre water before the turning of the tide ; and these intervals are the hottest parts of the day. The breezes are thus accounted for : when the sun is up, his heat takes more effect on the land than on the water, so that the heat is accumulated, and the air over the land is rarefied ; and as it mounts upward, the colder air from the sea comes in to keep up the equilibrium. In the evening the dews are so excessive, and the cold so sudden on the land, from the quick descent of the sun below the horizon, that the water in the night is warmer than the land ; and the air of the sea, being then most rarefied, the draught of air is contrary to what it was in the day. In the northern temperate zone the winds are variable. but the most general are the S. VV. and W. and the N. E. and E. In the northern-temperate and frigid zones, th( winds are more tempestuous in winter than in summer.* " In our climates, a tempest is but rarely known, am; its ravages are registered as an uncommon calamity; but. in the countries that lie between the tropics, and for i good space beyond them, its visits are frequent, and its effects anticipated. > In these regions the winds vary theii terrors, sometimes involving all things in a suffocating heat ; sometimes mixing all the elements of fire, air, wa ter, earth together ; sometimes with a momentary swift ness passing over the face of the country, and destroying all things in their passage ; and sometimes raising wholt sandy deserts in one country, to deposit them in another We have, therefore, very little reason to envy those cli mates, the luxuriance of their soil, or the brightness o their skies. Our own cloudy atmosphere, that wraps u round in obscurity, though it fails to. gild our prospect: with sunshine, or our groves with fruitage, nevertheles: answers the calls of industry ; the labourer toils in th< certain expectation of a moderate but happy return." * Dalton".^ Meteorological Observations, p. 88. AND MONSOONS. 463 The rains in the West Indies are by no means the things hey are with us. Our heaviest rains are but dews compa- -atively : they are rather floods of water, poured from he clouds with a prodigious impetuosity ; the rivers rise n a moment ; new rivers and lakes are formed ; and in l short time all the low countries are under water. It is in the rainy season, principally in the month of August, that they are assaulted by hurricanes, which de- stroy at a stroke the labours of many years, and prostrate he most exalted hopes of the planter, and that often when le thinks himself out of the reach of fortune. It is a sud- len and violent storm of wind, rain, thunder, and light- ling, attended with a furious swelling of the seas, and iometimes with an earthquake ; in short, with every cir- :umstance which the elements can assemble that is terri- ble and destructive. First they see, as a prelude to the msuing havock, whole fields of sugar canes whirled into he air, and scattered over the face of the country. The strongest trees of the forest are torn up by the roots, and iriven about like stubble ; their wind-mills are swept iway in a moment ; their works, the fixtures, the ponde- rous copper-boilers and stills of several hundred weight, ire wrenched from the ground and battered to pieces ; ;heir houses are no protection ; their roofs are torn off it one blast, whilst the rain, which in an hour rises five feet, rushes in upon them with irresistible violence. There are signs by which the Indians of these islands :aught our planters to prognosticate the approach of a lurricane. The hurricane comes on either in the quarter Dr at the full or change of the moon. If it come on at :he full, then at the preceding change the sky is troubled, :he sun more red than usual ; there is a dead calm below, and the mountain tops are free from those mists which usually hover about them. In the caverns of the earth and in wells, you hear a hollow rumbling sound, like the rush- ing of a great wind. At night the stars seem much lar- ger than usual, and surrounded with a sort of burs ; the north-west sky has a black and menacing appearance; the sea emits a strong smell, and rises into vast waves often without any wind. The wind itself now forsakes its usual steady easterly stream, and shifts about to the west, from 46 i OF TRADE-WINDS, &C whence it sometimes, with intermissions, blows violently and irregularly about two hours at a time. You hav< the same signs at the full moon ; the moon herself is sur rounded with a great bur, and sometimes the sun has th< same appearance. The nature of the soil over which the wind blows ha; a great effect upon the quality of the air : the vast sand] deserts of Africa and Arabia give a burning heat and blast ing quality to the air that passes over them. These horrh regions lie to the southward and eastward of the Medi terranean ; and hence it is that travellers, who have hac the opportunity of making the comparison, tell us, tha the air of the West India islands is nothing to the hot suf focating winds which blow in the night at Minorca ai Gibraltar, for these latter are scarcely supportable by tl human frame. At Goree, in the river Senegal, there an easterly wind from the inland parts, with which thos who are suddenly met by it in the face are scorched up a by a blast from a furnace. An extraordinary blasting wind is felt occasionally a Falkland Islands. Happily its duration is short ; it seldon continues above twenty-four hours. .It cuts the herbagi down as if fires had been made under them ; the leave are parched up and crumble into dust ; fowls are seize< with cramps, so as never to recover ; men are oppresset with a stopped perspiration, heaviness at the breast, am sore throat, but recover with care. But, beyond all others in its dreadful effects, is th samiel, or mortifying wind, of the deserts near Bagdad The camels, either by instinct or experience, have notic of its approach, and are so well aware of it, that they ar said to make an unusual noise, and cover up their nose in the sand. To escape its effects, travellers throw them selves as close as possible to the ground, and wait till i has passed by, which is commonly in a few minutes. A soon as they who have life dare to rise again, they exa mine how it fares with their companions, by plucking d their arms or legs ; for, if they are destroyed by the wind their limbs are absolutely mortified, and will come asun der. It is said of this wind, that when it happens to mee with a shower of rain in its course, and blows across it, it i OF THE AURORA BOREALIS. 465 once deprived of its noxious quality, and becomes mild and innocent. It is also said, that it was never known to pass the walls of a city. OF THE AURORA BOREALIS. No person has paid so much attention to this subject as Mr. Dalton ; he is also the only one that I know of who has given a clear and satisfactory account of this phenomenon. To this work I must refer you ; con- tenting myself here with laying before you his account of the appearances of the aurora borealis, without en- tering into his explanation thereof. The appearances of the aurora come under four dif- ferent descriptions. 1. A horizontal light, like the morning aurora, or break of day. 2. Fine slender lu- minous beams, well defined, and of dense light These often continue a quarter, a half, or a whole minute apparently at rest, but oftener with a quick lateral mo- tion. 3. Flashes pointing upward, or in the same di- rection as the beams, which they always succeed. These are only momentary, and have no lateral motion ; but they are generally repeated many times in a minute. They appear much broader, more diffuse, and of a weaker light than the beams : they grow gradually fainter till they disappear ; and sometimes continue for hours flashing at intervals. 4. Arcs, nearly in the form of a rainbow ; these, when complete, go quite across the heavens, from one point of the horizon to the opposite point. When an aurora happens, these appearances seem to succeed each other in the following order : ] . the faint rainbow-like arcs ; 2. the beams ; and 3. the flashes. As for the northern horizontal light, it appears to con- sist of an abundance of flashes or beams blended to- gether by the situation of the observer. The beams of the aurora borealis appear at all place? to be arcs of great circles of the sphere, with the eye in the centre ; and these arcs, if prolonged upwards, would all meet in one point. VOL. IV. so 466 THE SOURCES OF HEAT. The rainbow-like arcs all cross the magnetic meridian at right angles. When two or more appear at once, they are concentric, and tend to the east and west : also the broad arc of the horizontal light tends to the mag- netic east and west, and is bisected by the magnetic meridian ; and when the aurora extends over any part of the hemisphere, whether great or small, the line se- parating the illuminated part of the hemisphere from the clear part, is half the circumference of a great circle crossing the magnetic meridian at right angles, and terminating in the east and west : moreover, the beams perpendicular to the horizon are only those on the mag- netic meridian. That point in the heavens to which the beams of the aurora appear to converge, at any place, is the same as that to which the south pole of the dipping needle points at that place. The beams appear to rise above each other in suc- cession ; so that of any two beams, that which has the higher base, has also the higher summit. Every beam appears broadest at or near the base, and to grow narrower as it ascends ; so that the conti- nuation of the bounding lines would meet in the com- mon centre to which the beam tends. The height of the rainbow- like arcs of the aurora are estimated by Mr. Dalton to be above the earth's surface -about 150 English miles. OF THE SOURCES OF HEAT AND COLB.* If the changes of the weather depended on the course of the year, and the temperature of climates were go- verned by their situation with respect to the sun, that is, by their latitude, then the weather might be reduced to some regular theory. But this is so far from being the case, that the latitude of a place cannot be consider- * Kirwan % 8 Estimate of the Temperature of different Latitudes. Jones's Physiological Disquisitions* THE SOURCES OF HEAT. 46? ed as an index to the temperature of the climate : for we find the hottest days in the coldest climates ; and the coldest weather, and even perpetual snow, are found in countries bordering on and immediately under the equator : so that we must recur to some other causes besides the immediate influence of the solar rays. 1. But though the sun is not the only cause, its pre- sence is undoubtedly the principal source of heat, as well as light, and its absence the primary cause of cold. He is indeed the great spirit of the world : all things revive at his approach ; winter and frost lie behind him. 2. The second source of heat is the earth. Nobody has yet been found so absurd as to suppose that human perspiration was owing to the air that surrounds the skin ; it originates in an intertial cause ; it is occasion- ed by a heat within, not the air without. It is the same with respect to the earth ; which, by imparting its heat to the atmosphere, moderates the rigour of the winter's cold. Whether we suppose that this heat arises from a central source, or that the globe from its first crea- tion was endued with a heat sufficient for all the purpo- ses it was intended to answer ; yet it is evident that it is renewed and preserved by the influence of the sun, and that there is always a silent and imperceptible heat proceeding from the earth. M. de Luc shows, that our globe has a provision of fire spread through its whole mass ; so that, wherever there is no chemical operation to disengage or to ab- sorb it, this fire maintains the same degree of expansive force. From observation we also find, that the same degree of heat reigns in all subterraneous places, ex- cept in mines where there is reason to suspect some chemical operation. With respect to those parts of the globe which are nearest the surface, the fire passes therefrom into the air, when its expansive force exceeds that of the fire in the air, and mice versa. Thus a cer- tain equilibrium is preserved near the surface, though subject to certain vicissitudes. The solar rays exercise two distinct functions ; in the one acting as fire, in the other increasing the expan- 468 THE SOURCES OF COLD. sive force of the existing fire. Various combinations of fire are continually forming, as well upon the surface of the globe as in the atmosphere ; combinations which are afterwards under other circumstances destroyed. These compositions and decompositions occasion the greater part of terrestrial phenomena. 3. The next great source of heat is the condensation of vapour. Vapour contains a quantity of fire : it is this fire which causes it to assume, and supports it in an aerial expanded state ; when condensed into a liquid form, it lets go this fire, which warms the surrounu atmosphere : hence the sultriness frequently experi- enced before rain. OF THE SOURCES OF COED. 1. As the earth is one of the principal sources of heat in the atmosphere that surrounds it, so is distance from the earth a source of cold ; the greatest cold prevailing in the highest regions of the atmosphere : for, where the re-action is wanting by a superficial pressure, but little effect can be received from the rays of the sun ; and it is further proved by experiments with a burning glass, that a clear unclouded air receives no heat from these rays. Hence, when we ascend to a lighter air, at a distance from the surface, the heat is not sufficient to melt the snow ; and we find the highest mountains, even under the equinoctial, perpetually covered there- with ? thus, the mean height of the lower term of con- gelation in winter, in this latitude, may be considered in general to be at 6260 feet from the surface, and the mean height of the upper term at 1125 feet. We can- not in this lecture consider any of the minute ex- ceptions. Sir William Young gives a remarkable instance of the effect of hills in arresting vapours and producing rain, while the exhalations from the trees on its surface cool and temper the air ; observing, that the smooth polished parbadoes and our Leeward Islands are parched up, whilst the towering and rugged Dominica, St. Vincent, THE SOURCES OF COLD. 469 Grenada, and Tobago, enjoy incessant rains and deli- cious verdure. It is generally agreed, that the clearing away of wood in time lessens the vapours, and consequently the rain of a country. Several fine parishes in Jamaica, which used to produce large crops of sugar canes, and were Dnce the richest spots in the island, are now dry for nine months in the year, and are turned into cattle- pens, through the clearing away of the neighbouring woods. Water is very plentiful in those countries where woods and forests abound, and the purest springs are generally found beneath the friendly shelter of a grove. The natural history of every country shows, that in proportion as the woodlands are cleared, the water courses diminish. In America, unfortunately for the inhabitants, this truth is too well known ; for, since the woods in the vicinity of their towns have been cut down, many long established mill races have become dry, and others have been reduced so low, as to cause very great interrup- tions to the miller, who must wait a considerable time for the dams to fill between every few hours work. Hence we may learn the important necessity of pre- serving the trees, from beneath whose humid shades a water spring discharges its streams ; and hence, too, we may learn, that the smallest springs may be improv- ed by planting around them a grove of trees, particu- larly the oak, so highly valued by the Greeks, the Romans, and our ancient Druids ; who, considering the health of man and the fertility of the soil to be ab- solutely dependent upon plenteous streams of water, consecrated their groves to preserve their springs. 2. The next great source of cold is evaporation. The same cause which makes the condensation of va- pour a source of heat, makes evaporation productive of cold ; as it absorbs the fire in the latter instance, which it gives out in the former : it is this which gives the particles of vapour their aerial form. When fire passes from fluids which it has heated, its course is up- 470 OF EVAPORATION. wards, and it always carries with it a thin stratum of the fluid in the form of vapour : thus evaporation not only tempers the heat occasioned by the sun's rays, but i^ one great source of cold. OF EVAPORATION. Of evaporation it may be observed, 1. That in our climates the quantity of it is four times greater from the 21st of March to the 21st of September, than it is from the 21st of September to the 21st of March. 2. That it is greater in proportion as the difference in temperature between the air and evaporating surface is greater ; though, if the air be 15 degrees colder than the evaporating surface, there is no evaporation, but a deposit of moisture from the air. 3. The degree of cold produced by evaporation, is always much greater when the air is warmer than the evaporating surface, than that which is produced when this surface is warmer than the air. Hence warm winds, as the Serocco and Harmatan are more desiccative than cold winds. 4. Evaporation is more copious when the air is less loaded with vapours, and is consequently powerfully promoted by cold winds flowing into warmer countries. 5. That it is greatly increased by a current of air or wind flowing over the evaporating surface ; not only because the evaporating surface is thereby increased, but also because the vapour is thereby removed, and prevented from attaining its maximum : hence it is ge- nerally remarked, that calm days are the hottest. 6. Tracts of land covered with trees or vegetables emit more vapour than the same space covered with water : on this principle it is, that the air about a wood or forest is made colder by the evaporation from trees and shrubs, while the plants themselves are kept in a more moderate heat, and secured from the burning heat of the sun by the vapour perspired from the leaves. Thus, we find the shade of vegetables more effectual to OF ANNUAL TEMPERATURE. 471 cool us, as well as more agreeable, than that from rocks and buildings. 7. The heat and cold of different countries are trans- mitted from one country to another by the medium of winds. OF ANNUAL TEMPERATURE. Within ten degrees of the pole, there is very little dif- ference in the annual temperature, nor is there much within ten degrees of the equator. The temperature of different years differs very little near the equator, but they differ more and more as their latitudes approach the pole. It scarce ever freezes, unless in very elevated situa- tions, in latitudes under 35° ; and it scarce ever hails in latitudes higher than 60°. Between the latitudes of 35° and 60°, in places adja- cent to the sea, it generally thaws when the sun's altitude is 40°, and seldom begins to freeze until the sun's meri- dian altitude is below 40°. The greatest cold in all latitudes in our hemisphere is generally about half an hour before sun-rise : the greatest heat in all latitudes between 60° and 45° is found to be about half past two o'clock in the afternoon ; between latitudes 45° and 35°, at two o'clock ; between latitudes 35° and 25°, at half-past one ; and between latitude 25° and the equator, at one o'clock. The month of January is the coldest in every latitude, July is the warmest month in all latitudes above 48° ; but in lower latitudes, August is generally the warmest. December and January differ but little, and there is no great difference between June and July. In latitudes above 30°, the months of August, September, October, and November, differ more from each other than those of February, March, April, and May ; in latitudes under 30° the difference is not so great. The temperature of April approaches more every where to the mean annual temperature than that of any other month : whence we may infer, that the effects of natural causes, operating over a large extent, do not arrive at their maximum un- 472 OF ANNUAL TEMPERATURE. til the causes begin to diminish ; but that after these ef- fects have arrived at their maximum, the decrements are more rapid than the increments originally were, during their progress to that maximum. The differences between the hottest and the coldest months, within twenty degrees of the equator, are in- considerable, except in some peculiar situations ; but they increase in proportion as we recede from the equa- tor. In the highest latitudes, we often meet with a heat of 75° or 80° ; and particularly in the latitudes of 59° and 60° the heat of July is frequently greater than in latitude 51°. At the time of the equinoxes, when the sun passes from one hemisphere into the other, there is generally some disturbance in the weather ; the winds are then mostly higher : at the vernal equinox, they are for the greater part easterly, cold, dry, and searching. The sol- stitial point of the summer is more apt to be distinguish- ed by violent rains, and what w.e call a midsummer flood. The winter being less rainy than the summer, nothing particular happens at the winter solstice, except that the frost sets in more severely, with some continuance of snow, which lies long upon the ground. The temperature of a climate depends on many cir- cumstances, particularly on the disposition of the land ; as its elevation, its exposure to the winds, and the course of the mountains that are found in it. Thus the writer of Anson's voyage informs us, that while they coasted near the land of South America, which has those vast ridges of mountains, the Andes and Cordillieras, the air was rendered temperate by the wind that blew over them ; but when they had passed beyond this tract of land, and sailed by the isthmus of Darien, where the country is flatter, the air became insupportably close and sultry. All countries lying to the windward of high moun- tains, or extensive forests, are warmer than those to the leeward in the same latitude. The vicinity to the sea is another circumstance which affects the temperature of a climate ; as it moderates the heats from the lancj> and brings the atmosphere down to OF ANNUAL TEMPERATURE. 473 a standard best fitted to the human constitution. This is probably the reason why there is so great a proportion of sea on the terraqueous globe, and particularly why it is so largely distributed about the middle region of the ^arrh. In our hemisphere, countries that lie southward rf any sea, are warmer than those that have that sea to the south of them ; because the winds that should cool them in winter are mitigated by passing over the sea ; whereas, those that are northward of the sea, are cooled in summer by the breezes from it. A northern or sou- thern bearing of the sea renders a country warmer than an eastern or western bearing. Islands participate more of temperature arising from the sea, and are therefore warmer than continents. Most large islands have their greatest extent from north to south. With us, the southern parts are proportionably colder than the northern. A ridge of mountains generally traverses islands in the direction of their length. The soil of large tracts of land has its share in influ- encing the temperature of the weather : thus, stones or sand heat and cool more readily, and to a greater degree, than the earth or vegetable mould ; hence the violent heats of the most sandy deserts of Arabia and Africa, and the burning heat and blasting qualities of the wind that passes over them ; hence also the intense cold of Terra del Fuego, and other stony countries in cold latitudes. Living vegetables have a considerable effect in altering climates, and affecting the weather. Wooded countries are much colder than those that are open and cultivated ; thus, part of Guiana has only been cleared from wood since the beginning of this century, and the heat in that part is already excessive ; whereas, in the wooded parts, the inhabitants are obliged to light a fire every night. Every habitable latitude enjoys a heat of 60 degrees at least, for two months ; which heat seems necessary for the growth and maturity of corn. The quickness of ve- getation in the higher latitudes proceeds from the dura- tion of the sun above the horizon. Rain is little wanted, as the earth is sufficiently moistened by the liquifaction of the snow that covers it during the winter. In all this, we VOL. IV. 3 *> 474 OF ATMOSPHERICAL ELECTRICITY. cannot sufficiently admire the wise disposition of Provi- dence. It is owing to the same provident hand, that the globe of the earth is intersected with seas and mountains, in a manner that on its first appearance seems altogether irre- gular and fortuitous, presenting to the eye of ignorance the view of an immense ruin : but, when the effects of these seeming irregularities on the face of the globe are carefully inspected, they are found most beneficial, and even necessary to the welfare of its inhabitants ; for, to say nothing of the advantages of trade and commerce, which could not exist without these seas, we have seen that it is by their vicinity that the cold of the higher lati- tudes is moderated, and the heat of the lower. It is by the want of seas, that the interior parts of Asia, as Sibe- ria and Great Tartary, as well as those of Africa, are rendered almost uninhabitable ; a circumstance which furnishes a strong agreement against the opinion of those, who think these countries were the original habitations of man. In the same manner, mountains are necessary, not only as the reservoirs of rivers, but as a defence against the violence of heat in the warm latitudes. Without the Alps, Pyrenees, Apennine, the mountains of Dauphine Auvergne, &c. Italy, Spain, and France would be de prived of the mild temperature they at present enjoy without the Balgate hills, or Indian Apennine, India would have been a desert : hence Jamaica, St. Domingo, Sumatra, and most other intertropical islands, are fur- nished with mountains, from which the breezes proceed that refresh them. OF ATMOSPHERICAL ELECTRICITY. So little is known with any certainty concerning atmos- pherical electricity, that I shall detain you but a short time with what I have to say thereon. If every solution of continuity, every expansion and contraction of material substances, are sources of electrical appearances, we need not be surprized at finding it in great abundance among the clouds : in this view of the subject, the perpetual os- OF ATMOSPHERICAL ELECTRICITY. 475 dilations of the air must be also a means of rendering it sensible to us. Mr. Bennet's* electrometer, which I have already described, is the best and readiest instrument for observing the changes in the electricity of the atmos- phere. The following positions have been deduced from some observations on the electrical state of the atmosphere. 1st. That in the spring, when plants begin to grow, we are told that temporary electrical clouds begin to appear, and pour forth electric rain. 2. That the electricity of the clouds and of the rain increases, till that part of autumn, when the last fruits are gathered. It is hence supposed to actuate and animate vegetation, and to give to rain that power which renders it more propitious to vegeta- bles than any other kind of watering. Aerial electricity varies according to the situation ; it is generally strongest in elevated and insulated situations; not to be observed under trees, in streets, in houses, or any inclosed places ; though it is sometimes to be found pretty strong on quays and bridges. It is also owing not so much to the absolute height of the places, as their situa- tion ; thus a projecting angle of a high hill will often exhibit a stronger electricity than the plain at the top of the hill, as there are fewer points in the former to de- prive the air of its electricity. The intensity of the atmospheric electricity is varied by a great many circumstances, some of which may be accounted for, others cannot. When the weather is not serene, it is impossible to assign any rule for their varia- tion, as no regular correspondence can then be perceived with the different hours of the day, nor with the various modifications of the air. The reason is evident ; when contrary and variable winds reign at different heights, when clouds are rolling over clouds, these winds and clouds, which we cannot perceive by any exterior sign, influence however the strata of air in which we make our experiments, and produce those changes of which we only f See the Rev. Mr. Bennei's New Experiments on Electricity, Derby, 476 OF ATMOSPHERICAL ELECTRICITY. see the result, without being able to assign either the cause or its relation. Thus, in stormy weather, we see the electricity strong, then null, and in a moment after arise to its former force ; one instant, vitreous ; the next, resinous ; without being able to assign any reason for these changes. M. de Saussure says, that he has seen these changes succeed with such rapidity, that he had not time to note them down. When rain falls without a storm, these changes are not so sudden ; they are, however, very irregular, particu- larly with respect to the intensity of force ; the quality thereof is more constant. Rain or snow almost uniformly gives vitreous electricity. The state of the air, in which the electricity is strong- est, is foggy weather ; this is always accompanied with electricity, except when the fog is going to resolve into rain. The most interesting observations, and those which throw the greatest light upon the various modifications of electricity in our atmosphere, are those that are made in serene weather. In winter, during which most of M. de Saussure 9 s observations were made, and in serene wea- ther, the electricity was generally weakest in an evening, when the dew had fallen, until the moment of the sun's rising ; its intensity afterwards augmented by degrees, sometimes sooner, and sometimes later ; but generally before noon it attained a certain maximum, from whence it again declined, till the fall of the dew, when it would be sometimes stronger than it had been during the whole day ; after which it would again gradually diminish du- ring the whole night ; but is never quite destroyed, if the weather be perfectly serene. Atmospherical electricity seems therefore, like the sea, to be subject to a flux and reflux, which cause it to in- crease and diminish -twice in 24 hours. The moments of its greatest force are some hours after the rising and set- ting of the sun ; those, when it is weakest, precede the rising and setting thereof. The electricity of serene weather is much weaker in summer than in winter ; this renders it more difficult to erve these gradations in summer than in winter ; be- SIGNS OF THE WEATHER* 4/7 sides a variety of accidental causes, which at the same time render them more uncertain. In general, in summer, when the ground has been dry for some days, and the air is dry also, the electricity generally increases from the rising of the sun till three or four in the afternoon, when it is strongest ; it then diminishes till the dew begins to fall, which again re-animates it ; though after this it declines, and is almost extinguished during the night. But the serene days that succeed rainy weather in sum- mer, generally exhibit the same diurnal periods or states of electricity, that are to be observed in winter. ON PROGNOSTIC SIGNS OF THE WEATHER. There is no part of meteorology which interests man- kind so much, as the predictions it furnishes of the change of weather. The theory of it only engages the attention by animating us with the hopes of thereby bringing the knowledge of these predictions to perfection. And the far greater part of those who purchase mete- orological instruments, buy them, not so much to know the actual state of the elements, as to foresee the changes thereof. This science is, however, very imperfect ; for k is but of late years that we began to make observations on the changes of the weather ; but that its progress has been rapid and successful may be seen in the works of De Luc, De Sans sure , Jones, ' Marshall, and Kirwan. But these observations will be still more valuable to pos- terity ; for we can scarce expect them in sufficient num- ber in our own age to deduce from them a general and perfect theory. To attain this end it will be necessary to multiply ob- servations on as great a number of signs as possible ; for it is only by their combination and concurrence that we can expect to remove the uncertainty inseparable from each in itself. Thus the barometer is not always a certain sign ; the same may be said of the thermometer, the hy- grometer, and the action of winds. But if they all concur together, there is but little chance of being deceived ; and there would be still less, if to these were joined other 478 PROGNOSTIC SIGNS signs, which are easy to observe, and which, by their combination would render our prediction certain. No sign, nor any instrument of observation, should therefore be neglected, either from a love of ideal per- fection, or fears of inaccuracy. Thus, though the hy. grometer be at present a very imperfect instrument, yet one certain sign has already been obtained from its indications, and more may be reasonably expected. Even the words very dry, very moist., moderately dry, mo- derately moist, though of vague determination, may throw much light on the state of the atmosphere. It is necessary that the observer should enter into a precise detail of the various states of the sky and the clouds. What can we learn from the words covered, and cloudy, or half covered sky, &c. ? Nothing ; since it is well known, that a covered sky, in one case, is almost as certain an indication of fine weather, as in another it is an indubitable presage of rain. The accurate ob- server piques himself on a thermometer, with which he can observe within a degree, and a barometer that he can depend upon to less than the hundredth of an inch ; but is silent on the transparency of the air, on dews, on the elevation, the form, the sign, the disposition, the colour, and the density of the clouds ; things that may be observed with ease, and described without trouble ; being attended with no other inconvenience than that of extending the size of our meteorological tables. There is a phenomenon which has not been sufficiently attended to, namely, the undulating motion of the fir- mament, or that diurnal tumult in the air, which is kept up by the heat of the sun. What the sun raises from the earth by the heat of the day, is sustained in the at- mosphere by its heat, and the agitation, or expansive undulation of the air. This motion is often visible to the naked eye, but in the field of a powerful telescope it is very conspicuous ; all objects appear in violent agi- tation, and the line of the sensible horizon, which ought to be clear and well defined, is waved like a field of corn in the wind, or the surface of the sea in a storm. So long as this agitation continues, the vapours stay in the air; but when it subsides, and the sun ()• OF THE WEATHER. 479 parts, they are condensed, and fall down to the earth in the night as dew. In the present state of this part of science, when we are unacquainted with so many phenomena, and still more ignorant of their causes, general rules will often be found to fail, and particular ones will, without much circumspection, prove to be a source of error. Amongst the variety of means for predicting the changes of the weather, the barometer is undoubtedly one of the best ; and is in this, as well as many other respects, one of the greatest acquisitions to natural philosophy. The usual ranges of the mercurial column in this la- titude are comprized between 28 and 31 inches, of which the middle, or 29*, is considered as the variable : I think it should be placed somewhat higher. Near the pole, the variations of the barometer are much greater. OF PROGNOSTICS BY THE BAROMETER. Ever since the barometer has been invented, philoso- phers have endeavoured to account for the variations in the height of a local barometer, but hitherto in vain. M. de Luc, in the first volume of his Recherches sur les Modifications de 1' Atmosphere, has given a critical and very interesting account of the various physical opinions that have been invented for this purpose by Pascal, Beak, Wallis, Garden, Halley, &c. &c. and shown that they are all imperfect, and inadequate to the solution of the phenomena. He then proposes one of his own ; which with that candour that ever distin- guishes a lover of the truth he has since abandoned. To give a particular account of the various hypothesis would occupy a volume, and that to little purpose. As I know of none that can be depended on, I shall content myself with only relating the bare phenomena. The two great sources of error on this subject have been, 1st. The difficulty of observing the whole of the ap- pearances ; and 2dly, The facility with which the mind embraces and supports a favourite hypothesis. 480 dalton's general observations. There is one striking phenomenon in the variations of the barometer, which should be particularly attended to in every theory, because it is as great as it is certain and invariable ; namely, that the variations diminish in proportion as you approach the equator, and augment as you advance towards the poles. The countries, how- ever, that are situated about the equator, are subject to the changes of the weather, though it is more constant there than in the temperate climates : there are changes there of humidity and dryness, rains and fair weather, storms and tempests, &c. much more violent than with us ; and yet all these take place without any way af- fecting the barometer. MR. DALTON's GENERAL RULES AND OBSERVATIONS FOR JUDGING OF THE WEATHER. 2. The barometer is highest of all during a long frost, and generally rises with a N.E. wind ; It is lowest of all during a thaw following a long frost, and is often brought down by a S.W. wind. 2. When nearest the high extreme for the season of the year, there is very little probability of immediate rain. 3. When the barometer is low for the season, there is seldom a great weight of rain, though a fair day in such a case is rare. The general tenor of the weather at such times is, short, heavy, and sudden showers, with squalls of wind from theS.W., the W. or N.W. 4. In summer, after a long continuance of fair wea- ther, with the barometer high, it often falls gradu- ally, and for one, two, or more days, before there is much appearance of rain. If the fall be sudden and great for the season, it will probably be followed by thunder. 5. When the appearances of the sky are very promis- ing for fair weather, and the barometer at the same time low, it may be depended upon that the appear- ances will not remain such long. On these occasions the face of the sky changes very suddenly. FURTHER INDICATIONS FROM THE BAROMETER. 481 6. Very dark and dense clouds pass over, when the barometer is high, without rain ; but when the baro- meter is low, it sometimes rains almost without any ap- pearance of clouds. 7. All appearances being the same, the higher the barometer is, the greater is the probability of fair weather. 8. Thunder is generally preceded by hot weather, and followed by cold and showery weather. 9. A sudden and extreme change of the temperature of the atmosphere, either from heat to cold, or cold to heat, is generally followed by rain within 24 hours. 10. In winter, or during a frost, if it begin to snow, the temperature of the air generally rises to 32°, and continues there while the snow falls ; after which, if the weather clear up, expect a severe cold. 1 1 . The aurora borealis is a prognostic of fair weather. FURTHER INDICATIONS OF THE WEATHER BY THE BAROMETER. In general, when the barometer falls, there is rain ; but when the mercury rises, it is a sign of fair weather. If the mercury fall in a frost, we may expect snow, or a thaw ; but if it rise in winter, with a north or east wind, it generally portends a frost. If the mercury sink slowly and gradually, we may expect that the rain will be of some continuance ; and if the rise be gradual, we may judge that the fine weather will.be lasting. If it fluctuate much, rising and falling suddenly, the weather is unsettled and changeable: if it fall very low, there will be much rain ; but if its falls be low and sudden, a high wind generally ensues : when exceeding low, storms and tempestuous weather may be expected ; but if an extra- ordinary fail happen, without any remarkable change near at hand, it is probable, that there is a storm at a distance. The descent of the barometer is not, however, always an indication of rain, for it will often fall for wind ; nor VOL. IV. ,. 3Q 482 FURTHER INDICATIONS is its rise a certain sign of fair weather, particularly if the wind be northerly or easterly. If the fine weather be lasting, with a westerly wind, the mercury generally rests a little above changeable, but somewhat below thirty inches. In the summer months the barometer does not vary so much as in winter ; the greatest variations are in the first two, and the last two months of the year, but par- ticularly in the first and last. A northeast wind gen- erally makes the barometer in this country rise, and it is generally lowest with a south-westerly wind. If the mercury continue to fall while it rains, it will be likely to rain the next day : when the mercury is pretty high, and has fallen to foretel rain, and yet rises before the rain falls, it is an indication that there will be but little. In fair weather, when the mercury has con- tinued high and rising, if it fall about noon, and rise again towards the evening, a single shower may be ex- pected on the evening or noon of the next day, and then fair weather. When the mercury rises gradually about half a tenth, and continues to do so for many days, the fair weather may be expected to continue for some time, unless wind intervenes, particularly from the S. W. by S. FROM THE THERMOMETER. In winter, if the cold diminish suddenly, it in ge- neral portends rain ; in summer, a sudden augment- ation of heat is also a forerunner of rain. FROM THE BAROMETER AND THERMOMETER. If the air in foggy weather becomes hotter by the action of the sun alone, the fog generally dissipates and the air remains serene : but if the barometer fall, and the change of temperature be from a south or south- west wind, the fog rises and forms itself into clouds, and its ascension is generally a sign of rain. [ 483 J FROM THE BAROMETER, HYGROMETER, WIND, AND STATE OF THE SKY. The barometer being high and stationary, the natu- ral and factitious hygrometers indicating dry air, the canopy of the sky lofty, and the wind north-easterly, are the surest signs of settled fair weather ; while a light and moist atmosphere, the canopy of the sky low, and a south-west wind, certainly portend a wet season. FROM CLOUDS. When the clouds are formed like fleeces deep and dense towards the middle, and very white at the edges, with a bright blue sky about them, they generally soon fall in hail, snow, or in hasty showers of rain. In the north of England, such clouds are called woolpacks. There is no sign of rain more certain than two dif- ferent currents of clouds, especially if the undermost fly fast before the wind ; when this happens in sum- mer, there is seldom wind at the time, and thunder ge- nerally follows. In winter the light vapour, or scud as the sailors call it, often comes rapidly against the wind, and a gale is soon after to be expected. The transparency of the air is to the inhabitants of the Alps one of the most certain signs of rain ; when the distant objects appear distinct and well defined, when the sky is of a deep blue, they consider rain as near at hand, though no other signs appear. I have been informed by a gentleman, to whom I am under obligations for other observations, that this sign, from the transparency of the air, is by no means local, but is often observed in England ; that in such a state of the air, the sailors say the land, or other object, looms near, and expect bad weather. When the sky, in a rainy season, is tinged with a sea-green colour, particularly near the horizon, when it ought to be blue, the rain will continue and increase. If it 484 SUPERIORITY OF THE NORTHERN be of a deep dead blue, it will be showery : this is more particularly found to hold true near the sea coast. Clouds of a similar appearance produce thunder in summer, and snow in winter ; such clouds are broken, and irregularly shaped, heaped one on another, and from their uncommon density project towards the earth. After a thunder storm, when it has been of considerable duration, the wind generally, if not always, veers to the quarter from whence the first clap proceeded. A close sultry day, the current of air scarcely per- ceptible, is often succeeded by a change in the wind. The wind shifting from point to point round the compass, generally denotes rain. If, after a continued rain from a muddy sky, the horizon appear lighter in any quarter, expect the wind from that quarter. OF THE SUPERIORITY OF THE NORTHERN HEMIS- PHERE OVER THE SOUTHERN, FROM THE REV. MR. JONES* S PHYSIOLOGICAL DISQUISITIONS. The superiority of the northern hemisphere of the world, above the southern, is very manifest. It has more land, more sun, more heat, more light, more arts, more sense, more learning, more truth, more religion. The land of the southern hemisphere, that is, the land which lies on the other side of the equinoctial line, does not amount to one fourth part of what is found on the north side. The sun, by reason of the eccentricity of the earth's orbit, and the situation of the aphelion, makes our summer eight days longer than the summer of the other hemisphere ; which, in the space of four thousand years (for so long it is since any universal change has taken place in the earth), amounts to upwards of eighty-seven years ; and so much more sun has this hemisphere en- joyed than the other. What effects may have been arising gradually in all that time, we cannot ascertain ; but such a cause cannot have been without its effect : and I think it is allowed, that the temperature of the earth and atmosphere, in the highest latitudes of the OVER THE SOUTHERN HEMISPHERE. 485 north, is much more mild and moderate than in the correspondent latitudes of the south. The dreary face of Statenland, with the weather-beaten Cape of South America, a climate so severe as scarcely to admit of any human inhabitants, is no nearer to the pole than th_ j northern counties of England : but the difference in the atmosphere, and in the aspect of the earth, is almost incredible ; and this is the more remarkable, be- cause there is no mountainous country betwixt that and the pole to account for the icy blasts that prevail there. But it is also further observable, that the northern hemisphere is better provided for by night as well as by day. The stars of superior magnitudes are much more numerous on this side the equinoctial than on the other : we have nine stars of the first magnitude, and they but four ; and the stars of the Great Bear, so con- spicuous in this hemisphere, having nothing to equal them about the other pole. When the sun is remote from us in the winter, our longest nights are illuminat- ed by the principal stars of the firmament ; when the sun enters Capricorn, there comes to the meridian, about midnight, the whole constellation of Orion, the brightest in the heavens, containing two stars of the first magnitude, four of the second, and many others of inferior sizes ; and upon the meridian, or near it, there are four more stars of the first magnitude, Capel- la, Sirius, Procyon, and Aldebaran. No other portion of the heavens affords half so much illumination ; and it is exactly accommodated to our midnight, when the nights are longest and darkest. If the mid-winter of the southern hemisphere be compared, the inferiority of the nocturnal illumination is wonderful. Though it will carry us a little beyond the bounds of physics, the parallel is so glaring between the natu- ral and intellectual superiority of this part of the world, that your time will not be lost while we reflect upon it. Here the arts of war and peace have always flourished ; as if this part of the globe had been allotted to a supe- rior race of beings. Asia and Europe, from the re- remotest times, have been the seats of science, elo- quence, and military power ; compared with which, the 486 SUPERIORITY OF THE NORTHERN, &C. southern regions have ever been, as we now find them, beggarly and barbarous ; possessed by people stupid and insensible, illiterate, and incapable of learning. Where are the poets, the historians, the orators, the philosophers of the southern world ? We may as well search for the sciences among the beasts of the wilder- ness. All the inventions, by which mankind have done ho- nour to themselves in every age, have been confined to this side of the world. Here the mathematical sciences have flourished ; printing has been found out ; gun- powder and fire-arms invented ; navigation perfected ; magnetism and electricity cultivated to the astonish- ment of the wisest ; and philosophy extended by ex- perimental inquiries of every kind. There would be no end, if we were to trace this comparison through eve- ry improvement ; for here we have every thing that can adorn human life, and there they have nothing. But the difference is most conspicuous, when we compare the north and south in point of religion ; to which, indeed, that pre-eminence is owing on our side, which has extended to every branch of social civiliza- tion and intellectual improvement. It it notorious at this day, that arts and learning flourish to the highest degree, in those countries only that are enlightened by Christianity, and no where so much as in this kingdom, where that religion is established in its purest form. May it long continue ! and may we know our own fe- licity in the enjoyment of it ! for religion is undoubt- edly the sun that gives light to the mind ; the vital spirit that animates the human understanding to its highest achievements ; though many have been in- debted to it, without being sensible of their obligation, or without confessing it ; and others have turned against it that light which they borrowed from itself. The northern hemisphere then, whatever preference it may have in a physical capacity, has been much more honoured by the superior advantages of learning and religion : here knowledge first began to be diffused, and the world itself was first inhabited, in the finest climates of the earth, which are about the latitudes 36 CONCLUSION. s 487 degrees, &c. north : here the church was first settled ; and the Hebrew nation, rising by degrees till the reign of Solomon, formed a wise, wealthy, and splendid king- dom, long before the powers of Greece and Rome were heard of : here the light of Christianity was afterwards manifested, and with it the-lights of learning have been extended to the parts where they were never known be- fore, till both of them reach to the utmost boundaries of the west, in the once unknown regions of the At- lantic world. CONCLUSION. I have now finished my course of lectures, and have given you a general view of the principal phenomena in nature ; nor have I been inattentive to the discoveries made therein by man. I have endeavoured to point out the abuse that may be made of physical inquiries, and to guard you against the errors by which they may be perverted and rendered a prop to support the weak fa- bric of infidelity and falsehood. From these lectures it evidently appears, " 1. That man is composed of two substances, of which one perceives without being per- ceived by the senses ; and the other is perceived with- out having any perception in itself. 2. That man, in his present state, can perceive nothing more of the uni- verse than what is transmitted to him by his organs, whose faculties are very limited. 3. That there are evi- dently effects perceptible by man, which are occasioned by beings that he cannot perceive. 4. That man deprived only of one sense, sight, would have been ignorant of the greater part of what he knows of the universe, namely, of entire classes of beings, and of the relations of these beings to each other, and to those with which he is acquainted. 5, and lastly. By every rule of ana- logy, and from many phenomena, it is highly probable, that, there exist may classes of beings, related to each other, and to man, which he cannot in his present state perceive."* * De Luc , Lettres Physiques et Morales, torn. v. p. 11, and 689. 488 CONCLUSION. The spiritual powers of man are roused into action by the medium of the senses. His understanding ex- pands itself by the perceptions the senses transmit ; so that, notwithstanding the extent of his powers, he can make no progress in matters higher than sense, unless he take the creation for his lesson, and the Omniscient Creator for his Preceptor. It is therefore weak and perverse in him, without the very elements of know- ledge in his head, to desert such a wise and kind in- structor, and then set up for an independent discoverer. Put the philosopher to the trial, who pretends to know so much of a Deity without allowing him to discover him- self and explain his own works, and you will soon see the wise man confounded by his own wisdom. If this wanted proof, I need only mention the writings of Helvetius, Voltaire, Diderot, De la Metrie, and the whole school of Condorcet. In contradiction to these men, I have endeavoured to show that philosophy is illustrated, and just views of nature are exhibited by the sacred writings. What in- deed can we think of those who would have us believe they credit the scriptures, while they take upon them to correct its stile, as not philosophically just ? who would have us believe, that he who holds all nature in his hand, does not know how to accommodate his doc- trines to the capacities of the vulgar, without speaking with philosophical impropriety of his own works ? Will they, indeed, teach him to speak, who gave a mouth to man, whose word was sufficient to cause the mighty sun to shine, and daily diffuse his treasures of light around the heavens, irradiating the shifting hemispheres of the revolving earth, and at whose command it is surround- ed by the liquid air ? Shall the writings of men have excellencies in our eyes, and his have no beauty, who hath meted out the heavens, who knoweth the ba- lancing of the clouds, and by whose knowledge the deeps are broken up ? Both his word and his works prove, that he has employed and displayed infinite wisdom, power, and goodness, in the creation of this universe ; that he has with stupendous artifice stored our globe with every CONCLUSION. 489 thing necessary, not only for the support, but for the felicity of man : all his works are stamped with the characters of the infinite perfections, and overflowing goodness of the Author. He has given to man, and to him alone, a capacity to be entertained with the mag- nificence, the beauty, the harmony, and the order of the universe ; and has so moulded his heart and spirit, as to make pleasure attendant on admiration, and love and gratitude the necessary companions of the senses of favours received. Let us then praise the God of heaven, from whom we have received so much — whose mercy is extended over all. Let every thing that hath breath praise him ; and let man, the priest of the creation, offer up a sacrifice of thanksgiving unto the Most High. APPENDIX TO LECTURE LIL BY THE E. EDITOR. CONTAINING A FURTHER DESCRIPTION OF METE- OROLOGICAL INSTRUMENTS J WITH FIGURES. I HE barometer, as already described by our Au- thor, is called the chamber barometer. When the in- strument is constructed to be used at sea, on board a ship, it is called the marine barometer, and which is made somewhat different from the chamber one, in or- der to prevent the violent concussions of the mercury on the top of the tube, and the unsettled state of its al- titude, caused by the motion of the ship. There have vol. iv. J 3R 490 THE BAROMETER. been various contrivances to obviate these ; but the best appears to me, to consist in drawing about two feet of the lower portion of the barometer tube to a fine aperture, almost capillary : the resistance so occasioned to the mo- tion of the mercury in the tube, is found sufficient to re- tard and destroy a violent motion, and to produce a just altitude of the mercury. It requires a little longer time for the settling of the mercury to its true altitude, but this is of no consequence to the observation. The frame of the instrument is suspended on gimbals near to the centre of gravity, and occasionally to be screwed either to the ceiling or side of a cabin ; and from these positions the instrument has been found to answer sufficiently well all the purposes for which it is wanted at sea. Fortunate is it for the mariner, when by the alterations of the altitude of the mercury he can fore- tel the approach of a storm, or tempestuous state of the atmosphere. The instrument should be accompanied with a thermo meter. A BAROMETER TO MEASURE THE HEIGHTS OF MOUN- TAINS, DEPTHS OF VALLIES, HEIGHTS OF BALLOONS, &C. &C. The barometer has been found to be the most conve- nient and accurate instrument that can be used for these purposes. By experiments made by M. de Saussure, de Luc, and Sir George Shuchburgh, it appears that heights and depths have been ascertained to a few feet in several thousands. The instrument requires to be made with the utmost accuracy, and great diligence and attention paid to the adjustments, &c. during the observation. The tube of this sort of barometer has its lower extremity drawn out to a small aperture ; a floating index applied, so as to be depended upon to at least the 500th part of an inch, as a gage point ; the frame made very light either of wood or a brass tube ; the scale of inches extended downwards to about 17 or 18 inches, and a portable mahogany tripod having folding legs with gimbals, made DE LUC'S HYGROMETER, 491 to support it when in use, or serve as a case for the in- strument when not in use. To measure heights, &c. in the most accurate manner, the observer must be provided with two barometers, or in case of an accident, with a third : the nonius or slid- ing plate to the scale of inches should subdivide it into the 500th part at least. There should be a thermometer attached to each instrument, and two detached corre- spondent ones for the pocket. The manner of making observations and computing from these instruments, the reader will see in Sir George Shuchburgb 1 s account in the Philosophical transactions, vol. 67 and 68. Geometrical measurement with the assistance of good angular instruments is the best method, when a good base is afforded ; but, as few countries afford a suitable base, or favourable circumstances in the figure or situa- tion of the mountains to be measured, the barometer is the instrument most frequently adopted. DE LUC'S HYGROMETER. M. de Luc's hygrometer, made of a fine slip of whale- bone, is the most approved instrument of the kind, and in the most general use. It has been found by him of greater expansibility than any other substance, such a slip, lengthening about one-eighth of itself from extreme dryness to extreme moisture ; it is a substance easy to be cut into slips ; and they have been made so fine, as in a length of six or eight inches to weigh only one-tenth of a grain ; on this account it is the most suitable substance for a common hygrometer. Fig. 1 of the following page shows its form as now made for common use ; it is made of various dimen- sions, but the figure is about one-half the size of those generally made. The frame-work is of brass, lightly made, and can easily be understood by the figure, with- out a detailed description here. The slip of whalebone is. represented by a £, and at its end, f it, and the small one, on a moveable foot, near the mouth ; the eye-glass being placed in a small tube, outside of the large one, and near the perforation in the great mirror. The rays of light from any distant object entering the telescope and falling on the great mirror nearly parallel, will be reflected converg- ing to its focus, where an inverted image of the object will be formed. The rays, crossing each other in this focus, will proceed to the small mirror on which they will fall diverging, and will thence be reflected converging, and thus passing through the perforation in the great mirror will form a se- cond or erect image, to be viewed through the eye-glass, placed at its focal distance therefrom ; the magnifying power ©f this telescope, will be in the compound ratio of the focal di- stances of the great and small mirrors, and of the great mirror and eye-glass. P. Give a short account of the Cassegrain telescope. The Cassegrain telescope differs from the Gregorian only in 1 this, that instead of the small mirror being concave, it is convex, and placed between the great mirror and' its focus. The rays of light, therefore, from ihe great mirror, will fall on the small one converging, and from it be reflected less convergent, and form an inverted image in the focus of the eye-glass, from which it is viewed. The chief advantage of this construction consists in its being shorter than the Gregorian of the same magnifying power, by twice the focal distance of the smaller mirror. P. Give a short description of HerschePs reflector. S. This has only one reflecting mirror, viz. the great one, which is placed in the bottom of the tube, a little obliquely, so that its axis would pass near one side of the mouth of the tube, where the eye-glass is placed, at its focal distance from the image formed by the reflection from the great mirror ; the observer turning his back lo the object. In this, as in the Newtonian telescope, the object, with a single convex eye-glass, will appear inverted ; and will be magnified in the ratio of the focal distance of the mirror to that of the eye-glass. P. What is the peculiar advantage of this construction above the others ? S. The prevention of the loss of light, by having but one re- flection, and no perforation in the mirror; the objects, especially small celestial ones, will therefore be seen move distinctly and better defined. P. In what manner may the field of view be enlarged in any kind of telescope ? APPENDIX. 529 S. By using an amplifying lens, generally a plano-convex, as in the compound miscroscope, between which and the eye-giass the image is formed. A tripple eye-glass, has also the same advantage. Of Magnetism, P. What is meant by magnetism ? S. That species of attraction which, especially in any'consi- derable degree, is found only in iron, steel, or the calces or ores of iron. P. What is a magnet? S. Any piece of ore or steel, possessing this attractive power or magnetic virtue, in any considerable degree, is called a mag- net ; if a piece of ore, it is termed a natural magnet ', or load-stone ; if a piece of steel, to which this virtue has been communicated by art, it is termed an artificial magnet. P. Explain a few of the most common terms and phenomena relative to magnetism. For instance, — Magnetic meridian ? S. If a magnet, of an oblong form, be suspended by its centre of gravity, and suffered to move freely, it will finally settle in the plane of a vertical circle, called the magnetic meridian. P. Magnetic needle ? S. A small artificial magnet, balanced on a centre, which will then settle in the magnetic meridian ? P. Variation of the needle ? S. The angle which the magnetic meridian makes with the meridian of the place is called the variation of the needle, or of the compass. This is different in different parts of the world, and varies from time to time. P. Line of no-variation ? S. An irregular curve-line', surrounding the earth from north to south, and passing through all the places where the magnetic meridian coincides with the true, is called the line of no-varia- tion. On the east of this line the variation is west, and on the west of it it is east. This line at present passes through the western parts of Pennsylvania. P. Diurnal variation ? S. In the fore part of the day, especially in the summer sea- son, and in warm climates, the magnetic needle verges a little towards the west, and returns to its former situation in the after- noon ; and this is called the diurnal variation of the needle. It is, however, very inconsiderable, seldom exceeding a quarter of a degree. P. Dip of the needle ? S. A magnetic needle, balanced horizontally before it is ren- dered magnetic, will, after this, lose its equilibrium, the north end in the northern hemisphere, when it can move freely, dipping below the horizon, and vice versa. The quantity of this dip en- VOL. IV. 3 Z 5 SO APPENDIX. creases with the latitude, though according to some ratio not yet sufficiently ascertained. In Philadelphia, the dip is at present about 70°. P. Magnetic equator? S. An irregular curve-line, surrounding the earth from east to west, and passing through all those places where the needle has no dip. P. Poles, and equator of a magnet? S. The extremities of a magnet which, when it is suffered to move freely, point towards the north and south, are called the north and south poles respectively ; and a section of the magnet equidistant from the two poles is called its equator. P. Magnetic attraction and repulsion ? S. If one end of a magnet be brought near to a piece of iron, or any other ferruginous body, a mutual attraction will take place; and, if suffered to move freely, they will approach each other with increasing velocity, and finally adhere together: but if the equa- tor of the magnet be thus presented, no such attraction will be perceived. While a piece of iron, Sec. is in contact with a mag- net, or within the sphere of its attraction, it has itself all the pro- perties of a magnet ; but when removed without the sphere of magnetic attraction, it will, if iron or soft steel, lose its magnetic virtue, but if hard or tempered steel, it will retain its virtue, and thus become a permanent magnet. Permanent magnetism may, however, be more fully communicated to a piece of hard steel, by placing it, for some time, between the contrary poles of two strong magnets ; or by reiterated friction properly applied. The like poles of two magnets will repel each other, but their contrary poles will mutually attract. P. Does a magnet lose any of its virtue by communicating magnetism ? S. No, it is rather improved thereby. P. Does all pieces of iron naturally possess the magnetic vir- tue? S. An oblong piece of iron, in, or nearly in, the direction of the dipping needle, will, though most frequently in a small de- gree, possess magnetic virtue ; the lower extremity, in the nor- thern hemisphere, being the north pole, and the upper extremity the south pole. And if the piece of iron be of any considerable length, as a lightning-rod, a stove-pipe, or the like, it will be di- vided into a number of magnets, though of different lengths, and increasing upwards. But an oblong piece of iron, in the direc- tion of the magnetic equator, or at right angles to the dipping needle, will seldom exhibit any signs of magnetism. P. On what hypothesis may all the principal phenomena of magnetism be explained ? S. On that of M. ^Epinus, viz. 1. That there exists in all magnetic bodies a substance which may be called the magnetic fluid ; the particles of which strongly repel each other, with a force decreasing as the square of the dis- tance increases. , APPENDIX. 531 2. That, between the particles of iron, in any of its states, and those of this magnetic fluid, there exists a mutual attraction, whfch decreases according to the same law. 3. That the particles of iron mutually repel each other, ac- cording to the same law ; though this repulsion does not coun- teract the aggregate attraction of the particles, on which its tex- ture depends. 4. That the magnetic fluid moves without any considerable obstruction, through the pores of iron or soft steel ; but is more and more obstructed in its motion, as the temper is harder; and in hard tempered steel, as well as in the ores of iron, it is moved with the greatest difficulty. 5. In a piece of iron exhibiting no signs of magnetism, this fluid is conceived to be equally diffused, or in its natural state. 6. In a magnet, this fluid is conceived to be redundant in one extremity (most probably the northern) and deficient in the other extremity ; consequently, in the middle section, or equator of the magnet, the magnetic fluid will be in its natural state. 7. The earth probably contains a great magnet, continually acting, by its attractive and repulsive powers, on all bodies in which this fluid is contained ; this great central magnet having, its deficient extremity towards the north. P. Apply this hypothesis to the explanation of a few of the magnetic phenomena. S. 1. When the north or redundant end of a magnet is pre- sented to one extremity of a piece of iron or steel, the magnetic fluid contained therein wiil be repelled, by the redundant mag- netic fluid in the magnet, to the opposite extremity ; and the piece of metal, if tempered steel, will become a permanent mag- net, the fluid not readily returning through the pores of har- dened steel. But, for the opposite reason, if the piece of metal be iron or soft steel, the fluid will readily return on withdrawing the magnet, and no permanent signs of magnetism will conti- nue. 2. The contrary poles of two magnets will attract each other, from the mutual attraction between the redundant magnetic fluid in the one, and the deficient particles of metal in the other. 3. The like poles of two magnets will repel each other, from, the mutual repulsion between the particles of the fluid in the two redundant ends, or that of the metal in the two deficient ends. 4. The direction of the magnetic needle, both with respect to its azimuth and dip, is occasioned by the mutual attraction be- tween the deficient extremity of the great central magnet and the redundant extremity of the needle, and vice versa. 5. Heating a magnet will impair or destroy its magnetic vir- tue ; as the fluid will then find less difficulty in moving from the redundant to the deficient end. 6. The diurnal variation is probably occasioned by the heat- ing of the eastern hemisphere in the forenoon, which, weakening 532 APPENDIX. its magnetic attraction, will suffer the north end of the needle to verge towards the west, and vice versa. 7. The temporary magnetism of a piece of iron, while placed vertical, or nearly in the direction of the dipping needle, may be owing to the mutual attraction between the magnetic fluid in the piece of iron, and the deficient or negative extremity of the great central magnet. 8. A piece of iron will acquire a more powerful temporary magnetism while in contact with one extremity of a magnet than a piece of tempered steel, because the magnetic fluid will move more readily through the pores of the former than through those of the latter. Of Electricity, P. What is electricity ? S. That species of attraction, with other phenomena, which was first discovered in amber, electron, from which the term electricity is derived, but which is now known to belong to many other bodies ; friction being generally necessary to produce elec- trical phenomena. P. On what hypothesis may all the principal phenomena of electricity be satisfactorily accounted for? S. On that of Dr. Franklin, as more fully explained by M. JEpinus, viz. 1. All bodies are possessed of a certain elastic fluid, sui gene- ris, called the electric fluid, the particles of which, like those of every other elastic fluid, repel each other, with a power decreas- ing as the square of the distance increases. 2. Between the particles of this fluid, and those of some ingre- dients in all other bodies, there exists a mutual attraction, which decreases according to the same law. 3. Through the pores of some bodies the electric fluid passes with facility, or meets with little obstruction ; they are hence termed conductors of electricity. But through the pores of other bodies it moves with difficulty, or is wholly obstructed ; and these are hence termed van-conductors of electricity ; though among bo- dies there is a gradation from the most perfect conductors to the most perfect non-conductors. 4. By sundry operations, both of nature and of art, the equi- librium of the electric fluid is destroyed, becoming redundant in one body or part of a body, and deficient in another. Where the electric fluid is redundant, the body is said to be in a plus or fiositive state of electricity ; where it is deficient, the body is said to be in a minus or negative state ; and where it is neither redundant nor deficient, tl\e body is said to be in its natural state. 5. Friction weakens the attraction between the particles of the body rubbed and those of the electric fluid it contains ; hence, when two bodies are rubbed together, and one of them more af- APPENDIX. 533 fected by the friction than the other, the latter having its attrac- tion for the electric fluid less weakened than the former, will at- tract a portion of its electricity ; and if one of the bodies be a non- conductor, they will be found in different states of electricity ; but if they be both conductors, the equilibrium will be instantly res- tored as soon as disturbed. On this account all non-conductors are called electrics, and all conductors, non-electrics ; being more or less perfect in the latter respect, according as they are so in the former respect. P. What bodies, or media, are found to be non-conductors, or electrics ; and what, conductors, or non-electrics ? S. 1. All vitreous, and resinous substances, and gems; all animal excrescences, as hair, feathers, horn, silk, wool, &c. all vegetable substances when deprived of moisture, as dry- paper, baked wood, Sec. dry air, and some non-electrics, as ice, in a very low degree of temperature, are electrics or non-con- ductors. 2. All metals, water, or bodies abounding with moisture, animal fluids, charcoal, black-lead, a vacuum, flame, and elec- trics in a high degree of temperature, as melted glass, hot air, &c. are non-electrics or conductors. P. Please to describe and explain some of the most common phenomena of electricity, on the hypothesis just laid down. For instance, — exciting electricity by friction ? S. If a piece of smooth glass, as a tube, a globe, or the like, be rubbed with the dry hand, or, what is much better, with a piece of leather or oiled silk, smeared over with an amalgam of zinc and mercury with the addition of a little tallow, then, as the smooth glass will be less affected by the friction than the rough rubber, it will attract from this a part of its electricity ; and if the rubber be insulated, that is, cut off by a non-conduc- tor from any communication with the earth or any other con- ductor, it will be electrified negatively, and the glass, positively. But if a piece of rough glass, silk, sulphur, sealing-wax, or any other resinous substance, be rubbed with a piece of fine flannel, or rather of cat-skin, then, the finely-polished pile of the rubber being less affected by the friction than the rough glass, &c. the former will be electrified positively, and the latter, negatively. P. Electrical attraction and repulsion ? S. 1, If a positively-excited electric, as a smooth glass-tube, be brought near to one extremity of a conductor, as a gunbarrel, then, the redundant fluid in the excited electric will, by its re- pulsive force, drive the fluid in the conductor to the opposite extremity ; and, for a like reason, if the electric be excited ne- gatively, it will, from its attractive force on the fluid contained in the conductor, draw it towards itself. Hence, an excited electric will produce a contrary state of electricity in the part of a conductor nearest to it. 2. If an excited electric be brought near, or in contact with a conductor, and passed over it, the electric fluid will, from its pre- valent attraction to the body which has the least of it, be com- 534 APPENDIX. municated from one to the other ; and the conductor, if insu- lated, that is, suspended by silk lines or supported by glass pil- lars, or the like, will become electrified in the same state with the excited electric. For when the excited electric is positive, the electric fluid will pass from it to the conductor ; but when negative, it will pass to it from the conductor. 3. If two light bodies, as cork or pith balls, suspended by silk threads, and near, or in contact with each other, be electrified by an excited electric, they will repel each other. For if elec- trified positively, the redundant electricity forming an electric atmosphere round the balls, will be doubly dense between them, and, therefore, from the repulsion of its particles, the balls will be separated. If electrified negatively, the negative atmosphere surrounding the balls, (that is, the air deprived of its natural electricity) will be doubly rare between them, and, thertfore, from the prevalent attraction outwards, between the negative balls and the natural electricity of the surrounding air, they will still be separated. 4. If one be electrified and the other not, or if one be positive and the other negative, then they will attract each other. For, in both cases, as one body will have a greater proportion of the electric fluid than the other, a mutual attraction between this and the other body will take place. When they come into con- tact, the electric fluid will be equally distributed between them ; and if, in this case, they have either more or less than their na- tural quantity, they will again repel each other. Hence, 5. Any light body, as a feather, a tuft of cotton, or the like, will be alternately attracted and repelled, and thus move back- wards and forwards between two bodies in different states of electricity. P. The charging and discharging of the Leyden phial ? S. The Leyden phial, so called from its properties being first discovered in the city of Leyden, is one lined on the inside, and coated on the outside, within about two inches of the mouth, with tin-foil, or any other conducting substance. The inside com- municating, by means of wire or other conductors, with an ex- cited electric, as a glass globe, plate, or the like, and the out- side with the earth, or with the rubber ; while the excitation goes on, the electric fluid passes from the electric, if positive, to the lining of the phial, where, by its repulsive force, operat- ing through the glass, it will expel the electric fluid from the outside coating into the earth or rubber ; the inside lining will thus become eletrified positively, and the outside coating nega- tively ; and the glass phial, or intermediate electric, is then said to be charged. The thinner, therefore, the intermediate elec- tric, the higher may it be charged. If the negative side be now connected with the excited electric, and the positive side with the rubber, the phial will be gradually discharged ; and if the ex- citation be continued, it will be again charged, the sides assum- ing contrary states. If a communication, of any good conduc- tor, be formed between the opposite sides of a charged electric, APPENDIX. 535 the redundant fluid in the positive side will rush with violence to the negative side, and that even before the communication is complete, attended in its passage through the air with an elec- tric spark and explosion, proportioned to the quantity of coated surface, and the intensity of the charge ; and after this the coated electric will exhibit little or no signs of electricity. P. The electric shock ? S. If any part of a living body form the circuit of discharge in the Leyden experiment, a sudden spasmodic shock and painful sensation will be felt, greater or less, according to the quantity and intensity of the charged electric. P* The phenomena of the electric spark a»d explosion ? S. The electric fluid while passing, in any considerable quan- tity, from one body to another, through the air, exhibits the appearance of a spark of fire, accompanied with an audible ex- plosion; the spark, moving with prodigious velocity, and, when the distance is considerable, in a zig-zag direction. It will, in favourable circumstances, set combustibles on fire ; and, in . its passage through bodies that are but imperfect conductors^ rend them to pieces. The light and heat of the electric spark may possibly be ow- ing to the sudden condensation of the air, or other elastic me- dium through which it passes ; and perhaps a chemical com- bination of latent light and chloric, furnished by the electric fluid and the elastic medium, may take place in the production of the visible flame. The audible explosion is no doubt occasioned by the sudden, concussion of the air by the electric spark, in its passage, and its subsequent collapse. The zig-zag direction arises, in part, from the succes- sive changes in the figure of the electric spark or ball of fluid fire ; for when this becomes oblate in its direct course, it will, from its increased resistance, in this direction, glance off ob- liquely ; and thus, by successive changes of figure, as in a bubble of air moving through water, an angulated or zig-zag direction is produced. The condensation of the air, in the direction of the spark, will no doubt contribute to this phenomenon. P. The influence of metallic /wints in attracting and emit- ting the electric fluid ? S. A pointed conductor is, by experiment, found, to attract and transmit the electric fluid from one body to another, in dif- ferent states of electricity, at a greater distance, and in a more gradual and silent manner, than a conductor in any other form. This may be accounted for on the supposition, that a certain subtile elastic fluid, (the at her of Sir Isaac Newton) surrounds all bodies, and particles of matter, preventing them from com- ing into actual contact. This xtherial atmosphere will, there- for^, be the most dense on a concave surface, less on a plane surface, still less on a convex, and least of all, or in fact insen- sible, at a point. It follows, that the resistance from this subtile fluid, to the entrance or escape of the electric fluid will be 536 APPENDIX. less when the conductor terminates in a point, than when in any other form. P. The electrical aura, or sensible blast, from an electified point? S. While the electric fluid is passing either to or from a pointed conductor, it is accompanied with a sensible cold blast, frequently sufficient to extinguish a taper or small candle : For, the electric fluid in passing through the air produces a partial vacuum, in form of a cone inverted with respect to the conical conductor; the air, in consequence, will rush in from behind, to supply the vacuum, and thus the sensible current will be produced. P. Are there any phenomena in nature, that may be consi- dered as electrical ? S. Thunder and lightning are now well known to be electri- cal phenomena. P. Please to explain these, upon the general hypothesis. S. The clouds, by some operation of nature, not yet, perhaps, satisfactorily understood, are frequently put into a high state of electricity ; sometimes positive, sometimes negative. When an electrified cloud approaches near to the earth, which, from their mutual attraction, it will have a tendency to do, the part immediately under the cloud will, from the attrac- tion or repulsion of the electric fluid, be put into the opposite state of electricity, the stratum of air between the earth and cloud, thus becoming a charged electric ; and, in favourable cir- cumstances, a discharge will tyke place ; the redundant fluid in the one, rushing to the deficient or negative matter of the other, and thus exhibiting the awful phenomena of thunder and light- ning. P. How may buildings, ships, or other objects, be preserved from the dreadful effects of lightning ? S. By means of a metallic conductor, termed a lightning-rod ; extending a few feet above the highest part of the building or other object, and reaching to a moderate depth below the sur- face of the earth, or into the water: for if thus furnished, it will seldom, if ever, be struck with lightning. P. What are the best materials and construction of a light- ning-rod to defend a building ? S. The body of the rod may be of iron, say from \ to \ of an inch in thickness — the thicker kind of iron-wire or of nail-rod will answer the purpose very well. The top of the rod, or part rising above the building, which may, in general, be about six feet long, should be made a little thicker than the rest. The part under ground should also be made thicker, and descend as far below the surface as practica- ble, so as to reach a permanently-moist earth. The upper extremity of the conductor may terminate in a hol- low cone of thin sheet gold, or of copper, a few inches long, fill- ed with apiece of good black-lead, (which may be taken from a pencil) cut to a fine point, so as to reach to the vertex of the APPENDIX. 537 cone, and secured in its place by a paste made of calcined plais- ter of Paris and black-lead dust, rammed in round the piece of black-lead. And round the lower extremity of the rod, or the part under ground, there may be thrown a few bushels of charcoal. By these means, the conductor will be rendered much more per- fect: for, the black-lead at the upper extremity, being nearly as good a conductor as metal, and yet, in a manner, infusible^ will effectually secure it against the frequent accident of being melted off by a stroke of lightning ; and the charcoal round the lower extremity, from its conducting power, (being little inferior to that of metal) the angular or pointed figure of its parts, its quality of absorbing moisture, and its indestructibility by any agent, except fire, will afford a permanent, copious, and effectual means for the free passage of the electric fluid, between the conductor and the surrounding earth. P. What buildings, or parts of buildings, are most liable to be struck with lightning, and should therefore be secured by con- ductors ? S. Barns, after the in-gathering of the harvest, are observed to be more frequently struck with lightning than any other build- ings. This is probably owing to the ascent of vapour, generated by a slight fermentation taking place in the moist contents of the barn, which favours the descent of the lightning. For a similar reason, a chimney in which there is a fire, is more exposed to a stroke of lightning than any other. Hence, kitchen-chimnies* being the only ones in which fires are usually kept during the summer, (the season in which thunder most prevails) should, when other circumstances will admit, be furnished with light- ning-rods. As thunder-storms generally come from the westward, the lightning-rod, to secure the building, should, when other cir- cumstances are equal, be placed on the most western chim- ney. When the house is furnished with a metal gutter, or metal spout, to carry off the rain, this, so far as it goes, may be made a part of the lightning-rod, or metallic conductor. Of Astronomy* P. What is astronomy ? S. That science which describes and explains the various phe- nomena of the heavenly bodies. P. Give a short description of the general appearances of the heavenly bodies. S. The heavenly bodies appear to consist of a vast number of luminaries, of different magnitudes and degrees of brightness. The swn, whose light occasions our day, far surpasses in splen- dour all the other luminaries. The moon, which is visible chiefly, and shines only, during the night, emits incomparably less light VOL. IV. 4 A 538 APPENDIX. than the sun, yet much more than all the other luminaries toge- ther. The sun and moon are nearly of the same apparent mag- nitude ; and of the other luminaries a few only, called planet*, have any apparent magnitude; all the ixst, called Jixed t,tars f appearing only as mere luminous points, though differing in brightness. The sun always appears a complete luminous circle ; hut the moon is continually changing her phases', from that of a full cir« cle to that of the smallest crescent, accorcing to her greattr or less angular distance from the sun. Thus the moon, and, indeed, the other planets, (which, when viewed through a tele scope, exhi- bit similar phases) all appear to derive their light from the sun. These luminaries all appear to he in the concave surface of a great sphere surrounding the earth ; and exhibit the same phe- nomena as if the sphere containing them turned round on its axis daily, from east to west. The moon is continually changing her position, with respect to the sun, at the rate of about 12 1-2 degrees, from west to east, every day ; and thus returns to her former position, and under- goes all her changes of phases, in about 29 1-2 days, termed a lunar month. The sun is also continually changing his position, w ith respect to the fixed stars, at the rate of about a degree, from west to east, every day ; and thus goes through a whole circle in the heavens, in about 36j 1-4 days, termed a solar year. The planets are likewise continually changing their places, and return to any former position, in different periods. In their revolutions, however, through the starry heavens, they appear, as seen from the earth, to be sometimes progressive, sometimes stationary, and sometimes retrograde. Two of these planets, called Mercury and Venus, are never seen in opposition to the sun, nor at any very great angular distance from him ; but all the other plant ts are occasionally seen in op- position, and in ail other positions with respect to the sun. '1 he stars, however, newer appear sensibly to change their positions with respect to each other, and are on this account called Jixed stars. Neither the sun, moon, nor planets, appear constantly to de- scribe the same diurnal paths in the heavens; their places of rising, setting, and meridian altitudes, continually changing, and going through all their varieties of change in this respect, in different periods ; but the fixed stars undergo no such change in their diurnal paths. P. On what hypothesis may these phenomena of the heavenly bodies be explained ? S. These, and all other phenomena of the heavenly bodies, may he satisfactorily explained, on the Copernican or Newtonian hy- pothesis or system of the universe. P. Please to give a brief view of this system. S. 1. Round the sun, as a common centre, there revolves, in different orbits, and in different periodical times, a number of APPENDIX. 539 opaque bodies, nearly spherical, resembling the earth, (which is one of them) called primary planets. 2. Round some of the primary planets there revolve also in different orbits, and in different periodical times, other smaller and similar bodies called moo?is, satellites, or secondary filantts. 3. All the orbits of the primary planets round the sun, and of the secondaries round their primaries, are nearly circular, though, in reality, more or less ellifnical ; the sun and primary planets respectively being placed in one focus of the elliptical orbit. 4. The planets, both primary and secondary, revolve, in their respective orbits, in the same direction, namely, from west to east; and all, except in one instance, nearly in the same plane. 5. Besides the periodical revolution of the planets round their respective centres of motion, some of thtm it is certain, and all of them it is probable, perform a rotation round their oiim axes, in different times, from west to east ; tht ir axes of rotation making different angles with their respective orbits. 6. Besides the primary and secondary planets, there is another order of bodies, called comets, that revolve round the sun in very excentric elliptical orbits, in very different planes, and very dif- ferent directions. Few of the periodical times of these bodies are yet accurately ascertained; as they are never visible, unless for a short time when in or near their perihelion, (or nearest dis- tance from the sun) at which time they generally appear with a lucid or shining tail on the side opposite to the sun. 7. All the above bodies in the solar system, whether planets or comets, in their revolutions round their respective centres, ob- serve this general law, viz. " That the squares of their periodical times are directly proportional to the cubes of their mean dis- tances from the centre of motion." It therefore follows, from the doctrine of central forces, " that the gravity or mutual attrac- tion between any two bodies in the system, must decrease as the square of the distance increases." 8. The sun is a dense, and probably an opaque body, sur- rounded with a luminous atmosphere, which is the source of solar light and heat to the whole system. P. What account do you give of the fixed stars? S. They are, most probably, all great luminaries, resembling the sun, and each the centre of a system of bodies revolving round it, similar to those which compose the solar system. '1 hese stars are probably at as great a distance from each other as that of the nearest of them from the sun ; and this distance is truly immense. I P. What can prevent these systems from obeying the general laws of gravity, and falling together? S. The innumerable systems composing the stupendous fabric of the universe, are, it is more than probable, themselves also in motion round some common centre, and thus prevented from approaching each other; which, by their mutual attraction, they must otherwise do. This hypothesis is not only analogous to the 540 APPENDIX. general laws of nature in central motions, but is now, in some measure, verified by astronomical observations. P. Explain a few of the most common astronomical terms relating to the sphere. For instance — What is meant by a great circle of the sphere ? S. That circle, whose plane passes through its centre. P. The poles of a great circle ? S. Two opposite points on the surface of the sphere equidis- tant from every point in the circumference of the circle. P. The axis of a sphere ? S. That imaginary line passing through its centre, round which it performs its diurnal rotation. P. The equator? S. That circle which is at right angles to the axis of rotation. P. The eclifitic ? S. That great circle in the plane of which the earth performs its annual revolution'round the sun. P. A meridian ? S. Any great circle passing through the poles of the equator. P. The horizon? S. That great circle which, extended to the heavens, is the boundary of our vision. The poles of this are called the zenith and nadir, the one above, and the other below, the horizon. P. Azimuth circle ? S. Any great circle passing through the poles of the horizon ; that at right angles to the meridian being called the prime -vertical, P. The signs of the ecliptic, or of the zodiac ? S. These are twelve constellations through which the plane of the ecliptic passes ; they are named Aries T, Taurus c», Ge- mini n, Cancer 25, Leo SI, Virgo i»JJ, Libra =£=, Scorpio tit, Sagi- tarius ti Capricornus VJ, Aquarius ££, Pisces X. The begin- nings of Aries and of Libra, being those points in which the ecliptic crosses the equator, are called the vernal, and autumnal, equinoxial points, respectively ; and the beginnings of Cancer and of Capricorn, being those points farthest distant from the equator, are respectively called the northern and southern solstitial points. P. Tropics and polar circles ? S. The tropics are circles parallel to the equator, and touching the ecliptic in the extreme points, Cancer and Capricorn ; from which they are respectively named. The polar circles are also parallel to the equator, and at the same distance from the poles that the tropics are from the equator, and are denominated the north and south polar circles respectively. P. Zones ? S. These are divisions of the surface of the earth (or any other planet) by the tropics and polar circles. They are five in number, viz. the torrid zone, bounded by the two tropics ; the two frigid zones, lying round the poles, and bounded by their respective polar circles ; and the two temperate zones, lying be- tween the torrid and frigid. APPENDIX. 541 P. Latitude ? S. Latitude of a place on the surface of a planet, is its dis- tance, in degrees and parts, from the equator, measured on the meridian of the place ; and latitude of a star, which is fre- quently taken in a general sense to signify any of the celestial bodies, is its distance from the ecliptic, measured on a great cir- cle passing through the star and the poles of the ecliptic. P. Longitude ? S. Longitude of a place is the distance of its meridian from the prime meridian, or that from which longitude is reckoned; and longitude of a star is the distance, on the ecliptic, from the vernal equinoxial point, reckoned from west to east, or according to the order of the signs, to the circle passing through the star and the poles of the ecliptic. P. Right ascension of a star ? S. The distance, on the equator, from the vernal equinoxial point, reckoned from west to east, to the meridian passing through the star. P. Declination of a star ? S. Its distance from the equator, measured on the meridian passing through the star. P. Altitude of a star ? S. Its elevation above the horizon, measured on an azimuth circle. P. azimuth of a star ? S. The distance of the azimuth circle passing through the star, measured on the horizon, from the meridian. P. Amplitude of a star ? S. The distance of the point where it rises or sets from the east or west points of the horizon. P. Please to explain a few terms relative to the planetary or- bits : For instance — What is meant by Perihelion ? S. That point of a planet's orbit in which it is nearest to the sun. P. Aphelion? S. That point of a planet's orbit in which it is farthest from the sun. P. Perigee, and Apogee ? S. Those opposite points in the moon's orbit in which it is respectively, nearest to, and farthest from, the earth. P. Apuis or Apsides ? S. The extremities of the transverse axis of a planet's elliptical orbit. P. Node*? S. Those opposite points in a planet's orbit, in which it crosses the ecliptic. That point in which it passes from south to north, being called the ascending node or dragon's head (&) ; and the opposite point, the descending node or dragon's tail (£3). P. Excentricity of an elliptical orbit ? S. The distance between the centre of the ellipsis and one of its foci. 542 APPENDIX. P. Heliocentric filace of a planet ? S. Its place among the stars as seen from the sun. P. Geocentric place ? S. Its place as seen from the earth. P. Enumerate the several planets in the Solar system, in the order of their distance from the sun. S. 1. Mercury, nearest to the sun ; 2. Venus ; 3. The earth with one moon ; 4. Mars ; 5. Jupiter, with four moons ; 6. Saturn, with a double ring and seven moons ; 7. Herschel, with six moons*. P. What phenomena result from the order of the planets in the solar system in their annual revolution round the sun ? S. 1. Mercury and Venus, being inferior planets, that is, their orbits being within the earth's orbit, will never be seen in oppo- sition to the sun, nor at any great angular distance from him; the greatest elongation of Venus, exceeding, however, that of Mercury. 2. All the other planets, having their orbits without that of the Earth, will occasionally appear in opposition, and in all other positions with respect to the sun. 3. The motion of the planets among the stars, as seen from the earth, will be sometimes direct, sometimes retrograde; and sometimes they will appear stationary. Namely, the inferior planets will appear direct, when in the superior or opposite parts of their orbits ; retrograde, when in the inferior or nearest part ; and stationary, when at their greatest elongation. The superior planets will appear direct, when the earth is in the op- posite part of its orbit with respect to them ; retrograde, when the earth is in the nearest part of its orbit ; and stationary, when the earth, with respect to them, is stationary. 4. The planets will change phages and apparent magnitudes, according to their position with respect to the sun, and distance from the earth. P. What phenomena result from the earth's orbit being ellip- tical, and the sun placed in one of its foci ? 1. The sun will change his appareni magnitude, being great- est when the earth is in its perihelion, or about the miucile of winter, and least, when in its aphelion, or about the middle of summer. 2. The motion of the earth in its orbit will be unequable, be- ing slower or quicker, according to its greater or less distance from the sun; and this is one source of the equation Gf time, or ditference between the times pointed out by a well-regulated clock, and by the sun. * For a m >re particular view of the solar system, see the table at the tnd of this article. APPENDIX. 543 3. It follows, that the earth will be considerably longer in de- scribing the aphelion, then the perehelion part of its orbit ; and hence our summer half-year will exceed (by about eight days) our winter half-year. 4. From a sensible ratio between the velocity of the earth in its annual orbit, and that of light, the stars will each appear to describe small ellipses in the heavens, called their aberration, P. What phenomena result from the diurnal rotatory motion of the planets round their axes ? 5. 1. From this arises the spheroidal figure of the earth, and of all the other planets in which this motion has been discovered ; the matter of the pianet being thereby thrown out or rendered more protuberant in the equatorial, and consequently flatter in the polar parts. 2. The diurnal rotation of the earth from west to east, pro- duces the apparent diurnal motion of the sun, and other heaven- ly bodies, from east to west ; and hence the succession of day and night. P. How has the rotatory motion of the planets been discover- ed ? S. From the regular motion of certain spots on their surface, seen by the aid of the telescope: That of the moon is known from her always presenting the same face to the earth — Hence the time of her diurnal rotation is exactly equal to that of her monthly revolution round the earth c And there is reason to be- lieve that it is a general law — that all satellites or secondary pla- nets, constantly present the same side towards their primaries. P. What phenomena result from the obliquity of the equator to the ecliptic ? S. 1 . The continual changes in the apparent diurnal path of the sun, and consequently in the length of day and night; with the diversity of seasons. 2. This also, as well as the elliptical orbit of the earth, con- tributes to the difference between the mean time per clock, and the apparent solar time. P. What phenomena result from the planets' orbits, being all in different planes from that of the earth, or the ecliptic ? S. 1. Their greatest declinations, both north and south, will ex- ceed those of the sun, by the quantity of the angle which their respective orbits makes with the ecliptic. 2. From the moon's orbit crossing the ecliptic, eclipses of the sun and moon will be less frequent ; for an eclipse of the sun can never happen, when the moon is more than 17° from her node ; nor one of the moon, when she is more than 12° ; called their respective ecliptic limits : whereas, if the moon's orbit coin- cided wun the ecliptic, there would be an eclipse at every full and change. P. What phenomena result from the spheroidal figure of Ihe earth ? S. From the greater attraction of the sun, and of the moon, to the equatorial parts of the* earth, a great number of seeming irregularities in the motions of the heavenly bodies are pro- duced. 544 APPENDIX. 1. The recession, of the equinoxes, and firecession of the stars, a slow motion by which the equinoxial points of the ecliptic recede, or fall backwards, about 5Q\" per year, and, conse- quently, the stars increase in right ascension, more or less, accord- ing to their situation. 2. The nutation of the earth's axis, a slow motion in the axis of the earth, by which the extremities or poles describe in about 18 years, 7 months, (the lunar period or revolution of the moon's nodes,) a small ellipse whose transverse axis, = 19. 1" and con- jugate=14. 2" , thus producing corresponding apparent mo- tions in the stars. 3. The degrees of latitude, as well as gravity, will increase from the equator to the poles. P. How may the magnitude of the earth be ascertained ? S. 1. By proper instruments, let the meridian altitude of a star be accurately taken at two convenient places, on the same meridian, at a considerable distance from each other, about a degree, for instance ; and thus the arch in circular measure will be known. 2. Actually measure this distance on the surface of the earth ; or, by measuring a base-line, and taking angles, calculate the distance trigonometrically. 3. Then, having the length of this given arch of the meridian, we may, by the rule of proportion, find that of the whole cir- cumference, and thence the diameter and magnitude of the earth. P. From the earth's diameter, how would you find its distance from the sun ? S. From corresponding observations of the transit of Venus over the sun's disk, made at distant places, the sun's parallax, that is the angle under which the earth's semidiameter would appear at the sun, may be, and actually has been, pretty accu- rately ascertained ; and then, from this angle and the earth's semidiameter, the distance may be found by the solution of a right-angled plane triangle. P. From the distance of the earth from the sun, how may that of all the other planets be found ? S. Their several periodical times have already been accurate- ly ascertained, and hence their distances will be found from the general proportion, that the squares of the periodical times are as the cubes of the distances. P. How may the jnagnitudes of the several planets be found ? S. By the resolution of a plane triangle, from their respective distances and apparent magnitudes. P. How may their relative densities be found ? S. 1. By comparing the periodical times and distances of the satellites of one planet, with those of the satellites of another planet, the ratio of their attracting forces may be readily found. In the same manner, may be found the relative attracting force of the sun and of any planet having a satellite. 2. From the relative attracting forces of the planets on their satellites, may be found their relative attracting forces at their surfaces, and then from these and their respectives magnitudes* their relative densities may be readily computed. APPENDIX. 545 Of Eclipses. P. What is understood by an eclifise, and how is it produced ? S. The planets being all opaque, and nearly spherical bodies, but much less than the sun, will project a conical shadow on the side opposite to the sun ; while, therefore, any planet in its orbit passes through one of these shadows, it will be either par- tialy, or totally deprived of the light of the sun ; and this deprivation of light is called an ecli/ise. P. When is the sun said to be eclipsed ? S. When the moon, coming between the sun and the earth at the time of a conjunction or change of the moon, casts her shadow on the earth, then the sun is said to be eclipsed, with respect to that part of the earth on which the shadow or pen- umbra falls. P. When is the moon said to be eclipsed ? S. When the earth, at the time of an opposition or full moon, casts its shadow on the moon ; and thus, for the time, deprives it of the sun's light. P. In what circumstances will there be an eclipse of the sun, or of the moon, at the time of a conjunction, or opposition ? S. This can happen only when the moon is in or near one of her nodes; about 17° being the limit with respect to an eclipse of i the sun, and about 12° the limit with respect to an eclipse of the moon : for, when beyond these limits the shadow of the moon will pass by the earth, or the shadow of the earth will pass by the moon, and, consequently, no eclipse will take place. P. In what particular circumstances will an eclipse of the sun, or of the moon, be central ? S. When at the time of conjunction, or opposition, the spec- tator is in the same right line with the centers of the sun and moon. P. When will an eclipse of the sun be annular ? S. This will be the case, when it is central, or nearly so, and the apparent diameter of the moon less than that of the sun ; for then, at or near that part of the earth over which the axis of the conical shadow passes, an annulus or ring of solar light will appear round the body of the moon. P. When will an eclipse of the sun be total ? S. When it is central or nearly so, and the apparent diameter of the moon greater than that of the sun. P. When will an eclipse of the moon be total ? S. When the moon, at the time, is so far within the ecliptic limit, that she will wholly pass through the shadow of the earth ? P. When will an eclipse be partial ? S. When the moon is so near the ecliptic limit, that the axis, though not the whole, of the conical shadow, will pass by the moon or earth, at the time of the eclipse. An eclipse of the sun will also always be partial from any point within the pen* VOL. IV. 4 B 546 APPENDIX. umbra of the moon, and without this prenumbra the eclipse will be invisible. P. How frequently will eclipses of the sun and moon occur? S. In any year, the number of eclipses cannot be less than two, (both of the sun) nor more than seven ; very seldom more than six; the most usual number is four. P. How do you explain this irregularity in the number of eclipses ? S. 1. The moon's nodes move backwards annually, 194 de- grees ; and of course the sun will pass from one node to the other in 173 days. 2. When the sun is approaching either node and within 17 degrees of it at the tune of new mcon, there will be an eclipse of the sun; and, at the subsequent Opposition, the moon will be eclipsed in the opposite node, and come round to the next con- junction again, before the sun shall have got past the ecliptic limit on the other hide of the node ; and consequently the sun will again be eclipsed. When three eclipses thus happen while the sun is at one of the nodes, a like number will generally hap- pen, while at the other node ; for 173 days, the lime in which the sun passes from one node to the other, is wiihin 4 days of 6 complete lunations. But when the moon changes at or very near one of the nodes, and there is consequently an eclipse of the sun, she cannot be near enough to the other node at the next full to be eclipsed. There will therefore be no other eclipe till after six lunations, when the sun will have reached the op- posite node, and will then be eclipsed. In the former case, therefore, there will be six eclipses, viz. four of the sun, and two of the moon, in the course of the year; and in the latter case, only two, .and these both of the sun. P. Since there are more eclipses of the sun than of the moon, why are there more visible eclipses of the latter, than of the former ? S. Because eclipses of the sun are visible only over a small part of the earth's surface, but eclipses of the moon are visible over the whole hemisphere. P. In what time would there be a regular period of eclipses, ex- actly corresponding in circumstances to the eclipses of a former period ? S. In 223 mean lunations, after the sun, moon, and nodes have been once in a line of conjunction, the same eclipes, will again return with very little variation for many ages. P. Are not Jupiter's satellites frequently eclipsed? S. The magnitude of Jupiter being so great with respect to the distance of his satellites, the first, second, and third of them, are eclipsed in every revolution; but the fourth, on account of its great distance from Jupiter, and the inclination of its orbit, is seldom eclipsed. P. What is meant by an occultation ? S. When the moon, in her motion from west to east, comes between the spectator and any of the stars or planets, there APPENDIX. 547 is said to be an occultation of that star, or planet ; the com- mencement of the occultation being termed tiie immersion, uiid the end of it, the emersion^ P. What is meant by a transit ? S. When either Mercury, or Venus, at the time of its infe- rior conjunction, is in or near one of its nodes, it will appear to pass over the sun's disc, in the form of a black spot ; the be- ginning of the transit being termed the ingress, and the end, the egress. These transits, however, very seldom occur. P. To what practical use may the observation of eclipses, oc- cupations, and transits be applied? S. To the determination of the longitude of places. P* How may the longitude be found, by observing the eclips- es of Jupiter's satellites ? S. The times of all the visible immersions and emersions of those satellites are calcinated from tables of their motions, for the meridian of Greenwich, and inserted in the English nautical almanac always some years in anticipation ; and therefore the difference between iliis Greenwich time, and that observed un- der any other meridian when the same phenomenon takes place, will be the longitude, in time, of that meridian from Greenwich. P. How, by an eclipse of the sun, or an occultation of a star ? S. 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CM ^ -G 1> 3h "cU CO « N CO -^ »0 <0 N ^ °* 5t3 "? C ^2 CD gg c M CD EC Cj , satellites o ned to tlie oi are taken ch lave been lat 1.2 C3 en .8*8 +- u c o ei CO O CD * CD o g .2 Tj • h ^3 (D x cc: p. 5 ch ^ ^ CM — * o v B +* CD +T « «« * « ^* u cc ^ t) 01 u co tU ^3 ei ei rd ^ +J •-; r- « rt O ei .9.9 c .s.s ^ « « o +j - rcc = o t- .rr* HH Is parth ular obstruction, will be generally about three hours after .the moon has passed the meridian ; and the time of the highest spring-tide, about three tides after the full or change: for e\> i\ i itural cans must take some time to produce its ulti- mate effect ; or, the efTet c must always follow the cause. P. Why are there little or no tides perceptible in inland seas or lakes ? S. The natural extent of a tide from the place of high to that of low water is 90°. In inland seas or lakes, therefore, a tide cannot be raised, for want of a sufficient extent of water. P. Why arc the tides much less considerable on the shores of small islands in the open ocean, than on the shores of conti- nents, or in rivers, bays, or narrow seas? S. This difference is owing to the greater obstruction which the flowing tide meets with in the latter case than in the former; especially where the obstruction is opposed to the general direc- tion of the tide from east to west. Perhaps the obstruction or other influence of the Gulfih-stream, along the western coast of North America, may also contribute towards the height of the tides on that coast. P. Are there not considerable irregularities sometimes in the tides? S. At places where the tides come through two or more dif- ferent passages, and meet with less interruption through one passage than through another, it may arrive at different times; and in this case there may be two or more tides of flood succeed- ing each other at small intervals of time. Winds also and fresh- es will, in certain situations, have considerable effects. Of Winds. P. How many general kinds of wind are there ? S. They may chiefly be reduced under four different heads, viz. 1. Trade-winds, 2. Monsoons, 3. Sea and land breezes, 4. Irregular or variable winds. P. Give a brief description of the trade-winds. S. These prevail chiefly in the Atlantic and Pacific oceans, within the torrid zone. On the north side of the equator, or ra- ther of a space between the 2d and 5th degrees of north latitude, the wind blows constantly from the north-east, varying, however, a point or two on either side ; and on the south side of this space, the wind blows constantly from the south-east ; subject to a like APPENDIX. 553 variation. In the Atlantic ocean, the track of the trade-winds extends farther north, and they become more easterly, and de- crease in strength, as you proceed westward. P. How do you account for the phenomena of the trade-winds ? S. I. The surface of the earth under the torrid zone being more heated by the sun than the other regions, the air will there be rarefied ; and, ascending to the superior parts of the atmosphere, the colder and denser air, from the northern and southern re- gions, will rush in to supply the deficiency. 2. As the rarefying cause, the sun, is continually moving from east to west, the wind will, in consequence, have this general direction. S. But the air coming from northern and southern latitudes, where its eastern rotatory motion is less than that at or near the equator, will, when it arrives there, have comparatively a western direction, and this, combined with its real motion from the north or south, will, with the western motion of the sun, produce the N. E. and S. E. trade-winds. P. Why is the boundary between the N. E. and S. E. trade- winds north of the equator? S. This is probably owing to the sun's being longer north of the equator than south of it, and thus making the 'track of the sun's greatest influence somewhat north of the equator. Besides, the northern hemisphere containing much more land than the southern, will, of consequence, have a greater average heat. P. Give a short description of the monsoons. S. These winds are found chiefly in the Indian ocean, and blow generally six months in one direction, and six in the opposite di- rection. They may be reduced to two general classes, viz. those on the north side of the equator, and those on the south side. 1. Between the 3d and 10th degrees of south latitude, the S. E. trade-wind continues from April till October, but during the rest of the year, the wind blows from N. W. Between Sumatra and New-Holland, the monsoon blows from the S. while the sun is north of the equator ; and from the N. while it is south of the equator. Between Africa and Madagascar, the monsoons during the same periods are S. W. and N. E. 2. Over all the Indian ocean, to the northward of the 3d de- gree of south latitude, a N. E. wind blows while the sun is south of the equator, and a S. W. wind while it is north of the equator. From Borneo, along the coast of Malacca, and as far as China, the monsoon blows nearly from the S. while the sun is north of the equator, and from the N. while it is south. Regular monsoons are also found in the Red-Sea, blowing N. W. and S. W. the direction of the coast of Arabia. Monsoons also pre- vail on some parts of the coast of South America. P. How do you explain the phenomena of the monsoons ? S. 1. While the sun is passing daily over any region, the dry land will be more heated, and consequently the air more rare- fied, than the water or the air over it will be. This arises chiefly VOL. IV, 4,0 554 APPENDIX. from water being a transparent body ; and therefore admitting the solar rays readily to pass through it. It may also, in part, be owing to the copious evaporation from the surface of water, by which its increase of temperature will be moderated: to this end the continual motion of the water will likewise contribute. 2. The rarefied air over the land will ascend to the superior parts of the atmosphere, and a current from the colder and denser air over the neighbouring waters must of consequence take place, to supply the deficiency. 3. Wherever, therefore, two considerable tracks of land v one north and the other south of the equator, are separated by wa- ter, a monsoon must necessarily take place ; always blowing to- wards the land over which the sun daily passes ; and these are generally, if not always, the situations of those countries be- tween which the monsoons are observed to blow. P. Describe the phenomena of sea and land breezes. S. These are observable in all maritime countries of any con- siderable extent between the tropics ; and frequently beyond those limits. The sea-breeze, or that from the sea towards the land, generally sets in about 10 in the morning, and blows till 6 in the evening ; about 7, the land-breeze begins, and blows to- wards the sea till 8 in the morning, when it dies away. P. How do you account for these phenomena? S. On the same general principles on which the monsoons are explained; viz. 1. The land, during the presence and influence of the sun, will be more heated than the neighbouring water ; and, consequently, the rarefied air over the land will ascend, and its place be supplied by the colder and denser air over the water. Hence, the sea-breeze during the day. 2. During the night, in the absence of the sun, the air over the land will be more cooled than that over the water, ( 1) because the air that had ascended during the day, being cooled and con- densed in the higher parts of the atmosphere, or on the summits of the high lands, will now descend again to the surface ; and (2) because the surface of the water, as soon as it cools, will de- scend and be replaced by the warmer water from below : hence-, a land-breeze during the night will ensue. P. What account do you give of the irregular, or variable winds ? S. Such, with few exceptions, are the wiuds in the temperate and frigid zones ; though almost in all countries, certain winds are prevalent during certain seasons of the year; but the theory of these winds is still but imperfectly known. P. What do you say with respect to the velocity of the wind? S. It varies from the most gentle breeze, just perceptible, moving at the rate of about I mile per hour, to the most violent hurricane, with the velocity of 100 miles per hour. It moves, however, with different velocities, and sometimes in different di- rections, in different strata of the atmosphere: the velocity, as has been tully ascertained by aeronauts in their balloons, being: APPENDIX. 555 generally much greater in the superior than in the inferior parts of the atmosphere. . , Of Chronology, P. What does chronology treat of ? S. It treats of time with respect to its measures ; including its various distinctions, divisions, and subdivisions. P. How is time measured ? S. It is measured or regulated by the motion of the heavenly bodies. P. What are the usual distinctions of time ? S. It is usually distinguished into apparent.solar, mean-solar, sidereal, and lunar time. P. Give a brief account of apparent solar time, S. 1. Apparent solar noon, at any given place, is the moment of time when the sun's centre is on the meridian of the place ; an apparent solar day is the interval of time between one appa- rent solar noon and the next ; and the apparent solar time, hour of the day, or horary angle, is the angle (15 degrees to an hour) which the meridian passing through the centre of the sun makes with the meridian of the place. 2. The apparent solar days are not all equal to each other throughout the year. This inequality arises from two causes ; one is, that the earth, moving in an elliptical orbit, does not de- scribe equal arches in equal times ; and, therefore, as the earth, in a solar day, must make one complete rotation round its axis, and so much of another as will correspond with the earth's di- urnal arch in the ecliptic, and since these arches are unequal, being least when the sun is in its aphelion, and greatest when in its perihelion, it follows, that the days, on this account, must be unequal also. The second cause of this inequality is, the inclina- tion of the ecliptic to the equator ; whence equal arches in the ecliptic, in which the earth moves, will not correspond with equal arches of the equator, on which time is measured. P. What is meant by mean solar time ? S. It is that pointed out by a well-regulated time-piece, going with an equable motion throughout the whole year. P. What is meant by the equation of time ? S. It is the difference between apparent and mean. Four times in the year, viz. on the 16th April, 16th June, 1st September; and 25th December, this equation will be nothing, or, the appa- rent and mean time will be the same ; but when greatest, it will amount to upwards of 16 minutes. P. What is sidereal time ? S. That measured by the apparent diurnal motion of the stars. A sidereal day, therefore, is the interval of time from the pas- sage of any fixed star over the meridian, till it passes that meri- dian again. These days are all equal, and 3' 55 ".9 o+ time less than a mean solar day. 556 APPENDIX. P. What is lunar time? S. That measured by the motion of the moon. A lunar day is the interval of time between the moon's passing the meridian on any day and the next succeeding day. These days are une- qual, but, on an average, exceed a mean solar day about 48 mi- nutes. P. At what time is the day usually considered as beginning? S. The civil day begins at midnight ; ihe astronomical day at the succeeding noon ; and the nautical, or tea day, at thfe pre- ceding noon. P. What is a sidereal year? S. The time in which the earth performs a complete revolution through the whole circle of stars in the ecliptic = 365d. 6h. 9m. 17s. P. What is a trofiical year ? S. The time in which the earth performs a complete revolu- tion through all the artificial signs of the ecliptic, = 365d. 5h. 48m. 48s. P. What is the reason of this difference between the sidereal and tropical years ? S. The earth being of an oblate spheroidal figure, and the ecliptic inclined to the equator, the attraction of the sun and moon on the accumulation or redundance of matter at the equa- tor, will cause the equinoctial points to move backwards (called the recession > of the equinoxes) at the rate of 501" yearly, through which the earth will move in 20m. 29s* the difference between the sidereal and tropical years. P. Give an account of the origin of old and neiv stile. S. The mean Julian civil year consists of 365d. 6h. three years containing each 363, and every fourth year 366 days. The sup- plementary day was added to the month of February by counting twice the 23d day ; (which in the old Roman calendar was called the sixth of the calends of March) and hence this year was called bissextile, or, on another account, leap-year. This Julian year exceeds the true or tropical year, according to which the seasom take place, 11m. 12s. which in 131 years will amount to a whole day ; and this constitutes the difference between the Julian, or old stile, and the Gregorian, or new stile. P. When was the Julian calendar corrected, and on what oc- casion ? S. In the year 325, when the council of Nice appointed the time for the celebration of Easter, (viz. the first Sunday after the full moon, immediately succeeding the time of the vernal equi- nox) the equinoxes happened on the 21st of March and the 21st of September. In the year 1582, Pope Gregory XIII observing that the Julian had got a-head of the tropical year 10 days, ordered that so many should be then struck out of the calendar; and in the year 1753, when the British government adopted the new stile, 1 1 days were struck out. To prevent any irregularity from taking place in future, it was ordered, that three days should be struck out of the Julian calendar every 400 years, by reckoning APPENDIX. S5T 1700, 1800, 1900, 2100, &c. or every centurial year not divisible by 4, a common year, instead of a leap-year, which it would other- wise be. There is therefore now a difference of 12 days between the old stile and the new. P. How may the leap-years be found? S. The first year of the Christian aera was the first after leap- year ; hence divide the given year by 4, and, if nothing remain, it will be leap-year; except the centurial years not divisible by 4, which, according to the Gregorian account, must be considered as the 3d after leap-year: but if any thing remain, it will point out the year after leap-year, P. What is meant by the Dominical letter? S. In calendars, it has been customary to prefix the first seven letters of the alphabet to the several days of the year, succes- sively ; that opposite the first day of the year, being A, that op- posite the second, B, and so on to G. The same letter, there- fore, would continually correspond to the same day of the week throughout the year ; and thus the letter corresponding to the Lord's day (Uominicus dies) was called the Dominical letter. P. In what order will the Dominical letters occur, from year to year ? S. A common year containing 52 weeks and 1 day, and a bis- sextile 52 weeks and 2 days ; the Dominical letter will therefore fall back, in the order of the alphabet, one letter every common year, and two letters, after the last of February, every bissextile ; hence, on this account, it is frequently termed lea/:*year, P. How would you find the Dominical letter for any given year of the Christian sera? S. To the given year add its fourth part; rejecting fractions, divide the sum by 7, and the remainder taken from 7, will leave the number of the Dominical letter in the order of the alphabet ; viz. 1 = A, 2 = B, Sec. But in leap-years, the letter thus found will be the Dominical letter till the latter end of February, and the one next preceding this will be the Dominical letter for the rest of the year. This rule will hold good for any year of the 18th century; but for the 19th century, (1800 being a common, instead of a leap-year) the Dominical letter will be the next sue* ceeding that found by the rule. P. What, for example, will the Dominical letter be in 1807? b. 4)1807 451 7)2258 S24...4 7 3-f.l=4=D. P. Having the Dominical letter for any given year, how would you find on what day of the week anv given day of any month vould fall? 558 APPENDIX. S. By the following distich — " At Dover Dwells George Brown Esquire, Good Christopher Finch And David Friar." P. How is this to be applied? S. The first letter of each of these twelve words is the letter which, in the calendar, belongs to the first day of its respective month, from January (At) to December (Friar). Hence, count- ing from the letter of the first day of the month to the Dominical letter, v-e will have the day of that month on which the first Sunday falls. P. On what day of the week, for example, will the 4th of July fall in the year 1807 ; the Dominical letter being D ? S. July, the 7th month, Good-, G, A, B, C, D; — the first Sunday will be the 5th, therefore the 4th will be on Saturday. P. What is meant by a cycle-, in chronology ? S. A certain period of time, wherein the same circumstances, to which the cycle has a reference, will regularly return. P. What are the most noted astronomical cycles? S. The solar cycle, I he Metonic, or lunar cycle, and the Dionysian cycle or period. P. What is the solar cycle? S. It is a period of 28 years, after which the same day of any month will happen on the same day of the week as on the same year of a former cycle. P. How would you find what year of this cycle corresponds to any given year of the Christian aera? S. The first year of the Christian aera was the 9th of the solar cycle : hence, add 9 to the given year, divide the sum by 28, and the remainder will be the year of the cycle required; corre- sponding to the 28th, or last year of the cycle. P. What year of the solar cycle, for example, is 1807? S. 1807 9 28)1816(64 168 136 112 24, year of the solar cycle. P. What is the Metonic, or lunar cycle? S. A period of 19 years, in which the moon's age will be the same, as on the same day of the same month, in the same year of a former cycle. P. How would you find what year of this cycle corresponds to any given year of the Christian sera ? S. The first year of the Christian asra was the first year of this cycle ; hence, add 1 to the given year, divide the sum by 19, and the remainder will be the year of the cycle required ; corr* APPENDIX. 559 spomling to the 19th, or last year of the cycle. The year of the lunar cycle is also frequently called the prime-, or golden number* P. Find the golden number for the year 1807. S. 1807 1 19)1808(95 171 95 3, golden number. P. What is the Dionysian period ? S. A period compounded of the solar and lunar cycles, con- taining 532 years; (28x19) after which the day of the month, the day of the week, and the moon's age, will all return together in the same order as in the former period. P. What is the epact? S. It is the moon's age, reckoned from the change, at the beginning of the year. P. How may this be found ? S. A lunar year, or 12 lunar months, contains about 354 days, or 1 1 days less than a common solar year ; hence, should the solar and lunar years begin together, and consequently the epact=0, the next year the epact would be 1 1 , the next 22, the next (33 — 30) 3, and so on, through the cycle of 19 years; after which the epacts would again return in the same successive order. Hence, divide the year of the Christian sera by 19, multiply the remain- der by 1 1, divide the product by 30, and the remainder will be the epact. P. What is the epact for 1807 ? S. 19)1807(95 171 97 95 2 11 30)22 remainder, the epact. P. How may the moon's age be found, on any given day. S. By adding together the epact, the day of the month, and the number corresponding to the month, viz, Jan. Feb. Mar. Ap. M. Jun. Jul. Aug. Sep. Oct. Nov. De«. Com. year, 02 2244 6 7 8 10 10 Leap-year, 02 1 3345 7 8 9 10 11 and then the sum, or its excess above 30, will be the moon's age A nearly. 360 APPENDIX. P. Suppose the 4th July, 1807? S. 22 epact 4 day of month 4 num. for July 30, moon's age, or day of change. P. How would you find the time of the moon's passage over tl meridian on any given day of her age ? S. From her age subtract one-fifth part, and the remainder will be the time of her passage, nearly. P. Suppose the moon's age 30 days ? S. 5)30 6 24, or noon, the time of passage, nearly FINIS. GENERAL INDEX. JV. B. The Roman Characters denotes the Volume; thus i. ii. iii. iv. The Arabic Numbers, the Page : thus, 1, 2, 3, &c. ^LCID Liquors-, the variety in their freezing, ii. 62. Agency active in nature, iii. 213. Air, a fluid, the nature and properties of it, i. 45. Effects pro- duced by it, i.46. Its resistance, i. 47, 49. Its weight, i. 56. Its pressure in every direction, i. 50, 67. Its force equally extended, i. 52. By it is explained the suction of animals, i. 60. Its weight sustains the column of mercury in the baro- meter, i. 69. Its vast pressure on the earth, i. 71. Its elas- ticity proved, i. 72, 76. And differently accounted for, i. 30. Is caused by fire, i. 81. Is capable of raising great weights, i. 74. The air in timber causes it to swim, i. 79. Conti- nued varieties in nature caused by air, i. 80. Its elasticity is always equal to the force which compresses it, i. 83. Its elasticity undestroyed, i. 94. Air expands by heat and contracts by cold, i. 96. Is the cause of winds, i. 98. The benefits of fresh and cool air, i. 100. Carries off smoke, i. 101. Enlivens fire, i. 102. Different currents of air in chimnies in summer time, i. 109. Effects of con- densed air, i. 111. Produces fountains and jets, i. 116. Different calculations of the dilatation of air, i. 118. Raises water thirty-four feet, i. 120. Cannot be totally exhausted from the receiver of the air-pump, i. 133. The quantities of air exhausted at every stroke are not equal, but are per- petually diminished, i. 136. The different methods of ex- tracting it from the bodies which contain it, i. 178. By heat, by cold, by exhaustion, and by dissolution, i. 178. Air is contained in water, i. 178. In eggs, i. 179. In wood i. 180. The pressure of it may produce the finest anaton.ical injections, i. 182. Air condensed retards fermentation, i. 184. Air is a resisting medium, according to the surface VOL. IV. 4 D 562 GENERAL INDEX. exposed, i# 184. Supports the flight of birds, i. 186. Is of great importance in the theory of gunnery, i. Ib7. Its great resistance to cannon balls, iii. 169. Its use in the animal economy, i. 187, 189. Is necessary for combustion, i. 189. Is diminished by combustion, i. 192. Vast quan- tities are consumed by fires and various oth.tr means, i. 193. Nourishes the blood, i. 198. Passes into the buries of birds, i. 202. Is the medium of sound, i. 208, 209. Dense air conveys the sound best, i. 2u8. Air is thrown into an undulating motion by sound, i. 2u6. la what manner the pulses ol the air are propagated by sound, i. 206. The various effects in the atmosphere rest from the air, i. 241. Connexion between air and tire, i. 24fl The benefits resulting from air, i. 244. Is a general agent, i. 244. Hippocrates, his remark on it, l. 245. Is not a solvent of water, i. 547. Rarefied air a * ause of dry- ness, ii. 76. Very different currents of air in tne atmos- phere, iii. 403. Hot air, its power to raise weights, iii. 410. Air produced by fire and light, iv. 371. The upper regions of it extremely dry, iv. 445. See Fire, Light, Water. An account of the discoveries of the different airs i. 430. The manner of conveying them from one vess to another, i. 435. Methods of extracting air from seve ral subjects, i. 436. Air, alkaline acid, its nature and properties, i. 506. Air, at?nospheric, is a mixture of different fluids, i. ^59 • Particularly of two elastic fluids of opposite qualities, Contains about seventy-two parts phlogisticated air, am twenty-eight parts vital air, i. 474. Is a uniform eom- pound, i. 475. Air presses on fire, i. 360. Air supplies the fire, i. 362. Vital air, on inflammation, disengages much fire, i. 362. Explanation of Arganci's lamps, i. 3o3. Air, its efYecis on ignited iron, i. 363. Contains great quantities of fire, i. 423. Air, fixed, its nature and properties, i. 481, 490. Is found in subterraneous places, and is produced from fermentations, i. 482. From the respiration of animals, i. 482. Is com- bined with various substances, i. 483. Its effects whcH breathed by various animals, i. 483. Is absorbed by water, i. *434. Tiie analogy between this and phlogiston, i. 51 4, 5 16. Air, fiuor acid, an account of, i. o06. Air, hefiatic, an account of, i. 500. Its nature and properti es i. 501. Air, inflammable, its nature and properties, how obtained, i. 492. Is prouueed from water by means of fire, i. 494. Different manner of its burning, i. 498. Inflammable air from marshes, i. 503. A useful apparatus for making it, by the Editor, ii. 91. Air, cretaceous iuj-ammable, bow obtained, i. 502. Air, pure injiammable, its nature and properties, i. 497. Can form fire- works without smoke, i. 499. Being mixed with different airs, it produces different colours, i. 499. GENERAL IND£X. 563 Air, nitrous, obtained by the spirit of nitre poured on various metals, i. 476. Is a combination of phlogisticated and vital air, i. 477. Its nature and properties, i. 479. Air, phosphoric, its nature and properties, i. 501. Air, phlogisticated, called also azotic gas, or atmospherical me- phitis, is variously obtained, i. 468. Is light, tasteless, in- soluble, i. 469. Is improper for respiration or combustion, i. 476. Air, sulphureous acid, its nature and properties, i. 505. Air vital, its singular effects on fire, i. 364. Amazingly increas- es its power, i. 4 13. An account of it, i. 449. Is extricat- ed by heat from various substances, i. 450. Also by light from vegetables, i. 452. But in different quantities, i. 454. Water differently impregnated promotes the emission of it, i. 45 5. The quantity of it extricated is a test of the quan- tity of food taken in by the plant, i. 457. Is extricated from some metallic calces, i. 462. Its weight supports combus- tion, i. 459. The change it produces on phosphorus, i. 460. Many combustible substances become acid by vital air, i. 461. Metallic substances increase in weight as they ab- sorb vital air, i. 462. It forms one third of the atmosphere ; is the acidifying principle, i. 462. Is necessary for respira- tion, i. 464. It takes from the blood its superabundant mephitis, and imparts its own fire, i. 464. The quantity of it absorbed in respiration, i. 465. It gives the red colour to the blood in the lungs, i. 466. By respiration it passes from an aerial to a concrete form, and is the source of animal heat, i. 467. Its effects when taken medicinally, i. 468. Is a constituent part of certain bodies, i. 516. Air-gun, Editor's description of, i. 115. Air-pumps, description of, i. 39. History of, i. 43. An account of them, i. 39, 40. An account of their improvement, i. 127 By Mr. Smeaton, i. 129. By Dr. Prince, who removed the valves, i. 130. Means to obtain accurate exhaustion, i. 144. American air-pump, i. 38. Common double barrel, i. 29. Editor's caution in cleaning them, i. 41. Animals possess a natural language, i. 200. Retain the same de- gree of heat in different climates, i. 262. The wisdom ex- hibited in their form and magnitude, iii. 271. Ancients supposed that nature dreaded a vacuum, i. 64. But this was confined within certain limits, i. 65. Their knowledge in natural philosophy, ii. 420. Their knowledge of glasses, ii. 430. The just ideas which some of them entertained of God, iii. 61. Their opinion of the soul, iii. 61. Archimedes set fire to the Roman ships at Syracuse, by means of his burning glasses of plane mirrors, i. 410. ii. 205. His knowledge in hydrostatics, iii. 340. Aristotle, the influence of his authority, which impeded the pro- gress of truth, i. 65. His idea of a plenum, iii. 33. His just remark concerning the Creator, iv. 253. .irithmetic, mecanical, principles of. iii. 240. 564 GENERAL INDEX. Armillary sphere, iii. 482. The appearances of the starry hea- vens illustrated by it, iii. 496. Astronomy, observations on ; its design, iii. 457. Its general prin- ciples, iii. 458. Corrects appearances, iv. 1. Copernican system, iv. 2, 4, 145. Ptolemaic, iv. 3, 68. Remarks on phy- sical astronomy, iv. 214. The different analogies of the hea- venly motions which have been pointed out, iv. 215. Kep- ler's laws of it, iv. 225. Atmosphere, height of ; manner of ascertaining it, i. 84. Does not refract light above forty-five miles, i. 87. The height not accurately known, i. 87. Horseley's conjecture con- cerning its infinitude, i. 88. Is a mixture of all vegetable, mineral, and animal substances, i. 240. Great causes al- ways acting in it, iv. 408. Atoms, considerations concerning them, iii. 19. Are indivisi- ble, iii. 20. Are indefinitely small, iii. 22. Attraction, how differently used by authors, iii. 29. Observations on it, iii. 32. How falsely applied, iii. 33. Instances of at- traction otherwise explained, iii. 35. In a column of quick- silver, iii. 37. Attraction of cohesion examined, i. 275. Aurora borealis, the different appearances of, iv. 465. Its flashes cross the magnetic meridian at right angles, iv. 466. Where they converge, iv. 466. B Bacon, Friar, is said to have discovered the telescope, ii. 428. His superior character and attainments, ii. 429. Bacon, Lord, his reflexions on the philosophy of Aristotle, i. 18. Discovered the elasticity of the air, i. 75, His observations on the senses, i. 11. On the tendency of true philosophy, i. 281. On the scriptures and the creatures, i. 281. On se- cond causes, i. 282. His method of reasoning on natural phi- losophy ; his novum organum, i. 1,2. His uncommon me- rit, i. 2. His character of a true philosopher, i. 3. His ac- count of the idols of the mind, i. 5. The idols of the tribe, i. 5. Idols of the cave, i. 12. The idols of the market, i. 16. The idols of the theatre, i. 17. His account of different erroneous systems; the sophistical, c i. 17. The empirical, and the superstitious, i. 18. His new logic, or art of inter- preting nature, i. 23. His comparison of natural philosophy . to a pyramid, i. 33. His suggestions for a history of winds, iv. 455. Balance, its properties, iii. 241. How it should be constructed, iii. 242. Its fulcrum, iii. 244. The axis to be placed higher than the centre of gravity, iii. 245. Weights to be used with, iii. 247. Helsham's property of the balance, iii. 248. Balloons, air, iii. 399. Dr. Black and Mr. Cavallo disco\ered the principle of them, iii. 401. Were discovered and also GENERAL INDEX. 565 executed, by the Montgolfiers, with rarefied air, iii. 401. The firstf ascended from Paris with M. Pilatre de Rozier and the Marquis d'Arlandes, iii. 403. Of air balloons filled with inflammable air, iii. 404. Description of them, iii. 404, An account of different excursions made in them by Messrs. Charles and Roberts, and Mr. Baldwyn, iii. 406. Will rise from only the rarefaction of common air, iii. 412. Colleet many vapours in the atmosphere, iii. 412. Are unmanage- able from their size, iii. 413. Barometer arose from the Torricellian vacuum, i. 63. The man- ner of filling its tube, i. 66. Was first used by Pascal to mea- sure mountains, i. 67. Applied as a gage to the air-pump, i. 79. On the best form for it, iv. 413. The principal requisites of a good barometer, iv. 413. To boil the mercury in the ba- rometer tube, iv. 414. It requires a gage to regulate the quan- tity cf mercury in the cistern, iv. 416. Is influenced by heat and cold, iv. 416. Of the portable barometer ; how to use it, iv. 419. Its defects, iv. 419. Of the best portable barometer, iv. 420. How to use it, iv. 421. Of the scale of correction of the excesses of heat and cold, and their influence on the baro- meter, iv. 421. M. de Luc's remarks on them, iv. 415,479. The variations diminish as you approach the equator, iv. 480. Remarks from the barometer, iv. 481. Battery, electrical, iv. 3 1 3. Cautions in the using of it, iv. 313. Ef- fects produced by it, iv. 314 — 316. Battering rams of the ancients, iii. 99. Bell, the manner of its sounding explained, i. 209. Birds, the manner of their flying, i. 185 ; iii. 1 13. Provision made for this, in the air passing from their lungs into every part of their bodies, i. 202. Black, Dr. his doctrine of latent and sensible heat, i. 305. His experiments on the melting of ice, i. 309. His discoveries of airs, i. 432. Blindness, calamity of, ii. 250. Blood is purified in the lungs, and nourished by air; receives its Vermillion colour from vital air, i. 464. Boerhaave, his idea of fire, i. 257. Bones, their great strength, iii. 301. Boyle, Hon. Mr. his distinguished character, i. 44. Breezes, land and sea, an account of them, iv. 461. Buffon by plane mirrors burnt planks at a distance, ii. 206. Burning glasses, or convex lenses, collect the rays of the sun, ii. 172. Were known to the ancients, ii. 172. Effects produced by M.Tschirnhausen's burning glass, ii. 172. Mr. Parker ob- served a rotatory motion in the rays at the focus of his burn- ing glass, ii. 174. Camera obscura, its construction and use) ii. 175. Observations upon it, ii. 179. 566 GENERAL INDEX. Cataract. in the eye, how caused, ii. 302. Means used to disperse it, ii. 303. Catoptrics, on the laws of reflexion of light, ii. 135. Centre of the solar system, iv. 241. Charcoal, reasonings upon experiments made with it for and against phlogiston, i. 5 14. M. Lavoisier's mistakes, from con- sidering it as a simple principle, i. 5 18. Chimnies. High chimnies draw best, i. 103. Causes of chimnies smoking, i. 104. The want of a fresh current of air, i. 104. Chimnies too large, i. 105. Funnels too short, i. 106. The ac- tion of two chimnies on each other, i. 107. The position of the room door, i. 107. Communication of funnels, i. 107. Nar- rowness of funnels, i. 108. Low situation of the house, i. 108. Violence of winds, i. 108. Chimnies modern inventions, i. 1 1 1. — See Air, Fire. Count Rumford's improvements on, i. 153. Common fire-places capable of improvement, i. 153. All smoky chimnies may be cured, i. 154. Sketches of his improvements, i. 160. His practical directions for workmen, i. 163. His directions for laying out the work, i. 171. Clarke, Dr. his idea of the cause of motion, iii. 216. Clocks differ in different degrees of heat, i. 264. The principle! on which they are constructed, iii. 191. Their irregularities, iii. 192. Clouds, indications of the weather from them, iv. 483. Coals, objections against them at first, i. 111. The smoke from them considered by the Editor as injurious to the atmos- phere of cities, i- 111. Cohesion is produced by fire, i. 278. Cold, artificial, accounted for, i. 312. Cold is produced by the ab- sorption of fire, i. 338. It depends on the degree and the ra- pidity of the evaporation, i. 342. Cold is the sensation caused by fire passing from one body to another, i. 285. Effects of extreme cold on the animal frame, ii. 44. Produces great sierility, ii. 44. It is not extreme cold, but humidity, which is fatal to plants, ii. 49. The extreme degrees of cold, ii. 5 3. On the sources of cold, iv. 468.— See Fire, Evaporation. Collision, a means of exciting fire, i. 403. Effects produced by this, i. 404. Its use in New Holland, i. 405. Colours differently absorb the rays of heat, i. 55 1. The colouring substance is formed by the agency of light on the vegttabies, i. 381. On different teints of the rays of light} "■ S26 - sir . Isaac Newton's theory of light and colours, ii. 326 — See Newton, The seven colours exhibited by the prism, ii. 329. The order of the colours, ii. 330. The rays of different co- lours are of different refrangibility, ii. 332. The colours of the rays are not changed by refraction, ii. 333, 341. The due mixture of the primary colours produces whiteness, ii. , 340. Illustrated, ii. 341. The similarity between the seven primary colours and the seven notes in the scale of mtiMC, ii. 344. Different experiments on colours with the prism* GENERAL INDEX. 567 ii. 345. Illustrated by the phenomenon of the rainbow, ii. 3 lb. The different colours appear under different angles, ii. 35 1. The different colours observed in a soap bubble, ii. 356, 360. On the circles seen in glass plates, ii. 357. Different colours exhibited by reflected or by transmitted light, ii. 358. The colours of bodies depend in some degree on the thick- ness of the particles that compose them, ii. 361. The colours of different bodies arise from their reflecting one colour, and imbibing all the rest, ii. 365. The colours tinging shadows in summer explained, ii. 368. The colours of the atmosphere, clouds, kc. ii. 369. The excellency of the colours used by the ancients, ii. 370. Mr. Delaval found that the tinging matter of all vegetables was always black when viewed by reflexion, ii. 372. The colouring particles of bodies appear black when they are dense, ii. 374. When the colouring mat- ter is extracted from vegetables, &c. they appear white, ii. 376. Colours are destroyed by the rays of the sun, ii. 377. Animal and metallic colours are produced in the same man- ner as vegetable, ii. 378. The production of colour depends on fire, ii. 38 3. Colours are emitted from some phosphoric substances, ii. 395. They are produced by fire, ii. 395. Are not sensations, but secondary qualities of bodies, ii. 422. — See Light, Fire, Colure, equinoctial and solstitial, ii. 491. Combustion, an effect of fire, i. 356. The requisites for combus- tion, i. 358. In combustion the pure air is converted into fixed air and aqueous vapours, and gives out its fire, i. 424. Comets are regular parts of one great system, iv. 207. Their use unknown, and the knowledge of them imperfect, iv. 207. Their number, orbits and motion, velocity and size, iv. 208. Their form and tails, iv. 209. Conductors, and non-conductors, of electricity, iv. 263. Congelation, phenomena of, explained, i. 310. Constellations. — See Stars. Copernicus, his system, iv. 1. Was probably known to the an- cients, iv. 2. A view of it, iv. 4. The truth of his system proved by the planetarium, iv. 136. Crawford, Dr. his excellent treatise on animal heat, i. 415,423; and on combustion, i. 425. Creation, remarks on it, iv. 370. The order of it, iv. 371. Is a theatre for the divine goodness, iv. 373. Cruelty to animals justly condemned, i. 190. Crystallization explained, i. 400. CuJ./iing, the nature and operation of it, i. 54. Dalton, Mr. his account of the aurora borealis, iv. 465. His re- marks on the weather from the barometer, iv. 481. Darkness at our Saviour's crucifixion supernatural, iv. 107. Day, astronomical or solar, iv. 123. Sidereal, iv. 123. 568 GENERAL INDEX. Delaval, Mr. his experiments on the permanent colours of opake bodies, ii. 370. He found, that in transparent coloured sub- stances the colouring matter does not reflect any light, ii. 372. An account of his experiments, ii. 373. He concludes that the colouring particles do not reflect any light, but that objects are reflected by a medium diffused over the surfaces of the plates, ii. 374. His observations on the colours of animal, earthy, and metallic bodies, ii. 377.— See Colours, Light, Descartes, his error concerning the universe, iii. 215. Digits, iv. 94. - JDiodorus Siculus, his weak idea concerning the speech of men at the beginning, i. 200. Diofitrics, or the laws of refraction, ii. 135. Discoveries of printing, mariner's compass, gunpowder, ii. 389. Dista?ice, the appearance of, depends on the brightness of ob- jects, ii. 310. On the number of intervening objects, ii- 312. Different degrees suggested by different apparent magni- tudes of objects, ii. 313. Diving bell, description of Dr. Halley's, iii. 416. The improve- ment of this by Tried wald, iii. 418; by •Mr. Smeaton, iii. 418. Diving chest, invented by Mr. Smeaton, iii. 418. Dollond, Mr. John, the inventor of the achromatic telescope, ii. 463. The principles on which it is founded, ii. 465. The discovery attributed to Euler, ii. 467. The invention also ascribed to Mr. Hall, ii. 468. E Earth, the equatorial diameter greater than the polar diameter, iii. 45. Its size, motion, distance, &c. iv. 19. Its revolution ; its figure, iv. 40. The proofs of this, iv. 40. Its diurnal mo- tion, iv. 43. The reasons for this, agreeable to analogy, iv. 45. The phenomena arising from this diurnal motion, iv. 45. Is half illuminated at a time, iv. 46. Of the earth's an- nual motion, iv. 49. It partakes of various degrees of heat and cold, iii. 62. The earth enlightens the moon, iv. 90. The shadow of the earth forms a cone, iv. 93. The eclipses of it, iv. 99. Its monthly motion about the common centre of gra- vity between that and the moon, iv. 182. The matter of earth, how formed, iv. 433. The use of the earth, iv. 434. The magnetism of the earth, iv. 455. Earth a source of heat, iv. 467. Distance from it a cause of cold, iv. 468. The excellency of its distribution into seas and mountains, iv. 474. Ebullition is caused by the action of fire on the bubbles of air m any fluid, i. 329. Echo, nature'of, i. 218. Explanation of its effects, i. 220. — Sec Sound, GENERAL INDEX. 569 £clipses were formerly superstitiously regarded, iv. 91. Are ex- plained, iv. 92. Of the moon, iv. 92. Eclipses total and cen- tral, iv, 94. Of their limits, iv. 104. Of their periods, iv. 108. •Ecliptic, the sun's annual path, iii. 47 S. The obliquity of this, iii. 479. Is divided into twelve signs of thirty degrees each, iii. 490. Ecles, Mr. his system of electricity, iv. 264. Eggs, incubation of, greatly assisted by means of air, i. 180. Elasticity (see Air, Water) is caused by fire, i. 81. Elasticity of bodies, iii. 202. Supposed to lie caused by motion, iii. 203. Its effects in different handy-works, iii. 210. In gunnery and rockets, iii. 2 10. Electricity first discovered in amber, iv. 259. The uses derived from it, iv. 259. Is a fluid universally diffused, iv. 260. Electrical appearances, iv. 261. Electricity is vitreous or resinous, iv. 261,268, 281, 298, 299, 367, 369, 374. These are two distinct and active powers, iv. 265. They exist to- gether conjoined, in their natural state, iv. 265. Electricity is from the separation from these two powers, iv. 265. 268. Electrical atmospheres, iv. 267. Of the electrical machine, and its mode of action, iv. 267, 269. Cautions in using it, iv. 271, 272. Conductors must be insulated before they are electrified, iv. 269.- The brilliancy of the spark depends on tiie pressure of the atmosphere and medium, iv. 269, 288. The solar and electric fluid are probably the same, iv. 272. On the momentum of its force, iv. 272. On its attraction and repulsion, iv. 273. On the afflux and efflux of the two powers, iv. 276. Gives a rotatory motion to small light balls, iv. 27S. The electric fluid is universally disseminated, and in continued action, iv. 280. The smallest motion in nature disturbs its equilibrium, iv. 281. Observations on Franklin's system of electricity, iv. 282. On the electric • spark, iv. 285. On the use of points, iv. 286. The electric spark will fire spirits of wine, iv. 289. The sparks are of different colours, iv. 290, 306. Motions produced by elec- tricity, iv. 290. Promotes evaporation, iv. 292. Is a sub- ject of general curiosity, iv. 293. To ascertain the quantity of the electric matter, iv. 296. The powers are reciprocally exchanged, iv. 301. It melts wires, beginning in the mid- dle, iv. 305, 316. The electric matter is only luminous in a divided state, iv. 303. It perforated a quire of paper in. opposite directions, iv. 315. Is discovered in rain, hail, snow, iv. 322. The immense quantity of it, iv. 325. Is real matter, proved, iv. 331. Its resemblance to fire and light, iv. 331, 332. It produces heat, iv. 333; and accelerates evaporation, iv. 334. Is communicated by the same sub- stances which communicate heat, iv. 334. Electricity i;> procured by beat and liquifaction, iv. 335. Raises the ther- mometer, iv. 336. The electric state of the air in Russia, VOL. IV. 4F. 570 % GENERAL INDEX. iv. 338. Heat in summer becomes electric fluid in winter, iv. 338. Luminous experiments, iv. 339. The similar ef- fects on the solar and electric light, by different media, iv. 344. On animal electricity ; electricity the principal of ani- mal heat and motion, iv. 354. The electric fluid and the solar fire are the same, iv. 3 5 8. Its agency in animated nature, iv. 3j9. Its influence on health 'and our feelings, iv. 360. Remarks on animal electricity, iv. 360. Electricity may he the same as the animal spirits, iv. 362. Experiments -and results from animal electricity, iv. 365 — 368. Remarks . on atmospherical electricity, iv. 474. Its diurnal variations, iv. 476. — See Sun, J' ire. Electrometer, that described by Mr. Bennet, iv. 280. Experi- ments with it, iv. 28 1, 475. Elements^ active and passive, iv. 354. The importance of this agency, iv. 405. Engines, constructions of different sorts, iii. 272. Compound en- gines, iii. 273. How to compute their powers, iii. 374. Equator, of the, iii. 464, 488. Equatorial instrument or universal sun-dial, description of, iv. 180. Equinoxes, precessions of, iv. 12 1. Ether, by evaporation, is capable of freezing water, ii. 5 1. Re- duced mercury 29° below the freezing point, ii. 52. J'.vajioration an effect of fire, i. 323. Different effects resulting from it, i. 32 7, 339. Cools liquors, i. 330. Is the same in open air and in vacuo, i. 332. Evaporation of ether freezes water, i. 310. Effects of it, i. 543. Produces ice in the East Indies, i. 342. lis effects on the health of the body, i. 343. Spontaneous evaporations are also caused by fire, i. 347. And are assisted by motion, i. 348. Evaporation proceeds from grass, vegetables, trees, and shrubs, i. 34 9. In a cer- tain degree it contributes to health, i. 350. Eaws of evapo- ration by M. de Luc, ii. 77. Is a dissolution of water by fire, iv. 435. Is a great source of cold, iv. 469. Remarks, on evaporation, iv. 4 70. — See Fire, Water. Eudiometer tube, and measure, i. 437* Exhaustion, successive degrees of, i. 132. ' Experimentalist, character of, i. 175 — 177. Eye, greatly assisted by optical instruments, ii. 133. Can be adapted to very different degrees of light, and size of objects, ii. 149. Benefits derived from it, ii. 250. Short descrip- tion of the eye and its various parts, external and inter- nal, ii. 25 1. Its orbit, brows, ii. 251. Of the two eye-lids, ii. 252, 254. Of the lachrymal gland, in 254. Of the muscles of the eyes, ii. 255. Of the motions of the eye, ii. 257. Of its globe, ii. 258. Of its coats, sclerotica, choroides, ii. 259. Of the iris, ii. 260. The pupil, ii. 260. Of the retina, ii. 262. Of the optic nerve, ii. 263. The advantages derived from two eyes, ii. 2 56. Of the three humours, ii. 263. The aqueous, ii. 264. The crystalihe, ii. 254. And the GENERAL INDEX. 571 vitreous, ii. 265. Of theligamentum ciliafe, ii. 266. Of the artificial eye, ii. 270. The different degrees of sensation in different people, ii. 279. Distinct vision formed on the re- tina and near it, ii. 282. The eye accommodates itself to different distances, ii. 285, 305* Illustrated, ii. 286. How accounted for by various authors, ii. 287. The eye sees best when surrounded by* darkness, ii. 291. Is enabled (o see with a very small quantity of light, ii. 292. Of the de- fects of the eye ; of the long-sighted, ii. 294. Of the acciden- tal conformations of the eye by habit, ii. 295. Rales for pre- serving the sight, ii, 298. Maladies" 6f the eyes, how occa- sioned, ii. 299. Two remarkable cases, ii. 300. Of couched eyes ; require convex glasses, ii. 303. Its powers are limit- ed, ii. 310. Mistakes made by those who have but one eye, ii. 310. The analogy between the eye and the understanding, ii. 316. Reflexions on the wonderful powers of the eye, ii. S23. Lyes of animals differently phosphoretic, ii. 392. — See Light* Filtration, Mr. Peacock's new apparatus for, ii. 29. Fire, produces elasticity, i. 81. Fire and air are different condi- tions of the same elementary matter, i. 242. Plato's idea of it, i. 245. The different opinions entertained of it by the ancients and moderns, i. 249. Proved to be a real material substance, acting in a fluid form, i. 250, 256. Is not creaU ed by motion, i.25l. Is the cause of heat, i. 255. Absolute heat and relative heat, i. 256. Boerhaave's idea of fire; is universally diffused, i. 256. Penetrates all bodies, i. 257. Continually tends towards an universal equilibrium, except in animal bodies, i. 258,262. Is differently conducted in dif- ferent bodies, from their different capacities of retaining it, i. 258. Is retarded by soft substances, i. 260. Is rapidly conveyed by fluids, i. 260. Dilates all bodies, i, 263. Liberat- ed, manifested, or thermometric fire, i. 264. Different metals are differently expanded by fire, i. 265. The very great force of fire in expanding them, i. 273. Fire the grand agent in nature to dissolve and to unite all things, i. 276. Proved to be the cause of cohesion, i. 278. Acts in two different modes, by dilating and compressing, i. 279. Is the cause of cold, which is occasioned by fire passing from one body to another, i. 285. Latent fire, i. 290. A method to discover the quantity of fire contained in different bodies, i. 292. The action of fire depends on the re-action of the in- cumbent air, i. 300. Fire is the cause of fluidity, i. 303. Is extricated whenever water is congealed, i. 511. Was formerly made an object of worship, i. 322. Is a fluid never at rest, i. 322. Is the cause of vapours, i. 323. Its extreme violence when confined, i. £23. Is the cause of ebullition, i. 327. Much fire is absorbed in vapours, i. 333. Fire ex- 572 GENERAL* INDEX. panels or separates the parts of water, i. 335. Specific fire of any body, i. 337. Fire is differently absorbed by different coloured substances, i. 351. See Colours, Fire maintains its dimensions, although greatly pressed by the air, i. 361. The genera) effects it produces acting on different suhstanci!-- i. 384. Man alone understands the use of fire, i. 389. Its effect on gunpowder, i. 393. On fulminating powder, i. 395, and fulminating silver, i. 396. Produces solutions, i. 397. Crys- tallization, i.400. Clarification, i. 401. Odours, i. 402. Fire is collected by collision, 403. By fermentation, i. 406. By pi, "re- faction, i. 407. By the action of the solar rays, 407. B> the resistance of the parts of a body on which light falls, i. 409. "Methods of increasing or diminishing the action of fire, i. 412. Different bodies contain different quantities of fire, i. 417. Fire constantly tends to diffuse itself over al) hoc its, till they are brought to the same temperature, i. 417. It is contained in all bodies at the common temperature of thi atmosphere, i. 418. Atmospherical air contains a great quantity of fire, i. 423. Fire is distinguished into diffusible and constitutional, i. 426. Its agency in vegetation, i. 429. Causes the grand differences in bodies, i. 441. Is distin- guished into sensible heat, the latent fire of fluidity, and the latent fire of elasticity, 441.. Opinion of the Pythagoreans on fire, iv. 359. Fire is imponderable, ii. 58. Fire is the agent of all dissolution, iv. 69. Great quantity of fire is re- quisite to raise water into steam, iv. 70. Is subject to the same laws of inflexibility and refrangibility as the rays of light, iv. 215. Is the means of producing colours, iv. 383. Is retained in many bodies under the form of heat and light, iv. 379. Fire is necessary for producing the prismatic co- lours, iv. 395. The similarity between fire and light, iv. 413. Is the most important agent in nature, iv. 346. Natural life depends on its activity, iv. 346, 352. Is the active element within all bodies, iv. 35 1. Has been called by different names, iv. 350. Is every where present, iv. 351. Is only an instrumental cause, iv. 353. Its effects on the heart, iv. 356. Its influence in the animal economy, iv. 357. On this de- pends the health and activity of animals and plants, iv. 457. Theophrastus's opinion on fire, iv. 359. The various effects produced by it, iv. 360. Elementary fire, or the matter of light first formed, iv. 371. Fire, its expansive power, iv. 423. — See Light, Heat, Colours, Fishes, the manner of their breathing, i. 2C3. Their manner of swimming, iii. 1 17. Flame, an account of, i. 365. On the flames of candles and lamps, i. 366. Fluids of the least density expand most by heat, i. 275. Are caused by a degree of fire, iii. 342. Are not to be explained on mechanical principles, iii. 342. Their gravity in proprio loco, iii. 343. The parts of a fluid gravitate independently of GENERx^L INDEX, s'J each other, iii. 3-15. The surface of fluids is on a level, ill. 34 5. Their pressures, iii. 346. Fluids press in all directions, iii. 347. On their action against vessels of different sizes in which they are contained, iii. 348, 351. The bydrpstatic pa- radox, iii. 353. On the action of fluids on bodies immersed in them, iii. 357. Fluids, when deep, press equally on all sides of a body, iii. 360. Bodies sink if specifically" heavier, N or swim if lighter than the fluid, iii. 364. If of the same gra- vity, will remain in any part of the fluid, iii. 355. The Weight lost by a solid immersed in a fluid is communicated to the fluid, iii. 371. Their spouting through small orifices, iii. 420. The velocity is as that cf a heavy body falling from an equal height, iii. 421. The quantities di charged, iii. 422. Thewatet contracts in flowing out, iii. 42.3. Of the discharge cf fluids through additional tubes, iii. 428. Of jets d'eau ; they ne- ver rise to the height of the reservoir iii. 430. The position of the ajutage, iii. 430. Of the motion of fluids in' conduit pipes, iii. 443. The friction retards the velocity, iii. 444. If the pipes be curved, the discharge is less, iii. 445. Of the vibratory motion in fluids, iii. 448. Of the oscillatory motion of waves, iii. 449. Of the resistance of fluids, whether at rest or in motion, iii. 450. Mistakes of the moderns concerning this subject, iii. 45 1. Their ignorance of it, iii. 452. — See Fire, Neat, Fluids, elastic, are combinations of fire with given substances, i. 441. M. Lavoisier's mistakes concerning them, i. 443. M. de Luc's observations on them, i. 448. The difference be- tween these and vapours, i. 441. Fluidity is caused by heat, i. 303, absorbed or combined with the fluid substance, i. 306. Focus, real or imaginary, ii. 15 1. Forces, of the composition or resolution of forces, iii. 107. Forces, deflecting, of, iv. 226. Central forces, iv. 226. Fountain by condensed air, description of, i. 116. Frame, human, imbides water from the atmosphere to supply the animal moisture, ii. 17. The great quantities of this daily exhaled, ii. 17. Franklin, Dr. his system of electricity, iv. 282. Observations on his theory, iv. 283. His principles, iv. 2S5. The negative and positive electricity denied, iv. 282. Friction, iii. 290. Is a uniformly retarding force, iii. 292. It does not increase equally with the quantity of matter, iii. 292. The smallest surface has the least friction, iii. 293. General observations on frictions, iii. 295. Fulminating fiowders, their nature and operations, i. 395. Furs, their property of retaining heat, i. 261. • 5/4 GENERAL INDEX. Gages for the air-pump, i. 139. Their very different results, i. 140. Explained by Mr. Cavendish, i. 141. Galen, his just remark on praise, i. 194. Galileo couid not discover the reason why water coulq* not be raised by a pump above 32 feet, i. 64. Discovered the te- lescope, ii. 430. Applied the pendulum to measure time, iii. 178. Gas, inflammable carbonic, an account of, i. 502. Gas, muriatic acid, its nature and properties, i. 5G3. Gas, muriatic dcfihlogisticated, an account of its effects, i. 504, Destroys all vegetable colours, i. 504. Gases, permanently elastic fluids, i. 441. Are of different kinds, i. 49T. Gorgium Sidus was discovered by Dr. Herschel, iv. 29. Its size,- distance, revolution, iv. 29. It moons, iv. 30. The length of its year, iv. 138. Glass, its various uses, ii. 163. Different kinds of, ii. 465. Qlobe, 7iew terrestrial, the advantages of it, iv. 161. The absur- dities resulting from the old ones, iv. 1 62. The construction and use of this new globe, iv. 1 63. How to rectify this globe, iv. 164. New celestial globe, iv. 165. Editor's vindication of globes mounted in the common manner, iv. 176. Advan- tages of the new-mounted globes shewn by him, iv. 178, and of the common globe, iv. 179. Glow-worm, an account of the light in it, iv. 365. God, the author of speech, i. 199. His end in creating the uni- verse the greatest possible good, i. 243. The author of all good to man, i. 282. Is clearly indicated by final causes, i. 315. The immensity of his works, i. 27. Is the author of nature, ii. 244. The character of the Saviour, as delight- ing to communicate wisdom, ii. 371. His divinity and cha- racter, ii. 423. The infinity of his works, ii. 424. Has im- pressed on matter some faint characters of his own beauty, iii. 61. The ancients thought him to be goodness itself, and truth itself, iii. 60. The perfection of his wisdom, iii. 74. A hymn to his praise on the excellency of the soul of man, iii. 78. His wisdom and power, as the divine mechanic ; iii. 83 ; displayed in the natural and moral world, iii. 83. Time cannot be predicated of him, iii. 196. God the origi- nal cause of all motion, iii. 223. His mercy, wisdom, and power are discoverable in his works, iii. 456. He alone sees the whole of nature, iii. 457. He is glorified by the inhabi- tants of innumerable worlds, iv. 212. Has probably peopled all the planetary worlds, iv. 214. Is the real cause who go- verns the mundane system, iv. 224. The knowledge of him is the most excellent wisdom, iv. 254. He is one, Jesus Christ, iv. 254. His universal government, iv. 255# Is in- GENERAL INDEX. 575 visible to us, iv. 256. Is praised by all things which he has made, iv. 256. His perfections seen in his works, iv. 256. The manner and the end of creation, iv. 373. The perfection of his word and work, iv. 488. — See Providence, Nature* Man, ravlcjy and gravitation, an inquiry whether it be an essential property of matter, iii. 25. Considered as a fact, iii. 35. The proportion in which the force of gravity decreases, iii. 40. Gravity acts universally, but not uniformly in all places, iii. 41. A body falls about an hundred and ninety-three inches in a second of time, 44. The difference respecting gravity resulting from different positions on the globe, iii. 45. Gravitation extends to the planets, iii. 45. Reflexions on gravity as a law, iii. 46. Gravity considered as a resist- ing and as a moving power, iii. 47. The weight of bodies is not in proportion to their quantities of matter, iii. 48. Powers acting contrary to gravity, iii. 49 ; observed in plants and light, iii. 50. Of the centre of gravity in bodies, iii. 146. Of the centre of gravity in the human body, iii. 150. Cautions arising from it, iii. 151. Of the situation of the centre of gravity, iii. 154. To find the centre of gravity of a trapezium, iii. 155 ; of a pyramid, iii. 156. General observa- tions on gravity, iii. 161. It extends to the moon, iv. 230. It produces a small irregularity in the motion of the planets, iv. 234. Gravity, specific, of bodies, iii. 35S ; is as their density, iii. 358. is measured by water, iii. 359. All bodies immersed in fluids lose the weight of an equal bulk of that fluid, iii. 467. How to obtain the specific gravity of bodies, iii. 473. To find the specific gravity of solids, iii. 378. If heavier or lighter than water, iii. 380. Of fluids, iii. 382. Different methods of ascertaining the specific gravity of fluids, iii* 393. A table of specific gravities, iii. 390. Of several fluids in summer and winter, iii. 398. Green colour, the universality and excellency of it, ii. 324. Gunnery, the great difference between theory and practice, iii. 166. Mr. Robbing's application to this art, iii. 167. The imperfections in this art, iii. 170. Gunpowder, the substances which compose it, i. 392. Manner of making it, i. 392. Derives its force from vital air, i. 393.. An estimate of its expansive force? i. 395. H Hail is water suddenly congealed, ii. 46. Hallcy, Dr his weak solution concerning the saltness of the sea, ii. 32. His hypothesis to explain the variations of the needle. iv. 398. Hearty its situation and mutual action with the lungs, i. 197. 576 GENERAL INDEX. Hearts, of different creatures, in what manner affected by heat, iv. 355. Heat, the effect of fire, i. 255. Absolute and relative, the differ- ence between them, i. 25 6. Is conveyed through a medium more subtile than common air, i. 257. Animal heat re- mains universally the same, although under the most op- posite circumstances, i. 2G3. The degree of heat is mea- sured by the thermometer, i. 285. Different subjects re- ceive different degrees of heat, i. 295. The progression of heat' not easily ascertained, i. 295. The relative heat in different bodies marked by Mr. Jones, i. 297. Latent and sensible heat, i. 305, 337. More than eight hundred degrees of heat are absorbed in steam, i. 334. Heat counteracts the influence of gravitation, i. 387. The quantity of heat is di- minished by the change it undergoes in the lungs, i. 423. Light and heat are different modifications of the same mat- ter,^. $90. Without heat bodies do not emit light, ii. 401. The different sources of heat, iv. 466. — See Sun, Fire, Ther- mometer. Hersckel, Dr. his discoveries of new stars, ii. 427. His idea of the construction of the universe, iv. 192. That the visible universe is only a nebula, iv. 192. Concerning a sidereal stratum, iv. 193. The great powers of his telescope, iv. 196. On the origin of nebulous strata, iv. 197. Discovered volcanoes in the moon, iv. 203. — See Stars, Moon. Hipfiocrates, his admirable observations on air, i. 245. Hire, M. de la, his experiments on the distance which rainwater penetrates into the earth, ii. 25. Horizon, iii. -S62. Its uses, iii. 484. Is divided into rational and sensible, iii. 485. Horses, their advantage in drawing from their weight, iii. 306. Horselcy, Bishop, his conjectures concerning the infinitude of the atmosphere of the earth and the planets, i. 88. JTcw-ch'cle of the, iii. 489. Hurricanes, an account of those in the West Indies, iv. 463. The signs of their coming, iv. 463. Hydraulics treat on the motion of fluids, iii. 419. Hydrometer, of measuring the specific gravity of fluids by it, iii. 389. A description of, iii. 391. The requisites for a good one, iii. 392. Hydrostatics, their nature* iii. 338. The difference between theory and practice, iii. 339, Our ignorance relative to se- veral particulars on this subject, iii. 339. Aerostation simi- lar to hydrostatics, iii. 409. Hydrostatic balance, its use in determining the quantity of gold, &c. iii. 376. How it is constructed, iii. 377. It varies with the heat and colu of the weather, iii. 378. How to find the proportion of alloy mixed with gold, iii. 385. Hygrometer, its use: several substances affected by the moisture or dryness of the air, iv. 431, An account of M. de Luc's GENERAL INDEX. ,77 hygrometer, iv. 431. A further description and figure of it, by the Editor, iv. 491. The good effects of a hygrometer, iv. 432. The discoveries made by means of the hygrome- ter, iv. 438. Hygroscopic substances, different kinds of, iv. 436. Jiyfiothesesy observations on, i. 89. Conjectures discover no truths, i. 90; but confirm men in prejudice and ignorance, i. 91 See Truth. I Ice absorbs all lire until it is wholly melted, i. 293. It requires a hundred and forty degrees of heat to convert it into water, i. 210. Is a combination of water, when deprived of its fire, ii. 38. Freezing is promoted by air and by agitation, ii. 39. A bit of ice produces instant congelation in water cooled below the freezing point, ii. 40. Its great expansive force, ii. 41. The vast quantities of it in the northern seas, ii. 43. Ice is continually diminishing by the action of the air, ii. 45. The manner of rivers freezing, ii. 47. The great strength of ice, ii. 48. Frost does not penetrate deep into the earth, ii. 49. Ice may be produced by the evaporation of ether, ii. 51. The chemical properties of ice, ii. 58. — See Fire, Water* Idols of the mind, Lord Bacon's account of them, i. 4. Are of dif- ferent kinds, i. 5. • Ignition, a universal effect of fire, i. 355. — See Fire. Impulse the only material cause of motion, iv. 2 1 8. Ivgenhouz, Dr. first discovered the power which plants have of emitting vital air, i. 452. Flis experiments confirmed by others, i. 455. — See Light, Air, Vegetables. Insects, the manner of their breathing illustrated in the larva of the musca pendula, i. 204. Instruments, philosophical, described ; general remarks on them, i. 302. Air-pump, i. 38. An account of its improvements, i. 127. Philosophical hammer, i. 48. Cupping, i. 54. Mag- deburg- hemispheres, i. 55. Common pump, i. 56. Trans- ferrer of air, i. 56. Fountain of command, i. 57. Anti-gug- gler, i. 58. Common bellows, i. 60. Gage to the air-pump, i. 69. Bolt-head, i. 76. Smoke-jack, i. 101. Condensing engine, i. 112. Wind or air gun, i. 115. Artificial fountain by condensed air, i. 116. Common pump, i. 119. Forcing pump, i. 121. Water-works at London bridge, i. 123. Sy- phon, i. 123. Tantalus's cup, i. 124. Gages for the air- pump, i. 137. Pear-gage, i. 140. Pyrometers, i. 265. Calo- rimeter, i. 321. Wedgewood's thermometer for ascertaining intense degrees of heat, i. 294. Another, invented by Mr. Jones, i. 297. Papin's digester, i. 324. iftolipile or wind- ball, i. 325. Argand's,or cylinder lamps, i. 363. Pneumatic apparatus, i. 434. Eudiometer tubes and measure* i. 437. VOL. IV. 4 F 578 GENERAL INDEX. Dr. Nooth's machine to impregnate water with fixed air, i. 485. M. Bettancourt's contrivance to measure the force of steam, ii. 72. Steam engine, ii. 73. Inflammable air lamp, ii. 93. Animated optical balls, ii. 236. The boundless gal- lery ; the magical mirrors, ii. 234. Simple camera obscura, ii. 228. Reflecting, ii. 229. Dioptrical paradox, ii. 232. Op- tical paradox, ii. 234. Real apparition, ii. 239. Optical per- spective box, ii.242. Cylindrical mirror, ii. 243. The prism, ii. 329. Telescopes, ii. 255. Microscopes, ii. 478. Atwood's friction apparatus, iii. 125. Directions for construction, iii. 138. Spirit level, iii. 148. Plumb line, iii. 148. Odometer or way-wiser, iii. 152. Whirling tables, iii. 319. Hydrosta- tic paradox, iii. 353. Hydrostatic bellows, iii. 355. Hydro- meter, iii. 389. Discharging rods, iv. 394. Quadrant elec- trometer, iv. 296. Magic picture, iv. 31 1. Spotted bottle, iv. 3)2. Thunder-house, iv. 329. Jones, Rev. Mr. his observations on nature, i. 176. His just re- flexion on the origin of fire, i. 405. On the improvements in philosophy, iv. 310. His remark on the electric matter and animal spirits, iv. 364. His observations on the supe- riority of the northern hemisphere over the southern in se- veral particulars, iv. 484. Iris* — See Eye. Jupiter, his size, distance, revolutions, iv. 25. His four moons, iv. 26. His year, and motion round the sun, iv. 138. His belts, iv. 205. Their changes, iv. 205. K Kepler, his laws of astronomy, iv. 225. Knight, Dr. his discoveries in magnetism, iv. 386. K?ionvledge, its excellency, i. 61. Language, natural and artificial ; language was not invented by man, i. 199, 200. Latent heat, doctrine of, explained, i. 305, 337, 414. Is of two kinds, of fluidity and elasticity, i. 378. — See Phlogiston, Fire, Heat, Lavoisier, his opinion on fire, i. 256. His experiments with vital air, i. 413. His mistakes concerning elastic vapours, i. 445 — 446. A confutation of his system by Mr. Weiglib, i. 507 — 522. Laughter, how caused ; good effects of moderate laughter, i. 201. Le?is, may be formed of different substances, i. 409. Various ef- fects pro4uced by them, i. 410. Lenses of various sorts, plano-convex, plano-concave, double con- vex, double concave, concavo-convex, ii. 163. Their different GENERAL INDEX. 5T9 properties, ii. 165, 183. Methods to find their focal lengths by experiments, ii. 188. The properties and phenomena of single lenses, ii. 192. Levelling, the principle of it, iv. 42. Lever, its nature and properties, iii. 229. Levers are of three kinds, iii. 229—235. Of the hammer lever, iii. 232. Its va- rious applications, iii. 235 — 241. Its properties applied to various subjects, iii. 267. Leyden phial, iv. 293. How to charge it, iv. 294. The theory of it, iv. 296. On the discharge of this, the two electricities rush into union from opposite directions, iv. 304. Different shocks by means of it, iii. 304. Confirmed by the electric spark, iv. 306. The two powers are in contrary directions, iv. 307. Various effects produced by it, iv. 314. — See Elec- s tricity. Liberty, singular panegyric en, i. 390. The false pretenders to it, i. 39. . Genuine liberty, i. 392. Life, or the animating principle, iii. 59. The analogy between life and motion, iii. 216 — 224. Natural life depends on fire, iv. 346. Light travels at the rate of 72,420 leagues in a second, i. 216. Is the mediating substance between fire and air, i. 242. Light combined with fire and water produces an aeriform fluid, i. 447. Light extricates vital air from vegetables, i. 452. Rays of light are extremely minute, ii. 129. Its operations and analogy, ii. 130. The advantages of it to man, ii. 131. Light a property of fire, ii. 132. Light is a material real substance, is progressive, may be stopped and diverted, acts on all bo- dies, ii. 134. Light moves in a straight line, ii. 137. Is suc- cessive and contemporary, ii. 157. The rays of it indefinitely small, ii. 138. They carry the image of the point from which they proceed, ii. 139. Their refiexibility and refrangibility, ii. 140. In different mediums, ii. 141. The light of the moon is 300,000 times fainter than the light of the sun, ii. 146. The quantity of light decreases as it recedes from the radi- ant, ii. 148. Light is suffocated by various bodies, ii. 148. A table of the quantity of light dissipated in the atmosphere, ii. 150. Rays of light are parallel, diverging or converging, ii. 151. Are reflected before they touch the body, ii. 198. Light contracts the pupil of the eye, ii. 290. Reflexions on light, ii. 323. The rays of light are not homogenial, ii. 327. The rays of the sun consist of seven different coloured rays, ii. 327. The compound of all the rays exhibits whiteness, ii. 328. The rays are of different refrangibility, ii. 330. Homo- genial light suffers no alteration in any case, ii. 334. Rays which differ in their colour, differ also in their refrangibi- lity, ii. 335. Bodies reflect rays of one colour, and transmit rays of another, ii. 361. The rays of light are thought to 580 » GENERAL INDEX. be put in a transient state, and easily reflected and trans- mitted, ii. 363. This Sir Isaac Newton supposed was ow- ing to the vibrations of a subtile fluid, ii. 363. The ana- logy between the reflexion and refraction of the rays of light, ii. 365. Is imbibed by all bodies, except water and metals, ii. 391. Is matter moving- in a straight line from a body, ii. 399. Light and heat are different modifications of the same matter, ii. 399 — 405. Bodies are either luminous or illuminated, ii. 400. Without heat bodies will not emit light, ii. 401. The attractive gravitating matter in bodies has no power to resist light, ii. 405. Light is acted upon by ooches at a small distance by attraction and repulsion, ii. 407. The rays exhibit three fringes of coloured light round the shadows of small bodies, ii. 109, The influence of light in the vegetable kingdom ; it produces colours and smells, ii. 417. Its influence on animals, ii. 417. Its effects iu chemistry, ii. 418. Its effects on colours and or. wood, ii. 419. The opinions of the ancients concerning light, ii. 420. Questions concerning light, ii. 493. The opinions of the ancients concerning it, ii. 593. Of Plato, ii. 493. Its connexion with lire, ii. 496. And with electricity, iv. 332. Its energy and activity, iv. 347. Refraction of, iv. 111. Is different at different places, iv. 112. Effects re- sulting from it, iv. 113. Aberration of, discovered by Dr. Bradley, iv. 1 19. On the light which appears in the eyes of some animals in the dark, whence it comes? iv. 365. The - matter of light first formed, iv. 371. — See Fire, Heat, Co- lours, Kefraction^ Reflexion, .Veivton, Light ^inflammation cud light of ignition, their difference, i. 368. Lightning, on the phenomenon of; varieties of it, iv. 317. Its peculiar property, iv. 318. Its effects are limited, iv. 318. A remarkable instance of it, iv. 319. Produces whirlwinds, iv. 319. The identity of lightning and electricity, iv. 319. There is a reciprocal exchange from the earth to the cloud, iv. 320. The extent of their atmospheres, iv. 320. Causes concussions on the earth, iv. 323. Imparts magnetism, iv. 389. — See Fire, Electricity, Magnetism, Luc, M, o'f, his admirable reflexions on the true end of philoso- phy* i. 316. His remarks on infidelity, i. 319. His just ob- servations on elastic fluids, i. 448. His observations on the change of ice into water, and vice versa, ii. 57. On the state of aqueous vapour in the atmosphere, and laws of eva- poration, ii. 74—86. His excellent philosophical works ; his refutation of materialism, iii. 66. His observations on the hydrometer, iii. 393. An account of his whalebone hygro- meter, iv. 431. Was in a storm on the Buet, iv. 454. His remarks on barometers, iv. 479. — See Hygrometer, Lunarium, description of, iv. 155. GENERAL INDEX. 5 8i \ Lungs described, i. 195. Their situation and action, i. 195. How much unknown, i. 197. Receive great quantities of blood, i. ]97-» Their correspondence with thought, i. 197. Their con- nexion with the circulation of the blood, i. 198. Express va- rious affections, i. 199. M Magic lanthorn, its construction and use, ii. 182. Magnetism was observed by the ancients ; is unknown, iv. 374. Acts universally ; natural magnet ; its contents, iv. 375. The artificial magnet is preferred, iv. 375. Its poles, iv. 376. Its properties, iv. 376. Attracts iron. iv. 377. The sphere of its action is variable, iv. 379. The similarity between magnet- ism and electricity, iv. 402. On the magnetic centre, iv. 383. To render iron and steel magnetic, iv. '85. The most mag- netism may be communicated to steel, iv. 388. Is commu- nicated by lightning and percussion, iv. 389. On the magnet- ism of the earth, iv. 392. Effects from it, iv. 393. The great uses of it, iv. 394. An hypothesis concerning it, iv. 403. Is probably supplied by the sun, iv. 404. — See Electricity. Magnets, the manner of arming them, iv. 391. Man received an untaught language from nature, i- 199. Does not require the brightest evidence of truth at all times, i. 246. Is at first led by his senses, i. 247. Is a compound be- ing, i. 282. Is an imperfect judge of heat and cold from his sensations, i. 284. Is exposed to errors from various causes, i. 6. His knowledge is power, i. 24. His limited views of Divine Providence, ii. 388. Collects his knowledge from experiments or observations, ii. 412. The variety of experi- ments he necessarily makes, ii. 412. His pride, ii. 413. Re- ligion is adapted to his nature, ii. 423. His want of a Re- deemer, ii. 424. Is indebted to God for the discoveries which he makes, ii. 427. His unity ; he continues the same being, although he should lose different members, iii. 74. His organs only channels of conveyance, iii. 74. How much he is indebted to the mechanical powers, iii. 83. Men do not naturally swim, iii. 117. On walking in different directions, iii. 161. On jumping, skaiting, and running, jii. 163. Man considered as an artificial machine, iii. 297. The vain theo- ries for ascertaining the strength of man, iii. 298. The strength of his frame, iii. 30 1 . Is able to carry great weights, iii. 302. What depends on the posture of man, iii. 303. Me- thods by which he draws weights, and. instances of great strength, iii. 304. His dependance ; the advantages he de- rives from mechanics, iii. 317. A source of his errors, iii. 3^8. Of all animals, man is least able to swim, iii. 372. His limited powers and comparative ignorance, iii. 455. His rea- 582 GENERAL INDEX. son is to correct the fallacies of the senses, iv. 43. Is apt to be forgetful of the blessings he enjoys, iv. 63. The bene- fits he derives from the animals, iv. 170. General remarks on man, iv. 487. The means of his understanding the works of creation., iv. 488. — See" God, Providence, Mind. Mariner* a comfiass, a description of it, iv. 394. When discovered, and by whom, iv. 395. Its variations, iv. 397. When this variation was discovered, iv. 397. — See Magnetism. Mars, his size, distance, diameter, revolutions, iv. 23, 76. His year and motion round the sun, iv. 140. His atmosphere and poles, iv. 204. Materialism, danger from the system of, ii. 249. Considered as a system, iii. 63. Its danger and misery, iii. 63. Particularly examined and confuted, iii. 64 — 74. Particularly by the unity of the percipient being, iii. 74. Perceptivity cannot be annexed to a system of matter, iii. 76. Matter can never form an intelligent being, ii. 246. The use made of it by the ancient atheists, ii. 246 ; and some mo- dern philosophers, ii. 246. Is the object of the five senses, iii. 10. An inquiry concerning matter, iii. 11. The com- mon properties ascribed to it, iii. 1 1. The properties allow- ed to matter are, impenetrability, extension, divisibility, and hardness, iii. 1 1 — 14. Matter is not infinitely divisible, iii. 16. Illustrated, iii. 17. The great divisibility of matter, iii. 19. Sir Isaac Newton's opinion of matter, iii. 19. Mat- ter hath a capacity for motion, iii. 21. Concerning the in- ertia of matter, how understood, iii. 23. The absurdities re- sulting from this, iii. 23. Matter can only move as it is moved, iii. 24. Is gravity an essential property of matter? iii. 25. Matter and mind totally distinct, iii. 50. In what this difference consists, iii. 5 1. The opinions of the an- cients concerning matter; its visibility is supposed to arise from its form, iii. 53. The first matter homogeneous, iii. 54. This original matter was represented by Saturn and Ops, iii. 55. The primary forms of matter are extension, figure, organization, iii. 56. Matter is impressed with the marks of mind, iii. 5 8. Some have represented matter as without impenetrability and inertia, iii. 66. — See Mind. Mcyoro, Dr. his discoveries of airs in the last century, i. 433. lilcasures, philosophical, remarks on them, i. 301. Mechanics, their antiquity, iii. 82. The wonderful machines of the ancients, iii. 82. The object of mechanics is motion, iii. 87. Mechanical powers, on, iii. 224. Their use to man, iii. 225. Pos- tulata for the consideration of mechanical powers, iii. 226. The allusion of the Platonists and Pythagoreans to these, iii. 272. The advantages gained by them, iii. 282. Of power and time, iii. 282. Of the difference between practice and theory, iii. 288. Caused by the weight and friction, iii. 289. Their Use to manufactures and merchants, iii. 317. GENERAL INDEX. 583 Mercury congeals at 40° below 0, ii. 54. Mercury congealed by a frigorific mixture, ii. 55. A long column of it is sup- ported in a glass tube, iii. 37* Mercury, his size, distance, annual revolution, iv. 16. Meridian, iii. 465. The degrees on it. iii. 487. Metals, a table of the different expansions of different metals, i. 273. The analogy between them and transparent media, ii. 382. Meteorological diaries, importance of them, iv. 410. Meteors, their appearance at great heights in the atmosphere ; difficulty of accounting for them, i. 86. Microscopes, their several kinds, ii. 478. The advantages to be derived from them, ii. 479. Of their optical effects, ii. 480. Of the single microscope, ii. 485. Its properties, ii. 486. Of the compound microscope, ii. 487. Its properties, ii. 488. Of the solar miscroscope, ii. 489. General observa- tions on them, ii. 490. Their imperfections, ii. 492. Milky way, iii. 501. The computation of the number of suns in it, by Dr. Herschel, iv. 192. Mind and matter totally distinct, iii. 50. Mind always has some end in view, iii. 51. The powers and qualities of mind, iii. 52. Mind, its strong desire after truth, iii. 58. Forms exist in mind before they are exhibited in matter, iii. 60. Every- thing excellent is an emanation from mind, iii. 60. The mindofmanisnotacompound,iii. 75. Its immortality, iii. 79. Mirrors, plane, ii. 203. Their nature and properties, ii. 204. Observations on them, ii. 225. How to judge of their good- ness, ii. 227. Of convex mirrors, ii. 206. Of concave mir- rors, ii. 207. Deceptions and experiments by these, ii. 210. Increase heat and kindle fire, ii. 212. Of pictures seen in them ; to find the focal length of a spherical mirror, ii. 216. General properties of mirrors, ii. 217. Moisture is invisible water, iv. 437, Totally absent ; extreme, iv. 437. Monsoons, or periodical winds, iv. 458. An account of them, iv. 458. How caused, iv. 459. Montgolfiers, M. discovered the air-balloon, iii. 401. Their ex- periments, iii. 402. Moon, phenomena of ; her periodical motion, iii. 474. Her vari- ous uses, iv. 20. Her diameter, distance, revolution, ap- pearances, iv. 21. Her orbit, iv. 84. Her nodes ; her con- junction with the sun, iv. 85. The periodic month, and synodical, iv. 87. Her different phases, iv. 89. Eclipses of, when caused, iv. 93. The nodes of the moon, iv. 94. Is eclipsed by the shadow of the atmosphere of the earth* iv. 95. Sometimes the moon totally disappears, iv. 95. Her appearance in an eclipse, iv. 96. The beginning or end discovers the longitude, iv. 97. On what the quantity and the duration of the eclipse depends, iv. 98. She moves 2077 miles in an hour, iv. 103. Is about 240,000 miles from the earth, iv. 103 v The moon only intersects the plane of 584 GENERAL INDEX. . the ecliptic in two points, iv. 105. General phenomena the moon, iv. 156. Her diffe rent phases explained, iv. 157. Has always the same face to the earth, iv. 158. Is always half enlightened by the sun, iv. 159. Her clays and nights equal 14-| of our days, iv. 159. May be in conjunction or opposition without an eclipse; the cause of this explained, iv. 160. Her appearance when viewed through a telescope; consists of mountains and cavities, iv. 203. Volcanoes have been seen on her surface, iv. 203. Her atmosphere, iv. 204. She gravitates towards the earth, iv. 230. Is acted on with the greatest force when nearest the earth, iv. 233. Her orbit equal to 60 times the earth's semidiameter, iv, 236. Her irregularities, iv. 248. Whence caused, iv. 24£ — 253. Motion, improperly considered as the cause of fire, i. 352. On the communication of motion by collision, iii. 200. Is supposed to cause elasticity, iii. 203. The laws of the communication of motion, iii. 205. In elastic and non-elastic bodies, iii. 207. The inexhaustible source of motion and impuse, iii. 211. The cause of motion, iii. 218. Impulse is the material cause of motion, iv. 218. Motion, apparent, observation on it, ii. 316. In what degree it must be to become visible, ii. 317. Is change of place, iii. 87. Involves the idea of space and time, iii. 88. Velocity is the quantity of motion, iii. 89. The sources of motion, iii. 91. Of simple motion, iii. 93. Circumstances observed in this, iii. 93. Of the quantity of motion, iii. 97. To compute the momentum, iii. 98. The laws of motion, iii. 101. Ob- jection to the first law of motion, iii. 101. Motion is not a property of matter, iii. 24. Of compound motion, iii. 104. Its general laws, iii. 104. Instances of compound motion, iii. 113. Of accelerated motion, iii. 118. An inquiry whe- ther motion be a cause or an effect, iii. 184. On the perma- nent motions in nature, iii. 216. Fire and light are the in- struments of motion in nature, iii. 222. The permanency of motions, iii. 220. There is no motion independent of the action of any medium, iii. 223. Motion, whence produced ; varieties of motion, iv. 371. Munro, Dr. his objections to the nervous and electric fluid being the same answered iv. 363. Musical sounds, effects of, i. 365. Organs in man- to produce these, i. 238. N Xadn; iii. 463. Mature, the views of it infinite, i. 27. Is inexhaustible on every side, i. 27. Is a mere name, when considered as independ- ent of God, ii. 244. Is the benevolence of the Almighty pro- viding for all the inhabitants of the earth, ii. 412. Appears more excellent the more it is examined, iii. 9.- The opera- GENERAL INDEX. 585 tions in nature are carried on mechanically, iii. 22. There is nothing insulated in nature, iii. 214. A general circula- tion through all nature, iii. 220. The immensity of the works of npture, iv. 38. The perfection of them, iv. 137. All the works of nature are connected, iv. 257. Remarks on. the chemistry of nature, iv. 407. — See God, Providence, Man, Sun, Air, Water, Nebula of fixed stars, iv. 196. Planetary nebulse, iv. 198. Needle, magnetic, its diurnal variation, iv. 399. Is disturbed by the aurora borealis, iv. 399. Its dip ; by whom discovered, iv. 400. 'The variations in the dip, iv. 401. The needle is affected by the aurora borealis, iv. 402. — -See Magnetism, Mariner's compass, Newton, Sir Isaac, his first rule of philosophizing, i. 91. His discoveries of the aerial pulses ; the manner in which they are propagated, i. 211. His works ; his rules of philoso- phizing, i. 31. " His grand discoveries concerning light and colours, ii. 326. His optics, ii. 332. His experimentum crucis, ii. 335. An eulogium on him, ii. 347. He sup- posed that bodies of different densities reflected different rays of light, ii. 3->5. His conjectures on the fits of easy reflec- tion and transmission of a ray of light, ii. 362. He disco- vered that inflammable bodies, possessed the refractive power, more than bodies not inflammable, ii. 383. Constructed a reflecting telescope, ii. 470. His great modesty, ii. 471. His opinion concerning the original atoms, iii. 19. His discoveries of gravitation, iii. 45. Not very consistent in hydrostatics, iii. 341. His theories on the subject, iii. 451. An account of his principles, iii. 452. His observation on the curvilineal motion of the moon, iv. 232. His mathe- matical astronomy, iv. 225. — See Light, Colours, Gravita- tion. Nodes of the moon, iv. 105. Go backwards nineteen degrees and an half in every year, iv. 108. Nonius, scale to estimate the divisions on it, iv. 417. O Observer of Nature, character of i. 175 — 177. Opacity arises from the discontinuity of the particles of bodies, and the different density of the intervening medium, ii. 366. How destroyed, ii. 366. Different significations, ii. 397. Of opacity, considered as a positive quality in bodies, ii. 402. It does not depend on the solid matter in bodies, ii. 404. — See Light. Optics, the excellency of the knowledge of them, ii. 133. Oxygenation, or acidifying, is produced by the combination of any substance with vital air, i. 462. VOL. IV. 4 G 486 GENERAL INDEX. Parallax, annual diurnal, horizontal, iv. 113 — 119. The accu- racy necessary in finding it, iv. 118. Pascal^ M. his character ; he first applied the barometer to measure mountains, i. 67. Pendulum, its vibration explained, isochronous, i. 212. The analogy between a pendulum and a musical string, i. 212. Account of pendulums, iii. 175. Their oscillations, iii. 176. Their isochronism, iii. 179. Pendulums are simple and compound, iii. 181. Of the centre of oscillation in com- pound pendulums, iii. 181. Of the time of their oscillation, iii. 186. Are affected by heat and cold, iii. 187 ; by their place on the globe, iii. 187. Huygens adapted them to clocks, iii. 1 91. Wooden pendulums, their properties, iii. 193. The gridiron pendulum, its construction and advantages, iii. 193. Penumbra of an eclipse, iv. 9->. Percussion, centre of, iii. 187. Perspiration, great quantities of food carried off by it, i. 181. Philosopher, the universality of knowledge which ought to form his character, i, 2. A picture of a true philosopher, i. 2. His character, as draw r n by Lord Bacon, i. 19. Studies the intention of nature, i. 20. He proceeds by induction, i. 22 ; and thus forms general axioms, i. 22. He makes use of every help, particularly of analogy, i. 27. He proceeds with great caution, i. 30. The error of the modern philo- sophers, ii. 410. The weakness of vanity in a philosopher, ii. 425 — See Truth. Philosophy, natural, excellence and advantage of it, i. 37, 94. Ori* gin of the name, i. 37. Its tendency to elevate the mind, i. 126 ; and promote religion, i. 127. The business of it, i. 175. Its tendency to cultivate sublime taste, i. 243. Its grand object, i. 247. Is concerned with final causes, i. 385. Is continually presenting scenes of beauty to the mind of man, i. 281. It advances the cause of religion, ii. 9. The method of reasoning in it, i. 1. Leads us to the know- ledge of God, i. 20. Its connexion with religion, ii. 388. The discoveries of philosophy gradual, iv. 258. — See Air, Astronomy, Colours, Elastic Fluids, Electricity, Fire, Gravity, Light, Magnetism, Matter, Mechanics, Meteorology, Micros- copes, Phosphorus, Telescopes, Water. Philosophy, inductive, an account of, i. 25. Philosophy, false, its errors and dangers, i. 19. Phlogiston, or the principle of inflammability, i. 369. Denied by the French philosophers, i. 369. Is a substance sui generis ; the matter of light and heat, i. 370. Proved by the de- composition of water, and the luminous appearance then ex- hibited, i. 37 1 — 379. It is the solar substance detained in the GENERAL INDEX. 587 phlogistic composition,!. 378. Is restored by animal and vegetable substances, i. 380; particularly by the influence of light for the phlogistication of vegetable bodies, i. 382 ; and by the mass of colour which they obtain, i. 382. Is im- parted from vegetables to animals, i. 384. Is maintained by Mr. YYeiglib, in opposition to the French chemists, i. 507 — 522. Its existence proved, i. 508 ; particularly by the re- production of the metallic calces, i. 509. Other considera- tions in support of it, i. 517. Its universality and energy, i. 519. The analogy between phlogiston, or fixed fire, and fixed air, i. 519 — 522. — See Fire, Heat. Phosphorus, the several kinds of, ii. 385. The Bolognian phos- phorus was discovered by Vincenzo Cascariolo ; its proper- ties, ii. 390. Artificial phosphorus, how formed, ii. 392. Phosphori generally diffused, ii. 392. Of Canton's phos- phorus, how prepared, ii. 393. Imbibes its property from light, ii. 394. Mr. Wilson's phosphorus exhibited vivid co- lours, ii. 394 v Phosphorus is an incipient ignition in certain bodies, ii. 395. Different kinds of phosphori, ii. 396. They do not emit the identical light which they have received, ii. 397. The agreement and disagreement of phosphoretic and phlogistic bodies, ii. 398. The change it undergoes when burnt in vital air, i. 460. — See Fire. Physicians, the error into which some of them have fallen, ii. 245. Plane, inclined, descent of bodies upon it, iii. 139. Has a rela- tive and absolute gravity, iii. 141 — 146. Its use, iii. 257. Planets, on, iii. 480. They are spherical opake bodies, iv. 11. Inferior and superior planets, iv. 15, 23. A table of their diameters and distances, iv. 31. Revolutions round the sun, iv. 32 ; and their own axes, iv. 33. Their proportional mag- nitude, iv. 34. Their heliocentric and geocentric latitude, iv. 65. Their conjunction and opposition, iv. 66. Appear- ances of the inferior planets, iv. 73 ; and of the superior, iv. 76. Their direct and retrograde motion, and stationary si- tuation, iv. 77. Their satellites, iv. 79. Inferior planets, their superior and inferior conjunctions, iv. 141. Their ap- parent irregularities explained, iv. 143. Of the superior planets, as seen from the earth, iv. 144; are most probably inhabited worlds, iv. 214. They gravitate towards the sun, iv. 238. The irregularity produced among them by gravi- tation, iv. 243. Planetarium, its antiquity and use, iv. 135. Proves the truth of the Copernican system, iv. 145. How to rectify it for the true places of the planets, iv. 146. To use it as a tellurian, iv. 147. To explain the changes of the seasons, iv. 148. The parallel, direct, and right spheres, iv. 15 1. Plants are sensibly affected by light, ii. 417. Plants exposed to light emit vital air, ii. 417. Are aflected by vital air, ii. 419. — See Air, Light, Vegetables. Plato's idea of the intertexture of air and fire in the human frame, i. 245. His observation on colours, ii. 347. On the present state of human knowledge, iii. 55. 588 GENERAL INDEX. Plenum, a, necessary for motion by impulse, iv. 222. Bodies are able to move in a plenum, iv. 222. Plurality of worlds, reasons for them, iv. 211. Pneumatics* — See Air. Points, cardinal, and points of the compass, iii. 462. Pole star, iii. 460. Its position, iii. 460. How to be found, iii. 460. It describes a small circle round the pole, iii. 473. Poles, or arctic and antarctic circles, iii. 473. Poles of the magnet, iv. 379. Their action on each other, iv. 380. Their action on steel filings, iv. 381. The poles should always be left connected, iv. 39 1. Prayer, a, for wisdom and virtue, i. 33. Prejudice, its mischiefs and effects, i. 62. Priestley, Dr. his discoveries of airs, i. 433. His system of ma- terialism fully examined and confuted, iii. 63 — 74. Projectiles, motion of, iii. 165. Galileo's discoveries in them, iii. 166. Are opposed by the air's elasticity, iii. 174. The great quantity of motion which they lose, iii. 171. Providence, discoverable in the smallest as well as greatest events; no such thing as chance, ii. 386. Providence, reflections on the wisdom and goodness of, in the suction of animals and the swallowing of food, i. 60. In the pressure of the air, i. 72. In the universal good de- signed in all his works, i. 94. In the admirable provision made for breathing, i. 195. In the blessing of speech, i. 199. In the singing of birds, i. 206. In the admirable construction of the human ear, i. 240. In the creation of fhe universe, and particularly of the air, for the most uni- versal good, i. 243. In the provision made for the warmth of different animals, i. 262. In guarding against the too sudden changes of heat or cold, i. 313. In the continued agency of the Divine Mind, i. 318. In the insensible opera- tions of the rise of vapours, i. 548. In the provisions made for supplying heat and light, i. 38 4. In rendering evtry part of matter active and useful, i. 384. In the great and benevolent ends which are obtained in nature by simple means, i. 388. In the agency and operations of fire, i. 429. In the abundant productions of vital air, and in the preser- vation of the equilibrium of the atmosphere, i. 457. In the uses resulting from the vegetable kingdom, i. 45 7. In the provision made against cold, i. 467. In the ocean, and its inhabitants, ii. 90. In the various benefits bestowed by means of water, ii. 90. In the construction, form, and uses of the eye ; and in the blessings of sight, ii. 255 — 258, 321, 322. In restoring the purity of the air by means of vege- tables, ii. 417. In the simplicity and energy of his works, iii. 50. In the divine agency exhibited in nature, iii. 58. In the powers and excellency of the soul of man, iii. 79. In the regular order and establishment of the Divine Mechanic, iii. S3; both in the natural and moral world, iii. 84. In the starry heavens, iii. 5 10. In the gradual progress of arts and GENERAL INDEX. 589 sciences, iv. 3. In the immensity of his works, and their continual preservation, iv. 38. In the various changes of the seasons, iv. 62. In the clear discoveries of divine intel- ligence and design, and in the supplies of the numerous wants of man, iv. 166. In the wonderful structure of the human frame, iv. 168. In the vegetable kingdom, iv. 169. In the animal kingdom, iv, 169. In the universal distribu- tion and management of fire, iv. 353. In the degree of heat which every country enjoys in the course of the year, iv. 472. In the arrangement of mountains and seas, iv. 472. In the perfection of the word and works of God, iv. 488. — See God, Man, Natural Philosophy. Pulley, its properties, iii. 252. Are fixed and moveable, iii. 252. Of the moveable pulley r iii. 253. Of Smeaton's pullies, iii. 276. Of their immense force, Iii. 277. Pulses of the air, propagated by sound, i. 211. Are alternately condensed and rarefied, i. 213. All pulses move at an equal rate, 1142 feet in a second, i. 214. Pump, common, invented by Ctesebes, i. 119. Raises water thirty- four feet, i. 120. forcing, acts by condensed air, i. 122. Pumps, of. Of the chain pump, iii. 433. Its construction and use, iii. 433. Of the common pump, iii. 434. Its construc- tion and action, iii. 434. The piston must be less than thirty- three feet from the water, iii. 435. uf the forcing pump its construction and use, iii. 437. Of dc hi Hire's pump) iii. 439. Of Taylor's pump, in. 440. Of the Hessian pump, iii. , 441. Of Vera's pump, iii. 442. — See dir< Water, Pupil of the eye, an account of, ii. 289. lis motions; is naturally dilated, ii. 289. Is contracted by light, ii. 290. — bee Eye, Light. Pyrometer, an instrument for measuring theflegree of heat, i. 265. Quadrant, astronomical, its use and description, iii. 467. To find the altitude of any celestial body, iii. 471. Quadrant of altitude, iii. 490. R Rain is supposed to proceed from the decomposition of the air resulting from the aqueous vapours being converted into an aeriform fluid, ii. 78. Heavy showers caused by clouds of different electricities being driven together, iv. 320. How much we are ignorant of it; on what it depends, iv. 439. Rain is not the precipitation of water simply evaporated in the air, iv. 440. Is indicated by a hollow noise, iv. 457. Rains in the West Indies, iv. 463. Most rain falls in woody 520 GENERAL INDEX. and mountainous countries, iv. 46S. Rain and snow gene- rally give vitreous electricity, iv. 476. Generally follows sudden changes of the weather, iv. 481 — See Vafiour. Rainbow, the ignorance of the arrcients concerning it ; explained by Sir Isaac Newton, ii. 348. The order of colours; the varieties of them, ii. £50. Illustrated; the second bow ex- plained, ii. 351. On what the size of the bow depends^ ii. 352. — See Light, Colours, Refraction, Rain-gage, its construction and use, iv. 430. Explanation of, by the Editor, iv. 495. Read, Mr. his experiments on electricity, iv. 305, 306. Reaumur, M. his discoveries on eggs, i. 180. Rejiexion of light, ii. 198. All reflexion reciprocal, ii. 200. Laws of reflexion, ii. 202. No colours are displayed by reflected light, ii. 375 — See Light. Refraction of light, laws of, ii. 143. Refraction at a convex sur- face, ii. 15 6. At a concave surface, ii. 160. — See Light. Religion, it requires a sobriety of mind, i. 320. Religion and phi- losophy agree together, iv. 370. The arts and sciences flou- rish most where religion is cultivated, iv. 486. — See God, Providence, Man. Resinous electricity. — Sec Electricity. Respiration receives vital air, and mixes it with the blood in the lungs, i. 465. Respiration is similar to combustion; respi- ration explained, i. 188, 196. Concerned in smelling, laugh- ing, speaking, weeping, i. 201. Rods, conducting, iv. 326. Are the means of restoring the equili- brium, iv. 326. Observations against pointed conductors, iv. 328, They only draw off the electric matter when immersed in its atmosphere, iv. 329. Cannot attract the lightning out of its direction, iv. 329. Objections against them, iv. 329. — See Electricity. Salts produce great degrees of cold, ii. 5 3. How they form sa- line liquids, ii. 65. Saturn, his size, distance, revolution, iv. 26. His ring and moons, iv. 27, 206. His year, and motion round the sun, iv. 138. His belts, iv. 206. Screw, male and female, iii. 261. Of the endless screw, iii. 263. Of the micrometer screw, iii. 265. Sea. Saltness of the sea, inquiries into the cause of it, ii. 32. It was originally salt, ii. 33. Dr. Halley's weak opinion, ii. 33. The water is most salt where the sun is vertical ; an easy method to ascertain the saltness, ii. 34. The advantages derived from the sea to temperate the air, iv. 473. Seasons of the year accounted for, iv. 55. Summer is longer than winter, iv. 61. GENERAL INDEX. ' 59 1 Senses lead to all physical knowledge, i. 247. The imperfec- tions attending their information, i. 248. This to be judg- ed of from experiment, i. 249. Sight. Of imperiled sight, ii. 293. Of old, or long-sighted eyes, ii. 294. Of short-sighted eyes, ii. 296. How as- sisted, ii. 305. — See Eye. Smeaton, Mr. an account of his pyrometer, i. 271. Smoke. — See Chimnies. Smith, Dr. his observation on the division of labour illustrated, iii. 216. Snoiv keeps the ground warm in winter, i. 260, 262. The form of its flakes, ii. 46. Solution, an effect of fire, i. 397. Description of its operation, i. 398. Illustrated in the solution of salts, i. 399. Sound, benefits resulting from it, i. 205. Cause of, i. 206, 233. Of musical sounds, i. 235. Of sympathetic sounds, i. 237. Sounds of metals, how improved, i. 207. Classes of sonorous bodies, i. 207. Sound is besfeonducted in a dense medium, i. 208. May be conveyed through wood or water, i. 208. It does not proceed from a flux of air, but from a vibratory motion of the particles of air in their proper place, i. 210. Sound vibrates according to the motion of a cycloi- dal pendulum, i. 214. Differences among sounds, i. 214. The intensity is inversely as the squares of the distance, i. 2 1 5. The velocity of sound continues always the same, i. 215. S^und diminishes for want of perfect elasticity in the air, i. 215. Is more perfect in some winds that in others, i. 216. Its effects on solid bodies, iv. 349. — See Air. Sound judgment, the means to form it, i. 62. Space, the idea of it from extension, iii. 15. Space, absolute and relative, iii. 88. The analogy between time and space, iii. 88. , Sfieaking trumpet explained, i. 217. Spectacles, their use, ii. 297. Directions in the choice of them^ ii, 297. Directions to discover if they be wanted, ti. 301. Speech, the blessing of it ; the various parts which fofm it, i. 199. Spheres, right, parallel, or oblique, iii. 493. Spirit of man, the opinions which the ancients entertained of it, iii. 62. The gospel does not treat of its natural immorta- lity, iii. 64. Dr. Hartley represented the soul as uniform- ly passive, iii. 64. The excellencies of the soul, iii. 79. Springs of water, different opinions concerning them, ii. 23. Are not supplied by rains and dews, ii. 24. Some run the same in a wet or dry season, ii. 27. Springs are princi- pally supplied from the subterraneous stores of water, ii. 28. Stars, the numbers of, discovered by Herschel,ii. 427. Their ap- parent diurnal motion, iii. 459. Of the fixed stars, their twinkling, iii. 497. Are arranged in constellations, iii. 497. Are divided into different classes, from their size, iii. 498. The catalogues of them, iii. 500, by Hipparchus, by Bayer, by Flamstead, by de la Caille, bv Wollaston, *i. 500. De- 592 GENERAL INDEX. picted on a new 18-inch celestial ^lobe by W. Jones, iii 501. The immense number of the stars, iii. 502. The numbers discovered by Dr. Herschtl, iii. 503. How to obtain a knowledge of the constellations, in. 503. The vast distance of the fixed stars from us, of the first mag- nitude, iv. 36, of the second, iv. 36. Their parallax iv. 113. Their apparent motions, from the aberration of light, iv. 119. Their motion, iv. 121. Different stars appear at different times of the year, iv. 139. Their distance great beyond computation, iv. 187. Have a general motion, iv. 188. The variety in these ; some appearing, others vanish- ing, iv. 188. New stars; catalogues formed, iv. 189. Remark- able new stars, iv. 190. Stars, their proper motion, iv. 190. Stars of different lustre supposed to be at different distances from us, iv. 194. Nebulae of stars, iv. 196. A perforated nebula, iv. 198. All the universes of stars or suns connected together, iv. 199. Are probably suns; their use, iv. 21 1. There are more stars in the northern than in the southern hemisphere, iv. 485. Steam of boiling water occupies 1800 times more space than water, i. 331. Its nature and properties, ii, 80. Steel-yard, an account of the, iii. 247. Suction, improperly applied to account for some phenomena o air, i. 5 1. Sun. The emanation of matter from the sun one of the prime movers of the machine of the world, i. 384. Its influence under different forms, i. 384. The solar fluid is absorbed in vegetables, and is the cause of colour, flavour, &c. i. 45 3. The solar substance in one place is fire, in another light, in a third, electricity, ii. 401. The sun animates and quickens the globe of the earth, as the seminal bed of his rays, ii. 414. The source of natural life, ii. 415. His influence on the earth, ii. 415. particularly in the vegetable kingdom, ii. 415. His rising and setting, iii. 465. His annual motion, iii. 476. He rises and sets in different parts, iii. 476. He moves- about a degree every day, iii. 477. The centre of the system ;• the heart of heaven, iv. 12. His in- fluence, size, distance, motion, iv. 13. His supposed at- mosphere, iv. 14. Is the centre of the system, iv. 49. ijis apparent motion, iv. 50. The motion of the sun ap- pears differently to inhabitants of different planets, iv. 52. He appears to move in the ecliptic, iv. 53. His apparent diameter greater in winter than in summer, iv. 62. Eclipses of the sun, how caused, iv. 98, are visible to only a few in- habitants of the earth, iv. 99. Total eclipse remarkable, iv. 100. Are total, annular, or partial, iv. 101. On what the quantity and duration of the eclipse depends, iv. 102. His parallax, iv. 118. His tropical and sidereal year, iv. 122. A measurer of time, iv. 123. The inequality o:' his apparent motion, iv. 132. Appears to pass through the si^ns of the zodiac in a year, iv. 1 39. Dr. Herschel conjectures that our sun belongs to the milky way, iv. 192. The spots on its : GENERAL INDEX. sOo surface, their variety, iv. 200. Peculiarities of their nucleus and umbra, iv. 200. Sometimes they appear to burst, iv. 20 1 . Their directions different, iv. 201. Conjectures concerning them, iv. 201. His centre of gravity, iv. 242. Is the source of the electric fluid, iv. 339. Is the cause of natural life,'iv. 347. Probably supplies the magnetic fluid, iv. 404. Is a principal source of heat, iv. 467. His rays act as lire, and increase the expansive force of fire, iv. 467. Causes an undulating motion in the atmosphere, iv. 478. Shines more on the northern than on the southern hemisphere, iv. 484. — See Light) Heat-, Colours, Electricity, Magnetism. Sun-dial, universal or equatorial, description of, iv. 180. Swimmiiig, on, Hi. 369. Sympathetic inks, experiments with, ii. 122. Syphon explained, i. 123. Forms Tantalus's cup, i. 124. Ac- counts for intermitting springs, i. 125. Fuller account of; principles on which they act, iii. 446. Distiller's syphon, iii. 447. Of s'Gravesande's syphon, iii. 448. Telescopes, observations on their use, ii. 426. Lord Bacon's re- mark on them, ii. 427. Are supposed to have been disco- vered by Roger Bacon, ii. 428 ; and by Jansen, ii. 430. Were improved by Galileo, ii. 430. Of refracting telescopes, ii. 431. Their properties, ii. 434 — 440. Their apparent field, ii. 437. Of the astronomical telescope, its properties, ii. 440 — 443. Imperfections arising from the dispersion of the rays of light in them, ii. 448. Of the compound object glass, ii. 449. From the refrangibility of the light, ii. 452. How corrected, ii. 453. Of telescopes with several eye-glasses, ii. 457. Of achromatic telescopes, ii. 461, Were invented by Mr. Dollond, ii. 463. The invention has been ascribed to Mr. Hall, ii. 468. Are composed of different kinds of glasses, ii. 466. This discovery was claimed for Euler, ii. 467. Of the reflecting telescope, ii. 468. By whom discovered, ii. 469. Of the Gregorian telescope ; its properties, ii. 472. Of the Newtonian telescope, ii. 476» The most improved constructions of, described by the Editor, ii. 496. Transit telescope, ii. 508. Temperature of the earth, observations on it, iv. 471. On what it depends, iv. 472. Tests, chemical, list of, ii. 115. Thermometers, the principle on which they are constructed, i. 286. Of Fahrenheit, Keaumur, and Celsius, the relation between them, i. 287. Mercurial thermometer, an accurate mea- sure of heat, i. 288. Experiments on it, i. 340. May be reduced by ether, ii. 5 1. Its construction and use, iv. 422. The requisites for a good one, iv. 424. The manner of fill- VOL.IV. 4H 594 GENERAL INDEX. ing it, iv. 427. How to graduate it, iv. 428. To seal it hermetically, iv. 428. The thermometer is a scale of ex- pansion, indicating the transfusion of the igneous fluid, iv, 429. Six's thermometer, iv. 495. — See Fire, heat* Thunder, remarks on, iv. 452. Time, of, iii. 88. The analogy between time and space, Hi. 88. The measure of it, iii. 190. Observations on time, iii. 195. Mr. Locke's opinion of it, iii. 197. Dr. Clarke's mistake concerning successive and unsuccessive duration, iii. 197. Quotation from Tucker, iii. 198. Time, true and apparent, and mean, iv. 128. Equation of time, iv. 129. Whence the difference arises, iv. 130. Reflexion on the lapse of time, iv. 133. Tin, its peculiar quality in rendering other metals more sono- rous, i. 207. Torricellius, his invention of the barometer, i. 65. Transfxarency, the least part of all bodies are transparent, ii. 365. Transparency depends on homogeneity, ii. 367. Transpa- rent bodies reflect rays of one colour, and transmit rays of another, ii. 367. Transparency acquired, ii. 384. The ad- vantages from the transparency of glass, ii. 384. — See Light, Opacity, Tropics y iii. 491. Truth, love of, i. 03. In what manner it should be sought for, i. 63. Its gradual advances, i. 70. Nature of, i. 92. The cause of, injured by a deference to the authority of names, i. 255. Its analogy or correspondence with water, ii. 14. Its nature, ii. 249. Tschirnhauscn, 1V1. eft'tcls produced by his burning glass, ii. 172. Tubes, eudiometer, and measure, i. 437. Vacuum, no perfect vacuum of air can be produced by the air- pump, i. 135. A vacuum in nature disproved, iii. 221. Vapours, not permanently elastic fluids, i. 441. Are destroyed by pressure and cold, i. 442. The accidents occasioned by their sudden expansion, i. 324. Occupy 14,000 times more space as vapours than as water, ii. 73. Vapours, vesicular and concrete, an account of, ii. 83. In the form of spherical balls, ii. 84. Only the 3600th part of an inch in size, ii. 84. The difference between vapours and elastic fluids, iv. 432. Watery vapours are one half less than a like volume of air, iv. 433. Vapours consist of fire and wa- ter united, iv. 435. How they are decomposed, part with their water to hygroscopic substances, iv. 436. The conden- sation of them a source of heat, iv. 468. — See Fire, Air, Water. Vegetables, when acted upon by the solar light, afford abundance of vital air, i. 452 ; but in the shade, the air they yield is impure, GENERAL INDEX. 595 i. 454-. They imbibe mephitic and emit vital air, i. 457. Admirable reflexions on their uses, i. 457. They consume more water than falls in rain, ii. 26. Th,eir influence on the climate and weather, iv. 473. — See Lights Air, Water, Velocity, relative and absolute, iii. 95. Of the velocity of falling bodies, iii. 130. Venus, her size, distance, diurnal and annual revolutions, differ- ent appearances, atmosphere, iv. 17. Her conjunctions with the sun, iv. 67. When she appears stationary, iv. 71. Her phases, iv. 73. Vince, Mr. his observations on friction, iii. 290 — 295. His ob- servations on wheel carriages on a plain ground, iii. 311. Vision is caused by the refraction of the rays of light, ii. 267. Why are not objects seen in an inverted position \ ii. 272. Vision is not produced on the optic nerve, ii. 274. Of the extent and limits of vision, ii. 276. Is limited by various means, ii. 277. Vision is confused by the undulating mo- tion of the air, ii. 278. The angle of the least vision, ii, 280. Of distinct and clear vision, ii. 280. On what it de- pends, ii. 281. At what distance it is perceptible, ii. 284. The appearance of distance affected by light and colours, ii. 310. Mistakes concerning distances, ii. 314. Fallaciesof vision explained, ii. 319. Of vision by images, ii. 320. — See Light eye. Vitreous electricity. — See Electricity* Voice of man, wonders and variety of it, i. 238. W Walker, Mr. of Oxford, his experiments for freezing mercury, ii. 55. Water is converted into vapour whenever the pressure of the at- mosphere is diminished to a certain degree, i. 142. Gra- dually parts with its latent fire whilst it is freezing, i. 311, May be cooled several degrees below the freezing point, i. 311. Receives a less quantity of heat than quicksilver does, i. 316. Water boils with a small degree of heat when the pressure is removed, and vice versa, i. 329. Water is not dissolved by air, i. 333. One-thousand sixrhundred gallons of water raised from an acre of ground in a hot summer's day, i. 348. Water constitutes the ponderable part of all aeriform fluids, i. 418, ii. 59. Its nature and properties, ii. 10. Its various uses, ii. 10. Is not a com- pound of vital and inflammable airs, ii. 11. Of water in a fluid state, ii. 13. Is compressible in a small degree, ii. 13. Enters into the composition of all bodies, ii. 14. Its analogy to truth, ii. 14. The quantity of it suspended in the atmos- phere, ii. 15. It increases the weight of certain bodies ex- posed to it, ii. 16. Has a similar effect on the human frame, ii. 16. Water in mixture or combination with bodies, ii. 20. Is a general cement, ii. 20, Is never obtained pure. 596 GENERAL INDEX. ii. 20. Is of different degrees of softness, ii. 21. Is puri- fied by distillation, ii. 22, The water from rain is not a sufficient supply for springs, ii. 24. The subteraneous stores of water, ii. 2$, These supply springs and vegeta- bles, ii. 30. Peacock's filtration of it by ascent, ii. 29. Sea water deposits its salt by freezing, ii. 35, According to the quantity of heat is the quantity of salt which water can dissolve, ii. 36. Water becomes ice by losing its fire, ii. 3P. Boiled water does not so easily freeze as unboiled, ii. 39. Water may be cooled below the point of congelation without freezing, ii. 40. It increases in bulk just as it freezes, ii. 41. Ice is changed into water by means of fire, ii. 59. Is the ponderable part of all aeriform fluids, ii. 59. Its simple particles are of a certain -form, ii. 65. By means of acids the particles of water are brought nearer together, without losing the fire of liquifaction, ii. 66. Is probably the principal constituent in oils and salts, ii. 67. Is the universal menstruum, ii. 67. Water combines with all other substances, ii. 68. \\ ater expanded in vsipour is 800 times rarer than air, ii. 70 ; and 14,000 times rarer than itself, ii. 72. Is a principal ingredient in vegetable and animal substances, ii. 86. The varieties cf the neather depend on the changes of water, iv. 410. Can receive a greater degree of heat before it boils, than when ii boils, iv. 425. Is not held in solution by air, iv. 433. In what man- ner it is received by different hygroscopic substances, iv. 436. — See Fire, Air, Vapour, Evaporation, Waters, mineral, their nature and properties, ii. 86. Their dif- ferent qualities, ii. 88. Are artificially made, ii. 89. Weather, knowledge of, very interesting, iv. 405. But at pre- sent is uncertain, iv. 407. The phenomena which are to be observed, iv. 477. Depend on the circulation of matter, iv. 410. Inquiries concerning it ; instruments to be used, iv. 411. A barometer, iv. 411. How to attain a more perfect knowledge of the weather, iv. 478. Signs of the weather from the barometer, iv. 479. From the thermome- ter and hygrometer, iv. 483. From the appearance and different currents of clouds, iv. 483. Wedge, its use, iii. 256. A simple instrument to illustrate its theory, iii. 261. Whalebone, slips of, best substance for a hygrometer, iv. 491. Wheels, of their work, iii. 279. How to compute their forces, iii. 280. Wheel and axis, its properties, iii. 248. Acts as a perpetual lever, iii. 250. Crane-wheel, capstan, iii. 251. Watch spring, iii. 252. Of the fly-wheels, iii. 284. Wheel-carriages, on, iii. 306. Their utility, iii. 308. On the centre of gravity in wheel carriages, iii. 310. Observations on them on plain ground, iii. 311. On hard ground, with GENERAL INDEX. 597 obstacles, iii. 311. On sand, iii. 314. The advantages of springs, iii. 314. The reason of this, iii. 315. Whirling table, Ferguson's description of, iii. 319. Description of an improved one, iii. 336. Wieglib, Mr. a German chemist, an abstract of his Dissertation on Phlogiston, i. 507. Supported by the experiments of Mr. Green, i. 517. His analysis of mineral waters, ii. 97. Wilson, Mr. his experiments on phosphoric bodies in a dark chamber, ii. 392. Wind-gage, by Dr. Lind, iv. 495. Winds, cause, i. 99. Bacon's suggestion for a history of them, iv. 455. His queries concerning them, iv. 455. Different causes which affect them, iv. 455. Are influenced by the return of air to a state of vapour, iv. 457. On the origin of winds, iv. 457. Their irregularities, iv. 457. Are affected by the diurnal rotation of the earth, iv. 459. Various tem- pests produced by winds, iv. 463. Are affected by the soil over which thev blow, iv. 464. Remarkable unhealthy winds, iv. 464. Their indication of a change of weather, iv.481. — See Air, Wood is pervious to air, i. 180. Effects of this, i. 181. Woods, their utility in a country respecting rain, iv. 469. World, the great powers of it, heat and gravitation mutually counterbalance each other, i. 386. The influence of these, i. 387. The northern hemisphere of the world superior to the southern, iv. 484. Zenith, iii. 463. Zodiac , iii. 4-91. SUBSCRIBERS' NAMES, DAVID ALLEN, attorney, Winchester, Virg. Jonathan Aikin, student, D. College, N. H. Samuel Ayer, do. do. do. Dr. Joel Abbot, Washington, Georgia. Rev. Timothy Alden, Portsmouth, N. H. Alexander Addison, Esq. B Rev. Samuel Brown, V. D. M. Rockbridge. Hon. John Brown, (Chancelor). William Bernard, Port Royal, Virginia. Rev. James Blythe, Lexington, Kentucky. Thomas Billings, Philadelphia. Laurence Bassaile, Caroline county, Virginia. John C. Beeler. 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Neal, Chester, D. S. C. Francis Nichols, Philadelphia. O Samuel Olden, N. Jersey. Samuel Osgood, student, D. C. N. H. John Ormrod. (3) Walter Oliver, Carlisle. Joseph Olden, student, Princeton. P John Pratt, Cambden, Virginia. Gen. Robert Porterfield, Augusta. Comegys Paul> Trenton. Jacob Peck, Staunton. Thomas Park, A. B. Pee-Dee. Phineas Parkhurst, student, D. C. N. H. Albion K. Paris, do. do. James EL Parmeti, do. do. William Partridge, do. do. R Matthias Roush, No. 1 S, N. Front-street. Rev. John H. Rice, Prince Edward. John Reily, principal of the academy at Frankford. Alpheus Roberts, D. C. N. H. John Rosborough, Esq. C. D. William Reynolds, Esq. Bedford county, Penn. James Robb, Philadelphia. S David Steel, miller, Augusta. Godfrey Souders, C. P. Printer, Philadelphia. Valentine Sevier, Esq. Tennessee. Dennison Smith, student, D. C. N. H. Benjamin Sawyer, do. . do. Ruggles Sylvester, do. do. Amos Spaldins, do. do. William & Archibald Simpson, Esqrs. Wilkes county, Georgia. Rev. I. W. Stephenson, South 'Carolina. Rev. David Snodgrass, Greenville, Mississippi Ter- ritory. (2) William Snodgrass, Esq. do. John Stealey, Esq. Dr. Samuel M. Shute, Bridgetown, West N. Jersey. George Sullivan, Esq. Exeter, N. H. Samuel H. Smith, Clinton, near Zaneville, Ohioc Micajah Speakman, yeoman, do. T David Thomson, student, D. C. N. H. Silvanus Thayer, do. do. Arad Thompson, do. do^ David Terril, Esq. Wilkes county, Georgia, subscribers' names. Thomas & Andrews, Boston. (3) Abner Tatum, Esq. Lincoln county, Georgia* John Tappan, Boston. Thomas & Tappan, Portsmouth, N. H. (8) U Timothy Upham, Portsmouth, N. H. V Thomas Vickray, student, D. C. N. H. Dr. John Vaughn, Wilmington. W Rev. Abner Waugh, rector of St. Mary's, Bow- ling-Green, Caroline county, Virginia. Henry W. Weston, Philadelphia. Lawrence A. Washington, Winchester, Virginia. Lewis Wolf, attorney at law, do. John M. Whiton, student, D. C. N. H. George Wheeler, do. do. James R. Wheelock, do. do. , Samuel C. Webster, do. do. John Walker, Chester district, S. C. Henry Wood, Esq. Bedford, Pennsylvania. George Wood, Esq. do. do. Josiah Weigly, Esq. Greensburgh, Pennsylvania Dr. John R. Witherspoon, Charleston. Augustus Werninger, Morgantown. John Waggoner, do. . C. W. Wever, Prospect Hill, Virginia. David Worth, student, Princeton. Y William Young, Rockland, Delaware, John Young, Esq. Z Col. William Zimmerman, Pee-Dee. etiological an& JLtterarp 'Book'-Store* FROM W. W. WOODWARDS PRESSES. JUST PUBLISHED, VOLUME ONE, TWO, AND THREE, OF SCOTT'S Elegant and much Admired FAMILY BIBLE. This completes the old Testament....Vol. 4, containing the New Testament, is now in the press, May 20, 1807. This work is greatly esteemed by the friends of piety of various denominations ; about 1300 setts are already sold, and the demand increasing.. ..To subscribers before completed, jg 6 per vol. sheep binding ; g 7 calf, and § 5 25 in boards. The New Testament may be subscribed for separate, at g> 7, to be raised to $5 8 when finished«..one for .every nine subscribed or bought. VV. W. WOODWARD HAS ALSO JUST PUBLISHED, CONTEMPLATIONS ON THE SACRED HISTORY, Altered from the Works of the Right Rev. Father in God, JOSEPH HALL, D.D. Sometime Lord Bishop of Norwich By the Rev. John Henry Glasse, M. A. (late student of Christ Church, Oxford,) Rector of Han well, Middlesex, and Chaplain to the Earl of Randor,...From the third edition ; four volumes in two, neatly bound and lettered, Price $2 00. PROPOSALS ARE ISSUED FOR PRINTING DR. GILL'S COMMENTARY, On the whole of the Old and New Testaments, in ten 4to. vols. To subscribers, before the first volume is printed, the price pep volume will be the same as Scott's Family Bible ; one for every nine subscribed or bought. ...This publication, as well as Scott's Family Bible, have the strongest and most respectable commendations. Subscription papers are ready for any who wish to obtain subscri- bers. PROPOSALS ARK ALSO ISSUED FOR PRINTING, A THEOLOGICAL DICTIONARY, In two octavo volumes, Containing Definitions of all religious terms ; a comprehensive view of every article in the system of Divinity ; an impartial ac- count of all the principal denominations which have subsisted in the world, from the birth of Christ to the present day. Together with an accurate statement of the most remarkable transactions and events recorded in Ecclesiastical History By Charles Buck Price to subscribers, g> 2 25 per volume, neatly bound.. ..one for every five purchased or subscribed for....going to press. Commendations of the London Reviewers. " A very excellent and useful book, the result of much labour and investigation, and a remarkable talent for clearness of defini- tion and description. This undertaking, in its own nature very com- plicated and extensive, has not here fallen into unworthy hands. The diligence of the author has rendered it very copious ; and the soundness of his understanding has made it abundantly instructive. It is in general free from bigotry, and may be used advantageously by Protestants of all descriptions, and indeed by all Christians." Brithh Critic ** In former periods, when religious controversy was more in vogue than it is in these days, a publication like the present would have been in great request, and secure of a rapid sale. It possesses value however, independently of temporary circumstances. In these volumes a neat snd succinct account of various religious opinions is given, and which seems to us to possess much correctness." Monthly Reviev.\ " The compiler seems to have undertaken this work with a full view of the danger attending the enterprise. He observes, * that, while he has endeavoured to carry the torch of truth in his hand, he has not forgotten to walk in the path of candour.' To this decla- ration we give every degree of credit. He has laboured for his information, and in general has obtained it. He is strictly ortho- dox in his opinions, yet is candid in dilating on those of others. Ho is in general very clear, as well as candid, in the explanation of most opinions. The work possesses considerable merit." Critical Review. " A work of this nature has long been a desideratum in the Chris- tian world; yet this, we believe, is the first attempt that has been made to furnish the public with a compendious dictionary, explain- ing the various terms which have obtained general currency in di- vinity. It is very different from a Dictionary of the Bible ; such as Calmet's, Wilson's, Brown's, &c....Here we have an interesting im- partial account of the various sects and denominations that have arisen and flourished in the visible Kingdom of Christ, in every age and nation. The principal events in ecclesiastical history are briefly related with candour, fairness, and undeviating regard to truth. A mass of useful information is laboriously collected and judiciously compressed. The definitions of terms are, in general, concise and accurate ; and though we are not partial to the method of those who always lay a considerable stress on the etymology of words, in or- der to determine their precise meaning, yet we think Mr. B. has manifested considerable judgment, attention, and care, in the use he makes of it We also very highly approve and commend the Chi-istian Spirit which it uniformly breathes From a careful pe- iusal of this volume, we most cordially recommend it to our read- ers, as well calculated to inform the inquiring-, to instruct the ig- norant, and to establish the Man of God in his attachment to the Lord Jesus, as revealed in the Holy Scriptures." Theological Review. «« Though we have had various Dictionaries of the Bible, we have never before seen Divinity and Ecclesiastical History redu» ced to this convenient form. Mr. Buck is certainly entitled to much praise for the labour and care with which he has collected and arranged a body of information that will be found highly use- ful for ministers and private Christians, especially such as are not accommodated with extensive libraries." Evangelical Review. W. W. Woodward has likewise issued Proposals for printing by Subscription, [In three vols. 12mo. at §1 per vol. handsomely bound and lettered] ' THE MISCELLANEOUS WORKS OF THE REV. CHARLES BUCK, Minister of an Independent Church in London, and Author of the deservedly celebrated and highly useful Theological Dictionary, CONTAINING THE YOUNG CHRISTIAN'S GUIDE, OR I Suitable Directions, Cautions, and Encouragement) THE BELIEVER, On his First Entrance into Divine Life, This work contains, among- other things, Rules for understand- ing- the Scriptures ; Advice as to reading —Hearing — Joining a Church — Receiving the Ordinance of the Lord's Supper — The Im- provement of Time — Zeal — Leadings of Providence — Prayer — Usefulness, &c. Cautions as to forming* Connexions — Marriage — Novelty — Curiosity — Anger — Discontent — Bigotry, &c. — Dress — Recreations — Spiritual Declension, &c. Encouragement under Despondency — Temptations — Satanic Suggestions — Variety of Opinions — Persecution — Desertion — Fear of Death, &c. &c. &c. A TREATISE RELIGIOUS EXPERIENCE, In which its True Nature, Evidences, and Advantages, are considered, ANECDOTES, Religions, Moral, and Entertaining, Alphabetically arranged, and interspersed with a variety of Useful Observations. " Seize every opportunity of introducing or maintaining spiritual converse. In order to this, furnish your mind with an extensive stock of interesting anecdotes and striking hiiats." Brown. i LIKEWISE, Proposals are issued, for Printing by Subscription. (going to press) A COMPLETE HISTORY UF THE HOLY BIBLE, In two octavo volumes, as contained in the Old and New Tes- taments, including also the occurrences of four hundred years, from the last of the prophets to the birth of Christ, and the life of our blessed Saviour and his apostles, Sec. With copious Notes critical and explanatory, practical and devotional. From the text of the Rev. Lawrence Bowel, A. M. with considerable additions and im- provements, by the Rev. George Burder, author of the Village Sermons, Notes to Pilgrim's Progress, £cc. Price to subscribers,, g 2 25 per volume, bound and lettered one for every five sub- scribed for, Mr. Burder's Remarks. •« The History of the Bible, by the Rev. Mr. Howell, being much esteemed, and having become very scarce, I was desired by the publishers of this edition to prepare it for the press : in do- ing which, I found much more labour than I expected ; for Mr. Howell's style was frequently negligent, and required some im- provement to render it agreeable to modern and intelligent read- ers. Many events, recorded both in the Old and New Testaments, appear to me to have been passed over too slightly. To his account of these things I have made considerable additions; and have sometimes ventured to intermingle a few practical reflections. I have also endeavoured to throw' that light upon some of the ob- scurer passages of the Old Testament with which we are furnish- ed, by the New. The history of our Saviour's sufferings, death, and resurrection, is much enlarged, for which I am indebted chiefly to those excellent writers, Drs. Doddridge and Macknight; from whom, as well as from several other able critics, I have borrowed many explanatory notes, which, I trust, have contributed greatly to enrich the work; and throughout the whole, I have laboured to render the history uniformly Evangelical. In a word, if Mr. How- ell's original work received the approbation of the public, I hope this improved edition will still be more acceptable, and be found generally useful to Christians of all denominations. GEORGE BURDER London, January, 1807". The following valuable Works are aho printed. Scott's Essays on Important Subjects in Religion, price g l....Rev. Roland Hill's Village Dialogues, in two vols, price g 2.,..Village' Sermons, by George Bur-der, 2 vols*- g 2....Baxter's Miscellaneous Works, containing Call to the Unconverted, Walks in Solitude, and Dying Thoughts, price in one vol. g 1... Pocket Bibles in 12 different bindings.. ..Beauties of Evangelical Magazine, g 4 25 Songs of Solomon?. §c 1 75»...Bigland's Letters on the Study and Use of An- cient and Modern History, one handsome octavo volume, g2 This publication is highly commended by the Reviewers Bon-. nett's Inquiries into Christianity, 1 vol Witherspoon's Works, four 8yo. volumes, g 8 50. IM.ATK U.Y01..II: ASTUONOM l PlATEED VOZJK As i u(i\(i\n PLATE V (>'•./'.' ASTBO \ o MY. II \ I B vr VOI.1V. I'LATKYH JfaL-JV. Astronomy l'lulf Mil Vto. It Astro \ n mi PLATE IV I VSTRONOM1 \> 1'ikin o\n /'/.// v:\i.l ;//./i T ASTKONOMY Astronomy. pi.atk xm.Voz.JF. \S Til O N <> \l >» VI A 1 E \1V Vol. IV El K( I !