m^^ 17' '^m Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924031233848 Cornell University Library arV17387 A manual of the barometer containing an 3 1924 031 233 848 ® A MANUAL OF THE BAROMETEE; / CONTAIJHNG AN EXPLANATION OP THE CONSTRUCTION AND METHOD OF USING THE MERCURIAL BAROMETER, WITH APPROPRIATE TABLES ?0R CORRECTIONS FOR TEMPERATURE, AND RULES FOR OBTAINING THE DEW-POINT AND THE HEIGHTS OF MOUNTAINS : TO WmCH AKE ADDED, AN ORIGINAL TABLE OF THE MEAN OP THE HEIGHT BAROMETER FOR EVERY DAY OF THE YEAH, AND PHjENOMENA OF THE WINDS AND CLOUDS IN THEIR CONNEXION WITH THE CHANGES OF THE WEATHER: Ar,50, A DESCRIPTION OP THE ANEROID BAROMETER. BY JOHN HENRY BELVILLE, a. ' OF THE KOYAL OBSEBVATOBY, GEEENWICII. THIRD EDITION, LONDON: TAYLOR AND FRANCIS, BED LION COURT, FLEET STREET. 1858. PRINTED BY TAYLOK AND FRANCIS, RED tlON COURT, FLEET STREET. PREFACE. In the preliminary Chapter on the Atmosphere I have referred to De Luc's ' Recherches sur I'Atmosphere,' Robertson on the Atmosphere, and Dalton's Essays. The barometric table of the mean height of the mercurial column for every day in the year is, I believe, the first of the kind that has been deduced ; it is the result of thirty years'v observations, made by myself in one locality. In the name of " Heiiry " my table of daily mean temperatures was used for two or three years, at the Royal Observatory, as a standard of comparison for the temperatures in the w^eekly report of the Registrar-General, and it was only discontinued when the esta- blishment had accumulated sufficient data from which to deduce a standard of their own : that public acknowledgement of confidence in the accuracy of those results, together with my IV professional character as an observer, will give the value of authenticity to the Table now published. The phsenomena of the winds and clouds, in their connexion vdth the movements of the barometer, though deduced from obser- vation and experience, are not set forth as dogmas, but as helps, for the interpretation of the meteorological appearances of our very irregular and unsettled climate. The nomencla- ture of the clouds is Luke Howard's, by whom, when a young observer, I was favoured with a presentation copy of his valuable work on the Climate of London. J. H. B. Hyde Vale, Greenwich, April 1849. ON THE ATMOSPHERE. 1 HE Barometer is an instrument for measuring tte .weight of the atmosphere j it was invented in 1643 by Tbrricelli, who in investigating the cause of water as- cending in pumps to the height of 32 feet, and no higher, made the following experiment. He took a glass tube about four feet long, sealed at one end and open at the -other, and having filled it with mercury closed the open •end with his finger; he then inverted the tube, and placed the open end under the surface of a small quan- tity of mercury in a basin, and raising the tube perpen- dicular withdrew his finger; he observed the mercury in the tube suspended to the height of 27^ inches, -above the surface of that in the basin: he compared the height of the column of mercury with the height of the column of water raised by the pump, and perceiving those heights to be in an inverse ratio of the specific gravities of the water and mercury, he concluded they were kept in suspension by a common cause ; a, further consideration of the experiment led him to remark that .the upper extremities of the columns of water and mer- cury had no communication with the atmosphere, but ..that the lower, extremities.had a communication, and he 2 attributed the elevation of the columns in the tubes to the weight of the atmosphere. The curious may amuse themselves with the action of the weight of the atmosphere in the following manner : — Take a glass tube of uniform bore, open at both ends ; fit a cork to it, and cement a wire into the cork, which will form a piston to the tube; place the piston even with the lower end of the tube; and in that situation place the same end of the tube in mercury; hold the tube steadily and pull up the piston ; the mercury will follow the piston, and will fill that part of the tube which is below the piston. By this means the weight of the atmosphere is removed from ofi^ the mercury, which is forced into the tube as far as the piston, by the weight of the atmosphere on the rest of the surface of the mercury in the basin; when the mercury in the tube balances the weight of the atmosphere, it remains stationary ; and on pulling the piston higher, the space between it and the mercury is called a vacuum, or space void of air. In 1646 Pascal at Rouen repeated Torricelli's experi- ments with similar results. He also varied them by employing liquids of different specific gravities, and he perceived that the lighter the liquid the higher it as- cended in the tube ; but the agency of an invisible fluid was still doubted, and he therefore determined to make an experiment on the top of the mountain Puy de Dome, near Clermont in Auvergne, which should silence con- troversy. Two tubes filled with mercury, the columns of equal heights, were carried to the foot of the moun- tain, one of which was left there standing at 28 inches, and the other taken to the summit ; as they ascended, the mercury in the tube gradually sunk until it stood at 24'7 inclies ; as they descended, the mercury as gra- dually rose again ; and when placed by the side of the tube left below, their elevations coincided. As Pascal had anticipated, in ascending the mountain the weight of a portion of the column of the atmosphere equal to the height of the mountain being removed from the sur- face of the mercury in the bason, that which was in the tube fell, until its weight was again counterpoised by the atmosphere ; and conversely in descending, the weight of the column of the atmosphere being increased by the weight of the, portion equal to the height of the moun- tain, pressed upon the mercury in the basin, and forced it to ascend in the tube until both weights balanced each other. Pascal originated the idea of measuring elevations by the variations of the barometer, but he foresaw a diffi- culty. He compared the atmosphere to a mass of wool, the lowest parts of which were more pressed than those above ; and his sagacity led him to the fact, that from the dilatation of the atmosphere the rise and fall of the mercurial column would not be equal through equal spaces. This concluded his philosophical inquiries; he afterwards turned his attention to theology. In 1666 Boyle discovered that the atmosphere was elastic and compressible; ajid about the same period Mariotte proved its density was in proportion to the weight with which it was eompjjessed. The stratum of the atmosphere nearest the surface of the earth supports the weight of all above it, and is the densest; each stra- tum as we ascend becomes lighter or more rare, because its elasticity is less checked by having a less weight pressing from above. Pere Cotte deduced, that the ratio of the decrease of its density was in gebmetrical b2 4 progression, if we take the heights in arithmetical progression. Thus, if the density at 1 mile high was 1, and that at 4 miles high ^, then that at 7 miles high would he ^, at 10 miles high |., at 13 miles high ■^, &e. ; but this ratio is much disturbed by changes in the tem- perature of the strata of the atmosphere at different ele- vations. Heat expands the bulk of air, and forces it to occupy a larger space ; 1000 volumes of air at 33° of Fahrenheit become expanded into 1057"34 volumes at 60° ; thus heat is a cause of the unequal rise and fall of the barometer through equal spaces. Sir George Shuckburgh made numerous experiments upon the ef- fects of temperature on the atmosphere ; and from his labours we have a table, which shows in feet how much the spaces passed through may vary from temperature in a fall of yg^h of an inch of mercury, the barometer standing at 30 inches ; and by means of his theorem for its application, we are now enabled to ascertain the heights of mountains by the barometer as correctly as by geometrical measurement. There exists at all times in the atmosphere a certain portion of vapour, which exerts an influence, varying according to circumstances, upon the mercurial column ; it is derived from the spontaneous evaporation of water from the surface of the earth, and is called aqueous vapour. Evaporation is promoted by dry air, by wind, by a diminished pressure, and by heat ; the quantity evaporated is dependent upon temperature; for heat expanding the gaseous portion of the atmosphere, the spaces between its particles are enlarged and their ca- pacities for containing moisture augmented. Aqueous vapour is highly elastic ; its elasticity, which increases with an increase of temperature, has been determined by 5 Dalton, and its force measured by the height of the mer- curial column it is capable of supporting. Aqueous va- pour, raised at 32° of Fahrenheit, exerts a pressure on the mercury equal to 0*2 of an inch, at 80° to 1-03 inch, at 180° to 15-0 inches, and at 212° to 30-0 inches, — a pressure equal to the pressure of the whole atmo- sphere at the level of the sea. The quantity of vapour existing in theatmosphere is measured by an Hygrometer. The one now in general use consists of two thermo- meters, one bulb of which being covered with muslin and kept constantly moist, will, according to the quan- tity of evaporation at the time of observation, stand lower than the other bulb, which being left free gives the temperature of the air : from the difference of the readings of the two thermometers, we are able by a very simple rule to obtain the dew-point, or that degree of the thermometer to which the temperature of the air must fall for the atmosphere to become saturated with the quantity of vapour then actually existing in it, as will be shown by the following example : — Let free thermometer ... =63 63 = temp, by free therm. Wet thermometer =54 16-2 Difference = 9 46-8 = dew-point : Factor to multiply dif- 1 _ t.g """ ference J "" sgy TableO-337 of aninch»= , „ „ elasticity of vapour in u , . .V r.*^'. TT"" atmosphere, ber of degrees to be subtracted from free therm. If the readings of the two thermometers be alike, the temperature of the dew-point will be the same as the temperature of the air j and the air will then be saturated with moisture.- It is chiefly in the nights, and early in the mornings of the winter months, that the atmosphere is saturated * Page 36. with vapour, or that yapoar is at its maximum of elas- ticity for the temperature. In our climate, vapour never attains its greatest elasticity at a high temperature; for if in the summer months the atmosphere becomes saturated, it is caused by a declension of the heat, which, contracting the spaces between the particles of the air, squeezes the vapour contained in them closer, and thus brings its elasticity to a maximum for the temperature to which the air has fallen. It was upon the changes of temperature in the atmosphere that Dr. James Huttott founded his theory of rain. He considered rain to be formed by the mixture of two strata of the atmosphere of diflferent temperatures, and each saturated with moisture. The mean quantity of the vapour contained by the two strata before the mixture being more than the mean heat of the two {after the combination) can contain, the excess is precipitated : — o in. Let temp, of one stratum =65 its tension or elasticity =0-617* Let temp, of the other... =41 ... ... 0274 i|10B 0-891 Mean tenrperature after 1 -o Mean tension after"] q.aar combination J combination...... J By Table, the tension of vapour at 53° is 0'414 inch. / Therefore vapour of the tension of ■0'032 inch of the mercurial column is precipitated in cloud, fog or rain. Heat and moisture are the principal causes of the variations in the weight of the atmosphere, and neces- sarily of the variations in the barometer; the moon is considered to have some influence ; but if she exert any power in causing accumulations or tides in the atmo-. sphere, her action on the barometer, computed to be about xsu^h of an inch, is so small, that even with the * .Table, p. 36. most delicate instruments and tlie most accurate ob- servers we can scarcely hope to demonstrate it satis- factorily. The variations of the barometer are less within the tropics than in the temperate and polar regions; they vary in different countries in the same latitude^ and they are great in mountainous countries and islands ; in Peru the range of the mercury is about ^rd of an inch, in Lon- don 3|- inches, and in St. Petersburg it exceeds 3 inches. The pressure of the atmosphere at the level of the ^sea, the barometer at 30 inches, is 15 lbs. on the square ^inch, which amounts to 3160 lbs., or nearly a ton, upon every square foot. We cannot therefore be surprised at the effects of so elastic and compressible a body as air, when it is set in motion ; the soft breeze of summer and the furious hurricane of winter are instances of the effects of its different velocities. On the Construction and Method of Using the Mercurial Barometer. There are various forms of the Barometer, but the one best suited for meteorological observations consists. of a tube about 33 inches in length, the extremity of which is inserted into a small reservoir or cistern ; and in order to maintain the mercury in the cistern always at the same level, the cistern is constructed partly of leather ; that by means of a screw at the bottom, the surface of the mercury in it may be so adjusted, as to have it always at the place from which the scale com- mences. Some barometers are furnished with a gauge or float, that in great elevations and depressions the observer may perceive when the mercury in the cistern sinks too low or rises too high. 8 Kg. I. "Let a b, fig. 1, be the glass tube plunged into the mercury in the cistern C, and D the surface-line of the fluid in tlie cistern level with the commencement of the scale, and adjusted to the particular height of the, mercury in the tube, which has been actually measured from the surface of the cistern, in the construction of the instrument (which height is called its neutral point) : ■ when the mercury rises in the tube, a portion equal to that rise leaves the cistern, and the surface- line falls towards the dotted line e; and being lower than the surface from which its neutral point was measured, the actual varia- tion in the atmosphere is indicated too little r turn the screw / until the lines on the float h coincide, and the mercury then records the exact change : when depressions occur, the mercury sinking from the tube into the cis- tern raises the surface-line towards g ; in this ease the screw / must be unscrewed until the -leather at the bottom of the cistern be suffi- ciently loosened to allow the mercury to as- sume its proper level at the surface D. When there is not a gauge to the baro- meter, the relative capacities of the cistern and tube are ascertained by experiment, in the construction of the instrument, and ^ marked thereon ; as is also its neutral point. In this ease, when the mercury in the tube is above the neutral point, the difference be- tween it and the neutral point is to be divided. by the capacity, iind the quotient added to the observed height will give the correct height ; if the mercury be below the neutral point, the difference is to be divided as before. Jt^ ^ Observed height... =30-400 Neutral point =30000 Difference above 1 . ,/,« neutral point J Add for capacity ...+ -010 Correct heigKt 30-4 10 and the quotient subtracted from the observed height will give the correct height. Let capacity for every inch of elevation of the mer- cury in the tube be equal to -^^, which, reduced to a decimal, will be in. in. in. = 0-025 for 1 inch., 0-013 for iinch, 0-007 for ^inch. in. Observed height 29500 Neutral point 30000 Difference below neu- \ .cqjj tral point j" Subtract for capacity — -OIS Correct height 29-487 The scale of the standard barometer used in fixed observatories is made moveable, and terminates in an ivory point, which is brought down to the surface of the mercury : when this point and its reflexion appear to touch one another, the height indicated is correct. This kind of barometer requires no adjustment or correction for the cistern. The tubes of barometers vary in size : those of a large diameter are preferable, as the motion of the fluid is freer, and its friction against the sides of the tube is nearly inappreciable ; tubes of small diameters require correction for capillarity, or the depression of the mer- cury caused by its adhesion to the sides of the tube. The range of the barometer, or the spaces passed through by the mercury in its extreme depressions and elevations, being limited to 3^ inches, it is not usual to graduate the scale from the lower end of the tube : the divisions commence at 27 inches, and are continued to 31 inches. The graduations on Troughton's mountain b5 10 barometers for measuring great elevations, commenca at 15 inches and are carried on to 33 inches. Each inch is divided into ten equal parts, and these parts are subdivided into hundredths by means of a Vernier (so named from Peter Vernier, its inventor). The Vernier (A, figs. 2 & 3) is a moveable plate, one inch and one- tenth of an inch (together equal to yw) in length; these eleven-tenths are divided into ten equal parts, each part being equal to one-tenth of an inch and one-tenth of a tenth, together equal to eleven hundredths. When the pointer of the Vernier coincides with a division of the barometer scale, as in fig, 3, each division of the Ver- nier will exceed each division of the scale respectively by 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 parts, whose denominators are the number of parts between a, b ; the excess of each division being Yuoi & tenth or ^^, -5^ of a tenth or i-oir, ^ of a tenth or -^, -j^ of a tenth or y^, Ssc. The pointer in this position reads off to inches and tenths, viz. thirty inches and one tenth^ expressed in figures 30-10 inches. When the pointer does not coincide with a division of the scale as in fig. 3, observe which division of the Ver- nier does coincide ; and the number placed against that division of the Vernier will be the number of hundredths to be added to the inches and tenths. In fig. 3, 7 coin- cides with a division of the barometer scale, and there- fore 7 hundredths are to be added to the inches and tenths, and the reading is thirty inches, one tenth and seven hundredths, expressed in figures 30' 17 inches. By an alteration in the divisions of the Vernier, the mountain and standard barometer are read off to -g^th of an inch. II g-2 WSftft/ Fi g.3. '^ ,10 (f> r\ A < 1 2 — J U ) — — ? — ! — ii " : Y 2 3 4 i e 7 i / ^ !>0 A — V. lAW. gl kAAAW ■JJ^AAAAA^ WvnjV/ -30 -29.5 •29 A thermometer is attached to the barometer to indi- cate the temperature of the mercury in the cistern ; all bodies expand by heat and contract with cold ; the ex- pansion of mercury is easily tested by exposing a mer- curial thermometer to the heat of a fire, or by placing it in hot water : as the warmth increases, the mercury will expand and ascend in the tube ; as it diminishes, it will contract and fall towards the bulb : if the ther- mometer be plunged into a mixture of pounded ice and common salt, from the intense cold produced by the conversion of the ice into water, the mercury will sink 13 to zero, or 33° below the freezing-point of Fahrenheit ; if the tube of the thermometer should not be long enough to admit of so low a graduation, the mercury will shrink into the bulb. The expansion of mercury is -g-^-g-Q of its bulk for each degree of Fahrenheit between 33° and 313°. For conveniencej tables have been computed, from which may be taken out, at sight, the amount to be subtracted from the height of the mercurial column, on account of the expaiision of the mercury from tem- perature. The words Change, Fair, and Rain, engraved on the plate of the barometer, were placed there by the first observers of its variations : no great importance should be attached to them ; for from the observations of two centuries we find, that heavy rains, and of long conti- nuance, take place with the mercury at 39'5 inches or Change; that rain frequently falls when it stands as high as 30'00 inches, or Fair ; and, more particularly in win- ter, a fine bright day will succeed a stormy night, the mer- cury ranging as low as 39"00 inches, or opposite to Rain. It is not so much the absolute height as the actual rising and falling of the mercury which determines the kind of weather likely to follow. The late great elevation of 30'9 inches in February of the present year 1849, was suc- ceeded by a minimum of 39 "35 inches, which produced a storm of wind so violent that the horizontal pressure of many of the gusts amounted to 30 lbs. upon the square foot ; a pressure which is rarely exceeded, even when the barometer falls as low as 38'35 inches. This may appear extraordinary if we merely take into consideration the actual height of the column, and neglect the quantity of the fall which amounted to 1'65 inch. The mean height of the greatest observed elevations for the last 13 thirty-eight years is'30'61 inches^ and the mean height of the observed depressions for the same period is 28'69 inches; therefore a fall in the mercury of 1'65 inch from the mean of the elevations vrould give a minimum of 38'96 inches; a depression which is contemporary with violent stormsj as it is- within three-tenths of the mean of the lowest depressions of the barometer. In fixing the barometer great care must be taken to fix it perpendicular : a sitwation should be selected sub^ ject to the least change of temperature, for which reason a northern aspect is preferable to a southern ; the height of the cistern of the barometer above the level of the sea, and, if possible, the difference of the height of the mer- cury with some standard, should be ascertained, in order that the observations made with it should be comparative with others made in different parts of the country. Before taking an observation, the instrument should be gently tapped to prevent any adhesion of the mercury to the tube, the gauge should be adjusted to the surface-line of the cistern, and the index of the Vernier brought level with the top of the mercury. If the barometer have a Vernier which admits the light from behind, the lower part of the pointer must make a tangent with the convex part of the mercury in the tube. In reading off the observation the eye should be on a line with the mercury ; as by placing it above, the reading would be too low, and by placing it below, it would be too high. This difference in the manner of reading off is called error from parallax. It is indispensable that a reading of the attached thermometer be made simultaneously with the observation of the height of the mercury. Accuracy is the spirit of observation. A careful reading of inches, tenths and hundredths produces excellent results : the 14 T!^ place is better left to the skill of the old observer who is usually obliged to estimate it^ scarcely any baro- meter being graduated with suflScient precision to trust to the divisions for so small a quantity. The barometer is slightly affected periodically during y the twenty-four hours : at 9 a.m. and 9 p.m. it stands higher, and at 3 a.m. and 3 p.m. it stands lower ; the mean annual difference amounts nearly to "03 of an inch. These four periods of the day have been recommended for observation by the Committee of Physics of the Royal Society. It is usual, for the sake of comparison, to reduce the observations to 32° of Fahrenheit. in. Ex. If barom. stood at 29-900 therm, attached 54°, Correct for temp. —'057 (by Table), Height of barom. atT gq.Q.o temp, of 32° ...J The wheel-barometer, from its construction, cannot be trusted to for correct heights ; it merely shows if the mercury be in a rising or falling state : it may rather be considered as an ornamental piece of furniture than as having the slightest pretensions to a scientific instru- ment. 15 § <£. ri tt « 41 <"?, ts je: % '« "^5 •^ c •is •JJ *1 ^ 5».a «3 1 •fe.^ S^o. -S CO o ■M t. -S >*-. £ 1>« f^ Sir ;:• t^ ^ ^ hB?"^ OQO ?■ =3 *= 1^ ,o r?i ? »■:? « SCI 1^' ~ U Q •Sg • i '"='"'' ■°"°'-°°='SnS2 2SSS?59Sc3SSS[SSE?§5^§55 | i ^s.^s.(»coo>aocDOOooosc»o»o>OJO>ooc»t^«ooooooooc»oQO)O^Ole»ac .as 1 ssgSEEEsiSiliHgfes'lsHsiiigia CO : §■ 1 o gplIISi|i&lpE||iilliSiH5psHPl a -t -S (M O & a3c»a>0)CftcacDoocoaDce>cootooconc»cooic»aocDCoaococxicocswco .5^' ■ : ; ^ : S ti ssssssssgsgsssggs&sssasssssssaa^ • ^ .2S % t aaaag.g.g&aass&s&aasssssss'gsssssg i^ ■ g ' 1 t-a .iiiiigiSgsliillilliiiiliillali- 1 i?-ss5liSlllliislHiiEiiifeiaiHs CO 1 TO • 1 .5^ § i liSSI|iili|SSH?iligii|gil||ii' 00 .ss s hi 04 1 tssiiigsiifiiiiaiiisssigsssiiii .ss . • . s gEstllSalitlillsillSslsggggi 1 .ss s S sisisssi||2ii|f&iiiiiiii|§ifiii 1 Is |3 1 " " " " ■"■="^"» ""s - a 2 3 as $r2 s§ 5 s § s; s« ss ss s « 16 The foregoing Table of the daily mean heights of the barometer for Greenwich for every day of the year is the result of thirty years' observations made in one locality, andj with few exceptions for so long a period, by one person. The instrument by which the greater number were registered is by Watkins and Hill, the tube of which has a bore ^^gths of an inch in diameter. The Table is original, and it may assist in confirming that, at certain seasons of the year, great periodic atmospheric maxima and minima take place. The greatest daily mean pressure for the year, which a consecutive five years' observations J will not only verify bvjt increase, occurs about the 9th of January, and the minimum daily mean depression ^ towards the end of November. It is a remarkable co- incidence, that the lowest daily mean temperature for thirty years occurs on the 8th and 9tb of January, and the daily mean temperature for November rises suddenly '•■ four degrees in the last few days in November. y The greatest monthly mean pressure occurs in June, and the lowest in November. From June the monthly mean pressure declines till November, when it again rises and attains a second maximum in January; and again falling, comes to its , second minimum in March. The mean annual pressure for noon at Greenwich is ' 29-872 inches. 17 A Table of the greatest and least observed heights of the Barometer for the last thirty-eight years,, taken at Greenwich, and reduced to 32° of Fahrenheit. Date Greatest Least Great- '})ate Greatest Least Great- of the observed observed est of the observed observed est year. height of height of annual yea?.: .height of ieight of annual barometer. barometer. range, in. barometer. barometer. range. in. in. in. If in. in. 1811. 30-46 28:68- 1-78 1830. ^-63- 28-61 2-02 1812. 30-55 28-50 2-05 1831. 30-58 28-gS 1-65 1813. 30-47 28-65 1-82 1832. 30-64 29-12 1-52 1814. 30-40 28-2J 2 19 .1833. 30-71 .28-77 .. 1-94 1815. 30-63 28-86 1-77 1834. 3066 29-13 1-53 1816. 30-67 28-72 195 1835. 30-84 28-74 2-10 1817. 30-63 28-51 2-12 1836. 30-69 28-62 2-07 1818. 30-62 28-54 '2-08 :^1837.' 30-68 28-77 1-91 1819. 30-52 29- H 1-41 1838. .>50-58 ,28-61 1-97 1 820. 30-75 28-67 208 1839. 3057 28-97 1-60 1821. 30-82 2799 2-83 1840. 30-68 28-59 2-09 1822. 30-70 29-11- 1-59^ 1841. 30-49, 28-82 1-67 1823. 30-62 28-60 202 1842. 30-58 28-69 1-89 1824. 30-57 28-46 ?-ll 1843. 30-54 28-20 2-34 1825. 3089 28-74 2J5 1844. 3053 28-63 1-90 1826. 30-57 28-80 1-77 J845. 30-57 ' SIM7 1-80 1827. 30-70 28-77 1-93 1846. 30-66 . -28-66 2-00 1828. 30 55 28-92 1-63 1847. 30-53 .28-48 2-05 1829. 30-59 2892 1-67- 1848.. , JJ0-4S "28-40 2-08 In the preceding Table the maximum elevation for the period of thirty-eight years occurred, in 1825, when the mercury stood at 30'89 inches; in 1821 it however reached 30'82 inches; in, 1835, 3.0',84 inches ; and in February 1849, 30-86 inches. It is recorded that Sir George Shuckburgh,_ia 1778 in London, ohserved the barometer at 30935 inches, which he believed to be the greatest elevation ever seen. In the extreme depressions, those of 1821 and 1843 differ only by 21 hundredths : the first occurred on the 25th of December, when a Troughton's mountain-baro- meter at the Eoyal Observatory sunk as low as 27'89 inches. (See Pond's ' Greenwich Asti-onomicalObserva^ 18 tions, 1821.') A heavy rain of some hours' duration, ■with the wind at south-east, had preceded the miniraum pressure ; a gale from the north-west followed, in which the mercury rose a few tenths. The depression of 1814, 28"21 inches, happened at the close of the great frost, and was likewise preceded by a stormy wind from south-south-east and much rain. The difference of the extremes of the elevations is 0-49 inch. The difference of the extremes of the depressions is 1'14 inch. The following is the progress of the great depression of January 13, 1843. Hour of the day. Height of the barom. state of the weather. 2 a.m. 5. 7. 8. 8-45. 915. 10. 11. Noon. 1p.m. 2. 3. 4-30. 6. 10. in. r 2902-^ 28-72 28-54 1 28-48/ 28-41 28-40 2837 28-34 1 28-30/ 28-23 28291 28-34 28-37^ 28-48/ 28-68 Sky overcast ■with cirro-stratus : ground covered ■vrith snow. Temperature 30'. Rain and Wind. Direction S. Wind unusually violent. Dense nimbi. Thunder and lightning, l Wind Dense nimbi. Rain and Wind. J S.S.W. Tremendous squalls. Showers of rain discharged horizontally from nimbi. Wind 'blowing a hurricane from S.W. More moderate. Wind as violent as before, blowing down trees. Direction of wind W. Steady heavy gale from W. with scud. Temp. 38°-5. The horizontal pressure of the most violent gusts was SO and 35 lbs. on the square foot, the wind having a velocity of 60 and 80 miles an hour. It may be proper to observe, that neither extreme elevations nor extreme depressions occur suddenly, the mercury being usually for some few days preceding them in a gradually rising or falling state. 19 A Table showing the age, declination, and position of the Moon in her orbit, in some of the most remarkable elevations of the Barometer, Date of Day of tlie Heifiht Moon'B Moon's Position of the the year. month. barometer. age. declination. Moon in her orbit. in. days 1816. Dec. 1. 30-67 .14 UN. Apogee. 1820. Jan. 9. 30-75 25 15 S. Mean. 1821. Feb. 6. 30-82 5 ION. Perigee. 1822. Feb. 28. 30-70 8 27 N. Perigee. 1825. Jan. 9. 3089 22 Equator. Perigee. 1827. Dec. 28. 30-70 10 20 N. Apogee. 1833. Jan. 8. 30-71 18 15 N. Perigee. 183.1. Jan. 2. 30-84 3 18 S. Mean. 1836. Jan. 2. 30-69 14 26 N. Apogee. 1837. Oct. 14. 30-68 15 14 N. Past perigee. 1840. Mar. 8. 30-68 4 22 N. Perigee. 1849. Feb. 11. 30-86 18 Equator. Mean. A Table showing the age, declination, and position of the Moon in her orbit, in some of the most remarkable depressions of the Barometer. Date of Day of the Height Moon*8 Moon's Position of the the year. month. barometer. age. - declina^on. Moon in her orbit. in. days 1814. Jan. 29. 28-21 8 15 N. Near perigee. 1817. Dec. 8. 28-51 New. 26 N. Perigee. 1818. Mar. 4. 28-54 29 26 S. Mean. 1821. Dec. 25. 27-99 3 25 S. Perigee. 1824. Nov. 23. 28-46 4 20 S. Apogee. ■ 1830. Jan. 20. 28-61 27 17 S. Past apogee. 1836. Feb. 2. 28-62 Full. 16 N. Perigee. 1838. Nov. 28. 28-61 12 16 N. Perigee. 1840. Nov. 13. 28-59 19 24 N. Past perigee. . 1843. Jan. 13. ■28-20 13 24 N. Mean. 1847. Dec. 6. 28-48 28 17 S. Apogee. 1848. Feb, 26. 28-40 21 16 S. Apogee. Erom these Tables it does not appear that the moon exerts any influence on the extreme movements of the barometer. 20 Phcmomena of the Barometer. Strong winds in the winter from the west with a steady high pressure, invariably bring a high temperature and very little rain ; with winds from the east, a low temperature and sharp frosts. If the mercury fall during a high wind from the south-west, south-south-west, or west-south-west, an increasing storm is probable ; if the fall be rapid, the wind will be violent, but of short duration ; if the fall be slow, the wind will be less violent, but of longer continuance ; the disturbing cause is probably the same in each case, but its intensity unequal : nearly all our high winds from the south-west come with a falling barometer. If the depression of the mercury be sudden and considerable with the wind due west, a violent storm may be expected from the north-west or north, during which the mercury will rise to its former height. If the mercury fall with the wind at north- west, or north, a great reduction of temperature will follow ; in the winter severe frosts, in the summer cold rains. A steady and considerable fall of the mercury during an east wind denotes that the wind will soon go round to the south, unless a heavy fall of snow or rain immediately follow ; in this case the upper clouds usually come up from the south. The deep snow of the severe winter of 1814 was a notable instance. The lowest depressions occur with the wind at south and south-east, when much rain falls, and frequently short and severe gales blow from these points. In the winter months, sudden depressions of the mercury with the wind in these quar- ters are attended with electrical pheenomena. A fall of the mercury with a south wind is invariably followed by rain in greater or less quantities. A falling barometer with the wind at north brings the worst weather : in the summer, rain and storm follow ; in the winter and spring, deep snows and severe frosts. This case is of rare occurrence. A great depression of the mercury during a frosty period brings on a thaw ; if the wind be south or south-east, the thaw will continue ; if the wind be south-west, the frost will be likely to return with a rising barometer and northerly wind. In the winter season, a rapid rise of the mercury immediately after a gale from the south-west with rain (the wind going round 21 to north-west or north) is usually attended with clear $ky andi sharp white frosts. Great depressions in the summer months are attended with storms of wind and rain with thunder and hail : cold unseason- able weather generally succeeds these depressions. During a period of broken cold weather in the winter months, with the wind at north or north-north-west, a sudden rise of the mercury denotes the approach of rain and a southerly wind *. During a steady frost with the wind at north, north-east, or east, a continued slow rising of the mercury indicates snow and cloudy weather. If the mercury rise with the wind at south-west, south, or even south-east, the temperature is generally high. Observation does not show that extremes of temperature are contemporaneous with the greatest elevations and least depres- sions of the mercurial column. Meteors are not prevalent during very low pressures : the Au- rora Borealis has been noticed at all heights of the barometer. Small flashes of lightning are of frequent occurrence during stormy weather in the winter season when the mercury stands low. Great elevations in the summer are generally attended with dry, warm weather. Great depressions at all seasons are followed by change of wind, and by much rain. A rising barometer with a southerly wind is usually followed by fine weather. In the summer it is dry and warm ; in the win- ter, dry with moderate frosts. This is of rare occurrence. When the mercu ry is very unsteady during calm rainy weather, it denotes that the air is in an electrical state, and that thunder will follow. In the summer months, if a depression of two or three tenths of the mercury occur in a hot period, it is attended with rain and thunder, and succeeded by a cool atmosphere. Sometimes heavy thunder-storms take place overhead without any fall of the mer- cury ; in this case a reduction of temperature does not usually follow. Rain in some quantity may fall with a high pressure, provided the wind be in any of the northerly points ; and when much rain * Thaws also commonly set in during the night. 23 iFalls with a steady rising barometer and' the mercury attains a great elevation, a long period of fine weather usually succeeds. If after a storm of wind and rain, the mercury remain steady at the point to which it had fallen, serene weather may follow without a change of wind; but on the rising of the mercury, rain and a change of wind may be expected. During a series of stormy weather the mercury is in constant agitation, faUing and rising twice or thrice in the space of twenty* four hours, the wind changing alternately from south to west, and backing again to the south : this alternation of winds con- tinues until the mercury rises to a bold elevation, when it ceasesj and the weather becomes settled. Storms of wind, especially when accompanied with much rain, produce the greatest depressions of the mercury. No storm of wind on record has blown without some rain falling, although the time of its falling and its amount have been variable : sometimes the rain has increased with the increasing storm and sinking mercury ; at other times the rain has fallen suddenly at the close of the storm, or at the time of the minimum pressure. No great storm ever sets in with a steady rising barometer. As far as regards the locality of Greenwich, the most violent gusts of wind come from due south, and those next in violence from, due north ; in both instances the mercury remains stationary at its minimum point during the greatest horizontal pressure : the winds from these quarters are of short duration, and limited in their extent. The ordinary southi-west gales will blow unremit- tingly for twenty-four hours, and will sweep over the whole of the British Isles. Note. Although a rising mercury attends a northerly wind> great depressions occur previously to a great storm coming from that quarter. la England, the winds which blow for the greatest number of days together without intermission, are the west and west-south- west : they blow chiefly during the winter months, and are the principal cause of our mild winters. The east and east-north-east are the winds the next most prei- valent. The great antagonist winds, the north and south, are the origin of our most violent storms. The westerly winds surge mostly by nighti and their-average force is twice that of th& eastCTly winds. 23 The easterly winds ave generally calm at night, but blow with some power during the day. On an average, sunrise and sunset are the periods of the twenty-four hours in which there is the least wind. An hour or two after noon is the period when the wind is the highest. As a general rule, when the wind turns against the sun, or retrogrades from west to south, it is attended with a falling mer- cury; when it goes in the same direction as the sun, or turns direct from west to north, the mercury rises, and there is a pro- babiUty of fine weather. It never hails in calm weather. When hail falls, it is during sudden gusts of wind, and the mercury rises while the hail is actually falling. If the weather during harvest^time has been generally fine, and a fall of the mercury with a shower occur, — if the wind turn a few points to the north and the barometer rises above 30 inches, the weather may be expected to be fair for some days. The finest and most beneficial state of the atmosphere, more especially as regards the health of man, is with a uniform pressure at a mean height of the climate varying from 29'-80 to SO'OO. When there is only one current of 'air subsisting in the atmo- sphere, there is seldom much variation in the height of the mer- curial column. It is when two or more strata of the air are in motion in different directions at the same time, that great fluctuations of the mercury occur. In high pressures, the upper current usually sets from the northward ; in low pressures it sets from the south and south- west. The variations of the barometer are always greater in the winter than in the summer. In accounting for the different currents of the atmosphere, it must be remarked that the great heat of the torrid zone causes a constant ascent of air over it, which passes northward and southward ; while an under current of cold air flows from the poles to supply its place ; the diurnal rotation of the earth com- bined with these currents causes the trade-winds, whose direction is from east to west : these currents would from the same causes become in the north temperate zone north-east and south-west winds, and in the south temperate zone south-east and north- west winds; but the great itregul^ities of the temperature from 24 the seasons, the large tracts of ocean, and the different geogra- phical formations of the land, subject them to interruptions, and give to every country its prevailing winds, derived from local causes. In England, the south-south-west, south-west, and west-south-v^est winids set in towards the end of October, and blow with their greatest strength during November, December, and February, and are even powerful in June and July: the winds from the westerly quarters-prevail in March, but they then veer more towards the north, whence they blow with great vio- lence : in April, the east and north-east, and the west and north- west winds balance each other, and their comparative strength is nearly equal : in May, the east,north-east, and north-north- east winds preponderate; the latter blows the less frequently, but with the greatest violence ; in this month the average of the winds from the westerly quarters ranges low ; their average strength also decreases, with the exception of that from the west- south-west, which ranges higher than in April. In August the west and west-south-west winds prevail, but their power is mo- derate ; the stormy winds of this month blow irom the west- south-west and north-noi^-west. September is the calmest period of the year ; in thffljraonth the north and south winds, and the east and west wiifc, balance each other; in January the east and west winds upoPan average are nearly equal, both as regards the number of times they blow, and their average strength ; the winds from the south-south-west, west-south-west, and the north-westerly quarters are more rare, but they blow with great violence. As the winds from these opposite quarters predominate, so is the character of our winters determined as to mildness or severity. ^/ Sudden depressions of the barometer sometimes occur in weather apparently calm. It is almost an established fact that storms have a circular motion ; and if, when an exhaustion or sudden diminution of the atmosphere takes place, the mercurial column happen to be in the partial vacuum or centre of motion, the air will be at rest ; while the surrounding air at a greater distance from the centre will be violently agitated with a less fall of the barometer. This circular motion of the atmosphere is not confined to one spot where the storm may commence and expend its violence ; but it has a progressive cycloidal movement on- wards, changing constantly the situation of its centre of motion. 25 and, as it advances, enlarging its circumference, until, having tra- versed many hundred miles, it becomes exhausted as the air re- covers its equilibrium. These great rarefactions of the atmo- sphere are probably the effects of electricity ; they are common in their most terrific form in the Indian Ocean, on the western coast of Africa, and in the West Indies. In our own climate the approach of thunder-clouds produces violent squalls of wind ; and dense and highly electrified clouds will sometimes raise miniature whirlwinds as they pass overhead*. Of the Clouds. Howard's Nomenclature. Cirrus. Cumulo-Stratus. Cirro-Stratus. Stratus. Cirro-Cumulus. Nimbus. Cumulus. Scud. The Cirrus cloud is seen at all seasons of the year, and at all heights of the barometer. It occupies the most elevated regions of the atmospherCj and is sup- posed to be above the limit of perpetual congelation (in our latitude about 6000 feet). It is easily distinguished from all other clouds by its delicate, fibrous, thread-like, curling or feathery texture ; it lies in light patches on the blue sky, sometimes so faintly that the eye can scarcely discern it ; its motion is very slow, and in se- rene weather with a high pressure it will retain its form unaltered for many hours. If the mercury be falling, its changes are rapid ; and on the approach of rain its delicate texture becomes confused, and is ultimately lost in one dusky mass, resembling ground glass. During these changes the Cirrus has been descending ; and its * The mean annual horizontal pressure of the wind at Greenwich may be estimated at ^Ib. on the square foot ; equal to a velocity of 10 miles per hour. C 26 peculiar ctaracteristics having disappeared, it assumes a new nomenclature, the Cirro- Stratus. The progress- ive increase of the Cirrus cloud is generally from the west. The Cirro-Strattts is likewise in the higher regions of the atmosphere, and is seen at all seasons of the year : it is the immediate precursor of rain or wind and of a falling barometer. Sometimes it spreads itself over the heavens so attenuated, that the sun, though it shines through it, casts its shadows indistinctly; at other times it spreads itself in lurid darkness, threatening storm and tempest, but terminating in rain or wind. If, after a rapid rise in the mercury, this cloud make its appearance in bars, or streaks which seem to converge in the horizon, rain shortly follows. It is in the Cirro- Stratus cloud that hahs, parhelia, paraselenes, &c. are formed. The Cirro-Cumulus, or warm-weather cloud, attends a rising barometer. This pretty modification is often formed from the Cirrus, The Cirro-Stratus will also frequently after rain dissolve into Cirro- Cumulus, an in- dication that the frozen mass of which the Cirro-Stratus is formed is thawed on its descent into a warmer atmo- sphere ; where becoming attenuated, it breaks and splits, leaving clear blue sky between the small round patches of cloud, which take the name of Cirro-Cumulus. This cloud is often seen alone in the higher regions j it then assumes a dappled appearance, or what is popularly called a mackerel-back sky. Coloured Corona have their origin in this cloud. The Cumulus cloud is seen chiefly in the spring and summer months. Its form, when viewed sideways, in- creases from above in dense, convex heaps ; in showery 27 weather it is tufted with the Cirro-Stratus, and in the interval of the showers its texture is fleecy and its form changes rapidly. In hot weather it often appears sta- tionary with a flattened base, its rock-like summits shining with a silvery light. If during a fine morning this cloud suddenly disappear, and it be followed by the Cirro-Stratus cloud with the wind backing to the south, the mercury falls, and rain soon follows. The Cumulus is the day cloud: its great density keeps off the too scorching rays of the noonday sun ; it usually evaporates an hour or two before sunset. When it increases after sunset, and shines with a ruddy cop- per-coloured light, it denotes a thunder-storm. The Cumulus frequently attends a rising barometer. The Cumulus is uncommon during the winter months. The Cumulo-Stratiis cloud is most frequent in the spriiig and summer months. It indicates thunder- gusts, showers of hail and sudden changes of the wind. It is the densest modification of cloud, and as it passes overhead it causes a reduction of temperature. Its form is compounded of the rocky Cumulus, the Cirro-Stratus and Cirro-Cumulus ; its texture is puckered or corru- gated, and before thunder it becomes deeply fringed, so that it appears to touch the ground. It forms the basis of great thunder-storms, its electrical character attracting clouds and scud from all quarters of the heavens, which uniting confusedly, constitute that indescribable black mass always antecedent to storms of thunder and light- ning. The effect of the Cumulo-Stratus cloud on the mer- cury appears to be to give it a tendency to rise. The Nimbus is a modification of the Cumulo-Stratus cloud seen in profile during a shower. Its course can c2 38 be distinctly traced on landj by the dark mist occa- sioned by the rain then actually falling. The Nimbus is never seen with the barometer at great eleva- tions. The rainbow is the lovely attendant of the Nimbus cloud only. The Stratus is the cloud nearest the ground. It is formed from the sudden chill of certain strata of the at- mospherCj which condensingthe vapour containedin them, renders it visible in a misty cloud or creeping fog. Calm weather is essential to the formation of the Stratus ; it is frequent in fine autumnal nights and mornings, sometimes resting on the ground, sometimes hovering some hundred feet above it. It obscures the sun until his rays have raised the temperature of the air sufficiently to evaporate it, when it gradually disappears and leaves a clear blue sky. The Stratus deposits moisture > and when the temperature, from radiation or other causes, sinks below 33°, we find it fettered in icy spicules upon trees and shrubs, and sparkling in exquisite frostwork upon all nature. The Stratus is called the night cloud, and is most frequent from September till January. It has no sen- sible effect on the barometer. Scud is, with the exception of the Stratus, the lowest cloud. It is most commonly seen during the winter months, with every wind that blows and with all press- ures of the atmosphere. It always moves in the direc- tion of the wind, and apparently with great rapidity. It is more frequently seen after rain than at any other period. In our westerly gales in winter, it continues for days together, deforming the sky with its large, loose, shapeless masses. 29 The several modifications of cloud may be separated into two great divisions j the first comprising the Cir- rusj Cirrus- Stratus, Cirro-CumuluSj and Scud, which descend in the atmosphere; these produce rain, wind, &c., and affect the mercurial column : it is during their progressive movement downwards that the barometer is seen to fall. The second division consisting of the Stratus, which gives place to the Cumulus j they have their origin in the lower strata of the atmosphere, and are the ascending clouds ; they are the harbingers of fine weather, and have no effect on the movements of the barometer. It is not uncommon to observe two or three strata of clouds moving in different directions ; the lowest follows the direction of the wind blowing at the time near the surface of the earth; the upper strata follow the cur- rents in the upper regions of the atmosphere; which may be in opposite directions. Before thunder and heavy rain this is of usual occurrence, the barometer at the time being low or in a falling state. In hot sultry weather, especially after a slight fall of the mercury, small clouds sometimes suddenly form on a clear blue sky, and as suddenly vanish ; this is a sure sign of electricity. If the clouds collect without any progressive motion and increase rapidly, and a haze be observed above the clouds, a storm will in all probability be in the vicinity ; but if they move hurriedly towards any particular quarter of the heavens, the storm will be in the direction whither the clouds are seen to hasten : these signs of thunder are seen, though the storm may be 150 miles distant. Much has been accomplished towards gaining a know- ledge of the forms and modifications of the clouds by the classification of Luke Howard. Still, in certain states 30 of the atmosphere, when the clouds mix confusedly and change their forms ahruptly, it is difficult for the inex- perienced to class them ; the prevailing modification of the dayj in connexion with the movement of the baro- meter, is however sufficient to establish the character of the weather. The splendid crimson contrasting with the delicate azure of a fine autumnal sunset, and the golden flood encroaching upon the deep blue of a summer's sunrise, are chiefly referable to the lofty Cirrus and Cirro-Cumu- lus clouds. Perhaps no climate in the temperate zone can boast, during the fine period of the year, of clouds of so many beautiful and so varied forms as Great Britain. They are the production of Great Nature's hand, and are anticipated with equal delight by the painter, the meteorologist and the contemplative mind. A Table showing the average quantity of Rain at Green- wich, Kent, for each month of the year, deduced from thirty-four consecutive years from 1815 to 1848. Month of the year. Average quantity of rain for each month. Greatest quantity of rain recorded in one month. Least quantity of run recorded in one month. in. 1-68 1-S8 1-61 1-73 1-96 1-83 2-37 240 2-40 2-67 2-53 202 in. 4-83 3-69 3-45 4-79 416 4-26 6-65 4-65 4-79 5-37 4-33 4-72 in. 0-30 004 0-40 006 0-60 0-59 010 007 0-40 0-53 0-85 008 April May ..... July Sentember . ...■•■ Mean annual depth ... 24-781 31 From the above synopsis, it appears that the greatest average quantity of rain falls in October and the least in February. The heaviest rains, or those which yield the greatest quantity in the gauge, come down in the summer and early autumnal months. In the summer an inch and a half will sometimes fall in less than an hour in short but impetuous torrents; in the autumathe same quan- tity will occupy many hours in" falling. In winter the number of wet days exceeds that of the summer period ; the average fall of a winter's rain is seldom more than yu of ^.n inch an hour. The amount of snow for the thirty-four years is in- cluded in the above Table. Snow yields yu of water to 1 inch fall in depth ; or a fall of snow of 10 inches in depth on the level would be equal to 1 inch of rain. On the Vapour-Point. The dew- or vapour-point is to many a subject in- volved in so much mystery, and hence apparently of so little importance in meteorological investigations, that very little attention has been practically bestowed upon the registration of connected series of observations; when it is viewed as a means of unfolding the state of the atmosphere with regard to its moisture and dryness, and therefore, as a means of assisting not only the flori- culturist in the preservation of his exotics, but also each individual of the community in the protection of himself from the influences of a variable and humid climate, it will assume an interest which will overcome any sup- posed difficulty in its comprehension. The atmosphere derives its aqueous vapour from evaporation, and is 32 lighter than air in the proportion of 1-000 to 0-625 : as evaporation is caused solely by heat, the quantity of va- pour which any given portion of the atmosphere can con- tain must he dependent on the temperature of the air : Dalton's Table (page 36) of the elastic force of aqueous vapour, shows the quantity which can exist in it at every degree of temperature of Fahr. measured in the height of the mercurial column it can support. When the atmosphere is filled or saturated with vapour, if moisture be added, it will not increase the elasticity of the vapour already present, but it will collect into small drops and become visible in the form of mist or dew ; the temperature at the moment of this collapse of the vapour into dew is called the dew- or vapour-point, by which, and our knowledge of the elasticity of vapour for each degree of temperature, we are enabled to ascer- tain with accuracy the quantity of vapour actually ex- isting at any time in the atmosphere. If at the tem- perature of 50° objects be dewed with moisture, the elasticity of the vapour in the air is at its maximum j it then supports 0-373 in. of the mercurial column, and the dew-point is 50°, the same temperature as the tem- perature of the air : if the temperature of the air should rise to 60°, the moisture would disappear, and the va- pour might increase until its elasticity would support 0-523 in. of the mercurial column, when the tempera- ture of the dew-point would again become the same as the temperature of the air, and moisture would be again deposited. As however the atmosphere is not always saturated with vapour, we must contrive, in order to ascertain the quantity in it, to bring the vapour it does contain to a maximum of elasticity, and force it to de- posit moisture. This is efiected by evaporation; the liquid, as it passes into vapour, abstracting the heat con- 33 tiguous to the evaporating surfacej to maintain itself in the vaporous form, reduces the temperature of the air until the vapour contained in it deposits dew ; im- mediately the deposition takes place the dew-point is obtained ; the temperature of evaporation must then be observed, and referring to Dalton's Table (page 36) we shall find under that temperature the elastic force of the vapour actually existing at that time in the atmo- sphere. The higher the temperature of the air the greater the quantity of vapour it can contain ; if it ap- proach saturation at a high temperature, its closeness and sultriness are oppressive ; if, on the contrary, it be too dry, its harshness and chilliness are unpleasant tO' the sensations, as is experienced during the prevalence of the easterly wiuds in the spring months : the tem-. perature at which our atmosphere is most frequently saturated, is rather below the mean of the climate. These different hygrometric states of the atmosphere afford a satisfactory explanation of the low tempera- tures of spring producing little moisture during the nights, while the heavy loaded vaporous atmosphere of autumn not only deposits copious dews, but originates the Stratus cloud and fogs. A Daniell's hygrometer gives the dew-point by inspection and requires no com- putation, but its manipulation is so delicate and good ether so difficult and expensive to purchase, that for popular use the wet and dry thermometer known as Mason's Hygrometer has been universally adopted * : the following Table and. Rule will facilitate the reduc- tion of the observations made with it. * A very ingenious organic Hygrometer has been lately contrived by E. Simmons of Coleman, Street ; which shovfs approximately at sight, the dew-point, and relative humidity. 34 Table of Factors for deducing the Dew-point from the temperature of the air and the temperature of evapora- tion. [From the " Greenwich Magnetical and Meteoro- logical Observations," 1844.) Headings of the diy-bulb Factor. thermometer. Between 28 and 29 57 ... 29 ... 30 50 .. 30 ... 31 4-6 .. 31 ... 32 3-6 .. 32 ... 33 31 .. 33 ... 34 ' 2-8 .. 34 ... 35 2-6 .. 35 ... 40 2-4 .. 40 ... 45 2-3 .. 45 ... 50 2-2 .. 50 ... 55 21 .. 65 ... 60 19 .. 60 ... 70 1-8 .. 70 ... 80 1-7 .. 80 ... 85 1-6 .. 85 ... 90 1-8 Rule. — Multiply the difference between the two ther- mometers by the factor corresponding to the tempera- ture of the dry-bulb thermometerj and subtract the pro- duct from it ; the remainder will be the temperature of the dew- or vapour-point. o Let dry-bulb thermometer =66 Let wet-bulb thermometer =57 Difference = 9 1-8 Product =16-2 66° - 16°-2 = 49°-8 = dew-point. The elasticity of vapour at the temperature of dew- point, 49°"8=0'372 in. j the weight of a cubic foot of 35 vapour at the temperature of dew-point, 49°-8=4-13 grs. ; but as the temperature of the air is 66°, the elas- ticity of the vapour may increase to 0'638 in. ; and the weight of a cubic foot of vapour at 66° is 7'08 grs. ; therefore the air requires 2'95 grs. more of vapour to become completely saturated with vapour. The relative humidity of the air is found by consider- ing complete saturation as unity or I'OOO. In all other cases divide the number of grains of vapour contained in a cubic foot of air at dew-point, by the number of grains contained in a cubic foot at the temperature of the air ; the quotient wUl always be less than unity. In the above example 4'13 grs. divided by 7"08 grs. will give 0'583 for the relative humidity. To separate the gaseous from the aqueous pressure, subtract the elastic force of vapour at the temperature of the dew-point from the height of the mercurial column, the remainder will be the gaseous pressure. in. Reading of barometer = SO'OOO Elastic force of vapour at 49°-8 = 0-373 Gaseous pressure = 39'638 Note. — Dr. Apjohn's formula for finding dew-point is — Above 32°. Below 32°. ■^ -^ 88 30 -^ "^ 96 30" Where /" represents the force of aqueous vapour at temperature of dew-point. f represents the force of vapour at temperature of evaporation. d represents the difference between dry and wet thermometers. h height of barometer. 36 . Table showing the elastic force of aqueous vapour accm-ding to Dalton; and also the weight in grains Troy of a cubic foot of vapour as determined by Gay- Lussac/oT- every degree of Fahrenheit from 0° to 90°. Weight Weight Weight Tempe- F orce in grains Tempe- Force in grains Tempe- Force in grains rature. of Troy of rature. of Troy of rature. of Troy of Fahren- aq ueous cubic Fahren- aqueous cubic Fahren- aqueous cubic heit. va pour. foot of vapour*. heit. vapour. foot of vapour. heit. vapour. foot of vapour. o i" gr. o in. gr- in. gr- 061 0-78 37 0-238 2^80 64 0-597 6-65 5 074 0-93 38 ■246 2-89 65 -617 6-87 10 089 111 39 •255 2^99 66 -638 708 12 096 119 40 •264 309 67 -659 7-30 U 104 1-28 41 •274 319 68 -681 7^53 15 108 1-32 42 •283 3-30 69 -704 7-76 16 112 1-37 43 •293 3-41 70 -727 800 17 116 1-41 44 ■304 3^52 71 -751 8-25 18 120 1-47 45 •315 3-64 72 -776 8-50 19 125 1-52 46 •328 3-76 73 -801 8-76 20 129 1-58 47 •337 3-88 74 -827 904 21 134 1-63 48 •349 401 75 -854 9-31 22 139 1-69 49 •361 4-14 76 -882 960 23 144 1-75 50 •373 4-28 77 -910 9-89 24 150 1-81 51 •386 4-42 78 -940 1019 25 155 1-87 52 •400 4-56 79 0-970 10-50 26 161 193 53 •414 4-71 80 1001 10-81 27 167 200 54 •428 4-86 81 1034 U-41 28 173 207 55 •442 5 02 82 1067 11-47 •i^ 179 214 56 •458 518 83 1-101 11-82 30 186 2-21 57 ■473 5-34 84 1135 1217 31 192 2-29 58 •489 5-51 85 1-171 12-53 32 199 2-37 59 •506 5-69 86 1-208 12-91 33 207 2-45 60 ■523 5-87 87 1-247 13-29 34 214 2-53 61 ■541 606 88 1-286 13-68 35 222 262 62 ■559 625 89 1-326 1408 36 230 2-71 63 •578 6-45 90 ■ 1-369 14-50 * From the best authorities it is assumed that a cubic foot of dry air with a pressure of 30 inches and temperature of 32° of Fahrenheit weighs 563 grains. 37 * The following Table and Theorem are from Si}' George Shuckburgh, and will show how the Barometer is med for ascertaining the Height of Mountains. Eocplanation of Table I. This Table gives the number of feet in a column of the atmosphere equi- valent in weight to a like column of mercury -^^ of an inch high, when the barometer stands at 30 incheSj for every 5 degrees of temperature ranging from 32° to 80° ; .and from this Table II. has been constructed as more convenient for general use : — Table II. Table I. Thermometer. Feet. 'si 86'85 35 87-49 40 88-54 45 89-60 50 90-66 55 91-72 60 92-77 65 93-83 70 94-88 75 95-93 80 96-99 Thermo- Factor. Thermo- Factor. Thermo- Factor. meter. meter. meter. 30 864-4 47 900-2 el 936-1 31 866-5 48 902-3 65 938-2 32 868-5 49 904-5 66 940-3 33 870-6 50 906-6 67 942-4 34 872-7 51 908-7 68 944-5 35 874-9 52 910-8 69 946-7 36 877-0 53 913-0 70 948-8 37 879-2 54 9151 71 950-9 38 881-3 55 917-2 72 953-0 39 883-4 56 9193 73 9551 40 885-4 57 921-4 74 957-2 41 887-5 58 923-5 75 959-3 42 889-6 59 925-6 76 961-4 43 891-7 60 927-7 77 962-5 44 893-8 61 929-8 78 965-6 45 896-0 62 931-9 79 967-7 46 898-1 63 934-0 80 969-9 * To perform this operation accurately, two persons should take contemporary observations -with two barometers and thermometers, the one at the bottom of the hill and the other at the top. 38 Rule. — Let a?=height of mountain required. A=tlie mean height of the two barometers in inches. a=the difference of the two. S=the number in Table II. corresponding to the mean height of the two thermometers. (Barometer at 30 inches.) Then x-. QQab Example. — Suppose the barometer at the bottom of the mountain to stand at 30 inches, thermometer 60° ; the barometer at the top 36'36 inches, thermometer 46° ; required the height of the mountain, say Snowdon. The mean of the two barometers, or A, is 28'18 inches; their diiFerence, or a, 3 '64 inches; and the mean of the two thermometers, or h, 53°. In Table II. 913'0 is opposite to 53° ; therefore 30 X 3-64 X 913-0 28-18 -=3538-0 feet. A Table of the velocities and pressures of the Wind. Miles - per hour. Force in lbs. on square foot. 5 10 15 20 30 35 40 45 50 60 80 100 012 0-49 111 1-97 4-43 603 7-87 996 12-30 16-71 31-49 49-20 Gentle breeze. ■ A brisk gale. Very brisk. ■ High winds. j-Very high. A storm. A great storm. "] Tears up trees • and destroys J . aU before it. Depression of Mercury in glass tubes, or cor- rections to be added for capillary attrac- tion. 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