HI 1*51 New York State College of Agriculture At Cornell University Ithaca, N. Y. Library Cornell University Library QC 863.B86 1859 Elements of meteorology, with questions 3 1924 002 965 675 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/cu31924002965675 ■(?*'< y ^ ::. £','$ ^^ •/ ■ ! *y/** A^ /.fc~C~ < Volcanic Showers «?J Cause 259 Instances— Jorullo *™ Souffriere f- Tomboro f}> Cosiguina . *?* Yellow Rains— Pollen-rams ^j- Gossamer-shower z °" CHAPTER II. DRY POO AND INDIAN SOMMER EAZE. ft** :::::::::: SS Instances ogg Cause • ■ o Bfi Indian-summer Haze £ 6g Cause METEOROLOGY. 1. Meteorology is that branch op natural science which treats op the atmosphere and its phenomena. The subject may be properly divided into six parts. 2. Part I. THE ATMOSPHERE. Part II. AERIAL PHENOMENA— comprehend- ing Winds in general, Hurricanes, Tornadoes, and Water-spouts. Part III. AQUEOUS PHENOMENA— including Rain, Fogs, Clouds, Dew, Hoar-frost and Snow, and Hail. Part IV. ELECTRICAL PHENOMENA— com- prising Atmospheric Electricity and Thunder-storms. Part V. OPTICAL PHENOMENA— including the Color of the Atmosphere and Clouds, Rainbow, Mirage, Coronas, and Haloes. Part VI. LUMINOUS PHENOMENA— embra- cing Meteorites, Shooting Stars and Meteoric Showers, &nd the Aurora Borealis. Part VII. MISCELLANEOUS PHENOME- NA — including the Fall of Terrestrial Substances foreign to the Atmosphere, and Dry Fog and Indian ' Summer Haze. What ia Meteorology 1 Into how many parts is it divided? Rehearse the several parts with their subjects. PART I. OF THE ATMOSPHERE. 3. As the common properties of the air, viz., weight, fluidity and elasticity, are supposed to be already known, C. 502,) we shall proceed at once to the discussion of the entire body of air, termed the atmosphere ; and first of its pressure, which is ascertained by the barometer, an instrument so called from the Greek words, baros. weight, and metron, measure. BAROMETER. 4. This instrument is of the highest importance ir Meteorology, and requires a minute description. It is thus constructed. Into a glass tube, about three feet in length, open at one end and closed at the other, mer- cury is poured until it is full ; the open end being now closed by the finger, or any other means, the tube is in- verted, and the lower end immersed in a vessel of mer- cury. When beneath the surface of the fluid the end is unstopped, and the column of mercury within the tube then settles down, until its summit is about thirty inch- es above the level of that within the vessel. The space above the column in the tube is a void, and is called the Torricellian vacuum, from Torricelli, the name of the Italian philosopher, who first constructed this instru- ment. 5. The column of mercury within the tube is sup- ported above the level of that in the vessel, by the up- ward pressure of a column of the atmosphere, having the same base as itself What ia the atmosphere 1 How is its pressure ascertained 1 How is the barometer made 1 What supports the column of mercury 1 BAROMETER. 15 6. Thus, in fig. 1., the atmospheric F «- L column a a, of indefinite length, but of the same size as the barometric column Db, presses upon the mercury in the vessel. K, with a force equal to its own weight ; now since any force, acting upon a fluid, is communicated in every direction, this pressure will be transmitted through the mercury, in the direction of the arrows, and acts at D, within the tube, against the mercurial column Db. This upward force will be resisted at D, by the weight of Db, and the mercury will sink in the tube until the two pressures counterpoise each other, in exactly trie same manner as two equal weights in the opposite scales of a balance. 7. From these considerations, it is man- , : ,, ifest, that the weight of the atmospheric gn| hj_k column a a is equal to that of the mercuri- Jh§j S3| al column, Db of the same base ; and this |If weight can be estimated in the following Hill , ji manner. If the base at D contains one {SpsT square inch, the column Db, at its usual babometer. height, will contain, nearly, 30 cubic inches ; and since one cubic inch of mercury weighs 3426.76 grains, the weight of thirty will amount to 102802.8 grains. This product being now divided by 7004, the number of grains in a pound avoirdupois, the result will bo nearly 14.7 lbs. ; a quantity equal to the weight of the barometric column, and consequently to the pressure of the atmosphere on every square inch of surface. 8. Any increase in the density of the atmosphere will be denoted by an elevation of the mercury, and a de- crease by its depression. The cause of this is obvious, in the first case, a a becomes heavier, and requires more Explain Figure 1. How is the pressure of the atmosphere, on every square inch, computed 1 How does any change in the density of the air affect the height of th« barometer? 16 THE ATMOSPHERE. mercury to balance it ; therefore Db is lengthened. In the second case, a a is lighter, and, as a less quantity of mercury will then balance it, Db is shortened. Such changes are constantly occurring, but are very minute, and, in order that they may be accurately indicated, the instrument must be made with the nicest care. 9. To secure a perfeQt instrument, it is essential that the mercury should be free from any solid impurities, else the summit of the column will either be above, or below, its proper level, according as the foreign matter, mixed with the mercury, is lighter, or heavier, than the fluid. This end is attained by straining the mercury through chamois leather. If it is amalgamated with zinc, or lead, it is purified by washing it with acetic, or sulphuric acid. • 10. When the tube is filled, moisture and small bub- bles of air are found adhering to its interior surface, and are also contained in the mercury. These, if not expell ed, will ascend when the tube is inverted into the Torri- cellian vacuum, the moisture rising in vapor. By their united elastic force, the ascent of the barometric column will be checked, whenever any increase in the density of the atmosphere tends to elevate it. 11. This source of error is removed by boiling the mercury in the tube. When all the air and vapor are expelled, the tube, if gently struck, will give forth a dry, metallic sound ; but if a bubble of air remains, the sound will be dull and heavy. By connecting the open end of the tube with an air, pump, during the process of boiling, Dr. Jackson, of Boston, has still more effectually removed this imperfection. 12. By these means, the air may perhaps be totally excluded, when the instrument is first constructed ; but in the course of time, it will again insinuate itself be- tween the glass and the mercurial column. To pre- vent this evil, Prof. Daniell, of King's College, London, What precautions are adopted to secure a perfect barometer 1 How is the mercury purified, and why "! How are moisture and air expelled from the tube, and why 1 ? What is Prof. Daniell's improvement? BAROMETER. 17 welds to the open end of the glass tube a ring of plati- num, which possesses a greater affinity for mercury than glass. The mercury adheres closely to the platinum, like water, and the passage of air, according to all ex- periments, appears thus to be effectually prevented. 13. Since the constant changes in the weight of the atmosphere produce corresponding fluctuations in the height of the barometer, a scale is placed near the top of the tube, extending from twenty-seven to thirty-one inches, a space, which includes,' at the surface of the earth, all the fluctuations of the column. This scale is divided into tenths of an inch ; but, as the variations of the barometer are exceedingly minute, a contrivance, called a vernier, is annexed, by which a change, to the extent of one five hundredth of an inch, can be easily measured. 14. As the surface of the mercury, in the reservoir, is raised by the descent of the column, and depressed by its elevation, any change in the height of the barometer cannot be accurately estimated, while the scale remains in the same position; unless this surface is always brought to the same point, before taking an observation. The necessity of so doing will be obvious, from the fol lowing illustration. Suppose the surface of the mercury in the cistern K, figure 1., to be fifty square inches, while that of a hor- izontal section of the column is but one. Should the barometer sink one inch, the surface of the mercury in the cistern will rise one fiftieth of an inch, and the amount of the depression of the column, if measured from this surface, will be only forty-nine fiftieths of an inch, instead of one inch, its true depression. 15. The contrivance employed by Fortin, a celebrat- ed French artist, to remove this error, consists in ad- justing to the cistern K, fig. 1., a movable bottom, which can be elevated or depressed, by means of the screw What is the length of the barometric scale 1 How small a variation in height can be measured 1 What is Fortin's contrivance, and for what purpose adopted 1 18 THE ATMOSPHERE. P, until the surface of the mercury shall just touch the fixed ivory index L, at its lower extremity ; which point is the zero of the scale, or the place from which the height of the barometer begins to be reckoned. 16. When, by adopting the previous precautions, the barometer has been so far perfected, two corrections are still necessary, before recording observations ; the Irst for temperature, and the second for capillarity. That of temperature depends upon the expansion of -.he mercury and the scale ; the latter being partially corrective of the former, inasmuch as the divisions of measurement upon the scale, lengthen, at the same time, with the column of mercury. 17. Temperature.- Mercury dilates, for every de- gree Fah. about one ten-thousandth part of its bulk, taken at the freezing point. The expansion of the scale varies with the metal of which it is composed, but its amount is, usually, so small, that it may safely be neg- lected in the required correction. Hence the following practical rule has been adopted, for reducing any ob- served altitude of the barometer, to the corresponding altitude, at the freezing point. " Subtract the ten-thou- sandth part of the observed altitude, for every degree above the freezing point." Thus, if the barometer stands at 29 inches, and the thermometer at 52 3 , the required correction is 20 X .0001 X 29 = 058. If the temperature is below 32°, the correction must be added. To facilitate these calculations, a thermometer is always attached to the barometer. 18. Capillarity. By capillary attraction is (C. 53,) understood, the force exerted by the interior surface of small tubes, upon the fluids contained within them. When the fluid moistens the tube, it rises above its pro- per level ; but when it does not, as in the case of mer- cury, it sinks below it. From this cause, a depression, termed its capillarity, occurs in the barometer, the extent How is the barometer affected by a change in temperature 1 Give the rule for reducing the height to the corresponding height at tha freezing point. Why is capillarity a source of error 1 PRESSURE OF THE ATMOSPHERE. 19 of which is dependent upon the size of the interior diam- eter of the tube, and a correction for this must be added to the apparent height, in order to obtain the true alti- tude. In tubes of a small bore, the error from this source is considerable ; but when the diameter exceeds half an inch, it becomes so small, that it may safely be neglected. This will be rendered evident by the inspec- tion of the following table, which gives the amount of depression for tubes of various sizes. Diameter of tube. Depression. inches. inches. .10 1403 .20 0581 • .40 0153 .50 0083 19. When the instrument is not stationary, but is carried from clime to clime, and to different heights above the sea-level, two other corrections are necessary ; one for the varying force of gravity, in different latitudes, and the other for the change of pressure, which dimin- ishes with every increase of altitude above the ocean. Such is the barometer, an instrument of great prac- tical use, and of the highest value in meteorological re- searches. X PRESSURE OF THE ATMOSPHERE. 20. Variation in Latitude. The mean or average pressure of the atmosphere, as indicated by the barom- eter, is found to be nearly the same in all latitudes, when every essential correction is made. It increases a little from the equator to about the 30th degree of latitude, where itj.is greatest ; it then decreases to nearly khe 64th degree$\vhere it is least ; after this it again increases, and between the 75th and 76th degrees, the pressure is equal to that of the equatorial climes. All Is i&greater in tubes of a small or large bore 1 When the barometer is portable, what other corrections are necessary ? What is said of the barometer 1 In what manner does the pressure of the atmosphere vary in latitude J 20 THE ATMOSPHERE. this is obvious from the following table, founded upon observations, where corrections are made for gravity, altitude above the sea-level, and temperature. PLACES. LATITUDE. HEIGHT OF BAROMETER. Cape of Good Hope, . Christianburg, . . . Tripoli Godthaab, .... Spitzbergen, . . . 33° S. 5° 30' N. 33° N. 64° N. 75° 30' N. Inches. 29.955 29.796 30.127 29.598 29.801 21. The pressure of the atmosphere at any given spot is not invariable ; for the height of the barometer is perpetually changing throughout the year. The ex- tent of its fluctuations is, however, by no means the same in all places, being least at the equator, and great- est towards the poles. Thus its range within the tropics is but a little more than one-fourth of an inch ; at New York, 40° 42' 40" N. lat., 2.265 inches, from the observa- tions of five years ; at St. Johns, Newfoundland, 47° 34' 3" N. lat., 2.54 inches, during the same period ; while in Great Britain it amounts to three inches. The greatest fluctuations occur between the 30th and 60th degrees ot latitude. 22. There is also a constant daily variation in the atmospheric pressure, for the barometer, as a general rule, falls from 10 o'clock, A M. to 4, P. M. ; it then rises until 10, P. M., when it again begins to descend, reaching its lowest point at 4, A. M. ; from this time it rises, until it once more attains its highest elevation, at 10, A. M. These variations are exceedingly minute, and contrary to the annual range, ate greatest at the; equator, and decrease with the latitude ; disappearing about the parallel of 60°. 23. This variation amounts at Give examples. Where are the annual fluctuations of the barometer least t Where greatest 1 Give examples. Describe the diurnal variations. Where greatest 1 Where least ? PRESSURE OP THE ATMOSPHERE. 21 , PLACES. LATITUDE. INCHES. Rio Janeiro, . . . St. Petersburg, . . 22° 54' S. 12° 3' S. 22° 35' N. 59° 56' N. to .067 to .10 to .072 to .005 In the tropical regions, according to Humboldt, so reg- ular are the diurnal changes, that the barometer indi- cates true time, within a quarter of an hour. These daily fluctuations, in the atmospheric pressure, for a long time, perplexed meteorologists, but their cause has, at length, been discovered, by means of the late ob- servations, at the English observatories. They are found to arise from the stated variations in temperature, that occur during the day. 24. Variations in Altitude. As we ascend above the surface of the earth, we leave a portion of the at- mosphere below us, and are freed from its pressure. This fact is denoted by the fall of the barometer. When De Luc, a French philosopher, ascended to the height of 20,000 feet, his barometer sunk to twelve inches. In 1838, the aeronaut Green, rose from Vauxhall gardens, in London, to an elevation of nearly three miles and three quarters; the mercury in the barometer gradually de- scending, from thirty inches to fourteen and seven-tenths. 25. As a general rule, this depression, near the sur- face of the earth amounts to one-tenth of an inch for every eighty-seven feet in altitude ; but where perfect accuracy is required, several corrections must be made. The barometer then becomes, in the hands of skillful observers, an important instrument for determining alti- tudes, and so exact are its indications, that two inde- Give examples. What is said by Humboldt of their regularity in the tropics 1 How are they caused 1 How is the pressure of the air influenced by the altitude 1 What instrument indicates the ehanges of pressure 1 In the instances given, how low did the mercury sink ? What is the-law of depression ■? For what purpose is the barometer sometimes employed ? Give instances. 22 THE ATMOSPHERE. pendent estimates of the height of Mount J3tna, made by means of this instrument, differ only one foot ; that of Capt. Smyth being 10,874 feet, while Sir John Her- schel's is 10,873 feet, DENSITY OF THE ATMOSPHERE. 26. When one portion of the atmosphere is said to be more dense than another, all that is meant is simply this ; that a given volume, or bulk, of the first portion, as one gallon, contains more aerial particles than an equal volume of the second ; thus, if it contains twice as many particles, it is said to be twice as dense. 27. The density of the atmosphere decreases with the altitude. This result is caused by the diminished pressure of the air, and the decreasing force of gravity. Imagine the atmosphere to be divided into a vast num- ber of thin, concen- Fi? 2 trie strata, which in figure 2, are repre- sented by the spaces between the lines 1-2, 2-3, 3-4, 4-5, &c. Now it is clear, that the particles in each layer are pressed together by the whole weight of the atmosphere above them, while, at the same time, they are drawn together by the force of gravity. Vari- ations in the latter power are only appreciable at great distances from the earth, and the observed changes in density, at two or more stations, may therefore be as- cribed to the difference in the weight or pressure, of the superincumbent atmosphere. The height of the barom- eter, at different elevations, thus denotes the density of the air at these points. ATMOSPHERIC STRATA. When is one portion of the atmosphere denser than another 1 What two causes principally influence its density 1 Describe figure 2. Which cause may be neglected 1 What instrument measures the density 1 DENSITY OF THE ATMOSPHERE. 23 28. The density, however, is not exactly proportioned to the pressure, slight modifications arising, from sev- eral causes ; the most important of which is tempera- ture. The heat of the atmosphere decreases with the altitude, and since heat expands, and cold contracts, a given volume of air, Tsrrth part of its bulk, at 32°, for every degree Fah. ; or in other words, thus lessens and increases, its density, a correction must be made for this influence. 29. It has been found by calculation, combined with observations, that, if the altitudes are represented by an increasing arithmetical series, the densities of the at- mosphere decrease in a geometrical progression. Thus, if at the height of 18,000 feet the air, as the barometer indicates, is but half as dense, as at the surface of the earth ; at 36,000 feet it will be reduced to one-fourth, and at 54,000 feet to one-eighth of its original density. 30. The rarefaction of the air at lofty elevations, les- sens the intensity of sound, impedes respiration, and causes the minute veins of the body to swell and open. Thus, at a short distance, the report of a pistol upon the summit of Mont Blanc, can scarcely be heard. Gay Lussac and Biot, ascending from Paris, in a bal]oon y to the height of 25,000 feet, breathed with pain and diffi- culty, and upon the high table lands of Peru, the lips of Dr. Tschudi, cracked and burst ; while the blood flowed from the veins of his eyelids. In consequence of this diminution of pressure, water boils, in such situations, at a comparatively low tempe- rature ; thus, at Quito in Equador, 9,537 feet above the sea level, ebullition takes place at the temperature of 196° Fah. In what manner does temperature affect the density 1 What is the law of decrease in reference to altitude ? Illustrate. What are the effects of a rarefied atmosphere 1 Give instances. 24 THE ATMOSPHERE. WEIGHT OF THE ATMOSPHERE. 31. We have seen, that a column of mercury, about thirty inches in height, weighs, at the surface of the earth, exactly the same as a column of the atmosphere, possessing the same base. If then the globe was cov- ered with an ocean of mercury, thirty inches in depth, the latter would occupy the identical base that the at- mosphere does now, and their respective weights might be regarded as equal. 32. Under this supposition, the diameter of the earth would be increased five feet. The difference then, in cubic feet, between the solidity of the earth, and that of a globe, whose diameter is five feet greater, will equal the number of cubic feet in the sea of mercury. This number multiplied by the weight of a cubic foot of mer cury, viz. 848,125 lbs., will equal that of the whole mass, which is the same as the weight of the atmosphere. This calculation has been made, and amounts to more than five thousand billions of tons. t TEMPERATURE OF THE ATMOSPHERE. 33. The entire body of air surrounding the globe ap- pears to be warmed in two ways ; first by the luminous beams of the sun, secondly, by the radiation of heat from the earth. 34. According to Kaemtz and. Martin, the atmo- sphere absorbs nearly one-half the daily amount of heat, which emanates from the sun to the earth, even when the sky is perfectly serene. The remaining portion fall- ing upon the surface of the ground, elevates its tempera- ture, and. the earth sends back into the atmosphere rays of invisible heat. 35. Modern researches have shown, that all bodies, through which heat can pass, absorb a greater propor- How is the weight of the atmosphere computed 1 How many tons does it weigh 1 How is the atmosphere warmed 1 THERMOMETER. 25 tion of non-luminous, than of luminous calorific rays. The heat, therefore, that radiates from the earth, will not pierce the atmosphere, with the power of the solar ray ; all will be retained by the lower strata of air, which in their turn, diffuse invisible thermic rays, in every direction. 36. We . thus perceive, what all observations have proved, that the upper regions of the atmosphere must be colder than the lower. It is not, however, to be for- gotten, that the rarefaction of the superior strata con- tributes to this condition. THERMOMETER. 37. The temperature of the atmosphere is indicated by the thermometer, an instrument, which derives its name from the Greek words, thermos, warm, and metron, measure. It consists of a small glass tube, terminated by a bulb, and is partially filled with mercury. This fluid is usually preferred for several reasons, the most important of which are, its uniform dilation, its quick susceptibility to any change in temperature, and the great range of its expansion in the fluid state. If the instrument is to be exposed to extreme cold, alcohol must be used. 38. As mercury, like other fluids, expands by heat, and contracts by cold, its alternate elevations and de- pressions within the tube, can be made to indicate the corresponding changes in the state of the air, if two fixed temperatures can be found, whence to reckon the changes. These have been discovered. If a thermom- eter is immersed, at different times, in melting snow, the column of mercury invariably sinks to the same place in the tube, though many months may have elapsed between the experiments ; and, when exposed to the steam of boiling water, the mercury always as- ls it heated most by luminous or non-luminous heat 1 Are the upper or lower regions of the atmosphere the warmest t How is the temperature of the atmosphere measured'? Describe the thermometer. Why is mercury used 1 How are the two fixed temperatures obtained 1 26 THE ATMOSPHERE. 2/£ . . . Boiling point. cends to the same height, under the same atmospheric pressure. 39. These invariable positions, which are termed the freezing and the boiling points, are marked upon the scale to which the tube is affixed. In Fahrenheit's ther- mometer, figure 3., the interval between them is divided into 180 parts, each of which is called a degree (1°) and as the freezing point is marked 32°, the boiling is therefore 212°. The divisions are extended downwards from 32° to 0, or the zero point, and when extreme degrees of cold are to be measured, the range is con- tinued to 20°, 40°, and even 60° below zero. If the air is colder than 40° below zero, a spirit ther- mometer must be used, since mer- cury becomes solid at this tem- perature. When Simpson, a late northern traveller, wintered, in 1838, at Fort Confidence, 67° N. lat., he cast a bullet of mercury, the temperature being 49° below zero. Upon firing the ball, it passed through an inch plank, at the distance of ten paces ; but flattened and broke against the wall, three or four paces beyond. In addition to the mode of graduation adopted by Fahren- heit, several others prevail (C. 570), which it is not ne- cessary here to discuss. 40. The thermometer employed for meteorological purposes, should be made as accurate as possible, and in Fi g. 3. 2/£ 12 52 32 20 32_ . . . Freezing point Zero. FAHRENHEIT'S THERMOMETER Into how many intervals is the space between them divided in Fahren heit's scale ? What are the intervals called 1 How many degrees is the freezing point 1 How many the boiling point ■? What is the zero point 1 When must a spirit thermometer be used? Relate Simpson's experiment. SELF-REGISTERING THERMOMETER. 27 order to ensure its perfection, many niceties must be ob- served in its construction. 4L First. The tube should be of equal size throughout the whole stem ; else the same increase of temperature will not produce the same increase in the height of the mercury, throughout every part of the tube ; and so of the decrease. Secondly. The bulb should be large in proportion to the tube ; for then slight changes in temperature will be rendered perceptible, and the delicacy of the instru- ment increased. Thirdly. The mercury should be pure, dry, and recently boiled, in order to free it from air ; and, when in the tube, should there again be boiled, to drive off any air or moisture collected within. Lastly. When the mercury is at the summit of the tube, and every thing else has been expelled, the top of the tube must be perfectly closed by the fusion of the glass, leaving, when the mercury has cooled, a void space or vacuum above. 42. When a thermometer has been exposed to great changes in .temperature during the course of a year, the position of the freezing point upon the scale is found to be somewhat altered ; for, if the instrument is then placed in melting snow, the mercury is usually seen to stand a little higher than 32°, and less than 33°. This change would occasion a constant error in the ob- servations, and meteorologists therefore verify their thermometers at stated intervals, in the way just men- tioned. SELF-REGISTERING THERMOMETER. 43. The object for which this instrument is con- structed, is to obtain, in the absence of the observer, the highest and lowest temperature of the day, or of any other interval of time. What precautions must be taken to construct an accurate thermometer 1 What change occurs in the position of the freezing point 1 How are thermometers verified 1 For what purpose is a self-registering thermometer used 1 28 THE ATMOSPHERE. One of the most correct thermom- Fig - *■ eters of this kind, now in use, is that invented by Mr. James Six, of Col- chester, which is represented in Fig. 4. It consists of a long glass bulb, G, narrowing into a fine tube, which is first bent downward, forming the arm a b, and then upwards, forming the arm c d, which terminates in a small cavity, L. The two arms con- tain mercury, which extends down from a on one side, and up to c on the other : the bulb and the rest of the tube are filled with alcohol, ex- cept the upper part of the cavity L. Upon the top of the mercury in each arm rests an index (which is more perfectly seen at A), consisting of a piece of iron wire capped with ena- mel, and loosely twined with a fine glass thread ; when the mercury de- scends, the index would fall, were it not for the glass thread, which, press- ing like a spring against the sides of the tube, supports the index, in any position. 44. The action of the instrument is as follows : When an increase of temperature ex- pands the spirit, the mercury is depressed in the arm a b, and elevated in c d, carrying the index up with it. If the temperature now falls, the spirit contracts, and the mercury descends in c d ; but the index remains in its last position, from the pressure of the glass spring against the tube ; and, as it does not fit tightly to the latter, the alcohol above it flows readily by. As the cold augments, the mercury rises in a b, bear- ing up the index of this arm, until an increase of tem- perature occurs, when the mercury here falls, and the six's self-registerino THERMOMETER. Describe Six's, from fig. 4. MEAN DAILY TEMPERATURE. 29 index continues stationary. Thus, the highest point to which the index rises in the arm, a b, indicates the least temperature, and that in c d the greatest, that happens in any interval of time, as a day, or a year ; and the scale, as is evident from the figure, is graduated accord- ingly. 45. After every observation, each index requires to be adjusted ; this is done by means of a magnet, which, being moved down the side of the arm, draws the index after it. Another instrument of this kind was invented by Rutherford, (C. 575.) MEAN DAILY TEMPERATURE. 46. The mean or average temperature of the day, would be accurately found by observing the thermom- eter at intervals of an hour during the whole twenty- four, and dividing the sum of the temperatures by the number of observations, viz., twenty-four. This method is however too laborious, and meteorologists have en- deavored to arrive at the same result from two or three daily observations. 47. According to Kaemtz, a celebrated German me- teorologist, if, in Germany, the thermometer is noted at 6, A. M., 2, P. M., and 10, P. M., and the sum of the temperatures divided by three, the quotient will differ but little from the true mean. The rule adopted in the State of New York, under the direction of the Regents of the University, is as follows : Mark the temperature, first, between daylight and sunrise ; secondly, between 2 and 4, P. M. ; thirdly, an hour after sunset : add together the first observation, twice the second and third, and the first of the next day, and divide the sum by six ; the result will be the mean. The mean daily temperature at Philadelphia has been found, from the hourly observations of Capt. Mor- What is understood by the mean daily temperature ? How is it obtained? 30 THE ATMOSPHERE. decai, to be one degree less than the temperature at 9, A. M. 48. By taking the average of all the mean daily tem- peratures throughout the year, the mean annual tem- perature is ascertained. It is also obtained by the aid of the self-registering thermometer, the average of the two extreme temperatures being regarded as the mean of each day. 49. Variations op Temperature in Latitude. By comparing situations differing widely in latitude, it is found that the average annual temperature of the atmosphere diminishes from the equator towards either pole. This will be seen from the annexed table, whic h presents the results at the sea level, for nine places. PLACES. LAT. TEMP. PLACES. LAT. TEMP. Fahren. Fahren. Falkland Isles, 51° S. 47° .23 Calcutta, . . 22°35'N. 78° .44 Buenos Ayres, 34° 36' S. 62° .6 Savannah, 32°05'N. 64° .58 Rio Janeiro, . 22° 56' S. 73° .96 London, . . 51°31'N. 50° .72 Maranham, . 2° 29' S. 81° .32 Melville Isle, . 74°47'N. 1.66 be- Trincomalee, 8° 34' N. 81° .32 low zero. 50. From this table it is also evident, that places hav- ing the same latitudes, in the two hemispheres, do not necessarily possess the same average temperature. This is owing to a great variety of local causes, the effect of which cannot always be accurately estimated. 51. Variations in Altitude. The temperature of the air diminishes with the altitude, but the law of de- crease is very irregular, being affected by the latitude, seasons, hours of the day, and a diversity of local cir- cumstances. It may however be assumed, as a gen- eral rule, that a loss of heat occurs to the extent of one degree Fah.for every 343 feet of elevation. This is an How is the mean annual temperature found 1 How does the temperature of the atmosphere vary in respect to latitude 1 Give examples. Do like latitudes in different hemispheres have the same temperature 1 How is the temperature affected by altitude 1 What is the general law of decrease 1 MEAN DAILY TEMPERATURE. 31 average result, for the rate of decrease is very rapid near the earth, after which it proceeds more slowly, but at the loftiest heights is again accelerated. 52. During the winter of 1838, the French scientific commission stationed at Bossekop, in West Finmark, 69° 58 N. lat., found this law partially reversed, amid the rigors of a polar clime ; the temperature of the atmosphere increasing, nearly, 3° Fah. for the first 328 feet in height ; beyond this limit it began to decrease, at first slowly, but afterwards with greater rapidity. Dur- ing the summer, the temperature decreased with the altitude. 53. As a consequence of this gradual reduction of heat, a point at length may be attained, in any latitude, if we continue to ascend, where moisture, once frozen, always remains congealed. Hence, arise the eternal snows and glaciers, that crown the summits of the high- est mountains. 54. Since the mean temperature of the air is highest at the equator, and sinks towards either pole, the points of perpetual congelation are farthest removed above the ocean-level within the torrid zone, and gradually ap- proach nearer the general surface of the earth, with th increase of latitude ; as the following table shows. PLACES. LATITUDE. LOWES LIMIT OF PEBPETDAL BNOW. Straits of Magellan, . . . Isle of Mageroe, Norway," 54° S. 41° 00° 19° N. 37° 30' 56° 40' 71° 15' 3,706 feet. 6,009 " 15,807 " 14,763 " 9,531 " 5,248 " 2,362 " 55. A striking departure from the rule exists, how ever, in India ; for while on the south side of the Him- ■malehs, the snow line occurs at the height of about Was it found true at Bossekop 1 What results from this gradual loss of heat 1 Where are the points of perpetual congelation nearest to the ocean t Where farthest from it 1 Give examples. 32 THE ATMOSPHERE. 13,000 feet, on the northern acclivity it rises to the alti- tude of 17,000. Many explanations of this singular fact have been given, which admit not of discussion here. HUMIDITY OP THE ATMOSPHERE. 56. At all temperatures moisture resides in the atmo- sphere, self sustained, in an invisible state. Between the particles of air intervals are believed to exist, which are, either partially, or wholly, filled with the vapor that constantly rises from the earth. 57. This peculiarity in the constitution of the atmo- sphere is termed the capacity of the air for moisture, and when the intervals are full of vapor, it is said to be saturated. An increase of temperature, by dilating the air, separates the particles farther from each other ; the intervals are thus enlarged, and the capacity of the air increased. A diminution of temperature is followed by contrary effects ; the size of the intervals is then redu oed, and the capacity lessened. 58. The capacity increases, however, at a faster rate i han the temperature. A volume of air, at 32° Fan. is capable of containing a quantity of moisture, equal to the 1 60th part of its own weight ; but for every twenty seven additional degrees of heat, this quantity is doubled. Thus a body of air can contain, At 32° Fah. the 160th part of its own weight. " 59° " 80th " " " 86° " 40th " " " 113° " 20th " " From this it follows, that while the temperature ad- vances in an arithmetical series, the capacity is accel- erated in a geometrical progression. What departure from this rule exists 1 What does the atmosphere contain at all temperatures 1 What is meant by the capacity of the air for moisture 1 When is the air said to be saturated 1 What is the effect of heat upon the capacity 1 What is the effect of cold 7 Which increases at the fastest rate, temperature or capacity 1 Give instances. What is the rule in respect to temperature and capacity I HUMIDITY OP THE ATMOSPHERE. 33 59. Absolute Humidity. From the cause just men- tioned, it would naturally be inferred, that the quantity of atmospheric vapor, or the absolute humidity, is great- est in the equinoctial regions, and diminishes towards either pole'; a conclusion abundantly supported by facts as will be shown hereafter. 60. The air over the ocean is always saturated, and upon the coasts, in equal latitudes, contains the greatest possible amount of vapor ; but the quantity decreases aa we advance inland, for the atmosphere of the plains of Oronoco, the steppes of Siberia, and the interior of New Holland, is naturally dry. 61. The absolute humidity diminishes with the alti- tude, but the rate of reduction is not fully known. By comparing different seasons and hours, it is found to be greater in summer than in winter, and less in the morn- ing than at about mid-day. 62. Relative Humidity. This must not be con- lounded with absolute humidity. By relative humidity is understood the dampness of the atmosphere, or its proximity to saturation ; a state dependent upon the mutual influence of its absolute humidity and tempera- ture ; for a given volume of air may be made to pass from a state of dampness, to one of extreme dryness, by merely elevating its temperature, without altering, in the least, the amount of moisture it contains. Thus one hundred and sixty grains of air, containing one grain of vapor, would be damp at 36° Fah., but hot and withering at the temperature of 90°. By the reverse of this process, a body of hot air will not only become humid, but will even part with a portion 'of its original moisture, if it is cooled down to any great extent. 63. From the numerous observations of Kaemtz, at Halle, and on the shores of the Baltic, it appears that What Is absolute humidity'? Where is absolute hnraidity the greatest'? How does it diminish 1 Where is the air always saturated 1 What is said of inland regions'? What is the effect of altitude'? Compare summer and winter, morning and mid-day. What is relative humidity? Upon what does "it depend 1 ? Illustrate the effects of a change of temperature, the absolute humidity being the same. 2* 34 THE ATMOSPHERE. the relative humidity, in those situations, is highest in the morning before sunrise, and lowest, or farthest re- moved from the point of saturation, at the hour of the greatest diurnal heat. Corresponding results have been obtained in this country. )C HYGROMETER. 64. Those instruments by which the humidity of the atmosphere is measured are called hygrometers, from the Greek words ugros, moist, and metron, measure. Of these there exists a great variety, differing both in form and piinciple ; but those are esteemed the most accurate in their indications, that are constructed upon the prin- ciple of condensation, to which allusion has already oeen made, (Art. 62,) but a more extended explanation is here required. 65. Imagine a brightly polished metallic vessel, par- tially filled with water, at the temperature of 60° Fah., to be placed in a room at the same temperature. If pieces of ice are now thrown into the vessel, the water is gradually cooled clown, and as this reduction proceeds, the lustre of the exterior surface will be dimmed, at a certain moment, by a fine dew. This is caused by the deposition of moisture from the atmosphere, which, in contact with the cold surface of the vessel, is now cool- ed down just beyond the point of saturation. The tem- perature of the water at this instant, which is the same as that of the vessel, is termed the dew-point. 66. By marking the difference, in degrees, between the temperature of the air and the dew-point, the rela- tive dryness of the atmosphere, or its remoteness from saturation is obtained. But observations, like these, lead also to other important results ; for, by the aid of tables, giving the elastic force of aqueous vapor, at dif ferent temperatures, the absolute weight of the vapor, diffused through a given volume of air can be determin- Wha did Kaemtz observe in respect to relative humidity? What is a hygrometer 1 Explain the principle of condensation. What is the dew-point 1 How is the relative humidity obtained ? What other results can be deduced 1 HCMIDITY OP THE ATMOSPHERE. 35 ed, and likewise the proportion it contains, to that whicli would be required to saturate it. 67. The hygrometer of Prof. Daniell, which is extensively used, is thus constructed. A glass tube, e i, figure 5., is bent twice at right angles, and terminated by two bulbs, b and/, of the same material. The bulb b is partly filled with ether, into which is inserted the ball of a delicate thermometer, d, enclosed in one arm of the instrument. All air is excluded from the tube, which is filled with the vapor of ether; the other bulb,/, is cov- ered with a piece of fine mus- lin, a, and upon the pillar, g h, btobometbr. a second thermometer, k I, is fixed. 68. Observations are thus made. The instrument being placed by an open window, or out of doors, a few drops of good ether are suffered to fall upon the muslin-covered bulb, which, from the rapid evaporation of the ether, soon becomes cool, condensing the ethereal vapor with- in. In consequence of this effect, the ether in b evapo- rates, thus causing, not only in the ether, but also in the enclosing bulb, a reduction of temperature, which is measured by the interior thermometer, e d. As the evaporation at a proceeds, the temperature of b still continues to fall, and, at a certain point, the at- mospheric vapor will be seen gathering in a ring of dew upon the glass, and the difference in degrees, at this moment, between the external and internal thermome- ter, denotes the relative dryness of the atmosphere. Thus, if on one day the exterior thermometer stood at 65°, and the enclosed sunk to 50° ere the dew-ring ap- peared — and on another, the former was at 73°, and the latter had descended to 68° before the glass was dimmed Describe Daniell's hygrometer, fig. 5., and explain the mode of taking observations. 36 THE ATMOSPHERE. with moisture — in the first instance the dryness of the atmosphere would be indicated by 15°, and in the second by 5. 69. The action of this instrument is almost instan- taneous, for the enclosed thermometer begins to fall in two seconds after the ether is dropped. It is usual, where great precision is required, to read off the de- grees of the interior thermometer at the moment the dew-ring appears, and also at the moment it vanishes ; the average of the two observations being taken as the true dew-point. 70. In England the dew-point is seldom 30° Fah. below the temperature of the air ; the greatest differ- ence at Hudson, Ohio, as given by Prof. Loomis, is 36°. In the tropical regions its range is the most extensive ; for, in the burning clime of India, the dew-point has sometimes sunk as low as 29°, while the temperature of the atmosphere was 90° — a difference of sixty-one degrees. 71. Height of the Atmosphere. Whether the atmosphere is boundless or not, is a question which natural philosophers have been unable to determine. De Luc regards it as unlimited, and imagines the plan- etary spaces to be filled with a medium so exceedingly attenuated as not to retard the motions of the heavenly orbs. The earth and the various celestial bodies are supposed to condense this subtil fluid around them into an atmosphere, by virtue of their respective attractions. 72. Were this true, the densities of the atmospheres thus formed would differ, on account of the variations in the size and mass of these bodies. It therefore consti- tutes a strong objection to this hypothesis, that the den- sity of the atmosphere of Jupiter (as shown by the re- fraction of the light of his satellites, at the period of their eclipses) is not superior to that of our own ; although the force of attraction at the surface of this planet is al- How far below the temperature of the air does the dew-point descend in England 1 in Ohio 1 in India 1 Is the height of the atmosphere known 1 What is De Luc's opinion 1 What is the objection to this hypothesis 1 HEIGHT OP THE ATMOSPHERE. 37 moat three times greater than that of the earth. More- over, when Venus passes near the sun, she exhibits no atmosphere, according to Wollaston, notwithstanding her size is nearly equal to that of the earth. 73. Those who maintain that the atmosohere is lim- ited, suppose, that at a certain distance from the earth, the expansive energy of its particles is exactly balanced by the force of gravity, and that beyond this point, an infinite void extends. This distance has been computed to be not far from 22,200 miles from the centre of the globe. 74. Whichever theory may be adopted, it is certain that the atmosphere extends to very great heights. Dr. Wollaston has shown, by calculation, that the atmos- phere, at the altitude of nearly forty miles, is still suffi- ciently dense to reflect the rays of the sun, when this luminary is below the horizon. It is capable of trans- mitting sound at a loftier elevation, for in 1783, a vast meteoric body exploded at an altitude of more than fifty miles, the sound reaching the earth like the report of a cannon. Still farther ; if the combustion of meteors is truly assigned to the action of the atmosphere, the ex- istence of the latter, at the distance of one hundred miles from the earth, may be regarded as proved. What do the advocates of a limited atmosphere suppose 1 How far is this point from the earth's centre'? At what height does the atmosphere reflect light 1 At what altitude transmit sound 1 What inference is drawn from the combustion of meteors? PART II. AERIAL PHENOMENA. ^ CHAPTEE I. OP WINDS IN GENERAL. 75. Catise of Wind. Wind is air in Motion, occur- ring whenever the repose of the atmosphere is broken, from any cause whatsoever. It is usually the result of a change of temperature, and consequently of density, but the rush of an avalanche, causing a sudden displace- ment of a vast volume of air, has been known to pro- duce a momentary wind of great violence, along the borders of its path. 76. If two contiguous, upright columns of air, with their bases at the same level, are unequally heated, the colder is the denser, and at its base a current will flow towards the lighter column, (just as the compressed air within a bellows streams out into the rarer atmosphere,) but at the top, to supply this loss, a counter current pre- vails. 77. This is illustrated by Franklin's simple experi- ment; if a door is opened, communicating between a warm and cold room, and a lighted taper then placed at the bottom of the doorway, the flame is bent towards the warm apartment ; but if held at the top, its direction is reversed. 78. On account of the unequal distribution of heat What does part second treat of? What does chapter first treat of? Define wind. When does it occur 1 If two contiguous columns of air are unequally heated, what motion takes place 7 State Franklin's experiment. OP WlNUS IN GENERAL. 39 over the surface of the globe; phenomena like these occur in nature, on a widely extended scale ; for if two neigh- boring countries are unequally heated, the air above them partakes of their respective temperatures, and there arises at the surface of the earth, a wind blowing from the colder to the warmer region, while at the same time, a directly contrary current prevails in the upper strata of the atmosphere. 79. Velocity. Every gradation exists in the speed of winds, from the mildest zephyr, that scarcely bends the flower, to the most violent hurricane, which pros- trates the giant oak, and hurls to the ground the proud- est works of man. They have been classed as follows, by Smeaton, according to their rapidity and force. Velocity of the wind, Perpendicular force on one square Common appellation of miles per hour. toot in lbs. avoirdupois. such winds. 1 4 5 10 15 20 25 30 35 40 50 60 80 100 .005 .079 .123 .492 1.107 1.968 3.075 4.429 6.027 7.873 12.300 17.715 31.490 49.200 Hardly perceptible. > Gentle wind. I Pleasant brisk gale. I Very brisk. I High wind. Very high. Storm. Great Storm. Hurricane. Violent Hurricane. 80. The velocity of the upper currents of the atmos- phere, is as variable as that of the winds which sweep over the surface of the globe ; for the aeronaut, Green, who ascended from Liverpool, in 1839, to the height of 14,000 feet, encountered a current that bore him along at the rate of 'five miles per hour, but upon descending to the altitude of 12,000 feet, he met with a contrary wind, blowing with a velocity of eighty miles per hour. How does it explain the origin of winds t What is said of the velocity of winds'? Give the common appellations of winds, with their velocity and force. What is said of the apeed of the upper currents 1 Give examples. 40 AERIAL PHENOMENA. On one occasion, his balloon was carried over the space of ninety-seven miles in fifty-eight minutes. 81. Anemometer. The velocity of the wind is esti- mated by the anemometer, an instrument so called from the Greek words, anernos, wind, and metron, measure. One of the. best is Woltmann's. It consists of nothing more than a small windmill, to which is attached an index, in order to mark the number of revolutions per minute; the number of course increasing with the speed of the wind. Now if the atmosphere is still, and the anemometer is carried against it at the rate, for instance, of ten miles per hour, the number of its revolutions will be exactly the same as if the instrument was sta- tionary, and the vanes revolved by the force of a breeze possessing the same velocity. 82. If then, upon a calm day, the anemometer ia taken upon a railroad car, moving, for example, at the speed of twenty miles an hour, and the number of revo- lutions for half an hour accurately noted, we can obtain, (by dividing this result by 30,) the number of revolu- tions per minute, corresponding to those of a wind hav- ing a velocity of twenty miles per hour. In this manner, a table adapted to the instrument can be constructed for all winds, moving with a greater or less rapidity. The velocity of the higher aerial currents is ascer- tained by the speed with which the shadow of a cloud passes over the surface of the earth. 83. Force. The force of the wind is obtained, by observing the amount of pressure it exerts upon a given, plane surface, perpendicular to its own directions. If the pressure-plate acts freely upon spiral springs, the power of the wind is denoted by the extent of their compression, and that weight will be a "measure of it3 force, which produces the same effect upon the springs. This instrument, which is also termed an anemometer, What is an anemometer 1 Describe Woltmann's, and the mode of com- puting by it the velocity of the wind. How do we judge of the speed of the upper currents? In what manner is the force of the wind estimated t TRADE WINDS. 41 is constructed in exactly the same, manner as a letter weigher, where a weight of half an once compresses the spiral, bringing down the index to a certain division of the scale. 84. If, however, the velocities of the different winds are already known, and the force of one obtained, those of the rest can be found by the following rule, viz.- that their forces are as the squares of their velocities. For instance, if the power of a gale, possessing trie speed of twenty miles an hour, is known to be 1,968 pounds on a square foot, that of a storm with a velocity of fifty miles can thus be ascertained by a simple proportion. (20x20) (50x50) lbs.- lbs. 400 is to 2500 as 1,968 is to the answer 12,30. Should the forces be known, it is obvious that the velocities can be computed by reversing this process. Winds may be divided into three classes, constant, periodical, and variable. CONSTANT WINDS. TRADE WINDS. 85. The most remarkable instance of the first class, is that vast current, which, in the torrid zone, is ever sweeping around the globe, in a westerly direction ; and, from its advantage to commerce, in always affording a steady gale to the bark of the adventurous mariner, is denominated the trade wind. 86. So uniform is its motion, that on the voyage from the Canaries to Cumana, on the northern coast of South America, it is scarcely necessary to touch a sail ; and with equal facility, the richly laden Spanish galleons were accustomed to cross the Pacific from Acapulco to the Philippine Isles. 87. Origin. The cause of this wind has been thus explained by Halley, an English philosopher. From the vertical position of the sun, the regions near the equator If the velocities are known and one force, what else can be obtained 1 Give the rule and the example. If the forces are known, what can be computed 1 Into how many classes are winds divided 1 Name them. What is the trade wind 1 How does it originate 1 42 AERIAL PHENOMENA. are intensely heated, while those more remotely situated are less so ; the temperature gradually diminishing to- wards either pole. (Art. 49.) In accordance with the principles just unfolded, (Art. 78,) an upper current will flow from the equator towards the poles, and a cold cur- rent at the surface of the earth, from the poles and the higher latitudes, towards the equator. Here the air, becoming rarefied by the heat, rises, and mingling with the upper wind flows back again to the polar climes ; thus establishing a perpetual circuit. If then the at- mosphere was subject to no other influences, a north wind would prevail in the torrid zone, in the northern hemisphere, and a south in. the southern; but these directions are modified by the rotation of the earth, in the following mariner. 88. Every thing upon the surface of the globe at the equator, is carried towards the east, at the rate of about sixty-nine miles in four minutes ; but as we recede to the north or south of this line, the eastern velocity is so diminished, that at the latitude of 60° it is reduced to one-half, and at 83° to less than one-eighth of its original amount. A wind, therefore, blowing from the high latitudes towards the equinoctial clime, is constantly passing into regions where all terrestrial objects have a greater east- erly velocity than itself. They will consequently move against it, and as they are apparently stationary, it will thus acquire a relative westerly motion. Just as when a traveler, outstripping the wind that blows at his back, feels a breeze directly in his face. 89. Thus, the polar wind in the northern hemisphere is influenced by two forces at the same time, one of which carries it to the south, and the other to the west ; and the course it assumes by their combined action must be according to the laws of compound motion, (C. 249,) some intermediate direction, tending from the north- east to the south-west ; and such is the fact, according to all observations. What two forces influence the polar wind in the northern hemisphere ? What is the direction of the trade wind in this hemisphere 1 TRADE WINDS. 43 In a similar manner, the lower current in the south- em hemisphere, acquires a direction from the south-east to the north-west. The passage of a vessel across a river is an illustra- tion in point. If the vessel is steered before the wind, from east to west, while the stream is flowing from north to south, she will be seen by a spectator on shore sail- ing from north-east to south-west. 90. In the Atlantic and Pacific, the breadth of the trades increases as they flow towards the western shores of these vast oceans, the wind gradually changing to the east, by the mutual action of the two currents. 91. The land is heated by the sun far more intensely than the ocean. This is owing to the fact that the solar .rays warm only the surface of the earth, scarcely pene- trating an inch in the course of a day, while during the. same time they pierce the water to the depth of many fathoms. It has been computed that the beams of the sun communicate daily a hundred times more heat to a given extent of ground than to an equal surface of water. On this account, the proximity of highly heated continents produces local variations in the direction of these winds ; for the air, being more rarefied over the land, ascends, and to supply its place, the cooler air of the trades sets in from the sea towards these localities. 92. Thus, on the African coast, between Cape Baja- dor and Cape Verde, a north-west wind prevails within the limits of the north-east trade ; and off the coast, from Sierra Leone to the Isle of St. Stephen, the trade wind gradually changes to the south and south-west, veering to the west as it approaches the shore. From the same cause, the south-east trade becomes a south wind along the coasts of Chili and Peru. 93. Limits of the Trade Winds. In the Pacific, the north-east trade wind prevails between the 25th What in the southern hemisphere 1 Illustrate the subject. What is said of the breadth of the trades 1 Why is the land more intensely heated than the ocean ? How does this difference cause a local variation in the direction of the trades 1 Give instances of such changes. State the limit of the trade winds. 44 AERIAL PHENOMENA. aud 2d degree of north latitude. The extent of the south-east trade is not precisely ascertained, but it prob- ably ranges from the 10th to the 21st degree of south latitude. In the Atlantic, the former is comprised be- tween the 30th and 8th degrees of north latitude, and the latter within the limits of the 3d degree of north and the 28th degree of south latitude. 94. The limits, however, are not stationary, but are dependent upon the season — advancing towards the north during the summer of the northern hemisphere, and receding to the south as the sun withdraws to the southern tropic. Thus, on the west coast of Europe, the north-east trade has been found to extend as far as Ma- deira, and even to Mafra, in Portugal. 95. Calms. In the vicinity of the Cape Verde isles, between the 8th and 3d degree of north latitude, is a tract denominated by manners the rainy sea. This region is doomed to continual calms, broken up only by terrific storms of thunder and lightning, accompanied by torrents of rain. A suffocating heat prevails, and the torpid atmosphere is disturbed, at intervals, by short and sudden gusts, of little extent and power, which blow from every quarter of the heavens, in the space of an hour — each dying away ere it is succeeded by an- other. In these latitudes, vessels have sometimes been detained for weeks. In the Pacific, the region of calms is comprised within the 2d degree of north and south latitude, near Cape Francis and the Galapagos islands — a narrow belt of ocean separating the two trades. Here, likewise, dread- ful tempests prevail. 96. According to Humboldt, a similar state of the at- mosphere exists, during the months of February and March, on the western coast of Mexico, between the 13th and 15th degrees of north latitude, and 103d and 106th degrees of west longitude. A ship, richly laden with cocoa, was here becalmed for the space of twenty- Are the limits stationary "? Upon what do they depend 1 Give examples. Where is the rainy sea 1 Describe it. Where are the calms- in the Pacific 1 What instance is given 1 WINDS OF THE HIGHER LATITUDES. 45 eight days, when the water failing, the crew were com- pelled by their sufferings to abandon the vessel and seek the shore, eighty leagues distant, in an open boat. 97. The calms are supposed to be caused in the fol- lowing manner. The adjacent continents to the east of these stagnant regions being far more intensely heated than the sea, the air over the latter would rush easterly towards the land, were it not arrested by a con- trary impulse in the direction of the trade wind. , If these opposing forces are at any time equally strong, the atmosphere is motionless, and a dead calm ensues— just as a vessel, in ascending a stream, continues sta- tionary when the power of the wind is exactly balanced by that of the current. When, however, the relative strength of these forces rapidly changes, those short and sudden gusts which have been noticed will arise, as one or the othdr of these impulses prevails. 98. The presence of a highly heated region is strik- ingly marked in the case of the rainy sea. To the east lies the great African desert, from whose burning surface a vast volume of hot and rarefied air is perpetually ris- ing. Another cause must not be forgotten, which applies, more particularly, to the calms near Cape Francis. This tract is directly under the equator, and from its peculiar situation, the upward current of rarefied air is probably here so strong as to neutralize the action of the trade winds. The limits of the calms vary also with the seasons. Thus, in the Atlantic, the range in August is between 3° 15' and 13° N. Lat., but in February, extends from 1° 15' to 6° N. Lat. ^ WINDS OF THE HIGHER LATITUDES. 99. The upper equatorial currents, flowing off to- wards either pole, descend, on their passage, to the earth, and since they carry with them an excess of east- erly velocity, will become, upon the principles already How do the calms originate 1 What are their limits 1 What is the direction of the wind in the higher latitudes ? 46 AERIAL PHENOMENA. explained, (Art. 89,) south-westerly winds, in the north- ern hemisphere, and north-westerly in the southern. Such would be the course of these currents if left to themselves ; but as they meet on their passage with counteracting winds, and are influenced by a variety of causes, their direction is more or less changed ; yet not so much, but that a marked predominance in the fre- quency of westerly winds exists in both hemispheres. 100. That this is true, in regard to the northern hem- Fig. 6. North, ♦Russia, N. 87° W. West. France, S. 88° w'. 7 North America, S. 86° W. Germany, S. 76° W.- England, S. 66° W. Denmark, S. 62° W Sweden, S. 50° W... South PREVAILING DIRECTION OP THE WIND IN DIFFERENT COUNTRIES. * Kaemtz remarks of Russia, " that the number of observations hava not been sufficient to enable us to deduce any thing conclusive." What gives them this direction? Explain fig. 6., and give the course of the wind for the several countries WINDS OP THE HIGHER LATITUDES. 47 isphere, is obvious from the annexed cut, figure 6., which presents the results of a multitude of observations. A quarter of the circumference of a circle is here supposed to be divided into ninety parts, called degrees, and the inclination of the several lines on which the arrows are placed, to the north and south line, measures, in degrees, the average or mean course of the wind, in the several places mentioned. The degrees are reckoned from the south, in all cases except Russia, where they are counted from the north. The points of the arrows indicate the quarter towards which the wind blows. 101. The prevalence of westerly winds in the high latitudes of the north is also shown by the fact, that the average length of the outward passage, by packet, from New York to Liverpool, is but twenty-three days, while that of the return voyage is forty. It also appears, from the observations of Hamilton, during twenty-six voyages between Philadelphia and Liverpool, extending from 1798 to 1817, that, out of 2029 days on which the wind blew, it came from the west 1101 ; a result agree- ing with the observations of McCord, at Montreal, who found that, from 1836 to 1840, inclusive, the westerly winds at this station constituted moi'e than one-half of all the winds that blew, bearing the ratio of 54 to 100. 102. In the high southern latitudes,- the same fact has been observed. Lieut. Maury remarks, that at Cape Horn there are three times as many westerly as easterly winds, and that he has seen vessels arrive at Valparaiso and Callao, after having been detained off the Cape, by gales and head winds, for the space of eighty, and even one hundred and twenty days. In the late Exploring Expedition, the ship Vincennes remained at Orange Harbor, in Terra del Fuego, for the space of sixty days, during which time the weather was exceed- ing variable ; for thirteen days the wind blew from the north, eastward, and south-east, while for forty-seven, it prevailed from the west. State facts respecting westerly winds in the high northern latitudes. The same in regard to the high southern latitudes. 48 AKRIAL PHENOMENA. DPPER WESTERLY WIND OF THE TROPICS. 103. The prevalence of a westerly wind, above the trades, within the torrid zone, is shown by many con- clusive facts. In 1812, ashes from the volcano of St. Vincents were carried easterly, falling upon the island of Barbadoes; and the captain of a Bristol ship declared, that at this time volcanic dust descended to the depth of five inches, upon the deck of his vessel, at the distance of five hundred miles to the east of the former island. In 1835, an eruption occurred of the volcano of Con- sanguina, situated in Guatimala. The height of the crater is 3800 feet, and from it issued clouds of ashes, which obscured the sun for five days, and being borne along in a north-easterly direction, by the upper cur- rent, fell in the streets of Kingston, Jamaica, seven hun- dred and thirty miles distant. Even in the latitude of Teneriffe, nearly all travelers have found a westerly wind at the summit of the peak, while the regular trade was blowing in a contrary direction, at the level of the ocean. PERIODICAL WINDS. 104. Monsoons. In certain countries within and near the tropics, the regular action of the trade wind is destroyed by the monsoons, which are periodical gales, deriving their name from the Malay word moussin, sig- nifying seasons. These winds blow, from a certain quarter, for one half of the year, and during the other half from an opposite point ; and at the time of thejr shifting, dead calms, tempests, and variable winds alter nately occur. 105. From April to October, the south-west monsoon prevails north of the equator, and the south-east in the southern hemisphere ; but from October to April, the north-west monsoon blows south of the equator, and the north-east in the northern hemisphere. What is the direction of the wind above the trades 1 Give the proofs. What are the monsoons ? In what manner do they blow ? From April to October what monsoons prevail, and where 1 From October to April what monsoons prevail, and where J PERIODICAL WINDS. 49 This may be taken as a general rule, subject to the following modification ; that the south-west and north- west monsoons occur later in the season, according as the regions over which they prevail are farther removed from the equator. Thus, in India, at Anjengo, on the Malabar coast, 8° 30' N. Lat., the south-west monsoon commences as early as the 8th of April ; at Bombay, 19° N. Lat., about the 15th of May. In Arabia, it begins a month later than on the shores of Africa, and in the northern part of Ceylon, fifteen or twenty days earlier than on the Coromandel coast. 106. Origin. The cause of these regular changes is to be sought in the effect produced by the sun, during his apparent annual progress from one tropic to the other. In the Indian ocean, for example, as this lumi- nary advances towards the north, the zone of greatest rarefaction recedes from the equator, and the north- east monsoon (which is nothing more than the trada wind) then subsides, and is succeeded by calms and va- riable winds ; but as the summer approaches, and the sun arrives at the northern tropic, the southern portions of the Asiatic continent become hotter than the ocean, and the humid air from the equatorial seas flows north- ward to the land. South-west winds will therefore arise, (Art. 99,) which prevail from the peninsula of India to the Arabian gulf, until the time of the autumnal equinox. During the same period, the south-east monsoon, in the southern hemisphere, tempers the heat of Lower Guinea, and brings rain to the shores of Brazil. 107. The motions of the atmosphere, however, are reversed, as the sun crosses the equator and approaches the southern tropic. Pouring his fervid rays upon Southern Africa, the vast tract of New Holland, and the splendid clime of Brazil, the air flows in from the north and north-west, towards these highly heated re- gions, and winds from these quarters prevail for several What modifies the general rule 7 Give examples. How are the monsoons caused 1 3 50 AERIAL PHENOMENA. months: the monsoon extending along the coast of Brazil, from Cape St. Augustine to the Isle of St. Cath- erine. But now the influence of the sun is partially withdrawn from Southern Asia ; it glows no longer be- neath his vertical rays, and over the cooled earth the north-east monsoon resumes its wonted course. 108. Land and Sea Breezes. On the coasts and islands within the tropics a sea breeze daily occurs, about nine o'clock in the morning ; at first, gently blow- ing towards the shore, but gradually increasing in force until the middle of the day, when it becomes a brisk gale ; after two or three o'clock it begins to subside, and is succeeded at evening by the land breeze, which blows freshly off the coast during the night, dying away in the morning, when the sea breeze recommences. The extent of these winds is variable ; in some places they are scarcely noticed beyond the rocks that line the beach ; at others they are perceptible three or four leagues from land ; while such is their strength on the Malabar coast, that their effects are felt at the distance of twenty leagues from shore. These breezes are occasionally met with in every latitude. They are perceived upon the coasts of the Mediterranean, are sometimes felt at Bergen, in Nor- way, and even faintly discerned on the shores of Green- land. 109. Origin. During the day, the islands of the tropics acquire a far more elevated temperature than the adjacent ocean ; (Art. 91,) the atmosphere above them partakes of their warmth, and currents of rarefied air ascend from the interior of the land. To supply the par- tial void thus created, the cool, dense air of the ocean flows in from every quarter towards the shore, and the sea breeze then prevails. About mid-day the sea breeze is strongest, since the velocity and force of the ascending current is then at its height, for the sun now acts with its greatest energy j Describe the land and sea breezes. Where do they prevail 1 How do they originate 7 VARIABLE WINDS. 51 but as this luminary descends in the heavens, and sinks beneath the waves, the land rapidly loses its heat by radiation,, while the temperature of the ocean at its sur- face is scarcely changed. This is owing to the fact already stated, that the rays warm only the surface of the earth, but are diffused through the water to a con- siderable depth ; and besides, whenever the upper stra- tum of the fluid is cooled, it becomes heavier and sinks ; and a warmer stratum rising to the top the surface thus maintains an almost unvarying temperature. For these reasons, the land, at length, becomes colder than the sea, while the air above it, acquiring its temperature, is con- densed, and flowing off in every direction to the warm and lighter atmosphere that floats above the ocean, gives rise to the land breeze, which prevails throughout the night. 110. Variable Winds. From the extreme mobility of the air, the direction of the wind is affected by a countless variety of causes, such as the nature of the soil, the inequalities of its surface, the vicinity of the ocean and of lakes ; and the temperature, course and proximity of mountains. These local influences are, for the most part, con- trolled, where the great aerial currents exist in all their power ; but in the extra-tropical regions, where the force of the latter is diminished, a perpetual contest occurs between the permanent and temporary currents, giving rise to constant fluctuations in the strength and direction of the wind. 111. It appears from observations made at Toronto, and at Hudson, Ohio, that although the wind blows from every point of the compass during the year, yet, such is the force of the northerly gales, that, in these latitudes, there is a general motion of the atmosphere from N. W. to S. E. In England, on the contrary, from the hourly observations made at Plymouth, there seems In what regions do variable winds prevail 1 What appears to be the general course of the atmosphere at Toronto and Hudson 1 What in England? 52 AERIAL PHENOMENA. to be an annual movement of the atmosphere, from the S. S. E. towards the. N. N. W. 112. Physical Nature of Winds. Winds are hot, cold, dry or moist, according to the direction whence they blow, and the kind of surface over which they pass. In Europe the westerly winds are warm and moist, and the north-easterly cold and dry ; for the former come over the sea from the lower latitudes, and the latter sweep across the land from the polar climes : in our own cli- mate, a north-easterly wind is cold and moist. A south wind in the northern hemisphere is warm and humid, since it comes from warmer regions, and its capacity for moisture is constantly diminishing in its northward progress ; from opposite causes a north wind is keen and dry. In the southern hemisphere the nature of these winds would be interchanged. 113. Puna Winds. In Peru, between the Cordilleras and the Andes, at the height of 12,000 feet, are vast tracts of desolate table-land, known by the name of the Puna. These regions are swept, for four months in the year, by a piercingly cold wind from the snowy peaks of the Cordilleras, which is so extremely dry, and absorbs with such rapidity the moisture of animal bodies, that it prevents putridity. If a mule happens to die upon these plains, it is converted, in the course of a few days, into a mummy, even the entrails being free from the slightest evidence of decay. According to Prescott, the ancient Peruvians pre- served the bodies of their dead for ages, by simply ex posing them to the dry, cold, and rarefied atmosphere of the mountains. 114. Simoom. Upon the arid plains of Asia, and es pecially on the vast deserts of Africa, an intensely hot Whence arise the differences In the properties of wind 1 What is the nature of a south wind in the northern hemisphere, and why 1 Of a north wind, and why 1 Why would their properties be reversed in the southern hemisphere t Describe the Puna winds. What fact is stated by Prescott 1 NATURE OP WINDS. 53 Wind occasionally prevails. In Arabia and Syria, it is known by the name of the simoom, from the Arabic word samma, signifying at once hot and poisonous. In Egypt it is termed chamsin, fifty, because it usually continues fifty days ; while in the western parts of the greaJ Zahara, along the Senegal, and upon the coast of Guinea, it receives the name of harmattan. The stories of the Arabs, and the accounts of the earlier travelers, in regard to this wind, are clothed with marvelous fictions. It is described as a poison- ous, fiery blast, that instantly destroys life ; none ever surviving the effects of its deadly influence, if once in- haled. But these fables are now exploded, and the simoom is known to possess no other properties than those which naturally belong to an exceedingly hot and parching wind. 115. Cause. Its origin is to be sought in the pecu- liarities of the soil, and the geographical position of the countries over which it reigns. The surface of the Asiatic and African deserts is composed of dry quartz sand, which the powerful, ver- tical rays of the sun render burning to the touch. The heat of these regions is insupportable, and their atmos- phere like the breath of a furnace. In June, 1813, at Esne, in Upper Egypt, the thermometer of Burckardt rose to 120° Fah. beneath the roof of a tent, and in 1841, the British embassy to the king of Shoa, while advancing "from Tajura to Abyssinia, suffered under a temperature of 126° Fah. in the shade. When, under such circumstances, the wind rises and sweeps these burning wastes, it is at the same time hot, and extremely destitute of moisture ; and, as it bears aloft the fine particles of sand, the atmosphere is tinged with a reddish, or purple haze, the sure precursor of the simoom. 116. Though the blast of the simoom inflicts not in- stant death, it is yet a dreadful visitant to the traveler What is the simoom 1 Where does it prevail 1 By what other names is it known 1 What is the truth respecting it? How does it originate 1 54 AERIAL PHENOMENA. of the desert. Clouds of glowing sand, at times so thick that objects are invisible at the distance of a few paces, are driven with blinding force, against the face ; the moisture is rapidly absorbed from the body, the skin becomes parched, the throat inflamed, respiration is accelerated, and a raging thirst created. And in the midst of these horrors, the burning blast deprives its unhappy victims of the only means which they possess for alleviating their sufferings ; the water evaporates through the skins in which it is carried, and whole caravans have been known to perish, the prey of a con- suming thirst. 117. Sirocco. This name is given to a south-east wind which prevails in the Mediterranean isles, and along the Italian shores. During the summer and autumn it is peculiarly distressing to the inhabitants of these regions ; an oppressive sensation of heat is then felt, the skin is bathed in perspiration, the body becomes weak and languid, and the mind dispirited. These effects are attributed to the fact, that the sirocco, at this time, is both hot and moist; very little evaporation therefore occurs, and the sensations experienced, under these circumstances, are similar to thpse which are felt during a very sultry state of the atmosphere. While this wind prevails, the air is obscured by fine particles of dust, and is always hazy. The sirocco has been generally supposed to arise from a current of air flowing across the Mediterranean from the glowing sands of Africa. It acquires its heat from the desert, and its moisture from the sea- CHAPTER II. ^ OP HURRICANES. 118. Hurricanes are terrific storms, accompanied, at times, by thunder and lightning ; and differ from Describe its effects. Describe the Sirocco. What does chapter second treat of} What are hurricanes 7 HURRICANES. 55 Fig. 7. every other kind of tempest by their extent, their irre- sistible power, and the sudden changes that occur in the direction of the wind. Though known in other climes, they rage with the greatest fury in the tropical regions The rich products of the plantations are destroyed in a moment, forests are leveled, the firmest edifices pros- trated and their roofs whirled aloft into the air, which is filled with the flying fragments of a thousand ruins. Upon the coasts, the waves rush landward with appall- ing violence, lining the harbors and the adjacent shores with the cargoes and wrecks of shattered vessels. 119. From the late independent investigations of sev- eral eminent philosophers, it also appears that hurri- canes are extensive storms of wind, which revolve around an axis, either upright or inclined to the horizon ; while at the same time, the body of the storm has a progressive motion over the surface of the globe. J.20. We learn from the nu- merous observations collected by Mr. Redfield, of New York, that in the northern hemi- sphere, the Atlantic hurricanes generally originate to the east of the Carribean islands, and that their path is from south- east to north-west, until they have passed the northern tropic, when their course changes from south-west to north-east; the rotation of the storm being from right to left, contrary to the motion 'of the sun, (see fig. 7., where the arrows show the di- rection of the wind.) Where are they most violent 1 What do many philosophers now con- Bider them to be 1 State Mr. Redfield'a views in regard to the Atlantic hurricanes of the aorthern hemisphere. GENERAL DIRECTION AND ROTA- TION OP HURRICANES IN THE NORTHERN HEMISPHERE. 56 AERIAL PHENOMENA. The researches of Col. Reid, Tig 8 - the Governor of the Bermudas, have likewise shown, that the storms and tempests of the southern latitudes are vast whirlwinds ; moving, however, in a different manner from the hurricanes of the northern hem- isphere. Thus, south of the equator, the general course of the hurricanes is from the north- east to the south-west, within the southern tropic ; but after passing this limit they proceed from the north-west to the south- east ; revolving from left to r^fht, in the same way as the sun; a fact previously conjec- tured by Mr. Redfield. (See fig. 8.) The hurricanes of the southern hemisphere frequently occur in general direction and rota- .1 - • •. r Ti* -.' „ J TION OP HURRICANES IN THE the vicinity of Mauritius and southern hemisphere. Madagascar. 121. Path of the Storm. The distance traversed by these desolating tempests is immense. The memo- rable gale of August, 1830, which fell upon St. Thomas, in the West Indies, on the 12th, reached the Banks of Newfoundland on the 19th ; having traveled more than three thousand nautical miles in seven days ; and the observed track of the Cuba hurricane of 1844 was but little inferior in length. 122. Velocity. Their progressive velocity varies on the Atlantic Ocean, from seventeen to thirty miles per hour ; but at certain portions of the track it is sometimes much higher ; as in the case of the Cuba hurricane, where the State Col. Reid's views in respect to those of the southern. What is said of the distance traversed by, hurricanes '! What of their progressive and rotary velocity 1 HURRICANES. 57 average rate from the Bahamas to 45° N. Lat. was forty- miles per hour. Distinct from the progressive is the ro- tary velocity, which increases from the exterior boundary to the centre of the storm, near which point the tempest rages with terrific force ; the wind sometimes blowing at the rate of one hundred miles per hour. 123. Diameter. The surface simultaneously swept by these tremendous whirlwinds is a vast circle, varying from one hundred to five hundred miles in diameter ; but even the greatest of these dimensions was exceeded in the Cuba hurricane, for its breadth was computed by Mr. Redfield to be at least 800 miles, and the area over which it prevailed, throughout its whole length, 2,400,000 square miles ; an extent of surface equal to two-thirds of that of all Europe. 124. The rotary character of the hurricane accounts for the frequent changes that occur in the direction of the wind ; since, in order to preserve a circular motion, there must be a constant deflection from a straight course, and, at corresponding points in each half of the storm, the gale will blow from opposite quarters. The changes thus caused, will be perceived at any spot over which this fearful visitant passes. It also explains the fact, that the violence of the wind increases . towards the centre, and that, within the veiy vortex of the hurricane, the air is in repose. Here oc- curs that awful calm ; described by mariners as the lull of the tempest, in which it seems to sleep, only to gather strength for mightier conflicts. 125. Cases. Numerous instances of the facts above mentioned might be adduced, but one or two will suffice. In the Antigua hurricane of 1837, described by Col. Reid, it appears that Gapt. Newby of the Water Witch, first experienced its effects at St. Thomas, in the West Indies, on the morning of the second of August. The wind was then N. N. W., and at three in the afternoon How great is their breadth 1 How great the surface over which they prevail? What facts are explained by the rotation of the storm 1 Give instances. 3* 58 AERIAL PHENOMENA. became violent. At five P. M. it blew a severe gale, and at seven P. M., says Capt. Newby, "a hurricane arose beyond description dreadful. Soon after a calm succeeded for about ten minutes, and then, in the most tremendous screech I ever heard, it recommenced from the S. and S. W. At two o'clock on the morning of the third, the gale somewhat abated, and the barometer rose an inch. At daylight, out of forty vessels, the Water Witch and one other were the only two not sunk, ashore, or capsized." 126.. On the 12th of August, 1837, another hurricane commenced, in the same region, in 17° N. Lat. and 53° 45' W. Lon. At midnight on the 18th, in 31° N. Lat., the ship Rawlin, Capt. Macqueen, appears, according to Col. Reid, to have been in the very vortex of the storm. On the 17th, the wind blew strong from the N. E. by E. for twelve hours, then suddenly changed to the north, blowing with undiminished violence till the 18th at midnight when, in an instant, a perfect calm ensued for the space of one hour ; then, " quick as thought, the hur- ricane sprung up with tremendous force from the S. W. : no premonitory swell of the wind preceding the convul- sion." During the gale, the barometer was almost in- visible in the tube above the framework of the instru- ment. The sudden and extraordinary transition detailed in the cases just cited, are fully explained by supposing, that the vessels passed from one side of the whirl to the other, through the vortex of the tempest. 127. Fall of the Barometer. If the hurricane is indeed a vast whirlwind, the atmosphere, constituting the body of the storm, will be driven outward from the centre towards the margin (C. 171), just as water in a pail, which is made to revolve rapidly, flies from the centre, and swells up the sides. But the pressure of the atmosphere, beyond the whirl, checking, and resist- ing this centrifugal force, at length arrests the outward progress of the aerial particles, and limits the storm. If the hurricane is a whirlwind, in what manner should the barometer fell and rise 1 HURRICANES. 69 We should consequently expect to find (in accordance with the laws of circular motion) the density of the air increasing from the centre to the circumference of the storm, and even for some distance beyond its boundary : and likewise, that when a hurricane passed diametric- ally over any region, the atmospheric pressure would decrease, and the barometer continue to sink, during the first half of the storm ; but that the instrument would gradually rise, as the last half passed over. Such indeed is the case ; for, amid all the phenomena of storms., no fact is better established than this, that an extraordinary depression of the barometer in tropical climates is a sure forerunner of a hurricane. 128. Before the tempest of Aug. 2d, 1837, the harbor- master of Porto Rico warned the shipping in port to prepare against a storm, as the barometer was falling in an unusual manner ; having sunk one and a half inches since 8 o'clock in the evening of the preceding day. All precautions were however in vain ; thirty-three ves- sels at anchor were destroyed, and, at St. Bartholo- mews, two hundred and fifty buildings levelled to the earth. The following table of observations, taken at St. Thomas, over which island this hurricane passed, is full of instruction in regard to this important point. Is this the case 1 Relate the instances given. 60 AERIAL PHENOMENA 129. r HEIGHT OF TIME. WiHU THE BAROMETER. Hours and Minutes. ' Inches. Direction and Force. Aug. 2d, A. M. 6 29.95 P.M. 2 10 3 45 4 45 29.77 29.69 29.51 N. W. I N. N. J Increasing Tempest. 5 45 29.33 N. E. 6 30 29.16 N. W. 6 35 6 45 28.93 28.80 N. W. N. W. >Hurricane. 7 10 28.62 N. W. 7 30 28.18 N. W. 7 35 28.13 ' 7 52 8 10 28.09 28.09 >Dead Calm. 8 20 28.09 . 8 23 28.44 S. S. E. " 8 33 28.53 S. E. 8 38 28.62 S. E. 8 45 28.71 S. E. 8 50 28.80 S. E. 9 28.98 S. E. 9 10 29.16 S. E. • Hurricane. 9 25 29.24 S. E. , 9 35 29.33 S. E. 9 50 29.42 S. E. 10 10 29.51 S. E. 10 35 29.60 S. E. 11 30 29.64 S. E. Aug. 3d, A. M. 2 45 29.78 S. E. 8 29.91 S. W. 9 29.93 E. 130. In the case of the Water Witch, we have seen, that, when the centre of the tempest was past, and the gale abated, the barometer rose an inch. 131. Circuit Sailing. The gyratory motion of hur- ricanes is strikingly evinced by vessels sailing on a cir- cular course, when scudding before the wind. The most remarkable case is that of the Charles Heddle, related. by Mr. Piddington, which occurred in a storm, near Mauritius, in Feb. 1845. It appears from the log-book of this ship, that, in her course before the gale the wind changed completely What example is given of circuit sailing t HURRICANES. 61 round five times in the space of one hundred and seven- teen hours, having an average velocity of eleven miles and seven-tenths per hour. The whole distance thus sailed by the vessel was thirteen hundred and seventy- three miles; while her actual progress during this time in a south-westerly direction, was found to be only three hundred and fifty-four miles. 132. Axis op the Hurricane. The axis of the hurricane is not, necessarily, upright, but is usually in- clined to the horizon ; leaning in the direction which the tempest takes. This is owing to the friction of the base of the hurricane against the surface of the earth. Its velocity. is thus checked, while the upper portion is driven forward, and overhangs the base. This position of the axis is indicated by the circum- stance that the tokens of the approaching tempest often appear in the higher regions of the atmosphere, before it is felt below. The navigators of the tropic seas some- times behold, high in the air, a small black cloud ; rapidly it spreads down to the horizon, shrouding sea and sky, and the tempest then suddenly descends upon them in all its fury. 133. Remarks. Such are the opinions entertained by Redfield, Reid, Dove, and others, in regard to storms and hurricanes ; opinions based upon a vast assemblage of facts and observations, gathered from all points, within the track of a great number of these desolating gales. The numerous observations taken upon the American coast, commensurate with the extent of the Atlantic tempests, have been systematized by Mr. W. 0. Redfield, of New York ; while Col. Reid has investi- gated the West India hurricanes, and those of the southern hemisphere, with great success. The log- books of the British navy, in which the phenomena of the weather are recorded every half hour, have been What is the position of the axis of the hurricane 1 How is it caused f How is this position sometimes indicated 1 Detail the labors of Redfield, Reid, and Dove. 62 AERIAL PHENOMENA. placed at his disposal, and he has thus been furnished with an immense collection of valuable facts. Prof. Dove, of Berlin, has studied the laws of hurricanes in Europe, and gathered a large number of observations from every quarter of the globe. By noticing the time and place of each observation, storm-charts have been constructed for the use of mariners, and it is highly in favor of the rotary theory, that the conclusions result ing from these extensive and independent investigations are substantially the same. 134. Espy's Theory. The rotary character of hur- ricanes, including tornadoes and water-spouts, is how- ever denied by Mr. Espy, of Philadelphia, who main- tains that the wind blows from every quarter towards the centre of the storm. Espy asserts, that this law ob- tains without a single exception, in seventeen storms which he has investigated. The influx of wind towards the centre, he supposes to be caused by the development of heat, which occurs whenever atmospheric vapor is condensed in the form of a cloud. The heat, thus dis- engaged, rarefies the surrounding air, and establishes an upward current ; and so great an expansion is be- lieved, at times, to result from this cause, that the ve- locity of the ascending current has been computed to exceed three hundred and fifty feet per second. To this point of greatest rarefaction, the atmosphere rushes in from every side, just as the air of a room flows towards the heated current of the chimney ; the violence of the wind depending upon the rate of speed in the ascending column. Most of the phenomena of meteorology are also explained by Mr. Espy in accord- ance with his peculiar views. 135. The centripetal theory has found many able supporters ; but that of Redfield and Reid has been more generally adopted by men of science. 136. It may perhaps be found, when our investiga- tions are multiplied and more extended, that both these Detail Mr. Espy's theory. Which theory has been more generally adopted 7 May thest two motions co-exist 1 TORNADOES OR WHIRLWINDS. 63 motions often co-exist ; a circumstance which is by no means impossible. For when a whirlwind is once in motion, from any cause whatsoever, the great rarefac- tion of air that occurs at the centre, will create an influx of the atmosphere towards this point from all quarters, except where it is opposed by the centrifugal force. Now if the base of the whirl is above the sur- face of the earth, or when touching it, is inclined to it, (which is usually the case,) currents of air will flow beneath the base towards the vortex, and evidences of centripetal action will not be wanting, / CHAPTEE III. OP TORNADOES OR WHIRLWINDS. 137. Tornadoes may be regarded as hurricanes, dif- fering chiefly in respect to their extent and continuance. They last only from fifteen to sixty or seventy seconds, their breadth varies from a few rods to several hundred yards, and it is probable that the length of their track rarely exceeds twenty-five miles. 138. Facts. This phenomenon is usually preceded by a calm and sultry state of the atmosphere ; when sud- denly the whirlwind appears, traversing the earth with great velocity, and sweeping down by its tremendous power the mightiest products of nature, and the strongest works of man. Ponderous bodies are whirled aloft into the air ; trees of large dimensions twisted off or torn up by the roots ; buildings of the firmest construction pros- trated, and streams whirled from their beds and their channels laid bare. A whirlwind that occurred in Silesia, in the year 1820, carried a mass weighing more than 650 lbs., fifty feet above the top of a house, and What are tornadoes 1 Describe their effects. By what phenomena are they attended 1 1 64 AERIAL PHENOMENA. deposited it on the other side in a ditch, one hundred and fifty paces distant. 139. In 1755 a tornado fell upon the village of Mira- beau, in Burgundy, laying dry the channel of the small river by which it is traversed, and carrying the stream to the distance of sixty paces. In. the New Haven whirlwind of 1839, and in that which occurred at Chate- nay, near Paris, during the same year, trees eighteen inches in diameter were torn up by the roots. In one which happened at Maysville, Ohio, in 1842, a barn containing three tons of hay and four horses, was lifted entirely from its foundations. And such was the force of the wind during a tornado which occurred at Cal- cutta in 1833, that a bamboo was driven quite through a wall five feet thick, covered with masonry on both sides ; an effect which was estimated, by a person on the spot, to be equal to that produced by a cannon car- rying a six-pound ball. By the action of a tornado, fowls are often entirely stripped of their feathers, and light substances carriec 1 to a distance varying from two to twenty miles. 140. The whirlwind is attended by all the usual phenomena of thunder-storms ; showers of hail fre- quently occur, and, at times, it is the scene of very extraordinary electric appearances. In the one which happened at Morgan, Ohio, on the night of the 19th of June. 1823, a bright cloud of the color of a glowing oven, and apparently half an acre in extent, was seen moving below the dark canopy of the tempest. It shone with a splendor above that of the full moon, and ten minutes after its passage, the narrator of the phenomena was enabled to read his Bible by its light. Just before the Shelbyville tornado, which took place at midnight, on the 31st of May, 1830, two luminous clouds were seen approaching each other, of the color of red hot iron ; for a moment they united above the town, ex- tending over it like two fiery wings, and, at the next, rushed down to the earth : at this instant the whirlwind burst in all its fury upon the devoted spot. The writer What extraordinary appearances are sometimes seen 1 TORNADOES OR WHIRLWINDS. 65 of this work was informed by an eye-witness, that, dur- ing the prevalence of the storm, so incessant was the, play of the lightning, that the titles of books could be easily read, and the use of lamps was discarded in going to different parts of the house. 141. Origin. Several theories have been advanced to explain the causes of whirlwinds, but they are sup- posed to be generally produced by the lateral action of opposing winds, or the influence of a brisk gale upon a portion of the atmosphere in repose ; in a manner anal- ogous to the eddies that arise at the junction of two streams, flowing with unequal velocities, or the air- whirls that occur, when a wind sweeps by the corner of a building, and strikes the calm air beyond it. 142. The existence of such opposing currents is fully proved by the observations of aeronauts, as well as by those of observers at the surface of the globe. The whirl appears to originate in the higher regions of the atmosphere, and as it increases in violence, to descend ; its base gradually approaching until it touches the earth. Thus, when on the summit of the Rigi — a mountain in Switzerland — Kaemtz beheld two masses of fog ap- proaching each other, in the valley of Goldan, while the air around him was calm, and the sky serene. As soon as they united, a gyratory motion was perceived, the fog rapidly extended, accompanied with violent gusts of rain and hail. At the same time, (as appeared from subsequent information,) a furious storm fell upon the lake of the Pour Cantons, far below ; in the midst of which a water-spout was seen. (Art. 150.) 143. Whirlwinds excited by fires. Extensive conflagrations have been known also to produce whirl- winds, in consequence of the strong upward current, resulting from the great expansion of the heated air. A remarkable instance of this kind occurred between How do they originate, and where 1 What did Kaemtz witness 1 What is the effect of extensive fires t 66 AERIAL PHENOMENA. Great Barrington and Stockbridge, Mass., in the month of April, 1783, and is thus related by Mr. T. Dwight, who beheld it. " In an open field, a large quantity of Drush-wood was lying in rows and heaps for burning, perfectly dry and combustible. On a certain day, when the atmosphere was entirely calm, the brush was ignit- ed on all sides of the field at once. I was residing at this time, at the distance of about half a mile from the fire, when suddenly my attention was aroused by a loud, roaring noise, like heavy thunder. Upon going to the door, I beheld the fire covering the field, and the flames collected from every side into a. fiery column, broad at the base, tapering upward, and extending to the height of 150 or 200 feet. This pillar of flame revolved with an amazing velocity, while from its top proceeded a spire of black smoke, to a height beyond the reach of the eye, and whirling with the same velocity as the fiery column. During the whole period of its continuance, the column of flame moved slowly and majestically around the field. The noise of the whirlwind was loudei' than thunder, and its force so great, that trees six or eight inches in diameter, which had been cut, and were lying on the ground, were whirled aloft to the height of forty or fifty feet." 144. During the terrible conflagration of Moscow, in 1812, the air became so rarefied by the intense heat, that the wind rose to a frightful hurricane ; the roar of the tempest being heard even above the rushing sound of the conflagration. 145. Results of Centrifugal Action. By the centrifugal action of the whirl, the air is driven outward, as in the case of hurricanes, and at the same time spi- rally upwards, on account of the pressure of the sur- rounding atmosphere : a great rarefaction, therefore, occurs at the centre. As long as the base of the whirl- wind is above the ground, the warm air of the earth will stream under and upwards, into this partial void Give the cases. State the result of centrifugal action. TORNADOES OR WHIRLWINDS. 67 from every quarter ; while, at the same time, the cold air will descend into it from the higher region of the atmosphere. By this union, a powerful condensation of vapor oc- curs ; causing the precipitation of rain and hail, and the development of electricity. 146. These, however, constitute no essential part of a whirlwind ; for, if the currents of air that give rise to this phenomenon are very dry, the violence of the wind is the only remarkable circumstance. This was shown in the case of a small whirl, which involved two persons, who were going one cloudy day from Halle to Gie- bichenstein. Suddenly they were separated by a gust of wind ; one being driven against a wall, and the other thrown into a field ; while the people who were near had not discerned the slightest disturbance in the at- mosphere. 147. When the base of the whirlwind descends to the earth, it touches the surface, either partially or wholly, according as the axis is inclined or vertical. In the first case, the inward flowing currents will be partially, and in the second entirely, arrested by the centrifugal action of the storm. The same results often occur when it covers a build- ing. Hence, the atmosphere becomes exceedingly rare- fied, both above, and around the edifice ; and if it hap- pens to be closed, and the tornado is violent, its walls will be burst outward by the sudden expansion of the air within, (C. 509.) Just as a sealed bottle of thin glass, under the exhausted receiver of an air-pump, is shivered by the elastic force of the enclosed air. 148. Effects of Expansion. In the tornado that happened at Natchez, in 1840, the houses exploded wherever the doors and windows were shut; the roofs shooting up into the air, and the walls, even of the strongest brick buildings, bursting outward with great Are rain, hail, and electricity necessary to the production of awhirlwind t Give the case. Why are buildings burst outward by the action of tornadoes 1 Give instances. b8 AERIAL PHENOMENA. force; but no such destruction occurred when a free outlet was afforded to the air within. One gentleman as the storm approached, caused all the windows and doors of his house to be opened, and though its struc- ture was frail it experienced no injury ; not even a single pane of glass being broken. 149. On the 18th of June, 1839, a whirlwind (to which we have alluded) fell upon the village of Chate- nay, near Paris. In the room of a house, over which it passed, several articles of needlework were lying upon a table : the next day some of them were found in a field, at a great distance from the house, together with a pillow-case taken from another room. They must have been carried up the chimney by the rush of air out- wards, as every other means of exit was closed. An- other singular illustration of the fact before us took place in the Shelbyville tornado. Soon after its occur- rence, a lady missed a bonnet, which, the day before the storm, was lying enclosed in a bandbox in her cham- ber ; some weeks afterwards, she accidentally observed a ribbon hanging from the chimney, which proved to be the string of her bonnet. The house had been closed during the storm, and the expansion of the air within the bandbox had forced off the lid — the lost article had been borne by the outward flowing current up the chim- ney, which afforded the only mode of egress, and there it had lodged. CHAPTER IV. K WATER-SPOUTS. 150. A water-spout is a whirlwind over an ex- panse of water, as the sea or a lake, differing from a land-whirl in no other respect than that water is sub- jected to its action, instead of the bodies upon the sur- face of the earth. 151. A water-spout usually presents the following Define a water-spout. WATER-SPOUTS. 69 successive appearances. At first it is seen as an invert- ed cone, either straight or slightly curved, extending downward from a dark cloud to which it seems to be attached. As the cone approaches the surface of the water, the latter becomes violently agitated, and, rising in spray or mist, is whirled round with a rapid motion. As the cone descends lower the. spray rises higher and higher, until both unite, and a continuous column is formed extending from the water to the clouds. The spout is now complete, and appears as an im- mense tube, possessing both a rotary and progressive motion ; bending and swaying under the action of the wind as it advances on its course. When the observer is near, a loud, hissing noise is heard, and the interior of the spout seems to be traversed by a rushing stream. After continuing a short time the column is disunited, and the dark cloud gradually drawn up ; for a while a thin, transparent tube remains below, but this at last is also broken, and the whole phenomenon then disap pears. These successive changes are represented in figures 9, 10, 11, which are taken from sketches of water- spouts actually seen. Fig. 9. WATER-SPOUT FORMING. What are its successive appearances 1 70 WATER-8POUT FORMED. Fig. 11. WATER-SPOUT ENDING. 152. Facts. A watev-spout occurred at Cleveland, Ohio, in September, 1835, which, from the description Describe the one which was seen at Cleveland. WATER-SPOUTS. 71 well illustrates the origin and characteristics of this phe- nomenon. " A heavy storm-cloud, driven by a north~ west gale, was met by a strong opposing current; when an arm of the cloud appeared to drop down, and drag the waves up towards the sky. The whirling and dashing of the spray at the surface of the lake, and the column of water and mist extending, in a tall and tor- tuous line, to the cloud, were so well defined as to ex- cite the admiration of all who observed them. At the expiration of about seven minutes, the north-wester tri- umphed, and swept the cloud to the south-east of the city." 153. The water-spout does not always pass through the various changes that have been detailed ; sometimes the upper portion only is developed, depending from a masss of black clouds, like a huge, tapering trunk, with- out ever reaching the water ; at other times, nothing is seen but the cloud of spray and mist that forms the base. On the voyage of the Exploring squadron from New Zealand to Tongataboo, a spout was beheld in the act of foiming, at the distance of about half a mile. A cir- culai motion was distinctly perceived, the water flying off in jets from the circumference of a circle, apparently fifty feet in diameter. A heavy, dark cloud hung over the spot, but no descending tube appeared, nor was there any progressive motion. In a short time the cloud dis- persed, and the surface of the sea resumed its former state. 154. It is by no means uncommon for several water- spouts to appear at the same time. In May, 1820, Lieutenant Ogden beheld, on the edge of the Gulf stream, no less than seven in the course of half an hour : vary- ing in their distance from the ship from two hundred yards to two miles. 155. Dimensions. The diameter of the spout at its base ranges from a few feet to several hundred, and its Does the water-spout always undergo these changes 1 Under what forms is it sometimes seen 1 How many have been seen at once 1 What is the breadth and height of water-spouts ? 72 AERIAL PHENOMENA. altitude is supposed by some to be at times as great as a mile. In the account given by the Hon. Capt. Napier, of a spout which he beheld in 30° 47' N. Lat., and 62° 40' W. Lon., the diameter was judged to be 300 feet ; and the height of the column to the point where it en- tered the hanging cloud, was computed, from observa- tions taken by the quadrant, to be 1720 feet, or nearly one-third of a mile. 156. Popular error. It is a common belief, that water is drawn up by the action of the spout into the clouds ; but there is no proof, whatever, of a continuous column within the whirling pillar, and the fact, that the water, which sometimes falls from a spout upon the deck of a vessel at sea is always fresh, sufficiently re- futes the idea. The torrents of rain, by which this phe- nomenon is often accompanied, can be fully accounted for by the rapid condensation of vapor that occurs, when the warm, humid air of the sea flows inward to the vortex of the whirl, and there combines with the cold air of the upper regions of the atmosphere, which de- scends to fill the partial void. From this union the electric phenomena of water-spouts arise, and the vio- lent hail-showers that at times prevail ; the mode, how- ever, in which they originate, will be explained here- after. When a vessel is in the vicinity of water-spouts, can- non shots are usually fired for the purpose of destroying them ; lest the vessel should be injured if a spout were to pass over it. It is not improbable that such an effect may be produced when the spout is either struck by the balls, or violently agitated by the concussion of the air arising from the discharges. 157. Sand Pillars. Another form of the whirlwind is exhibited in the pillars of sand, which are not unfre- quently seen in the deserts of Africa and Peru. Bruce, on his journey to Abyssinia, beheld eleven vast columns of sand of lofty height, moving over the plain at the What popular error exists in regard to this phenomenon 1 For what purpose are cannon discharged 1 Where do sand pillars occur 1 WATER-SPOUTS. 73 same timt. ; some with a slow and majestic motion, and others with great velocity. Now, with their summits reaching to the clouds, they rapidly approached the terrified observers, and, the next moment, were borne away by the wind with incredible swiftness. Their tops, at times, were seen separated from the main pillars, and the latter were often broken in two, as if struck by a cannon shot : the diameter of the largest was about ten feet. While Mr. Adanson was crossing the river Gambia, a sand-whirl, twelve feet in breadth and two hundred and fifty in height, passed within forty yards of his boat. 158. The same phenomena are seen upon the Peru- vian coast. " The sand," says Dr. Tscfiudi, " rises in columns from eighty to one hundred feet high, which whirl about in all directions, as if moved by magic. Sometimes they suddenly overshadow the traveler, who , only escapes by rapid riding." 159. Beneficial effect of Winds. The utility of winds must be evident to all. By their aid vast oceans are crossed, and the products of distant climes wafted from shore to shore. Different nations are linked to- gether by social and commercial ties, the blessings of civilization diffused, and the glad tidings from a better world borne to every land. The growth and decay, both of animal and vegetable life, vitiates the atmosphere, and renders it unfit for respiration ; but the winds prevent the deadly effects that would flow from this source, and the air becomes pure and salubrious, from its constant circulation. Even the fierce tempest may be a messenger of mercy, by sweeping from the air the seeds of pestilence and contagion. The advantage of winds in distributing moisture to the earth, will be seen in the following pages. Describe those seen by Bruce and Adanson. What does Dr. Tschudi relate 1 What are the advantages arising from winds 1 PART III. AQ.UEOUS PHENOMENA. CHAPTER I. /• OF RAIN. 160. Rain is produced by the rapid union of two or more volumes of humid air, differing considerably in temperature ; the several portions in union being inca- pable of holding the same amount of moisture that each can separately retain. This circumstance is the result of the law, that the capacity of the air for moisture decreases at a faster rate than the temperature. 161. This effect may be thus illustrated : 4000 cubic inches of air, at the temperature of 86° Fah., can con- tain no more than 31i grains of moisture, and an equal volume, at 32° Fah., only 7-fth grains. Now, if the two volumes are mingled together, their average temperature will be 59° Fahrenheit, and the weight of moisture they unitedly possess will be 39ith grains. But, at this tem- perature, 31£ grains is all the moisture that 8000 cubic inches of air can possibly retain ; since the first portion, by its union with the second, diminished its capacity one-half while that of the latter was only doubled. The excess, therefore, of 7-f grains will be condensed, and descend in the form of water. What does part third treat of? What subject is discussed in chapter first 1 How is rain produced 1 Give the illustration. OF RAIN. 162. Rain is the result of such combinations on an extensive scale, and the quantity that falls at any par- ticular dme or place, depends upon the difference in the several temperatures of the combining volumes, and the amount of moisture which each separately possesses. 163. Winds are the great natural agents by which such combinations are effected, and these occur most readily, when the currents of air are 'shifting and vari- able. Constant winds, blowing steadily from the same quarter and possessing an unchanging temperature, can produce no such admixture, and they are consequently attended with dry weather ; except in the case where they strike the sloping sides of lofty mountains, carrying the warm air of the sea and the vales far up into the colder regions of the atmosphere. 164. Rain-gauge. The quantity of rain that falls at any station during a given time, is ascertained by means of the rain-gauge, an instrument which is constructed in a variety of ways. One of the best consists of a cylindrical, copper vessel, furnished with a float ; the rain falling into the vessel raises the float, the stem of which is so graduated that an increase in depth, to the extent of one-hundredth of an inch, can easily be measured. The greatest annual depth occurs at San Luis, Ma- ranham, 2° 30' S. Lat. ; and Vera Cruz ranks next in this respect. At the former place, 280 inches have been observed to fall in the course of a year, and at the latter 278 inches. 165. Distribution of Rain in Latitude. Since the. capacity of the air for moisture increases with its temperature, we should naturally infer, that the higher What circumstances influence the amount of rain which falls at any place 1 ? What great natural agents effect these combinations 1 What is said of variable and constant winds ? What is the rain-gauge 1 Where does the greatest yearly depth of rain occur 1 What is the law of distribution in respect to latitude 1 /O" A0.UE0ES PHENOMENA. (he mean temperature of any region, the greater would be the amount of rain which descends upon it. This is true as a general rule, for the annual depth of rain is found to decrease with the increase of latitude, as will be seen from the annexed list of seven localities, where the rain has been measured. North Latitude. Annual depth of Rain in inches. Grenada, . . 12° 126 Cape Francois, 19° 46' 150 Calcutta, . . . 22° 35' 81 Rome, . . . 41° 54' 39 London, . . . 51° 31' 25 St. Petersburg, 59° 56' 16 Uleaborg, . . 65° 1' 13.5 1C6. Exceptions. Although this general relation to latitude exists, it is by no means to be supposed, that the same amount of rain descends yearly upon all re- gions lying within the same parallels ; local causes will have their influence, and create, in many cases, extra- ordinary departures from the common rule. Thus, Bombay and Vera Cruz possess, nearly, the same position in latitude ; but while at the former city, the annual depth of rain varies from 61 to 112 inches, that of the latter ranges from 120 to 278 inches. This is owing to the following circumstances. Vera Cruz is backed by a chain of lofty mountains, rising be- yond the limits of perpetual frost, and hither the hot and humid tropical air is constantly driven by the trade winds, as they sweep from the sea. Hence a great and sudden reduction of temperature occurs amid these icy regions, and the air, no longer capable of absorbing its vast stores of moisture, precipitates an immense quantity of rain. 167. At Bergen, in Norway, it has also been found, that in consequence of the moist south-west winds be- ing checked in their course by the mountains, more than 88 inches of rain descend in a year : a quantity greater Illustrate. State the exceptions and the cause. OF RAIN. " 77 than that which falls at Calcutta during the same period. 168. Days of Rain. Though the annual amount of rain is greater in the low than in the high latitudes, the rainy days are not so numerous ; as appears by the fol- lowing table, which presents the average yearly num- ber, within the latitudes indicated. » N. latitude. Mean annual number of rainy days. Prom 12° to 43° 78 u 43° " 46° 103 brook, which had been a foot deep when they began U cross. It came down with surprising violence, carrying away some loaded camels that were crossing at the time, and rising about ten feet within a minute. Such was its force, that it ran in waves, like the sea, and rose against the bank in a ridge, like the surf on the coast of Coromandel." 171. Distribution in Altitude. The great stores of atmospherical humidity reside in the inferior strata What is the rule in respect to days of rain 1 Where are rains most powerful 7 78 Aau'EOUS PHENOMENA. of the air, and, for this reason, less rain descends upon lofty table-lands and mountains, than upon regions situated lower down in the same latitude. Thus, in India, on the Malabar coast, twelve degrees from the equator, the annual depth of rain is 136 inches ; while at Ootacamund, in the Nhilgerries, a region lying a short distance to the east, in the same latitude, but 8/100 feet above the ocean, the yearly amount of rain is only 63.88 inches. Likewise, at Sante Fe de Bogota, New Grenada, a city that enjoys an elevation of 8,800 feet, in the fourth degree of north latitude, the annual quan- tity of rain is nearly the same as that which falls in Germany, which is about twenty-one inches. Even slight variations in altitude cause perceptible differences in the quantity of rain. At the Paris Observa- tory, a rain-gauge is placed in the court, and another upon the terrace, eighty-nine feet above. The mean annual depth of the rain which fell in the court for a space of ten years, was found to be 22.44 inches, and of that which descended upon the terrace during the same period, only 19.68 inches. 172. Rain upon Coasts. We have remarked, that the air above the ocean is always saturated, and that its humidity decreases as we penetrate from the sea-shore into the interior of a country. Conformably to this law, other things being equal, more rain descends upon the coasts than upon the central regions of a country ; inas- much as a less reduction of temperature will here pro- duce a precipitation of moisture. Besides, when the warm, humid air is borne inland by the winds from the sea, its course is marked by descend- ing showers, and its inherent moisture decreases with its progress. Thus, on the west coast of England, 37 inches of rain fall in the course of a year ; while in the interior, upon the eastern side, the annual depth is 25 inches. The maritime and inland regions of France Give the rule in regard to distribution in altitude. Illustrate. Compare the rain upon coasts and inland regions. Why is there a difference 1 Give instances. RAINS WITHIN THE TROPICS. 70 and Holland differ, in this respect, one inch. In this country, the yearly average fall of rain at Boston, for a period of 22 years, is 39.23 inches ; at Hanover, New Hampshire, 38 inches ; in New York State, 36 inches ; and in Ohio, 36 inches. A diminution occurring as we advance into the interior, notwithstanding the influence of the great northern lakes, in the last two instances. RAINS WITHIN THE TROPICS. 173. Upon the oceanj in the region of calms, where the gusts of wind are ever changing their direction, tor- rents of rain frequently descend. On the land, in all places where the trade wind blows constantly seaward, no rain falls, and the sky is always serene ; but, wher- ever disturbances occur in this current and the mon- soons prevail, the rains are periodical, and the year is divided into two seasons, the wet and the dry. These are so marked in their character, that whole months pass away without a cloud obscuring the sky, or miti- gating the fierce heat of the sun : then the face of na- ture entirely changes, the heavens gather blackness, the rain comes down like a deluge, and the parched earth is refreshed, for many successive weeks, by copious showers. 174. Rainy Season. The rainy season commences, in all the countries within the tropics, at the shifting of the monsoons ; and as this change is dependent upon the position of the sun, it begins earlier in those regions that lie near the equator, than in those more remote. At Panama, 8° 48' N. Lat., the rain falls early in the month of March; but it seldom appears at St. Bias, California, before the middle of June. In Africa, near the line, the rainy season begins in April, both upon the sea-coast and in the interior ; but in the countries watered by the Senegal, it commences in June, and lasts till November. How are rains distributed within the tropica % How ia the year divided where the mqnsoona prevail 1 When does the rainy season occur? In what regions early 7 In what late ? Illustrate. 80 AQUEOUS PHENOMENA. In India, the rains occur in May, at the southern ex- tremity of the Malabar coast, but do not reach Delhi until nearly the end of June. 175. Cause. These stated rains originate in the change of the periodical winds, by which the union of vast volumes of air, differing in temperature, is rapidly effected. The subject cannot be better illustrated, than by recurring to the origin of the monsoons of India. (Art. 106.) Early in the month of June, the soil of the penin- sula becomes intensely heated by the vertical rays of the sun, and powerful currents of rarefied air then ascend from the earth. To supply the deficiency thus created, the Warm and humid atmosphere of the equatorial seas flows in. constituting the south-west monsoon ; this wind now mingles with the cool, dry air, which the north- east monsoon, for the six previous months, has been constantly bringing to the peninsula from the polar and temperate climes, and thus produces a combination favorable to the precipitation of rain, upon a most exten- sive scale. ,K 176. Periodical Rains of India. On the Malabar coast, the south-west monsoon is ushered in by terrific storms of thunder and lightning, the water pours down in torrents, and, when the thunder has ceased, nothing is heard for several days but the rush of the descending rain, and the roar of the swelling streams. In a few days, the storm ceases, and the earth, which before was withered by the glowing atmosphere, is now, as if by magic, suddenly clothed with the richest verdure ; the air above floats pure and balmy, and bright tropical clouds sail tranquilly through the sky. After this, the rains fall at intervals for the space of a month, when they again return with great violence. In July, they attain their. height, and from that time gradually subside until the end of September, when the season closes, as it began, in thunders and tempests. 177. The following table, the result of the observa- How do these rains originate '? Describe those ot India. RAINS WITHIN THE TROPICS. 81 tions of twelve years, shows the mean monthly average for the rainy season, at Bombay ; and serves to elucidate the preceding remarks. June, Inched. 24 July, 23.95 August, 18.87 Sept., 14.06 Oct., 1.06 178. The south-west monsoon does not, however, bring rain to the whole of India. Parallel to the west- ern coast runs a chain of high mountains, termed the Ghauts: here the monsoon is arrested in its course, and most of the moisture with which it is charged, is precipitated, ere it arrives at the central table-land of Mysore. On the eastern, or Coromandel coast, its in- fluence is not felt, and the seasons are here reversed. From March till June, the winds are hot and moist, blowing mostly from the south, over the Bay of Ben- gal ; from June to October the heat is very great, but about the middle of the latter month, .the cool, north- east monsoon commences, bringing the periodical rains, which terminate by the middle of December ; the mon- soon continuing to blow until the beginning of March. 179. Periodical Rains op Congo. We trace the rainy and dry seasons of Congo, in the southern hemi- sphere, to the same cause. In general, from about March to September, no rain descends, but gales from the south and south-east temper the burning atmosphere. In October, hot and humid winds blow from the north- west over the Gulf of Guinea, and the country is then flooded by frequent rains, which continue to increase until January. Slight showers then fall at intervals until March, when the rains recommence and continue for a short time. Illustrate from the table. What is the influence of the Ghauts upon the south-west monsoon t What is said of the seasons on the eastern coast t What wind brings the rains to this region 1 Describe the periodical rains of Congo. 4* 82 AQUEOUS PHENOMENA. RAINS IN THE HIGHER LATITUDES. 180. Beyond the tropics, the rains no longer occur at stated periods, but are distributed throughout the sea- sons without regard to any law. . , Thus, in the west of England, the amount of rain in winter is eight times greater than in summer ; but in Germany, it is one-half of what falls in summer, and at St. Petersburg a little more than one-third. In Italy the greatest quantity descends in autumn. There is the same irregularity in the number of rainy days ; for in the west of England, there are more rainy days in win- ter than in summer ; but in Siberia, it rains four times as often in summer as in winter. 181. Rainy Winds. The rains in the higher lati- tudes, as well as within the tropics, depend upon the changes of the wind ; though one wind may be more productive of rain than another, and, in different regions, the rainy winds do not always blow from the same di- rection. In Europe, north of the Alps, the north-east wind is dry and cold, since it sweeps over the land from the higher latitudes ; but the south-west wind brings the rain, for, coming over the Atlantic from southerly climes, it is warm and humid, and its capacity for moist- ure is constantly decreasing. Out of one hundred showers that were noted at Ber- lin, scarcely any occurred when the north-east wind pre- vailed ; while nearly half were brought by the winds from the south-west and west. Moreover, it rained only once for every nine timr? that the easterly winds blew, but thrice for the same number of times in which the south- westerly breezes predominated. 182. The reverse of this occurs on the eastern coast of the United States, for here the north-east winds give rise to the long storms of the fall and spring. At these seasons, as appears from the observations of Dr. Hale, Where are the rains irregular"! Give cases. What is the rainy wind of Northern Europe ? Why is it rainy 1 Give instances. Whence comes the rainy wind, on the eastern coast of the United States , REGIONS WITHOUT RAIN. 83 of Boston, continued through a period of twenty-two years, the winds are colder than the atmosphere of the land, and as they come from the sea charged with moist- ure, the cause of the rain is readily discerned. REGIONS WITHOUT RAIN. 183. Egypt. In Egypt it scarcely ever rains. At Cairo, there is an average of four or five showers a year : but, as we recede from the coast, it becomes more rare, until in Upper Egypt, under the cloudless sky of Thebes, a man's life may pass away without his ever beholding a single rain. 184. The cause of this scarcity of rain is to be sought in the peculiar conformation of the surface of this coun- try. It is a narrow valley, bounded by two mountain ridges on the east and west; the first prevents the moisture exhaled from the Red sea from reaching the ▼alley, and, as the African deserts extend beyond the western range, no source of rain exists in this quarter. 185. The northerly winds, which blow from May till October, bearing off the vapors of the Mediterranean, pass over the whole length of the valley of the Nile, with- out meeting any obstruction ; and it is only when they are driven up the high range of the Abyssinian moun- tains, that they become sufficiently cooled to precipitate rain. Here it descends most copiously during the sum- mer months, swelling the tributaries of the Nile, and producing its annual inundation. 186. Much of the humidity brought by these con- stant winds, can be retained by the atmosphere of Egypt, without being' precipitated ; since it is far below the point of saturation, in consequence of the prevalence of hot, dry winds from the desert, (Art. 114,) and the ex- treme aridity of the soil. So free from moisture is the ground, that myriads of human bodies have rested for centuries within its bosom Why is it rainy 1 What is said of Egypt 1 Why is it that rain rarely falls in this country ? What is said of the dryness o( the soil 1 84 AQUEOUS PHENOMENA. without suffering the least decay ; and in a collection of antiquities, now in the British Museum, there is an an- cient model of an Egyptian house, the store-rooms of which, when first discovered, were full of grain that had remained uninjured for ages. 187. Peru. Along the coast of Peru is stretched a plain of sand, five hundred and forty leagues in length, and varying from three to twenty in breadth, upon which no rain descends. So rare is the occurrence of a real shower at Lima, that it is a source of terror ; and when such an event happens, religious processions parade the streets, imploring the protection of heaven for their en- dangered city. The want of rain in this region is thus explained. Parallel to the coast of Peru, and at a short distance from the sea, extends the lofty range of the Andes, the peaks of which rise far above the limit of perpetual frost. The constant east Wind, sweeping from the Atlantic to the Pacific, across the extreme breadth of South America, gradually ascends the slope of the Andes ; but by the time it has passed their summits most of the vapors with which it is charged, are precipitated, and it comes to • the shores of Peru comparatively destitute of moisture. 188. Moreover, as a sandy soil is naturally dry, scarcely any evaporation occurs, and the hot air of the plains possesses but little humidity. For these reasons, the difference in the temperature of two or more com- bining volumes of air is rarely sufficient to produce rain. 189. A similar destitution of rain exists on the north- west coast of Africa, where the desert of Zahara reaches the Atlantic. In this region, intervals of six or seven years occur between the showers. 190. Constant Rains. In Guiana, it rains for a What is said of Peru 1 Is a copious shower regarded as a blessing at Lima ? Explain the cause of this scarcity of rain. What other region is destitute of rain 1 CONSTANT RAINS. 85 great portion of the year ; nor is this surprising, when we reflect that this country is a low and marshy region, over- spread with immense forests ; situated but a few degrees north of the equator, and subjected to the influence of the north-easterly trade. The fierce heat of the sun fills the atmosphere with vapor, which returns to the earth again in incessant showers, as the cool air from the ocean flows in from the higher latitudes. In the interior, amid the primeval forests, the sun and stars are seldom visible, and the rains not unfre- quently continue for five or six months, with scarcely any intermission. 191. According to Darwin, rain thus prevails at the Strait's of Magellan. " At Port Famine," says the writer, " we have rounded mountains, concealed by impervious forests, which are drenched with rain brought by an endless succession of gales : rock, ice, snow, wind and ■water, all warring with each other, here reign in abso- lute sovereignty." It is a proverbial saying, in the Isle of Ohiloe, 43° S. Lat., that it there rains six days of the week, and is cloudy on the seventh. 192. Excessive Showers. The quantity of rain that falls during a single shower is sometimes amazing. At Cayenne, Admiral Roussin found, on one occasion, that ten inches and three quarters fell in the course of ten hours. There fell at Genoa, Oct. 25th, 1822, thirty inches in twenty-four hours; and at Geneva, on the 20th of May, 1827, six inches in three hours. In the famous Catskill storm of July 26th, 1819, a tub, very nearly as large at the bottom as at the top, was filled to the depth of fifteen inches and a half in. four hours. 193. Rain without Clouds. Singular as it may appear, there are yet many well-attested instances of showers occurring when the sky was clear. This phe- nomenon was several times observed by Humboldt ; and, What is said of the rains of Guiana 1 What of those at Port Famine 1 Give instances of excessive showers. Does rain ever fall from a cloudless sky 7 86 AaUEOUS PHENOMENA. according to Kaemtz, it happens in Germany twice or thrice in a year. On the 9th of August, 1837, a shower fell at Geneva, when the sky was cloudless, that lasted two or three minutes ; and at Constantinople, rain was seen to fall by M. de Neveu, for the space of ten min- utes, when the heavens were perfectly serene. Accord- ing to Le Gen til, this occurrence is by no means un- common in the island of Mauritius, during the preva- lence of the south-east winds ; slight showers falling in the evening, when the stars are shining brilliantly. 194. Cause. The following explanation has been given of this phenomenon. When rain is produced by the intermixture of different volumes of air, the precipi- tated moisture usually assumes, at first, the form of small globules of vapor ; an assemblage of which in the higher regions of the atmosphere constitutes clouds. As the process of condensation advances, more moisture is precipitated, and the globules uniting in rain-drops, de- scend to the earth. Now it is supposed, that, at times, the humidity of the atmosphere is condensed at once into rain, without passing through the intermediate state of cloud ; and under these circumstances a shower might fall from a cloudless sky. X CHAPTER II. OF FOGS. 195. Fogs, or mists, are visible vapors, that float in the atmosphere, near the surface of the earfh. They originate in the same causes as rain ; viz., the union of a cool body of air with one that is warm and humid ; when the precipitation of moisture is slight, fogs are produced ; when it is copious, rains are the result. Give cases. How is this circumstance explained 1 What is the subject of chapter second 1 Define fogs. In what do they originate 1 DISTRIBUTION IN LATITUDE. 87 196. Constitution. When a mist is closely exam- ined, it is found to consist of minute globules, and the investigations of Saussure, and Kratzenstein, lead us to suppose, that they are hollow ; for the latter philosopher discovered upon them rings of prismatic colors, like those seen upon soap bubbles ; (C. 79,) and these could not exist if the globule was a drop of water, with no air or gas within. The size of these globules is greatest when the atmosphere is very humid, and least when it is dry. DISTRIBUTION IN LATITUDE. 197. Tropical Regions. Fogs are not generally common in the equatorial clime, its high mean temper- ature being favorable to the dissolution of vapor. They are however, by no means, unfrequent at certain seasons, and in particular localities. Thus, in India, just before the commencement and at the close of the rainy season, when the air contains an excess of moisture, but not enough to produce rain, clouds of mist so dense and thick obscure the atmos- phere, that they are not dissipated until late in the morn- ing. During the month of December, the towering sum- mits of the Abyssinian mountains are also shrouded in impenetrable fogs. Peru is remarkable for its misty atmosphere, of which we shall soon speak more par- ticularly. 198. Temperate Regions. In the temperate climes, mists frequently occur ; but are of comparatively small extent. 199. Polar Regions. In the polar regions they spread far and wide, over sea and land, and prevail both in winter and summer. At the beginning of winter, the whole surface of the northern ocean steams with vapor, denominated frost smoke; but as the season advances, and the cold in- What does a mist consist of? Where do fogs prevail least 1 When do they appear in India 1 Where do they occur frequently 1 Where most 1 SO AQ.UEOUS PHENOMENA creases, it disappears. Towards the end of June, when the summer commences, the fogs are again seen, mant- ling the land and sea with their heavy folds. By the middle of summer, these also disappear, to return again at the approach of winter. So dense are these mists, that they render the naviga tion of the polar seas extremely dangerous, and the nar ratives of the hardy explorers of these inhospitable climes are full of the perils arising from this source. Simpson, who penetrated by land to the Arctic ocean, in 1837, speaks of the dense fog that often involved his party in imminent danger, while coasting along these ice-bound shores. 200. Cause. The phenomena of the polar fogs are explained in the following manner. During the short Arctic summer, the earth rises in temperature with much greater rapidity than the sea : the thermometer sometimes standing, according to Simpson, at 71° Fah. in the shade, while ice of immense thickness lines the shore. Flowers also bloom at the surface of the ground, when the soil is firmly frozen four inches below. The air, incumbent upon the land and water, partakes of their respective temperatures ; and on account of the ceaseless agitations of the atmosphere, a union of the warm air of the ground with the cool air of the ocean will necessarily occur, giving rise to the summer fogs. But, as the winter approaches, the land becomes colder than the sea ; since the heat acquired during the sea- son of summer is lost far more slowly by the latter than by the former ; and then, upon the warm surface of the ocean, will float the frost smoke, as the cool air flows down upon it from the adjacent shores. LOCAL DISTRIBUTION. 201. Fogs are found along the course of rivers, upon the sides of mountains, and over shoals and capes. It is not difficult to detect the cause of their appearance in these situations. Describe the polar fogs. Explain the cause of their formation. In what localities are mists found 9 LOCAL DISTRIBUTION. 8i» 202. Rivers. The banks of a river, during the night, lose more heat by radiation than the stream itself, and to the air, which rests upon each, a similar difference in temperature is imparted. By the fluctuations of the at- mosphere, an intermixture is readily effected ; and the superfluous moisture is seen, in the morning, floating in fog over either bank, and tracing in a wreath of mist the devious windings of the stream. Fogs usually oc- cur over rivers in the early part of the day ; for the rea- son, that soon after the sun rises the equality of tem- perature is restored, and the vapor is then rapidly dissi- pated. 203. When Sir Humphrey Davy descended the Dan- ube in 1818, he observed that mist was regularly formed, when the temperature of the air on shore was from three to six degrees lower than that of the stream ; and, at the junction of the Inn and Ilz with the Danube, at six o'clock on a morning in June, he found the distri- bution of temperature, and the state of the mist, to be as follows. Temperature of the air over the land. Temperature of the rivers. State of the atmosphere over the livers. 54° Pah. \ Danube, 62° Fah. Inn, 56° Ilz, 55° ■ Thick fog all over. Dense mist all over. Light mist. 204. It is not essential to the production of fogs, that the air of the stream should be warmer than that of the land ; it may be colder, and then fogs appear, if the dif- ference of temperature is sufficiently great. This is the case on the Mississippi. During the spring and fall, mists form over the river in the day time, when the tem- perature of the water is several degrees below that of the air above, and the air above cooler than the atmos- phere upon the banks". These diurnal fogs, though often extremely dense, are chiefly confined to the river, and seldom extend beyond its banks. Why do they occur along the course of rivers 1 State Sir H. Davy's observations. Under what other circumstances can mists occur f Give instances. 90 AQ.UEODS PHENOMENA. 205. On the 31st of Dec. 1847, as the writer was standing upon a bridge, which Crosses one of the tribu- taries of the Connecticut, he was unable to perceive a mill, 140 yards distant, in consequence of the dense fog which covered the river. Upon examination, the tem- perature of the water was found to be 32° Fan., and that of the air, close by the stream, 46° Fah. : a difference, here existing, of fourteen degrees. 206. Mountains. Fogs appear upon mountains, be- cause the warmth of the atmosphere diminishes as we ascend, (Art. 51,) and the cool and shady forests, that clothe their sides, contribute still further to lower the temperature. Hence, when the warm air of the vales is gradually driven up by the wind into these regions, ils capacity for moisture is continually reduced, until at length a precipitation occurs, and clouds of mist involve both cliff and forest. 207. At the Mountain-House, on the Catskill range, the temperature in summer is ten degrees lower than in the valley of the Hudson : and often when a breeze sets towards the mountain, a spectator upon the summit be- holds, at first, a wreath of mist extending along the base; soon the lower belt of forest is concealed from view, and the fog continuing to ascend, thickening and spreading on every side, the landscape ere long is com- pletely veiled, and a chilling wind sweeps past, loaded with moisture. A fact related by Sir John Herschel, affords a striking illustration of the influence of trees in condensing moisture. During the residence of this gen- tleman at the Cape of Good Hope, he observed, that on the side of the Table-Mountain from which the wind blew, the clouds were spread out and descended very low, and often without any rain falling ; while on the oppo- site side they poured over the face of the mountain in dense masses of vapor. Sir John discovered, when walk- ing beneath tall fir trees, while these clouds were closely overhead, that he was subjected to a copious shower, but on coming from beneath the trees, the rain ceased. On Why do fogs appear upon mountains 1 Illustrate. What feet is related by Sir John Herschel 7 LOCAL DISTRIBUTION. 91 investigating the cause, he found that the clouds were condensed into rain on*the cool tops of the trees. 208. Capes. The reason for the existence of fogs over capes and headlands has already been given, in accounting for the prevalence of mist in the polar climes. The soil of these places becomes warmer in summe? than the ocean that washes their shores ; but in the winter, colder; and the difference in temperature is usually sufficient to produce a constant succession of mists. 209. Shoals. A similar state of the atmosphere occurs over shoals, inasmuch as their waters are colder than those of the main ocean. Thus, Humboldt found near Gorunna, that while the temperature of the water on the shoals was 54° Fah., that of the deep sea was as high as 59° Fah. Under these circumstances, an intermixture of the adjacent volumes of air, resting upon the waters thus differing in temperature, will naturally occasion fogs. 210. Newfoundland. Mists of great extent shroud the sea on the banks of Newfoundland, and particularly near the current of the Gulf Stream. The difference in the warmth of the waters of the stream, the ocean, and the banks, fully explains this phenomenon. This current, flowing from the equatorial regions, possesses a temperature 5-£° Fah. above that of the adjacent ocean, and the waters of the latter are from 16° to 18° warmer than those of the banks. The difference, in tempera- ture, between the waters of the stream and banks, has even risen as high as thirty degrees. 211. England. England, surrounded by a warm sea, is subject to thick fogs, that prevail extensively in the winter. In London they are often so extremely dense, that it is necessary to light the gas in the streets and houses in the middle of the day. On the 24th of Why over capes 1 Why over shoals t How are the fogs of Newfoundland explained 1 Describe those of England. 92 AdUEOUS PHENOMENA. February. 1832, people in the streets were unable, at mid-day, to see distinctly on acdbunt of the fog ; and in the evening, the city having been illuminated, as this day was the birth-day of the queen, boys went about with torches, saying, "that they were looking for the illumination." Similar fogs have been observed at Paris, and Amsterdam. The smoke, arising from the exten- sive combustion in such large cities, is regarded, by some, as contributing to the density of these extraordinary fogs. 212. Garuas. We have seen, (Art. 187,) that along the coast of Peru, the atmosphere scarcely ever possesses sufficient moisture to produce rain ; it contains, how- ever, enough to create widely extended and continued fogs. The wintry season, in this country, lasts from April to October, and, throughout the whole of this period, a veil of mist shrouds sea and shore. At the beginning and end of this season, it rises between nine and ten in the morning, and disappears about three in the after- noon, at the hottest portion of the day. But, during the months of August and September, the vapor is ex- tremely dense, and rests for weeks immovably upon the earth. In October and November, the misty canopy begins to rise, and gradually growing thinner, at length yields to the piercing lays of the sun, and is entirely dissipated. 213. These fogs, termed by the natives, Garuas, are said to be at times so heavy, that the moisture falls to the earth in large drops, which are formed by the union of small globules of mist. There is, however, this dis- tinction, between them and rain-drops ; that the latter descend from the more elevated regions of the atmos- phere, while the garuas do not extend higher than twelve hundred feet ; their average altitude varying from seven to eight hundred. 214. Passing eastward from the Puna table-lands of Peru, across the lofty ridges of the Andes, the traveler, Describe the Garuas. Describe the state of the atmosphere east of the Puna regions. LOCAL DISTRIBUTION. 93 after descending a few hundred feet, arrives at a region totally different from that which he has just left. He no longer breathes a pure and refreshing atmosphere ; for the air is loaded with vapors, and the wooded ranges, called the Ceja de la Montana, or mist of the mountain, are clouded with fogs throughout the year. In the dry season, these are dissolved, during the day, by the powerful influence of the sun ; but in the winter they condense upon the hills, and descend in ceaseless tor- rents of rain. 215. Proceeding in the same direction, from the Ceja de la Montana, the magnificent slope of the Andes soon opens upon the sight ; not reposing beneath a clear and azure sky, but overshadowed by a thick veil of mist, impenetrable to the rays of the early sun, and yielding only to his noontide .beams. 216. The explanation of these phenomena is to be found, in the constant advance of the humid trade wind, from the eastern shores of South America to the tow- ering summits of the Andes. Rising continually in its onward progress into higher and colder regions, its ca- pacity for moisture is ever diminishing, and the atmos- phere is always near the point of saturation. Its inland course will thus be marked by abundant rains, and when these abate, fogs and mists succeed in their turn. By the time this great aerial current has arrived at the more elevated ridges, most of its humidity has been discharged ; during the dry season enough only remains to produce extensive mists ; and when, at length, it has reached Peru, it possesses scarcely any moisture. (Art. 187.) 217. A powerful auxiliary cause exists, in the rich and luxuriant vegetation, that springs up every where throughout this boundless region. The light of a trop- ical day, in its meridian splendor, can scarcely pierce the massive foliage of those mighty forests, which stretch away for leagues from the base of the lower Andes ; while the lighter forms of vegetation, spreading in wild Explain the cause. What is the effect of the vegetation in this particular'? 94 AQUEOUS PHENOMENA. exuberance over the higher belts, effectually shield the earth from the fierce rays of the sun, and check the pro- gress of evaporation. The soil, thus shaded, is always moist, and the air warm and humid; and from the causes already stated, such results are here produced as we should readily infer — excessive rains in the lower forests (190), and clouds of mist .upon the more elevated ranges. J' CHAPTER III. OF CLOUDS. 218. The name of clouds is given to those collections of vapor, that float at. a lofty altitude above the earth. 219. Though differing from fogs in situation, they originate in . precisely the same causes ; being formed, in the higher regions of the atmosphere, by the union of warm and cold air, when the combining volumes are over saturated. The excess of humidity, when slight, then appearing in the atmosphere in the form of clouds. 220. During the daily process of evaporation, warm, humid currents of air are continually ascending from the earth; the higher they ascend, the colder is the at- mosphere* into which they enter ; and, as they continue to rise, a point at length will be attained, where, in union with the colder air, their original humidity can no longer be retained ; a cloud will then appear, which in- creases in bulk with the upward progress of the current into colder regions. If the cloud however, in its ascent, either meets with a warmer stratum of air, or descends towards the earth into a region of a higher temperature, a portion of the What is the subject of chapter third ■? Define clouds. How do they originate 1 In what manner do ascending currents produce clouds 1 OF CLOUDS. 95 minute globules of water which compose it, perhaps all, will be re-dissolved, and the cloud will either contract in size, or completely vanish, according to the increase of •heat to which it is subjected. 221. The entire atmosphere, to the altitude of many thousand feet, is constantly traversed by numerous hori- zontal currents of air, flowing in different directions, and at different heights. Combinations of vast volumes of air, varying in temperature, must therefore at times inevitably occur, as well in the higher as in the lower regions of the atmosphere ; and when the excess of moisture resulting from this union is but small, clouds, with their ever-changing forms, obscure the serenity of the sky. 222. When Clayton, on the 31st of July, 1837, as- cended in a balloon from Louisville, Ky., the direction of his course was altered no less than Jive times, in the space of fourteen hours. Once, when at a very great height, he beheld, a mile below him, a snow-white cloud of a mountain shape drifting in an opposite direction to that in which he was traveling. 223. The upward impulse given to the warm atmos- phere near the earth, when driven by the wind against the sloping sides of mountains, is also a fruitful source of clouds. (230.) 224. Strata of Clouds. When an extended range of clouds settles down towards the earth, its under sur- face often copies the outline of the landscape immediately beneath, assuming a horizontal direction. This is owing to the high temperature of the air below the cloud, in consequence of which the latter would cease to be visi- ble, were it to descend lower ; for the globules of vapor would then be dissolved by the warm atmosphere. 225. Above the first range of clouds, the temperature is often much higher than in the region of vfipors be- What is said as to the existence of horizontal currents, and their effects 1 Relate the illustration. What is the influence of mountains 1 What is said as regards the figure of the under surface of clouds 1 How is the existence of successive ranges of clouds explained 1 96 AQ.UEOUS PHENOMENA. neath. Here the air will be clear, and a tract of con- siderable thickness frequently intervenes before we ar- rive at a second range of clouds ; to this may succeed another body of pure air, and still higher a third range of clouds, and so on, alternately. 226. The following account, given by Jolliffe, of his ai'uial voyage, which took place in England, in 1826, is instructive in this connection. "Our progress, during the first quarter of a mile, was so gradual, as to be nearly imperceptible ; but on dis- charging a portion of the ballast, the balloon ascended with a rapidity, which, in a few minutes, buried us in the vapors of a dense mass of clouds. The temperature was here cold and raw ;• such as I have felt on a moun- tain-top, when enveloped in fog. We loitered here for some time ; but at length rose with uncontrollable velo city, and burst, almost suddenly, out of this dark barrier into realms of light and glory. The stratum of clouds from which we had emerged, seemed depressed to a vast distance below us, involved in radiant folds, which com- pletely shut out all view of the earth/' 227. Thickness. The thickness of clouds is some- times immense. On the 29th of Sept. 1826, Peytier and Hossard, two French engineers, were upon the Pyre- nees, and so stationed, that they beheld, at the same time, the upper and lower surfaces of the same cloud. As the altitude of each station was known, the thick- ness of the cloud was readily determined, and found to be 1,476 feet. On the succeeding day, the thickness of the clouds was 2,788 feet ; or more than half a mile. ■ 228. Height. The height of clouds has been vari- ously estimated. According to observations given by Dalton, two-fifths of all the clouds observed in England for the space of five years, were more than 3,150 feet above the'earth. By noting when the upper and lower surfaces of the clouds touched the peaks of the Pyre- Relate the account given by Jolliffe. What is said respecting the thickness of clouds 1 What of their height 1 OF CLOUDS. 9? nees, which had previously been measured, Peytier and Hossard obtained no less than forty-eight altitudes. It was thus found, that the low-r surfaces here varied in height from 1,476 feet to ^,200, and the upper from 2,952 feet to 9,840. 229. The computations of many distinguished ob- servers have been collected by Kaemt.z ; and from these it appears, that clouds range, in height, from 1,300 feet to 21,320. The extreme elevation here given is, however, not sufficiently great ; for clouds are sometimes seen float- ing above the summit of Chimborazo, which rises 21,480 feet above the sea-level ; and when Gay Lussac, in the month of September, 1804, ascended in a balloon to the altitude of 23,000 feef, he beheld clouds still soaring above him, apparently at a great height. 30. Clouds on Mountains. When a mountain range is viewed from a distance, the various peaks are frequently seen capped with a cloud ; while the atmos- phere between them is perfectly clear. This appear- ance sometimes continues for hours, and even entire days ; and was often noticed amid the Alps by the cele- brated Saussure. It is caused by the wind impelling up the sides of the peaks the warm, humid air of the vales, which, in its ascent, gradually sinks in capacity, until it is over-saturated, when the excess of moisture becomes visible, and appears as a cloud. 231. This phenomenon is illustrated by figure 12. Let ABC represent the outline of a mountain peak, up the sides of which a warm current flows, in the direc- tion of the arrows. Above the line D E, the tempera- ture is below the dew-point of the current, and its hu- midity is condensed into a cloud at B. As the wind sweeps over the summit, the cloud B is carried below the line D E, on the opposite si.le, and re-dissolved in the warm atmosphere beneath ; but its place, mean- while, is occupied by a fresh cloud, caused by the ascent What is the appearance sometimes presented by distant mountains 1 How is this accounted for? 5 98 AftUEOUS PHENOMENA. Fig. 12. A CLOUD UPON A MOUNTAIN PEAR. of the warm air on the side A B. It thus occurs, that though the cloud upon the mountain is stationary for hours together, yet the particles which compose it are continually changing. 232. The appearances just described are finely dis- filayed upon the St. Gothard, a mountain in Switzer- and, about 6000 feet high. Dark, heavy clouds that have formed on one side of the mountain, are frequently seen, passing rapidly over its summit, and descending in dense masses into the vale of Tremola, on the oppo- site side ; but, instead of filling the plains beneath with thick vapor, the clouds are dissolved by the warm air into which they are precipitated. 233. A singular instance of the alternate appearance and disappearance of a cloud occurred, not long since, upon the boast of England. A cloud was seen, borne along by the wind, apparently passing from one side of an arm of the sea to the other, but not extending across the water. It was visible over the land, on each shore, but the sky above the water was perfectly serene. This phenomenon may be thus explained. Over the land, in the region of the cloud, the air was below the dew-point ; but over the water, the sea being warmer than the land, Explain from the figure. Give the illustrations. CLASSIFICATION. 99 the temperature of the air was higher, and above the dew-point. When, therefore, the wind carried the cloud over the sea it vanished, its moisture being re-dissolved by the atmosphere ; but when the body of air in which the cloud had previously existed, arrived at the opposite shore, a second precipitation of moisture took place, and the cloud reappeared. CLASSIFICATION. 234. Clouds have been divided into seven kinds ; three original, viz. the cirrus, the cumulus, the stratus ; and four formed by combination, viz. the cirro-cumulus, the cirro-stratus, the cumulo-stratus, and the nimbus. 235. Cirrus or Curl Cloud. This cloud is so oalled, from the Latin word cirrus or curl, because it usually resembles a distended lock of hair. It is dis- tinguished from the other kinds by its fibrous structure, the lightness of its appearance, and the variety of figures it is capable of assuming. After a period of fine weather, slender filaments of the cirrus are frequently seen, stretching like white lines across the azure sky. Some- times these threads, of clouds are arranged in parallel bands, which in the northern hemisphere, (wherever observations have been taken,) are either directed from south to north, or from south-west to north-east ; at other times they separate, resembling the tail of a horse ; a form which is known in Germany by the name, of wind- trees. These filaments are also not unfrequently seen crossing each other, and investing the sky with a deli- cate net-work of gauze-like vapor. One of the most beautiful forms of the cirrus occurs, when the fibres curl from each side of a band of light cloud, and the whole appears like the feathered grain of a rich piece of ma- hogany, (figure 13, a.) 236. The white color of the cirrus renders it difficult, in all cases, to detect its peculiar structure ; for the eye Into how many classes are clouds divided 1 What are they 1 » Describe the cirrus. 100 AQ.UEOUS PHENOMENA. is dazzled by its excessive light. The cloud may, how- ever, be viewed at leisure, by reflection from a bladkened mirror, which diminishes the brightness. 237. The cirrus soars the highest of all clouds. Its altitude, at Halle, in Germany, has frequently been esti- mated, by Kaemtz to be not less than 21,300 feet ; and, from the observations of ten years, this distinguished meteorologist has been led to believe, that it is entirely composed of snow-flakes. Indeed, the temperature of the elevated regions in which it floats, must be often far Delow the freezing point. 238. Cumulus. This kind of cloud acquires its name from the Latin word cumulus or heap ; the vapor seeming to be piled or heaped together. It is usually seen in the form of a hemisphere, resting upon a hori- zontal base ; but at times detached masses gather into one vast cloud upon the horizon ; their radiant sum- mits gleaming like the snowy peaks of distant moun- tains, (figure 13, b.) 239. The cumulus is the cloud of day, and is produced by the ascending currents of warm air, caused by the solar heat. During the fine days of summer, its peculiar figure is most perfect, and its formation and decline oc- cur in the following manner. Although the sun may have arisen in a cloudless sky, a few solitary specks of vapor may be seen towards eight or nine o'clock ; these, as the day advances, enlarge from within, become thicker, and accumulate in rounded masses, which continue lo increase in number and siz°., till the hottest part of the day. After tlvs time they gradually lessen, and often entirely vanish, leaving the sky at sunset again perfectly serene. 240. The ov.mulus floats low i« th", rroniing; but its How may its peculiar structure be best discerned '? How far above the general surface of the earth does the cirrus riie t Of what does it consist according to Kaemtz 1 Describe the cumulus. How does it originate ? Describa the mode of its formation and the changes it underpoeO CLASSIFICATION. 101 altitude increases \vith that of the ascending currents, which attain their highest elevation soon after mid-day ; towards evening the current? subside, and the cloud descends. This circumstance has often been remarked by meteorologists, when stationed on elevated moun tains. In the morning, the cumulus has been seen be- neath them; it enveloped them towards noon; then soared above them for several hours, and descended to the vale at the close of day. Fig. 13. CIRRUS (a), CUMULUS (6), AND STRATUS (C). What is said of its height in the morning, at mid-day, and in the even- ing? Account for the facts stated in IT 240. 102 AftUEOUS PHENOMENA. 241. It is not difficult to account for the facts just de- tailed. The cumulus begins to be formed, when the warm currents, in their upward progress, arrive at a temperature so low that they become over-saturated with moisture ; and the excess is then condensed into a cloud. The higher the currents rise, the colder is the atmos- phere, generally speaking, and the cloud must necessa- rily enlarge ; but when in the afternoon the strength of the currents abates, the clouds which are buoyed up by their force, sink down into warmer regions of the at- mosphere, and are either partially or completely dis- solved. 242. The rounded figure of the cumulus is attributed by Saussure to the mode of its formation ; for when one fluid flows through another at rest, the outline of the figure assumed by the first will be composed of curved lines. This may be seen, by suffering a drop of milk, or ink, to fall into a glass of water ; but the shape of a cloud of steam, as it issues from the boiler of a locomotive, presents a far better illustration. 243. Stratus. This cloud derives its name from the Latin word stratus, or covering ; it forms about sunset, increases in density during the night, and dis- appears at sunrise. It is caused by the vapors which have been exhaled during the heat of the day, but re- turn again to the earth towards the evening, when the temperature has declined, and are then condensed, into a sheet of clouds, which stretch along and rest upon the horizon (figure 13., c). This class likewise includes those light and spreading mists, which gather in mead- ows and vales in the evening of a warm summer's day, floating like a veil over the surface of the ground, and extending but a short distance above it. 244. Cirro-stratus. This cloud is so called, be What causes the rounded figure of the cumulus 1 Describe the stratus. When does it form, increase and vanish 1 How does it originate ? CLASSIFICATION. 103 cause it partakes of the characteristics of the cirruj and stratus ; originating usually in the cirrus. It is remark- able for its great length, in proportion to its thickness ; but though preserving in the main this peculiarity, it assumes many varieties of form. 245. At one time it consists of a number of parallel bars of vapor, in close proximity, blended together at the middle, but separated at the edges (figure 14, b), or it may appear as a streak of vapor, broadest at the mid- dle, and tapering towards either end (c). A third va- riety consists of small rows of clouds, parallel to one another ; each successive row becoming shorter, from the widest part of the- cloud to the extremities, (d.) The name of cirro-stratus is also given to that thin, gauze-like- cloud, which sometimes overspreads the whole sky, and through which the sun and moon are dimly visible. 246. Cirro-cumulus. It not unfrequently happens, that the heavens appear as if sown with little round masses of clouds, lying near to each other, but perfectly separated by intervals of sky (figure 14., a). This cloud is the cirro-cumulus, and often arises from a change in the cirrus and cirro-stratus ; the bars of the latter being divided across the direction of their length, and the dif- ferent parts rounding into the cirro-cumulus. Some-, times the reverse occurs, and the cirro-cumulus is seen changing into the cirrus and cirro-stratus. 247. The structure of the cirro-cumulus is not always the same : at one time the masses are very dense and well-rounded ; at another their form is irregular, and the sky often presents a curdled appearance, when cov- ered with this cloud. Sometimes the cirro-cumulus is so light and fleecy, that the rays of the sun, as they tra- verse it, are scarcely dimmed. Humboldt found them Describe the cirro-stratus. How is it produced'? What are some of its varieties 1 Describe the cirro-cumulus. Whence does it arise 1 State some of the peculiarities of this cloud. 104 AQUEOUS PHENOMENA. even so delicate that he was able to discern through them the spots on the moon. The last two classes of clouds, like the cirrus, float at a very lofty height. Fig. 14. OIRRO-STRATUS (b, C, d), CIRRO-CUMULUS (fl), NIMBUS (e), CUJIULO-STRATUS (/).. 248. Cumulo-stratus. The variety of cloud to which this name is given, combines the characteristics of the cumulus and stratus. Its base consists of a hor- izontal stratum or layer of vapor, from which rise large, overhanging masses of cumulus (figure 14.,/). Some- what is said respecting the height of the eirro-stratus and cirro-cumulus 1 Describe the cumulo-stratus. Of what does it consist 1 CLASSIFICATION. 108 times contiguous cumulus clouds unite, and passing into the state of cumuknstratus, form groups of immense size. This cloud is seen in perfection upon the ap- proach of a thunder-storm, when the cumulus clouds, driven together by the wind, are piled upon each other, and assume those peculiar forms commonly known by the name of thunder heads. 249. This modification also frequently arises, when the cumulus is pierced by the cirro-stratus ; and it is by no means unusual to see four or five parallel bars of the cirro-stratus, one above the other, passing through the same pile of clouds, which then present successive tiers of the cumulo-stratus. 250. Nimbus, or Rain-Cloud. This cloud is so called from the Latin word, nimbus, a rainy dark cloud ; it possesses no peculiarity of form, but is distinguished by its uniform gray ^nt and fringed edges (figure 1.4., e). It is usually composed of some of the preceding classes of clouds, so blended together that they cannot be dis- tinguished, and is produced by a change in their struc- ture, the result of an increase in density. 251. The nimbus often originates in the cumulo-stra- tus, which, as it increases in thickness, frequently as- sumes a black or bluish tint. In a short time this hue changes to gray, a circumstance which indicates that the nimbus is formed and rain descending. When is this cloud most perfectly formed t Under what other circumstances is the cumulo-stratus seen 1 Describe the nimbus. How is it distinguished 1 Of what does it consist 1 ! How is it caused 1 In what, cloud does it often originate 7 What does a gray tint indicate 1 5* '06 AQUEOUS PHENOMENA. CHAPTEK IY. V OP DEW. 252. Dew is the moisture spontaneously deposited upon the surfaces of bodies exposed to the atmosphere, when the latter is free from the presence of fogs ana rain. 253. The whole subject of dew was most happily illustrated by the observations and experiments of Dr. Wells, in 1812 ; and the theory which he then advanced is now generally received, supported as it is by a vast assemblage of facts. 254. Deposition. The deposition of dew is caused by the unequal radiation of heat from the atmosphere and the substance bedewed. During the day, the bodies, that either compose the solid crust of the earth or clothe its surface, become heated by the solar rays, and the lower stratum of that portion of the atmosphere which is directly above, is then likewise raised in temperature, and its capacity for moisture increased. When, however, the night comes on, and even before, the earth and air, radiating their acquired heat into free space, sink in temperature ; but the loss of the former is greater than that of the latter, since, during the night, as experiments show, the air a few feet above the ground, is sometimes warmer than the surface of the soil, by fif- teen degrees. It thus occurs, that the stratum of air immediately in contact with the earth is cooled down by the latter, be- yond the point of saturation ; and the excess of humidity is condensed, upon the substances that form its surface, in drops of dew. (Art. 65.) 255. It may therefore be assumed as a principle, that dew never begins to be deposited upon the surface of What ia the subject of chapter fourth 1 Define dew. Whose theory is generally received 1 How is the deposition of dew caused 1 Explain the process. How much warmer is the air sometimes than the ground 1 What principle may be assumed 1 INFLUENCE OP THE ATMOSPHERE. 107 any body, until it is colder than the contiguous atmos- phere ; and, other circumstances being the same, the greater this difference in temperature, the greater the amount of dew. The quantity of dew deposited within any given time, depends, chiefly, upon the humidity, serenity, and tran- quillity of the atmosphere ; and the constitution, form, surface, and location of the bodies receiving the moisture INFLUENCE OF THE CONDITION OF THE ATMOSPHERE. 256. Humidity. That the quantity of latent vapor in the air must regulate, in a great measure, the amount of dew, is perfectly clear, since the latter is nothing else than condensed atmospheric vapor. 257. Serenity. Every thing that favors radiation from the earth, and consequently produces an increase of cold, contributes to the formation of dew. Thus it is copiously deposited on serene nights ; for the radia- tion from the earth then proceeds unchecked : while, on the contrary, little or no dew is seen after a cloudy night ; since the canopy of the clouds reflects back to the earth the heat that is proceeding from it, maintain- ing its surface and the contiguous air at nearly the same temperature. If, however, the clouds separate only for a few mo- ments, the heat escapes from the earth through the in- tervals, and dew is rapidly deposited ; but if the sky is again suddenly overcast, the radiation is arrested, and the heat reflected back to the earth, raising the tem- perature at its surface, and speedily evaporating the dew just formed. 258. These singular changes in temperature were observed by Dr. Wells. On one night, the sky being clear, the temperature of the grass, at half past nine, was 32° Fah. ; in twenty minutes afterwards, the heav- ens being suddenly overcast, it rose to 39° Fah. ; in What circumstances influence the quantity of dew ~i What is the effect of humidity 1 What of serenity 1 What is the influence of clouds 1 Give instances. 108 AQUEOUS PHENOMENA twenty minutes more, under a serene sky, it sunk again to 32° Fah. It was also found, that a thermometer ly- ing upon the grass, would rise several degrees, if the sky directly above it was covered by a cloud only for a few minutes. The influence of clouds upon the tempera- ture of the air is by no means as great ; for while, on one evening, when the sky was obscured for the space of forty-five minutes, a thermometer placed upon the turf rose fifteen degrees, another, suspended in the atmos- phere just above, rose but three and a half degrees. 259. Tranquillity. In a calm night, other circum- stances being the same, more dew will be deposited than when it is windy ; for the wind will not suffer any one portion of air to remain long enough in contact with the cold surface of any body to condense much of its moisture, hurrying it away before it is sufficiently cooled down for this purpose. 260. A slight agitation of the atmosphere, however, is of advantage ; since, after one portion of air has de- posited upon any surface its exuberant moisture, it re- moves it from the spot, bringing up a fresh portion to the same place, and so on successively ; giving time to each to sink to the temperature of the surface bedewed. As the night advances, and the earth becomes still colder, the same volumes of air, renewing their contact with the same surface, may be again surcharged with humidity, and deposit more dew. 261. Evening and Morning. Dew is often formed towards the close of the afternoon, in consequence of the earth then losing more heat by radiation than it receives from the slanting rays of the descending sun. It also frequently forms in shady places just after sun- rise ; for the surface of the globe, which has been grad- ually sinking in temperature during the night, is not Which is most affected by clouds, the air or the ground 1 Illustrate. Why does a wind lessen the amount of dew "? What is the effect of a slight agitation of the airl Why does dew begin to form towards the close of day 1 Where does it form after sunrise 1 SUBSTANCE BEDEWED. 109 immediately influenced by the warm beams of the sun. Indeed, at this time, more dew is deposited than at any other equal period in the twenty-four hours. INFLUENCE OP THE SUBSTANCE BEDEWED. 262. Constitution. Since the production of dew requires that the body bedewed must be colder than the surrounding atmosphere, all substances, which rapidly lose their own heat and slowly acquire that of others, are susceptible of being copiously bedewed. On the con- trary, substances possessing the opposite qualities con- tract but little dew. Under the first class may be included glass, silk, down, wool, and, in general, all bodies of a porous tex- ture ; while metals and rocks belong to the second divi- sion. 263. If similar plates of polished glass and metal are exposed alike upon the soil during a favorable night, in the morning the glass will be drenched with dew, but the brightness of the metal will be scarcely dimmed. These different results arise from the fact, that, while the glass is deprived by radiation of ninety hundredths of its original heat, twelve hundredths is all that the metal loses. Besides, the glass, being a bad conductor, draws but little warmth from the earth to supply its loss ; while the metal, being a good conductor, can easily restore any reduction of heat from the warm soil imme- diately below. Large plates of metal, exposed throughout the night, never acquire a temperature more than three or four degrees below that of the atmosphere. 264. Surface and Form. A polished surface does not radiate so well as one that is rough and uneven; and the latter is always found, under a like exposure, to receive more dew. Points radiate heat most rap«dly, Account for its deposition at this time. What substances are capable of being copiously bedewed 1 What not 1 Give examples. Account for the unequal deposition of dew on glass and metal. What is said of polished and rough surfaces in this particular 1 110 AQUEOUS PHENOMENA. and are therefore the coldest portions of a radiating body ; hence, of all the globules of dew that form upon blades of grass, the largest are found at the very ex- tremities. Grass is well known to be copiously bedewed ; its form, as just mentioned, causes it to lose its own warmth with great rapidity, while its porous texture prevents it, at the same time, from replenishing its loss from the soil. 265. Location. If a body is screened from the open sky, it contracts less dew than when fully exposed ; for the screen arrests radiation in the manner of clouds ; and the difference in temperature between the sheltered body and the contiguous air, is less than that which would exist between the same body and the surround- ing atmosphere, were the substance bedewed, entirely unsheltered. This has been proved by the experiments of Dr. Wells. 266. In four trials, made with two parcels of wool, in all respects alike, the first of which was placed upon the upper side of a board, four feet from the ground, and the second loosely attached to the under side, the gain, in dew, was as follows : 1st night. 2d. 3d. 4th. gTS.gT8.gTfl. gra.^ 1st parcel, 14 19 11 20 2d do. 4 6 2 4 We hence perceive, why, beneath the shelter of trees, and on the under surfaces of leaves, but little dew is found. 267. Dew has never been found upon the surface of large bodies of water ; for whenever the aqueous parti- cles at the surface are cooled, they become heavier than those below them, and sink ; while warmer and lighter particles rise to the top. These, in their turn, become What of points 1 Why are the largest beads of dew upon the end of the blades of grass 1 Why does an exposed body contract more dew than one which is shel- tered 1 Give the results of Dr. Wells' experiments. Why are the surfaces of large bodies of water free from dew 9 LOCATION AND COLOR. Ill heavier and descend ; and the process continuing through- out the night, maintains the surface of the water and the air at nearly the same temperature. Dr. Wells ascertained, by experiment, that even a small quantity of water gains no weight by exposure during a single night. It appears, from the narrative of the U. S. Exploring Expedition, and from other sources, that on the ocean heavy deposits of dew sometimes occur upon the decks of vessels. 268. The exposed parts of the human body are never covered with dew ; since the vital heat, varying from 96° to 98° Fah., effectually prevents such a loss of warmth as is necessary to its production. 269. Color. A few experiments were made by Dr. Wells, in order to ascertain the effect of color upon dew; but without any decisive results. In 1833, Dr, Stark, of Edinburg, made two experiments, from which he inferred, that under like exposures, more dew was de- posited upon dark-colored bodies, than upon light-colored. But the author of this work, from an investigation pros- ecuted by himself during the summer of 1846, arrived at. the conclusion, that color exerts no influence what- ever upon the quantity of dew. This fact might also be • inferred from the experiments of Dr. Bache on heat, which clearly' show, that the amount of radiation is not affected by color. 270. Observations. The observations, which have been made in various regions of the globe, in regard to the occurrence of dew, strongly corroborate the theory of Dr Wells. In Bengal, during the month of Novem- ber, the nights are beautifully serene, and accompanied with heavy dews ; but in April and May, when high winds prevail, with a close and cloudy atmosphere, no What experiment was made by Dr. Wells ^ What is stated in the narrative of the Exploring Expedition 7 Why is dew never found upon the human body 1 What is said as to the influence of color 1 What do the observations made in different regions attest 1 Give instances. 112 ' AQUEOUS PHENOMENA. dew is deposited. From September to March, the sun glows like an orb of fire over Southern Guinea ; but the nights are cool, and the parched soil is refreshed with abundant dews. In Egypt, profuse dews, like rain, occur in the summer, when the nights are resplendent with stars ; while at Thebes, where the sky is con- stantly serene, abundant dews are the only moisture that the earth receives from above, during the lapse of many years. 271. Facts explained. The explanation of several familiar facts, depends upon the foregoing principles. Thus, for instance, if, in a warm summer's day, a glass is filled with cold water, the exterior surface is seen covered with moisture ; for the reason, that the glass, being colder than the air in contact, cools the latter be- low the dew-point. In summer, caves and cellars are damp ; because the warm air that enters them from without is cooled down, and its humidity either floats in the atmosphere, or is condensed in beads of dew upon the stones. 272. Beneficent Distribution. The mode in which the blessing of dew is dispensed to the earth, beautifully exemplifies the benevolence of our Creator. In the temperate climes, where the frequent inter- change of sun and shower preserves the earth from the extremes of heat and moisture, very little dew is needed, and but comparatively little is deposited. The regions however within the tropics are deprived of rain for months, and this destitution is partially supplied by the dews, which precisely at these seasons are most abundant. 273. The lake and the river are not visited by dew, for no form of vegetation exists within them that needs its presence. To the naked rock it comes but in scanty measure ; for there is nothing here to nourish — save, perhaps, the thorny cactus, which, from its very form and What facts are explained upon the foregoing principles'! What does the distribution of dew exemplify 1 Give the various illustrations. DISTRIBUTION. 113 nature, is adapted to its situation ; and though spring- ing from the cleft of a rock beneath a tropic sun, or striking its roots in the sands of the desert, is capable of deriving from the air an adequate supply of moisture. 274. Upon the foliage of the grove very little dew is deposited, in consequence of the inclined position of the leaves, their frequent motion, and the shelter they afford each other. Nor is it needed ; for the natural supply of moisture rises deep from the soil through the parent trunk, diffusing itself throughout every branch to the remotest, extremity of the slenderest bough. 275. The dew, however, blesses, in all its invigorating exuberance, the humble plant and tender herbage, a vast class of vegetable life, at once the most perishable and the most useful ; it is the first of all to feel the effects of drought, and yet it is that which is necessary to the very existence of man. From the field, not from the forest, comes our support ; and the failure of a single plant, the grass or the bladed grain, may bring upon a nation scarcity and famine. /\ CHAPTER V. OF HOAR-FROST AND SHOW. 276. Hoar-Frost. Hoar-frost is produced in the same manner as dew. Late in the spring, and early in the fall, the surface of the earth, during serene nights, sinks in tempe. ature below the freezing point, while the atmosphere, a few feet above, is higher by several degrees. The moisture which is then deposited becomes con- gealed in sparkling crystals ; and the stems of plants and the branches of low shrubs are often adorned with fringes, formed of the most beautiful and delicate crys- tallizations. What ia the subject of chapter fifth 1 How is hoar-frost produced 1 Describe its appearance. J 14 AQUEOUS PHENOMENA. 277. A species of hoar-frost occurs when a warm south wind succeeds a continuance of cold weather. Stone columns and buildings are then covered with a snowy incrustation, composed of an assemblage of 'minute crystals, caused by the influence of the low temperature of the stone upon the condensed vapor of the air. The effect of a cold body upon moist air is well shown by the following facts related by Ballantyne, who resided for two years at York Factory, in the vicinity of Hud- son's Bay. After narrating the adventures of a hunting expedition in the depth of winter, he thus describes an incident that occurred upon the return of himself and his companions to their dwelling. " It was curious to observe the change that took place in the appearance of our guns after we entered the warm room. The barrels and every bit of metal upon them instantly became white, like ground glass. This phenomenon was caused by the condensation and freezing of the moist atmos- phere of the room upon the cold iron. Any piece of metal, when brought suddenly out of such intense cold into a warm room, will in this way become covered with a pure white coating of hoar-frost. It does not remain long in this state, however, as the warmth of the room soon heats the metal and melts the ice. Thus, in about ten minutes our guns assumed three different appear- ances. When we entered the house they were clean, polished, and dry ; in five minutes they weie as white as snow, and in five more were dripping wet." 278. Every thing that prevents the ratriation of heat, arrests the formation of hoar-frost. During the chilly nights of spring, plants that are sheltered by trees are less liable to be frozen than those which are fully ex- posed ; and a slight covering of straw, or even of paper, will often afford an effectual protection. Vineyards have frequently been saved from the effects of frost, by enveloping them during the night in a cloud of smoke. What effect is caused by a warm south wind, after a period of cold wea- ther 7 Relate the facts related by Ballantyne. What arrests the formation of hoar-frost 1 HOAR-FROST AND SNOW. 11? 279. The effect of a screen in checking radiation, and thus preventing frost, has been finely illustrated by the experiments of David Scott, of India. Throughout the whole region of Upper India, ice is artificially pro- cured by placing upon a layer of dry straw, in the bottom of small pits, and fully exposed to the clear sky, broad, shallow earthen pans, filled with water. Such is the radiation during the night, that a thin crust of ice will sometimes form upon the water, when the tempera- ture of the air on a level with the pits is as high as 41° Fah. On qne occasion, Mr. Scott extended a muslin turban across a pit, three feet above the pans. No ice was formed in the vessels immediately under it ; but, in sev- eral that were partially covered, ice appeared upon the part of the water beyond the shelter of the muslin ; while the surface beneath the turban remained in a fluid state. Two strings, crossing each other at a lower height above a pan, under favorable circumstances, divided the ice into four quarters, the water beneath the strings continuing unfrozen. SNOW. 280. Snow is the frozen moisture that descends from the atmosphere when the temperature of the air at the surface of the earth is near or below the freezing point. 281. Snow-Flake. At moderate heights, and in the temperate regions, snow commonly falls after several days of severe frost when the weather has moderated. The largest flakes occur when the air abounds with vapor and the temperature is about 32° Fah. ; but as the moisture diminishes, and the cold increases, the snow becomes finer. In the former case, it is not unusual to observe flakes an inch in diameter ; and in the latter, they only measure a few hundredths of an inch. Illustrate the influence of a screen, by the experiments of Scott. Define snow. When does it usually fall 1 Under what circumstances do the largest flakes occur 1 ? Under what circumstances do the smallest 1 How large are they 1 116 AQUEOUS PHENOMENA. At Bossekop a fall of snow occurred when the thermo- meter stood at 10° Fah., and the diameter of the flakes then scarcely exceeded seven hundredths of an inch. The snow-flake, is composed of regular crystals, and its beautiful figures and rich diversity of forms have ever excited the admiration of observers. In solid ice, the crystals are so blended together that their symmetry is lost in the compact mass ; but in snow, they are per- fectly developed, when the flakes descend through a calm atmosphere. Any agitation of the air, or an in- crease of moisture or temperature, destroys their deli- cate structure. If the crystals of snow were solid, they would be transparent, like other crystallized bodies; but they contain air, and to this circumstance is attributed their brilliant whiteness ; for the air preventing the ready transmission of light through the snow-flake, the rays are copiously reflected from the assemblage of crystals The bulk of snow which has just fallen is ten or twelve times greater than that of the water obtained by melting it. 282. Though single crystals always unite at 'angles of 30°, 60°, or 120°, they nevertheless form, by their different modes of union, several hundred distinct varieties. Scoresby, a celebrated Arctic navigator, has enu- merated six hundred, and delineated ninety-six ; and Kaemtz has observed twenty more, not figured by Scoresby. 283. Snow-Crystals. Although the varieties are so numerous, they are all comprised under five principal classes. 1st. Crystals in the form of thin plates ; they are generally very thin, transparent, and of a. delicate Hnw small are they ? Of what is the snow-flake composed? How is the whiteness of snow caused ? What is said of the bulk of snow? State the number of varieties of snow-crystals. In how many classes are they comprised ? Describe them. SNOW-CRYSTALS. 117 structure. This class includes many remarkable vari- eties, which are represented by the first twenty-five figures of the annexed cuts, (15., 16.) , 2d. Flakes" either possessing a spherical nucleus, ox a plane figure, studded with needle-shaped crystals, (figure 26.) 3d. Slender prismatic crystals ; usually six-sided, but sometimes having only three sidgs. 4th. Pyramids with six sides ; (figure 27.) 5th. Prismatic crystals, having, perpendicular to their length, both at the extremities and in the middle, thin, six-sided plates ; (figures 28., 29. and 30.) The last Fig. 15. SNOW-CRYSTALS. 113 AftUEOUS PHENOMENA. Fig. 16. 6N0W-CRYSTAI.S. two classes are extremely rare, Scoresby having ob- served the fourth but once, and the fifth only twice, in all his voyages. Flakes belonging to two consecutive falls of snow, possess different figures ; but those which descend during the same storm, are usually alike in this par tieular. 284. Natural Snow-Balls. Balls of snow are sometimes formed by the action of a high wind upon light snow. Prof. Cleaveland, of Brunswick, in Maine, What is said of the crystals that fall during the same storm 1 RED SNOW. 119 ODserved, on the first of April, 1815, a great number of enow-balls scattered over the fields, varying from one to fifteen inches in diameter. They had evidently been caused by the wind rolling up the snow, as the track of the balls was distinctly visible. In 1830, similar balls were seen by Mr. Sheriff, in East Lothian, scattered over a wide extent ; some of the masses being eighteen inches in diameter. 285. But the most remarkable exhibition of this kind was beheld by Mr. Clarke,, of Morris county, New Jer- sey, in January, 1808. A crust having formed upon the snow that had previously fallen, a light snow soon after occurred, covering the glassy surface to the depth of three-quarters of an inch ; the sky then suddenly be- came serene, and a high -wind arose. Beneath the force of the gale, small portions of snow would slide along for the distance of ten or twelve inches, when they would begin to revolve, rapidly increasing both in length and diameter. Where the descent of the ground favored their formation, masses rolled up to the size of a barrel, and, as far as the eye could see, the dazzling surface Was covered with balls and cylinders of snow; varying in magnitude from ten inches to three feet in diameter. Upon examination they were found to be hollow at each end, almost to the centre, and as round as if they had been so many logs of wood turned in a lathe. The cylinders covered nearly 400 acres, and their number was judged to be nearly 40,000. 286. Red Snow. In 1819, Capt. Ross beheld snow of a brilliant crimson hue, clothing the sides of the mountains at Baffin's Bay ; rising, according to his re- port, to the height of several hundred feet, and extending " to the distance of eight miles. Snow of this tint is not, however, confined to the Arctic regions. Raymond had previously observed it in the Pyrenees. In 1818, vast masses were spread ovei the Italian Alps and Apennines, and five years before. Relate the several accounts of natural snow balls. What is said of red snow 1 120 AQUEOUS PHENOMENA. the whole range of the last-mentioned chain was covered with rose-colored snow. The same phenomenon was seen by Scoresby, Parry and Franklin, in high northern latitudes, and the navigators of the southern hemisphere have found red snow in great quantities at New Shet- land, 62° S. Lat. 287. In snows of great depth, the accounts differ in regard to the thickness of the colored stratum. Ross conjectured, that, in the Arctic mountains, the crimson hue penetrated to the depth of several feet below the surface ; while others could not detect its existence be- yond one or two inches. Among the Alps, the red snow is usually discovered in little sheltered hollows, in layers not exceeding twc or three inches in thickness : though these are some- times situated far beneath the general surface of the snow. 288. Green Snow. When the French meteorolo- gists, Martin and Bravais, traversed a field of snow at Spitzbergen, in 1838, it appeared of a green hue, wher- ever it was pressed by the foot. The coloring matter seemed to reside just below the surface, which was bril- liantly white. Upon another excursion, the first observer beheld the green particles spread like dust over the snow, which was also tinted green beneath the surface, and upon the sides of the field. 289. Cause. These singular hues are produced by the presence of an infinite number of a certain class of microscopic plants, which from their great tenacity of life, are capable, not. only of existing at a very low temperature, but even of flourishing with extraordinary vigor. These minute vegetable forms are composed of glob- ules, which vary in diameter from one-thousandth of an inch to one three-thousandth. Each globule is divided into seven or eight cells, filled with a liquid, in which What of green snow '? To what cause are those colors attributed 1 USES OF SNOW. 121 * live a great n imber of animalcules. The cells are gen- erally red, which is supposed to be their original color, the green tint being probably acquired by exposure to the air and light. These extraordinary hues may, therefore, be regarded as originating in the same plant, in different stages of development. 290. Uses of Snow. Snow subserves many impor- tant purposes. Gathered in exhaustless stores upon the high mountains of the globe, it feeds, as it gradually melts beneath the heat of summer, thousands of rivers, which, flowing on from clime to clime, enrich the soil and crown the land with plenty. The snow-capped mountains are the natural refriger- ators of the glowing regions that lie within the tropics ; cooling the winds that pass over them, and mitigating the fierce temperature of the atmosphere. In the higher latitudes, where the winters- are severe, the snow forms a warm covering for the soil, and de- fends vegetation from the rigors of the frost. It is well known, that grain, during an open winter, is frequently destroyed by the cold ; and, in the mild climate gf Eng- land, Alpine plants have perished, in consequence of being deprived of their natural covering of snow. During the long night of the polar climes, the inten- sity of the darkness is diminished by the presence of the snow ; inasmuch as it reflects, instead of absorbing, like the bare ground, the faint, light that there proceeds from the sky. Nor is it to be forgotten, that, in these incle- ment regions, the wretched natives would be unsheltered during the winter, were it not for the snow ; since this, when cut into blocks, supplies the Esquimaux with the means of constructing their huts. What is said in regard to the uses of snow ? 6 182 AQUEOUS PHENOMENA. CHAPTER VI. X OF HAIL. 291. Hail. The ice that descends in showers, and usually in summer, is called hail. It is different from sleet, which is nothing more than frozen rain, and oc- curs only in cold weather. 292. Structure. Hailstones are generally pear- shaped, and if they are divided through the centre, they are found to be composed of alternate layers of ice and snow, around a white, snowy nucleus, resembling the coats of an onion. The surface is rough, and is some- times studded with icicles. 293. Size. Hail varies in size, from stones as small as a pea to those which are several inches in circumfer- ence. Benvenuto Cellini relates in his memoirs, that during his journey from Italy to France, he was over- taken by a terrible storm in the vicinity of Lyons ; hail- stones falling of the size of lemons, and with sufficient force to kill even men and cattle. At Roncesvalles, in August, 1813, there fell upon a division of the British army a storm of hail, in which the stones ranged in size from a bean to a he?i's egg. The tin camp-kettles of the soldiers were indented by the masses of ice, some of which were round, and armed with icicles three inches in length. In May, 1847, hailstones of immense size descended near the town of McDonough, in Georgia ; one of them was measured an hour after it fell and found to be ten inches in circumference. During a terrific storm, that occurred at Cazorta, in Spain, on the 13th of June, 1829, the roofs of the houses were broken in by the hail. Some of the stones are stated to have weighed nearly four pounds and a half. It is probable that such extra- Define hail. What is the form and structure of the hailstone ? What is said of its size ? Narrate the facts stated. HAIL. 123 ordinary masses as those which have been mentioned, are formed by the union of several hailstones frozen to- gether. GEOGRAPHICAL DISTRIBUTION. 294. Hailstorms are most frequent in the temperate climes; and rarely occur within the tropics, except in the vicinity of mountains whose summits tower above the limit of perpetual frost. Although by no means common, they are not unknown in the high northern latitudes. Simpson, on the 12th of August, 1839, was exposed in the straits of Boothia, in 68° 32' N. Lat., to a tremendous thunder-storm, accompanied with torrents of rain and heavy showers of hail. It is mostly in summer, and usually at the hottest part of the day, that hail is observed to fall. Scarcely any occurs in the night. ORIGIN. 295. The structure of the hailstone shows that it is not formed at once; for the concentric layers around the snowy nucleus, consist of different accessions of moisture, successively condensed and congealed upon the surface of the stone. The light, porous texture of the snowy centre, seems to indicate, that the place of origin must be some region in the atmosphere where the air is rare, and the cold in- tense ; since the appearance of the centre is similar to that presented by a drop of water, when frozen under the exhausted receiver of an air-pump. 296. It is necessary then for the production of hail, that a warm, humid body of air should mingle with another so extremely cold, that their temperature, after uniting, shall be below the freezing point. This com- bination must also take place during the warmest period Where do hailstorms frequently occur ? Where rarely 1 When do they usually prevail 1 What indicates that the hailstone is not formed at once ? Where must it originate 7 What conditions are necessary for the production of hail'? 124 AQUEOUS PHENOMENA. of the year and the day. In accounting for an intense degree of cold under such circumstances, consists the great difficulty of explaining the phenomena of hail- storms. 297. Until within a few years, almost every meteor- ologist attributed the cold of hailstorms to the agency of electricity. It is well known that air, when electri- fied, is expanded, and that expansion pioduces cold. It was therefore imagined, that the electrified state of the atmosphere before a storm, caused such a reduction of temperature as to freeze the falling moisture and pro- duce hail. Volta, a distinguished philosopher of France, sup- posed the cold to be the result of evaporation, but em- ployed electricity in a singular manner, as explained below. 298. Volta's Theory. According to this theory, two clouds, differently electrified, are supposed to extend through the sky, one directly above the other. The cold, caused by evaporation from the upper surface of the lower cloud, is considered to be so intense, that the vapor is frozen, and the nucleus of the hailstone then formed. Its size is afterwards increased by the humidity it gathers in vibrating backwards and forwards between the two clouds, like the dancing figures upon electrical plates. (C. 969.) At last it becomes so large, as to break through the lower cloud, and fall to the earth. 299. The sanction of a great name gave weight to this fanciful view, and in 1821, throughout the southern districts of Prance, which are peculiarly liable to hail storms, hail-rods were erected, in order to draw the electricity from the clouds, and thus protect the vine- yards. Their efficacy, however, is exceedingly question- able. The electric hypothesis is, moreover, at variance with facts. The forests, which constitute a vast assemblage of hail-rods, are often ravaged by hail ; and in the tor- What effect has been attributed to electricity 1 Explain Volta'» theory. ORIGIN. 125 rid zone, where the development of atmospherical elec- tricity is greatest, hailstorms are almost unknown. 300. Olmsted's Theory. Prof. Olmsted, of Yale College, considers electricity as an effect, and not the cause of hailstorms. According to his theory, which has been very extensively received, the cold body of air derives its low temperature, not from electricity, but from some known source of cold ; and the combination, which occasions the hail, may arise in various ways, the prin- cipal of which appear to be the following. 301. First. An exceedingly cold wind, coming from a region far above the limit of perpetual frost, may •meet with a current of warm air, blowing from a point many thousand feet below this limit. Secondly. By the force of whirlwinds, large volumes of warm air from the surface of the earth may be sud- denly transported into the higher and colder regions of the atmosphere. Thirdly. In the vicinity of lofty mountains, cold blasts are frequently known to sweep down their sides from the snowy peaks and glaciers, and mingle with the warm atmosphere of the vales. Each of these methods we will discuss separately. 302. Curve of Perpetual Congelation. In Art. 53, we have seen that a point can be reached in every latitudes, where moisture, once frozen, always remains so. An imaginary line passing through these points, and extending from pole to pole, forms what is termed the curve of perpetual congelation, which possesses the peculiar figure shown in the annexed cut. Does the electric theory agree with facts 1 What are Professor Olmsted's views in regard to electricity 1 Whence comes, according to his theory, the cold of the hailsto>-»> In what three ways may hailstorms arise 1 What is the curve of congelation? 126 AQ.UEOUS PHENOMENA. Fig. 17. Feet. 14,000 / ~V 12,000 » ^ ./ N 10,000 8,000 C*s->- c" 6,000 4,000 \ j£ ^ iA \ 2,000 ^rr >-\ 90° 80° 70° 60° 50° 40° 30° 20° 10° 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° CURVE OF PERPETUAL CONGELATION. 303. The heights of the curve from the surface of the globe vary but little from the equator to Lat. 30° ; but from 30° to 60° the change is very great, and the line rapidly approaches the earth. The difference in the height of the points of congela- tion, for -every five degrees of latitude, is presented in the following table : Lat. Difference of height in feet. 0° to 5° 122 5° to 10° 388 10° to 15° 569 15° to 20° 779 20° to 25° 689 25° to 30° 1,438 30° to 35° 928 35° to 40° 1,648 40° to 45° 1,358 45° to 50° 1,398 50° to 55° 1,348 55° to 60° 1,228 60° to 65° 1,168 65° to 70° 959 70° to 75° 809 75° to 80° 628 304. Action op Opposite Currents. We are now to imagine, for the sake of illustration, that a north Describe its oeculiarities. ORIGIN. 127 •wind, originating in 50° N. Lat., moves horizontally, at the rate of sixty miles per hour, at an altitude of' ten thousand feet ; while a south wind blows simultaneous- ly from 30° N. Lat. with the like velocity, and at the same height. If they are upon the same meridian, they will meet in ten hours at 40° N. Lat., and since the first wind commences its course at M, three thousand feet above the limit of constant frost, it will be extremely cold ; while the south wind will be comparatively warm, as it proceeds from a region, N, two thousand feet below the boundary of perpetual congelation. By the union of air, thus widely differing in temper- ature, the inherent atmospheric vapor is both condensed and frozen, and the central portion of the hailstone formed, which, in its descent to the earth, is gradually enlarged by constant accretions of frozen moisture. 305. The prevalence of such opposite currents as have just been supposed, has already been shown (Art. 222) ; and it is by no means improbable that, in their ceaseless circuits, there are times in which they encoun- ter each other. It may be asked, how can the different winds preserve their respective temperatures, in traversing a distance of ten degrees ? To this it is answered, that a fluid in motion can pass through a fluid of the same kind in repose, and differing in respect to heat, without suddenly changing its own temperature. The waters of the Gulf-stream, flowing through the North Atlantic from the torrid zone, are warmer than the ocean, even in the latitude of Newfoundland. 306. The occurrence of hailstorms, under these cir- cumstances, substantially agrees with facts. It will be seen, by referring to the figure, that the mingling of opposite winds, at a lofty elevation, in the tropics, C, C 2 , would occasion nothing but a combination of warm cur- Explain the action of opposite currents. Why can the currents preserve their respective temperatures 1 Show to what extent the occurrence of hailstorms, under these circum- stances, accords with facts. 128 AQUEOUS PHENOMENA. rents, and in the polar climes of cold currents, A, A 2 ; in neither case could hail be the result of the union. In the temperate regions, the admixture of warm and intensely cold currents can only be found, and precisely within these limits hailstorms are most prevalent. Their frequency in summer is attributed to the cir- cumstance, that the opposing winds are then most easily set in motion by the increased energy of the solar rays. 307. The space ravaged by hailstorms, often indicates the presence of aerial currents, the devastations being frequently confined to a long and narrow strip of coun- try. Sometimes the storm proceeds in two parallel tracks, leaving the intervening region entirely unin- jured. Thus a hailstorm once commenced in the south of France in the morning, and in a few hours reached Hol- land. The places desolated formed two parallel paths from S. W. to N. E. ; the length of one was 435 miles ; and that of the other 497 miles. The average width of the eastern track was five miles, and that of the western ten ; and upon the space comprised between them, which was twelve miles and a half in breadth, no hail fell, but only a heavy rain. 308. Action of Whirlwinds. It has been stated, (Art. 132,) that whirlwinds are not always vertical, but frequently inclined towards the earth. In consequence of this position, the gyratory motion of the whirl (if its diameter is considerable) will, doubtless, often carry up hot and humid air from the surface of the earth into the higher regions of the atmosphere, bringing down in return large volumes of cold air from the upper strata ; thus causing such a combination as results in the pro- duction of hail. This action will be more extensive and energetic if, as some suppose, whirlwinds at times exist whose axes are parallel to the horizon. 309. It must also be remembered, that in the vortex of the whirlwind the air is rarefied, and into this partial Explain the action of whirlwinds. ORIGIN. 129 void the cold air from above will descend, by reason of its superior -weight; while below, on account of the pressure of the surrounding atmosphere, warm currents will stream under the base into the vortex. Here, then, may evidently occur a union of hot and cold air, differ- ing so greatly in temperature that the condensed moist- ure will freeze into hail. The cold, arising from the rarefaction of the air m the centre of the whirlwind, also contributes to the forma- tion of hail.' 310. Influence of High Mountains. In the vicit. ■ ity of those lofty mountains, whose peaks are always covered with ice and snow, destructive hailstorms fre- quently occur. The south of France, which lies be- tween the Alps and Pyrenees, is annually ravaged by hail ; so great is the ruin to the productions of the soil, and especially the vineyards, that the yearly loss to the national revenue was estimated, by the Linnean Society of Paris, at fifty millions of francs, or nine millions three hundred and seventy-five thousand dollars. In Peru, hail has been seen to fall ; and on the 17th. of August, 1830, it covered the streets of Mexico to the depth of several inches. 311. That such phenomena should arise in these and similar localities, is by no means surprising: for cold blasts of wind descending from the snowy summits of the neighboring mountains, and mingling with the warm »air of the plains, could doubtless occasion these results ; and the existence of such breezes is fully established. 312. Hail in Southern India. Hail sometimes occurs within the tropics, even at a distance from those mountain-chains that rise above the limit of perpetual frost. Thus in India, in 16° 30' N. Lat., during the year 1825, hailstones fell at Darwar, of the size of pigeons' eggs ; and in a similar storm, which happened ai In what localities do hailstorms occur 1 Give instances. What is the cause of hail in these regions'? Does hail ever occur at a distance from snow-capped mountains J 6* 130 AQUEOUS PHENOMENA. Trinconopoly, in 1 805, the stones were as large as wal- nuts. 313. In conclusion, we may say in regard to this sub- ject, that at present it is not fully understood. Much valuable information has been gathered, but hitherto no theory has been advanced, which completely accounts for all the facts that arise. Give instances. Is this subject fully understood 7 PART IV. ELECTRICAL PHENOMENA. i CHAPTER. I. OP ATMOSPHERIC ELECTRICITY. 314. The atmosphere is usually electrified. The means employed for collecting its electricity differ ac- cording to the object proposed ; for we may desire to conduct our investigations at one time in the lower regions of the atmosphere, at another in the higher ; or the air may be explored to a great distance horizontally. 315 Electrometers. For ascertaining the elec- tric state of the atmosphere near the surface of the earth, Volta's electrometer is sufficient. An electrometer is an instrument which serves to indicate and measure elec- tricity. The one just mentioned consists of a glass jar, surmounted by a pointed, metallic rod ; and to the lower end of the rod, which enters the jar, two fine straws are loosely attached. The pointed rod, collecting the elec- tricity from the air, the two straws become similarly electrified and recede from each other, (C. 957) ; the amount of divergence measuring the intensity of the fluid. 316. Insulated rods of iron are erected for testing What is the subject of part fourth? What of chapter first 1 What is the usual state of the atmosphere t Why are different means employed for collecting its electricity 1 What are electrometers 1 How is the electric state of the atmosphere near the earth ascertained 1 132 ELECTRICAL PHENOMENA. the air at greater elevations. By means of its pointed summit, the entire conductor becomes charged with atmospheric electricity, the nature of which is easily determined by the electrometer. At the Kew Observatory, near London, the conductor is a conical tube of thin copper, raised sixteen feet above the roof; to the top of the tube a lamp is affixed, its ascending stream of smoke and heated air being an excellent collector of electricity. Where a fixed apparatus is not at hand, observations may be made by discharging metallic arrows into the air, in the way hereafter to be described. 317. Experiments are made in the higher regions of the atmosphere by the aid of kites and balloons. The string of the kite must be wound with fine wire, in order to convey the electric fluid from the sky ; and it must also be insulated, by attaching the lower end either to a silken cord or glass pillar. Small, stationary balloons are sometimes employed, the strings of which are arranged and fastened in the same manner. Occasionally meteorologists ascend in balloons for the purpose of making observations. 318. The method adopted by Mr. Crosse, of Broom- field, near Taunton, for exploring the atmosphere in a horizontal direction, is the following. Upon some of the loftiest trees on his estate, strong poles are firmly fastened, and a copper wire extended from tree to tree ; its length was, originally, a mile and a quarter, but is now about 1600 feet. The wire, being perfectly insu- lated, forms a conductor, conveying the electricity of the atmosphere to the room of the observer ; where one end of it terminates in an insulated brass ball, near which is a receiving ball, connected with the ground. In all apparatus for collecting atmospherical electri- city, the most careful and certain arrangements should be made for conveying harmlessly to the earth any excess that may accumulate. How at greater elevations ? For what purpose are kites and balloons employed 1 What was the object of Mr. Crosse's apparatus 1 Describe it. ATMOSPHERIC ELECTRICITY. 133 By the aid of the instruments just described, much important knowledge has been acquired in regard to the electric condition of the atmosphere. 319. Electric Condition of the Atmosphere. In the ordinary state of the atmosphere, its electricity- is invariably positive ; but when the sky is overcast, and the clouds are moving in different directions, it is subject to great and sudden variations ; rapidly chang- ing from positive to negative, and back again, in the space of a few minutes. Upon the first appearance of fogs, rain, hail, snow and sleet, the electricity is gen- erally negative ; it then changes to positive, gradually increasing in strength, and then decreasing in the same manner ; the alternations both in the strength and na- ture of the electricity occurring every three or four minutes. Similar changes are observed upon the ap- proach of a thunder-cloud. The atmosphere is highly electrical, either when hot weather succeeds a series of wet days, or wet weather follows a series of dry days. 320. Annual Yariation in Intensity. The electricity of the atmosphere is stronger in winter than in summer, and, by comparing observations from month to month, a gradual decrease in intensity is perceived from January to July, but an increase from July to January. During the winter the electricity strengthens with the cold. 321. Daily Variation. At sunrise the electricity of the air is weak, but as the day advances, it increases in power, until 6 or 7 o'clock, A. M. in summer, 8 or 9 in spring and autumn, and 10 or 12 in winter ; it then begins to diminish, and by 2 P. M. is hardly stronger than at sunrise. In summer, it continues to decrease till some time between 4 and 6 P. M., and in winter is weakest about 5 P. M. After this period the electricity again becomes strong- State the facts in regard to the electric condition of the atmosphere. What is said respecting the annual variation in intensity 1 What of the daily variation '? 134 ELECTRICAL PHENOMENA. er, advancing in intensity until about two hours alter sunset ; when it once more begins to abate, growing more and more feeble until sunrise. Thus, during the day, there is a regular fluctuation in the strength of the atmospheric electricity ; two peri- ods occurring when its intensity is greatest, and two when it is least. 322. Variation' in Altitude. The electricity of the air increases in strength with the altitude. This is shown by the following experiment, made by Bequerel and Breschet, at the monastery upon the Great St. Bernard. Having extended upon the ground a piece of gummed silk, ten feet long and seven wide, the experimenters placed upon it an electrometer ; to this they attached one end of a silk cord, into which was twisted a fine wire, the other end of the cord being fastened to an iron arrow. By means of a bow, the arrow was shot upwards to the height of 250 feet ; and as in its ascent the elec- tricity of the air was gradually collected and conveyed along the wire to the electrometer, the straws of the latter were seen to diverge more and more, and at length to strike the sides of the glass jar. When the cord was detached, the electricity of the straws was discovered to be positive. 323. In order to determine whether this increased divergence was really caused by the superior energy of the electricity residing in the higher regions of the atmos- phere, the arrow was discharged horizontally to the same distance as before ; but, as it speeded on its course, no increased electric action was manifested by the elec- trometer. 324. Experiments for the same purpose were made by two celebrated French philosophers, Gay Lussac and Biot, during their aerial voyage in 1804. From the car What is said of variation in altitude 1 Relate the experiment of Bequerel and Breschet. Describe that of Gay Lussac and Biot. What inference Is drawn from both these experiments 1 ORIGIN. 135 of their balloon was suspended a wire 170 feet long, to the lower end of which a metallic ball was attached ; the upper end being connected with an electrometer in the car. By means of this apparatus, these observers were enabled to note the electrical changes occurring in the atmosphere at different heights ; and, from their various observations, arrived also at the conclusion, that the electricity of the atmosphere was positive, and in- creased in strength with the altitude. • * ORIGIN. 325. Evaporation. One of the most abundant sources of atmospherical electricity is evaporation. It was shown by Volta, whose experiments were confirmed by those of Saussure, that electricity was produced when water was evaporated. But it appears from the late researches of Pouillet, that this is only the case when the water is not pure, and chemical decompositions occur. If distilled water is evaporated, no electricity is developed ; but if a little chalk, lime, salt, or other foreign matter is dissolved in the water, the rising vapor is then positively electrified, and the vessel containing the fluid negatively. 326. Now the waters of the earth are generally in this latter condition, being seldom pure, and the vapors, which are constantly ascending from the ground, will therefore possess positive electricity, and the earth negative. The briny waves of the ocean also contribute their share, and supply the air with a great amount of posi- tive electricity. 327. The process of evaporation advances invisibly and in silence ; and, for this reason, we might easily undervalue its agency in accumulating those vast stores What is the first source of atmospheric electricity 1 What is said in regard to the experiments of Volta, Saussure and Pouil- let? In consequence of evaporation, does the air become positively or nega- tively electrified 1 Is the amount of electricity thus developed supposed to be great 1 136 ELECTRICAL PHENOMENA. of electric matter which arm the storm with such ter- rific power. But when we reflect, that more than two hundred millions of hogsheads of water are computed to rise daily in vapor from the Mediterranean, we shall find no difficulty in believing, that this influence is one of the most energetic causes of atmospherical electricity. 328. Condensation. Condensation, or the change which vapor undergoes when returning to a fluid state by a decrease of temperature, is another fruitful source of electricity. This is shown from the great amount of electricity occasioned by the condensation of steam, as it issues from the boiler of an engine. In one instance, the steam which rushed from the safety-valve of an insulated locomotive, was found to develop seven times the amount of electricity produced by an electrical machine, having a plate of glass three feet in diameter, and making seventy revolutions in a minute. Machines in which the electricity was gene rated by steam, have been constructed of such power, that a spark twenty-two inches long has been obtained from the prime conductor, (C. 964,) of sufficient energy to inflame shavings. 329. Vegetation. The vegetable kingdom also supplies the air with a great amount of electricity. Plants during the day exhale oxygen gas ; in the night, carbonic acid gas — and from the experiments of Pouillet it appears that positive electricity rises with the latter when the seeds first sprout, leaving the earth in which they are placed negatively electrified. The same results probably occur during the life of the plant. 330. Combustion. Combustion is still another source of electricity. When any substance is burning, positive electricity escapes from it, while the substance What calculation would lead us to this conclusion 1 What is the second source ? What is condensation 1 How is it shown that condensation produces electricity 1 Illustrate. What is the third source of atmospheric electricity 1 Explain. What is the fourth 1 During combustion, does the air receive positive electricity or negative 1 THUNDER-STORMS. 137 itself is negatively electrified ; the atmosphere is there- fore the reservoir of al] the positive electricity originating in this manner. 331. Friction. In accounting for the electricity of the atmosphere, the effect of friction is not to be disre- garded. If a> piece of silk is shaken in the air, it be- comes electrified ; and it is highly probable, that when masses of air, moving in contrary directions, encounter each other, electricity is developed by the friction of their surfaces. Such will be the effect, according to Kaemtz, when the masses differ in respect to moisture and temperature ; the warmer then becomes positively electrified, and the colder negatively. The action of the wind upon terrestrial objects, as rocks, buildings, trees, and hijls,' may possibly in like manner produce electricity. CHAPTER II. )>{ OF THUNDER-STORMS. 332. General Distribution. Thunder-storms prevail most in the torrid zone, and decrease in fre- quency towards either pole. During a residence of six years in. Greenland, 70° N. Lat., Gisecke heard the rolling of thunder but once ; and, according to the testimony of the arctic navigators, Scoresby, Parry and others, thunder-storms rarely occur between the 70th and 75th degree of north latitude ; and never beyond the latter parallel. As respects time, they are more frequent during the summer months. What is the effect of friction ? If two bodies of air differ in temperature, in what manner will the elec- tricity, developed by their friction, be distributed 1 What may possibly be the effect of the friction caused by wind '? Of what does chapter second treat ? How are they distributed in regard to latitude and time ? 138 ELECTRICAL PHENOMENA. The prevalence of these laws will be seen from the observations contained in the following table. Places. Latitude. Period of Observation. No. of days of thunder in the year. No. of days of thunder during 6 summer-months. Buenos Ayres, Rio Janeiro, . Calcutta, . . Padua, . . . Paris, . . . St. Petersburg, 34° 30' S. 22° 54' S. 22°30'N. 45° 15' N. 48° 30' N. 59° 56' N. 7 years. 6 " 1 year. 4 years. 51 " 11 " 23 51 60 18 14 9 13 43 45 14 12 8 333. Thunder-storms are most violent within the torrid zone. Here the play of the lightning is inces- sant, and the crashing bursts of thunder most terrific ; and none but those who have actually witnessed a trop- ical tempest, can form an idea of its awful power. Occa- sionally, in the higher latitudes, fierce ^torms occur, like that which was seen by Simpson rh the Straits of Boothia. (Art. 294.) 334. Origin. The thunder-storm is produced in the same manner as the common rain-storm ; namely, by the condensation of atmospheric vapor ; but it differs in two respects ; first, in the rapidity of this condensation, and secondly, in the accumulation of electricity resulting therefrom. 335. Wo have seen that when vapor is condensed, electricity is developed (Art 328) : the cloud then in the very process of formation becomes electrified, and to its own electricity is added that which collects upon its sur- face from the amiosphere ; whet he'' derived from evapo- ration, combustion, vegetation, friction, or any other source. This condensation must be copious, or the electricity would be weak ; it must also be rapid, else it will es- Repeat the table. Where are'thunder-storms most violent 1 How does the thunder-storm differ from the common rain-storm t Whence is the electricity of the thunder-cloud derived 1 Why must the condensation be both copious and rapid ? THUNDER-CLOUDS. 1 39 cape too fast from the cloud, and never collect in suffi- cient strength. 336. Thunder-storms are usually attended by a change in the direction of the wind, which accounts for the con- densation of atmospheric vapor ; indeed, one of the most sublime elements of a storm of this nature, is the conflict and raging of opposing currents. In the Meteorological Register of Yale College are recorded 116 thunder-storms, which occurred between 1804 and 1823. Of this number, ninety-nine were either preceded ox followed by an alteration in the direc- tion of the wind ; the change in fifty instances being from a south-westerly breeze to a north-westerly. Since the air abounds with vapor when its tempera- ture is high, the condensation will be most copious if a loss of heat then suddenly takes place. We there- fore easily perceive the reason, why thunder-storms are more frequent in summer than in winter, in low than in high latitudes, and their intensity greatest in the tropic climes. For the same reason they happen more frequently after mid-day than in the morning. 337. Electrical State op Thunder-clouds. Since the air surrounding it is a non-conductor, a single thunder-cloud floats in the atmosphere a vast insulated conductor (C. 963) ; its electricity being spread over the surface of the globules of which it is composed, and there retained by the pressure of the atmosphere. 338. Thunder-clouds may be either positively or neg- atively electrified ; and the observations of Mr. Crosse lead to the conclusion, that at times a cloud of this kind is complex, consisting of a series of concentric bands or zones, alternately positive and negative ; the electricity being weakest at the edges of the cloud, and strongest at the centre. How is this condensation effected 1 What fact is stated in proof? Explain the cause of the differences that exist in the frequency and via lence of thunder-storms. What is the electric state of a single thunder-cloud t State Mr. Crosse's opinion. 140 ELECTRICAL PHENOMENA. iiiuv it ill \\8T Thus in figure 18., which rep- F 's- m resents a section of such a cloud obliquely seen, P P' P", &c, are positive zones, N N' N", &c, neg- ative, and the number of dashes show the increase of intensity. 339. Electric Action op Thunder-clouds. The earth may be regarded as a reservoir of electricity : when, therefore, an electrified cloud floats near its sur- face, it induces the opposite elec- tricity upon the ground immedi- ately beneath it. The cloud may approach so near, that the mutual at- traction of' the two electricities overcomes the pressure of the. atmosphere ; a union then occurs, and the light- ning, at the same moment, is seen darting between the cloud and the earth, and soon after the rolling of thun- der is heard. 340. A similar inductive action arises between the clouds themselves ; for. if two clouds differently electri- fied approach each other, the electricity upon the near- est opposite surfaces augments in intensity, and often increases to such a degree that a discharge takes place, the lightning then flashing from cloud to cloud. It may sometimes happen, that the path of least resistance will not be directly through the air, but from the first cloud to the earth, and from the earth to the second cloud, and under these circumstances the lightning will take the latter roMte. • 341. Return-stroke. When a highly charged thunder-cloud approaches the earth, it induces, as al- ready stated, the opposite kind of electricity upon the ground below, and repels that of the same kind. Should Describe the electric action of thunder-clouds. When does a flash occur 1 What is the influence of one cloud upon another 1 Why does the lightning in passing from cloud to cloud sometimes talco •he earth in its course 1 What is the return-stroke 1 RETURN-STROKE. 141 the cloud be extended, and come within striking dis- tance, either of the earth or of another cloud, a flash at one extremity is often followed by a flash at the other. This is called the return-stroke, which sometimes oc- curs with such violence as to destroy life, even at the distance of several miles from the place of the first dis- charge. The mode of action may be explained by means of the following figure. Fig. 19. 342. Let D B represent a thunder-cloud, positively electrified, and within striking distance of the tree A ; the cloud, at D, being near the summit of the hill, C. By the inductive action of the cloud, the positive elec- tricity will be repelled from the tree, A, and the sum- mit, C : and both will be highly charged with negative electricity, just before the flash occurs. The moment this happens at B, the cloud becomes unelectrified, its inductive action upon C suddenly ceases, the positive electricity, which had been repelled, instantaneously returns, and, uniting with the negative electricity at C, produces an explosion. If, at this time, a person should, unfortunately, be standing upon the top of the hill, his death might ensue. 343. In this manner the following singular facts have How is it caused 1 Illustrate from the figure. Relate the instances given in Art. 34?. 142 ELECTRICAL PHENOMENA. been explained, which happened on the 10th of July, 1785, in the vicinity of Coldstream, in Berwickshire. After a fine morning, clouds were seen in the north- west by the observer Brydone, at about eleven o'clock. Between twelve and one o'clock, the storm being still distant, lightnings were seen darting from cloud to cloud, followed by thunders. Immediately after, Bry- done was startled by several loud explosions near his house, like the reports of a gun. At this moment two carts loaded with coals were passing by. The driver and horses of the first were instantly killed, and the coal scattered in all directions, while the driver of the second wagon, which was about twenty yards behind, neither perceived any lightning nor experienced any shock. Upon examination, the hair on the legs and bellies of the horses was found to be singed, and where the wheels rested at the time of the explosions, the tire was melted, and two round holes were discovered in the ground. A quarter of an hour before this event, and at a spot nearly a mile and three-quarters distant, a shepherd of the name of Bell perceived a lamb suddenly fall, while a flame passed before his face. Upon raising the lamb he found it to be dead. A woman, who was cutting grass upon the bank of the Tweed, felt a violent shock upon the soles of her feet, and was thrown to the ground. During a storm which happened near Manchester, in June, 1835, loud discharges were heard at different points of a road, like the reports of a pistol, and electric flashes distinctly seen ; a person is said to have been killed at this time, by an explosion under his right foot. 344. Height of Thunder-Storms. Though thun- der-storms prevail in the lower regions of the atmos- phere, they have often been seen at a very great alti- tude. A storm, observed by Kaemtz, amid the moun- tains of Switzerland, rose to the height of more than 10,000 feet, and the dwellers in the vale of Chamouni assured him, that storms frequently swept over the What is said respecting tha height of thunder-storms ? LIGHTNING, 143 summit of Mont Blanc. On the peaks of the Cordille- ras, a violent thunder-storm was encountered by La Condamine and Boguer, at an elevation of even 16,000 feet. Vitrified rocks have at times been discovered at lofty heights, and as this change is supposed by some to have been effected by lightning, they have sought to determine the altitude of thunder-storms from facts of this kind. The reasoning, however, is inconclusive, for these vitrifications may be owing to other causes, and were we even to grant that they are produced by light- ning, the case is by no means proved; since a flash sometimes passes between the clouds and the earth, when the former are below the point that is struck. Thus, on the first of May, 1800, a church situated on Mount St. Ursula, a lofty peak in Styria, was struck ; and seven persons were killed by a flash of lightning darting upwards from a thunder-storm below. 345. From the observations of Peytier and Hossard among the Pyrenees, it appears, that the upper and lower surfaces of thunder-clouds bear no resemblance to each other, for while the latter are perfectly level, the former are broken and uneven, presenting the appearance of mountains and ridges ; whence, during seasons of great heat, lofty peaks and pinnacles of clouds shoot far up into the sky. LIGHTNING. 346. Origin. When a portion of air is subjected to a very sudden and powerful compression, a spark is elicited (Art. 551) : that electricity produces such a com- pression can be proved by experiment, and to the ener- getic condensation of the atmosphere before the electric fluid, in its rapid progress from point to point, is at- tributed the vivid flashes that illumine the stormy sky. 347. Kinds. Lightning has been divided by Arago into three kinds, principally distinguished by their form What did Peytier and Hossard observe 1 What is the cause of lightning 1 Into how many kinds has it been divided by, Arago 1 144 ELECTRICAL PHENOMENA. viz., zigzag-lightning, sheet-lightning, and ball-light ning. 348. Zigzag-Lightning. This kind is so called, from the peculiarity of its figure, which is thus explain- ed. As the electricity passes through the atmosphere, the air is supposed, at length, to be so powerfully com- pressed before it, that a great resistance is presented, and the electric fluid then finds an easier route in some other direction. In this it proceeds, until it once more meets with a like opposition, and is compelled again to change its course ; and thus it continues glancing from side to side, until at last it reaches the place it seeks. Zigzag-lightning appears as a narrow, jagged line of intensely vivid light, traversing space with extreme velocity. On account of the unequal conducting power of different portions of the atmosphere, the flash some- times divides, branching out in several different direc- tions ; the lightning is then said to be forked. A divi- sion into three distinct lines is of rare occurrence ; but even more have been seen, for Kaemtz beheld, at Halle, in June, 1834, a flash of lightning which threw out nu- merous branches at the sides ; the whole presenting the figure of a spine, with its supporting ribs. It is said that zigzag-lightnings usually pass between the clouds and the earth, seldom flashing from cloud to cloud. 349. Sheet-Lightning. This kind is the- most common, and appears during a storm as a diffuse glow of light, illuminating the edges of the clouds ; and at times breaking out from the central mass. When it occurs, the clouds are said to open. The flashes of sheet-lightning often follow each other in rapid succes- sion, for the space of many hours ; their intensity is by no means great, and the thunder which attends them is low and distant. 350. Ball-Lightning. Lightning of this class is What are they 1 To what is the peculiar figure of zigzag-lightning owing 1 What is its appearance t Describe sheet-lightning. Describe ball-lightning. BALL-LIGHTNING. 145 extremely rare, and so singular are its attendant phe- nomena, that we might well doubt its' existence, were not the instances of its occurrence fully authenticated. In a storm that happened at Steeple Aston, Wiltshire, in 1772, the Rev. Messrs. Pitcairne and Wainhouse, while in the vestry of the church, saw suddenly before them, at the distance of a foot, and at about their own height from the floor, a ball of fire, nearly the size of a ma?i'sfist, surrounded by a black smoke. It burst with an explosion like the discharge of several cannon Pitcairne was dangerously wounded, and his person and clothes showed the usual marks of lightning. During a thunder-storm that- occurred in 1809, at Newcastle on Tyne, the house of David Sutton was struck : the lightning descending the chimney. After the explosion, several persons who were assembled in a room, saw at the door a globe of fire, which, after re- maining stationary for some time, advanced into the middle of the room, where it burst into fragments, with a report like a rocket. 351. On the fourth of November, 1749, in 42° 48' N. Lat., 2° W. Long., the crew of the ship Montague be- held, a little before noon, and beneath an unclouded sky, a globe of bluish fire, like a millstone, rolling rapidly upon the sea. At a short distance from the vessel, it rose perpendicularly from the water, and struck the masts with an explosion louder than the discharge of a hundred cannon. Five sailors were thrown senseless upon the deck, one of whom was severely burned. In the midst of a storm in Scotland, two globes of fire, connected together like chained cannon-s,hot, were seen by a Mr. Lumsden, passing through the sky revofving one about the other, and striking at last upon the summit of a hill. Philosophers have not yet been enabled to account for lightning of this description; it has, however, been supposed to arise from an uninter- mitted discharge of electricity. 352. Heat-Lightning. It not unfrequently hap- Relate instances. How is ball-lightning supposed to arise 1 What i6 heat-lightning 1 7 L46 ELECTRICAL PHENOMENA. Eens, during the serene evenings of summer, that tha orizon is illumined for many hours with successive flashes of light, unattended with thunder. This is called heat-lightning, and has much perplexed meteorologists. It is affirmed by some, that this illumination is the reflection from the atmosphere of the lightnings of re- mote storms ; the storms themselves being so far dis- tant, that their thunders cannot be heard. Others assert, that during warm, sultry weather, when the air is highly rarefied, its pressure upon the clouds is so much dimin- ished, that the electric fluid can never accumulate upon their surface beyond a certain point, when it escapea in noiseless flashes to the earth. . 353. Multiplied observations have proved, that heat- lightning generally originates in the first-mentioned cause; but the instances are by no means rare, when silent flashes of electric light play between the earth and the clouds. These cases occur when the* weather is sultry, the air being then both rarefied and moist ; two conditions which lessen its non-conducting power ; the atmosphere thus becomes an imperfect conductor between the clouds and the earth, which are in opposite electrical states^ and opposes just sufficient resistance to the passage of the electric fluid as to render it visible. 354. Velocity of Lightning. By a very ingenious piece of apparatus, Prof. Wheatstone, of King's Col- lege, London, has been enabled to show that the dura- tion of a flash of lightning is less than the thousandth ■part of a second, and Arago has demonstrated that it does not exceed the millionth part. Now the duration of a flash, is the time it occupies in traversing the space between two cloiids, or between a cloud and the earth ; if we then estimate thi? distance to be equal sometimes to a quarter of a mile, which is a low computation, the velocity of lightning, in such cases, according to Arago, could not be less than 250 000 miles per second. The electricity developed by the How does it originate"! 'Vhat is said in regard to the velocity of lightning 1 EFFECTS OF LIGHTNING. 147 electrical machine, has been shown by another beau- tiful contrivance of Prof. Wheatstone, to possess a speed of 288,000 miles per second : the rapidity of lightning is probably not less. The preceding remarks apply only to lightnings of the first and second class. Ball-lightnings, on the contrary, often move slowly, and are visible for many seconds. 355. Color. When thunder-clouds are near the earth, the flashes are of a brilliant white ; but when the storm is high, and the lightnings play through a rarefied atmosphere, their color approaches to violet. A spark of electricity assumes the same hue, when it is made to pass through the exhausted receiver of an air-pump. 356. Effects of Lightning. These are precisely similar to those of common electricity in kind, though far exceeding them in degree. Life is destroyed by the shock, the stoutest trees shivered to pieces, ponderous weights displaced, combustibles inflamed, metals soft- ened and fused, sand vitrified, and iron and steel ren- dered magnetic. It is needless to multiply instances in proof of these particular points, but a few cases may tend to impress them upon the mind. 357. On the night of the 21st of June, 1723, a tree in the forest of Nemours was struck by lightning. The trunk was split -into two fragments, one seventeen feet long, the other twenty-two ; and though the first required four men to lift it, and the second eight, yet both of them were hurled to a distance of seventeen yards. On •the 6th of August, 1809, a flash of lightning struck a house, at Swinton, near Manchester. The wall of a building attached to the house was loosened from its foundation a foot below the ground, and raised in a mass to the surface, still maintaining its upright posi- tion ; one end of it was moved nine, and the other four feet from its original place. The wall thus moved was eleven feet high and three feet thick, and contained 7000 bricks, which, exclusive of the mortar; were estimated to weigh nearly twenty-six tons. What of its color ? What of its efecU? 148 ELECTRICAL PHENOMENA. On the 20th of April, 1807, at Great Mouton, in Lan- cashire, a windmill was struck by lightning ; the fluid passed along a large iron chain, the links of which were so softened, that by their own weight they became welded together ; and the chain was converted into an inflexible bar of iron. In Sept. 1845, a house at New Haven, Ct., was struck during a thunder-storm. Several articles of steel were rendered magnetic, and a razor, lying in a case near the spot where the lightning entered, was found capable of sustaining a key, weighing half an ounce. 358. Fulgurites. When a flash of lightning falls upon sand, its path below the surface is often marked by a fulgurite, so called from the Latin word fulgur, lightning. It is a tube composed of sand, vitrified by the action of the lightning. Fulgurites were first discov- ered in Silesia, in 1711, and specimens were forwarded to the museum at Dresden, where they are still preserved : they have since been found in great numbers, in Germany, England, and amid the sands of Bahia, in Brazil. The fulgurite is winding in its form, often throws out lateral spurs or branches, and contracts in size towards the lower extremity, which usually terminates at a spring of water, or in some substance that is a good conductor of electricity. 359. These tubes are generally hollow, the interior surface being coated with a brilliant glass. Their di- ameters vary from four-hundredths of an inch, to three inches and a half and the thickness of their sides from one-fiftieth of an inch, to nearly an inch. The branches of the fulgurite, differ in length from three quarters of an inch to a. foot, but the main tube often extends to the depth of many yards. Several of considerable length, which had been taken from the sandy 'plains of Silesia, were exhibited at London, some Give instances. • What are fulgurites "! Where have they been discovered 1 What is their form ? State their dimensions. VOLCANIC LIGHTNING. 149 years ago, by Dr. Fiedler, of Germany. One, discovered at Paderborn, in Westphalia, was forty feet long. 360. That these tubes are really produced by light- ning, has been proved by actual observation. A num- ber of sailors, being upon the isle of Amrum, in Den- mark, saw a flash of lightning fall upon the sand ; upon examining the spot, they found a fulgurite : a similar circumstance happened on the borders of Holland. Savart and others have obtained artificial fulgurites, by passing powerful electric sparks through powdered glass, and a mixture of sand and salt ; tubes were thus formed an inch in length, and the tenth of an inch in thickness, the inner diameter being the twenty-fifth of an inch. 361. Volcanic-Lightning. The clouds of smoke, ashes, and vapor, that issue from volcanoes during their eruption, are the scene of terrific lightning and thunder. Pliny the younger, in his letters to Tacitus, mentions the lightning that was seen above Vesuvius, during its eruption, in the year 79, A. D. In that which occurred in 1767, the inhabitants at the foot of the mountain as- sured Sir William Hamilton, that they were more terri- fied at the lightning which flashed around them, than by the burning lava, and all the other attendant dangers. During the eruptions of the same mountain in 1779 and 1794, there appeared, in the midst of the dark vol- canic clouds, globes of fire, which, bursting like bomb- shells, darted on every side vivid flashes of zigzag-light- ning. In the latter eruption were heard loud and con- tinued peals of thunder. 362. The cause of volcanic-lightning is found, in the rapid condensation of the vast volumes, of heated vapor, which are carried up from the crater of the volcano into the higher and colder regions of the atmosphere. In like manner, in the midst of vrater-spouts and How is it known that they are actually caused by lightning'? In what manner have they been artificially made 1 Relate the instances given of volcanic lightning and thunder How are volcanic lightnings caused 1 150 ELECTRICAL PHENOMENA. whirlwinds, an abundant condensation of vapor suddenly occurs, which frequently develops such an amount of electricity, that the lightning here displays itself in all its fearful energy. 363. Thunder. In consequence of the lightning passing through the atmosphere with an amazing ve- locity, it leaves a void space behind it, into which the surrounding air instantly rushes, with a loud report. This noise is thunder. When the lightning is near the observer, the report is sharp and quick, but when at a distance, it is long and rolling. 364. The rolling of thunder is frequently occasioned by the reverberations of the sound, from clouds and adjacent, mountains ; but this is by no means always the case. When the lightning-flash darts to a great dis- tance, sucli is its velocity, that the thunder may be considered as occurring at every point of the flash at the same time. But sound has a progressive motion of 1142 feet per second, and all the thunder will not reach the ear at the same instant. It will be first heard from the nearest point, in the path of the flash, and later and later from points more remote ; and the combined effect will be a continued peal. The zigzag form of the flash, and its division into several streams, is regarded by Herschel as affording an adequate explanation for all the changes that occur in the sound of the thunder-peal. 365. The time that elapses between the lightning and the thunder, enables us to form an estimate of the dis- tance of the former, which is a little more than a mile for every five seconds. This interval usually varies from three to sixteen seconds ; but cases have occurred, where it has amounted to fifty, and even seventy-two seconds. 366. Identity of Lightning and Electricity. What is the cause of thunder 1 How is its rolling occasioned 1 How can we estimate the distance of lightning 1 How great an interval of time sometimes occurs 1 LIGHTNING AND ELECTRICITY. 151 The resemblance between lightning and electricity was noticed by the earlier electricians, Wall, Grey, and Nol- let ; but their identity was first established by Dr. Frank- lin. The strong points of similarity which convinced him of this fact, were the following. 1st. Lightning and the electric spark are both zig- zag inform. 2d. Lightning strikes trees, chimneys, spires, masts of vessels, mountains and elevated points upon the sur- face of the earth; Electricity is likewise attracted by pointed bodies. 3d. Both choose the best conductors. Ath. Both ignite combustibles. 5th. Both fuse metals. 6th. By the action of each, a bad conductor is shiv- ered when struck. 7th. Lightning reverses the poles of a magnet, and renders iron magnetic. Electricity does the same. 8th. Animal life is destroyed by each. 9th. Blindness is produced by both. 368. Franklin, however, did not stop here. He re- solved to test the truth of his reasoning, by drawing lightning from the clouds, and in June, 1752, made the hazardous experiment in the vicinity of Philadelphia. 369. Franklin's Experiment. Having made a kite, by tying the corners of a large silk handkerchief to the ends of two light strips of cedar that crossed each other, and pla'ced upon it a pointed iron wire connected with the string, Franklin went out into a field upon the approach of a thunder-storm, accompanied by his son, "When the kite was raised, he attached a key to the lower end of the hempen string ; to the key one end of a silk ribbon was now tied, the other being fastened to a post. The kite was thus insulated, and the. experimenter, for a considerable time, awaited the result with intense solicitude. A dense cloud passed over, but no indica- By whom was the identity of lightning and electricity first established 1 What points of similarity did Franklin observe 1 Relate Franklin's experiment. 152 ELECTRICAL PHENOMENA. tions of electricity appeared upon the string ; when, just as Franklin began to despair of success, he beheld the loose fibres of the cord starting asunder, and immedi- ately presenting his knuckle to the key he received an electric spark. The rain now descending, increased the conducting power of the string, and vivid electric sparks issued from the key in great abundance. By means of the lightning thus obtained, all the cprtimon electrical experiments were performed, and jhc'laentity of light- ning and electricity thus indubitaWy proved. 370. Romas' Experiment. No sooner was this wonderful discovery made knowti, than men of science were eager to repeat the experiment. With a kite eleven feet high and three feet wide, Romas obtained in France the most brilliant and aston- ishing results. In one instance, when the kite was raised during a storm, such an accumulation of electrici- ty occurred, that streams of electric fire nine or ten feet long, and an inch in thickness, flashed spontaneously from the string, with reports, like those of a pistol. Thirty streams of this magnitude burst forth in the space of an hour, without counting a multitude of others, seven feet in length. 371. Richman's Death. That such experiments are, however, attended with great danger, unless every precaution is strictly observed, is proved by the unfor- tunate death of Prof. Richman, of St. Petersburg, who was killed by lightning, on the 6th of August, 1753. He had erected, upon the top of his house, an iron rod from which proceeded a chain that entered his study. The whole apparatus was entirely insulated. On the day in question while examining the electrometer, as a thun- der-storm was approaching, a large globe of blue fire flashed from the conductor to his head, instantly depriv- ing him of life. Relate Romaa' experiment. What error did Richman commit in the construction of his apparatus ? LIGHTNING-ROD. 153 ^ LIGHTNING-ROD. 372. The invention of the lightning-rod for the pro- tection of buildings was the fruit of the brilliant discov- ery of Franklin. Even before his decisive experiment he had been led to suppose, from the analogies existing between lightning and electricity, that pointed metallic rods might possibly disarm the thunder-cloud of its ter- rific power.. ,' 373. In order that the lightning-rod, or conductor, may afford atf effectual protection, regard must be had to the material of which it is made, its size, and the towde of erection. 374. Material. Wrought iron is usually employed, and forms a good conductor ; but copper is preferable, inasmuch as it is less liable to be corroded or fused, and possesses a greater conducting power. 375. Size. The rod, if made of iron, should be three- quarters of an inch in diameter, and its upper extrem- ity should terminate in one or more points. Each of these points (which are usually three in number) ought to be capped with some metal which does not rust, as silver, gold, or platina ; for the conducting power of the points, if made of iron, would be weakened by the rust. 376. Mode of Erection. The rod should be con- tinuous from the top to the bottom ; an entire metallic communication existing throughout its whole length. This law is violated, when the joints of the several parts that form the conductor are imperfect, and the whole is loosely put together. The parts may be screwed one into the other; or the rod may be formed of wires twist- ed together. 377. The conductor should be fastened to the build- ing by wooden supports, but if masses of metal, as By whom was the lightning-rod invented 1 To what particulars must attention be directed, that the lightning-rod may afford an effectual protection 1 What is 'said in regard to the material ? To the size 1 To the mode of erection 1 r 154 ELECTRICAL PHENOMENA. leaden pipes and troughs, are connected with the build- ing, it is best to attach them to the rod by strips of metal ; for, unless this is done, lightning may pass from the rod to the metal, and enter the edifice, especially if the rod is in any way defective. By adopting the above precaution, the metallic masses are made a part of the conductor, and if the lightning strikes them, it is con- veyed through the rod to the earth. 378. The lower end of the rod should be 'divided into two or three branches, so bent as to pass away from the building ; and it is highly essential that these branches should extend so far below the surface of thg ground, as to reach either water or a permanently moist stratum of earth. The rod should be surrounded with powdered charcoal, which at once preserves the iron from rust, and facilitates the passage of electricity be- tween the metal and the earth, in consequence of its conducting power. For the same reason, the conductor should be painted with black paint, made of charcoal. 379. Extent op Protection. According to the in- vestigations of M. Charles, the lightning-rod protects the space around it to a distance equal to twice its height. Thus, if the conductor extends ten feet above the sum- mit of a house, it affords protection to a circular space forty feet in diameter ; the rod being in the centre. The experience of nearly one hundred years has shown that, where the above rules and precautions are observed, an effectual security has been provided against the effects of lightning ; so far as human means can avail to disarm the elements. 380. It is an error to suppose that conductors attract the lightning towards the building upon which they are erected. They simply direct the course, and facilitate the passage of the electricity between the clouds and the earth, when a discharge must inevitably occur, where the building is situated. How great a space is protected by a lightning-rod 1 Have we any proof of the utility of lightning-rods'! Is a building more or less liable to be struck when furnished with a good conductor 1 ELECTRIC FOGS. 155 It is indeed highly probable, that a. silent and gradual discharge of a thunder-cloud, is often effected by the points of the rod, and an explosion thus prevented. This is the opinion of Arago, who expressly states, that " lightning-rods not only render strokes of lightning inoffensive, but considerably diminish the chance of a building being struck at all." 381. Electric Fogs. Fogs are at times highly electrical; a most extraordinary instance is thus related by Mr. Crosse, of Broomfield, whose apparatus has al- ready been described. " Many years since I was sitting in my electrical room, on a dark November day, during a very dense, driving fog and rain, which had prevailed for many hours, sweeping over the earth, impelled by a south-west wind. I had at this time 1,600 feet of wire insulated, which crossing two small valleys, brought the electric fluid into my room. From about 8 o'clock in the morning until four in the afternoon, not the least appearance of electricity was visible at the atmospheric conductor, even by the aid of the most delicate tests. Having given up the trial of further experiments upon it, I took a book and occupied myself with reading, leaving by chance the receiving ball upwards of an inch from the ball in the atmospheric conductor. About four o'clock in the afternoon, while I was still reading, I sud- denly heard a very strong explosion between the two balls, and shortly after many more took place, until they became one uninterrupted stream of explosions, which died away and recommenced with the opposite electri- city in equal violence. The stream of fire was too vivid to look at for any length of time, and the effect was most splendid, and continued without intermission, save that occasioned by the interchange of electricities, for upwards of five hours, and then ceased entirely. The least contact with the conductor would have occasioned instant death, the stream of fluid far exceeding any- thing I have ever witnessed, excepting during a thun- der-storm." ' What instance is given of an electric fog 1 156 ELECTRICAL PHENOMENA. SPONTANEOUS ELECTRICITY. 382. St. Elmo's Fire. When in a darkened room a needle is brought near to the charged conductor of an electrical machine, the point is tipped with a vivid light, caused by the flow of electricity from the conductor to the needle. In the same manner when thunder-clouds approach very near the earth, lightning does not always occur; but the electricity becomes so intense, that it escapes from one to the other by points upon the surface of the earth, which then glow with a brilliant flame. This phenomenon has received the appellation of St. Elmo's fire. It was known to the ancients by the name of Castor and Pollux, and many instances have been re- corded by classic writers. On the night before the bat- tle that Posthumius gained over the Sabines, the Roman javelins emitted a light like torches ; and Caesar relates that during the African war, in the month of February, there suddenly arose, about the second watch of the night, a dreadful storm that threw the Roman army- into great confusion, at which time the points of the darts of the fifth legion appeared to be on fire. 383. The fire of St. Elmo is often finely displayed upon the masts of vessels. An extraordinary instance, which happened in 1696, is thus related by Count For- bin : " In the night it became extremely dark, and thun- dered and lightened fearfully. We saw upon different parts of the ship about thirty St. Elmo's fires ; among the rest was one upon the top of the vane of the main- mast, about eighteen inches long. I ordered one of the sailors to take the vane down, but he had scarcely re- moved it when the fire again appeared upon the top of the mast, where it remained for a long time, and then gradually vanished." When Lord Napier was on the Mediterranean, in June, 1818, he observed, during a dark and stormy night, a blaze of pale light upon the mainmast of his vessel. It appeared near the summit, What is the cause of spontaneous electricity 7 What name has been given to this phenomenon 1 Relate the several instances. SPONTANEOUS ELECTRICITY 157 and extended about three feet downward, flitting and creeping' around the surface of the mast. The heads of the other two masts presented a similar appearance. At the end of half an hour, the flames were no longer visible. 384. This phenomenon frequently occurs on the sum- mits of mountains, when thunder clouds pass near them. SaussUre observed it upon the Alps, in 1767. On ex- tending his arm, he experienced slight electric shocks, accompanied by a whistling sound, and obtained distinct sparks from the gold button of a hat belonging to one of his party. It is often noticed at Edinbu rg castle, which stands upon a high rock, 250 feet above the surrounding country. Upon the approach of a storm, the bayonets of the soldiers mounting guard are frequently seen cap- ped with flame, and an iron ramrod, placed upright upon the walls, presents a like appearance. A singular instance of spontaneous electricity took place at Algiers, on the 8th of May, 1831. During the evening of this day, as some French officers were walk- ing with their heads uncovered, each was surprised at seeing the hairs upon the heads of his companions erect, and tipped with flame. Upon raising their hands, they perceived a similar light flitting upon the ends of their fingers. A remarkable case of this kind was observed by Pres. Totten, of Trinity College, at Hartford, Ct., in the mouth of Dec. 1839. As this gentleman was walking one evening in the midst of a heavy snow-storm, protected by an umbrella, his attention was arrested by moment- ary flashes of light, which at intervals illumined his path. The source of the light was detected upon meet- ing another person, the point of whose umbrella was seen covered with flame, which was constantly escaping in flashes. The light first noticed by Pres. Totten, pro- ceeded from his own umbrella. 385. Electric Rain, Hail, and Snow. Numerous When does it occur on the summits of mountains ? State the cases in Art. 381. 158 ELECTRICAL PHENOMENA. and well attested instances have occurred, in whicn rain, hail, and snow, have displayed flashes of elec- tric light, but we will confine ourselves to a few. On the 22d of Sept. 1773, in a thunder-storm which fell upon Skara, in Sweden, the drops of rain were seen to strike fire and sparkle as they touched*the ground. On the 28th of Oct. 1772, as the Abbe Bertholon was traveling between Brignai and Lyons, in the midst of a heavy storm, he was surprised at seeing the rain-drops and hail-stones emitting jets of light, as they fell upon the. metallic parts of his horse's trappings. It is also recorded, that the miners of Freyburg, on the 25th of January, 1822, beheld the sleet which fell during a storm flash with light as it struck the earth. 386. It is not difficult to explain these phenomena ; we have only to suppose, that the electric intensity of the atmosphere and the earth is at these times very great, and that the electricity of the falling bodies is the opposite in kind to that of the ground and of the objects upon it. At the moment of contact the two kinds of electricities combine, their union (as is always the case when their intensity is great) being indicated by a sud- den flash. 387. Electric Action upon Telegraphic Wires. It is not unusual for the electricity of the atmosphere to exert an extraordinary influence upon the wires of the electrical telegraph. According to Prof. Henry, this influence may arise, as follows, in several different ways. 388. First. The wire may be struck by a direct dis- charge of lightning from the, clouds. An instance of this kind occurred on the 20th of May, 1846, when the lightning struck the wire of the telegraph, at the place where it crosses the Hackensack river. From the point Relate the instances given of electric rain, hail and snow. How are these facts explained 1 What are Prof. Henry's views respecting the influence of atmospheric electricity upon telegraphic wire 7 What is the first mode of action 1 Give the instance. ACTION UPON TELEGRAPHIC WIRES. 159 where the discharge took place, the fluid passed along the wire each way for a distance of several miles, strik- ing off at irregular intervals down jhe supporting poles. Wherever a pole was struck, a number of sharp explo- sions were successively heard, like the rapid reports of several rifles. 389. Secondly. The state of the wire maybe disturb- ed by the conduction of a current of electricity from one portion of space to another, without the presence of a thunder-cloud ; and this will happen in the case of a long line, when the electric condition of the atmosphere which surrounds the wire at one place is different from that at another. 390. This difference in the electric condition of the atmosphere may result from a difference in elevation. (Art. 322.) A wire, raised by means of a kite, gives sparks of positive electricity, in a perfectly clear day ; hence, if the telegraphic wires pass over a high mountain-ridge, a current of .electricity will be continually flowing, during serene weather, from the more elevated to the lower parts of the wire. A current may also arise in a long, level line, if a. fog exists at one end, while the sky is clear at the other ; or if a storm of rain or snow occurs at some portion of the line, while the remainder is free from its presence. Currents of electricity have been produced by some of these causes, of sufficient power to set in motion the working machine of the telegraph. In one case it began to operate spontaneously, without the aid of the battery, when a snow-storm prevailed at one end of the line, and clear weather at the other. 391. Thirdly. The inductive action of a thunder- cloud may also change the natural electric condition of the wire. If, for example, a cloud positively electrified, is moving across the direction of the wire, it will con- . * What the second 1 When will this disturbance happen in the case of a long line 1 How may this difference arise t What is stated respecting the energy of the currents thus produced t What is the third mode of influence i 160 ELECTRICAL PHENOMENA. tinue, as it gradually approaches the line, to drive to the remote extremities of the wire, more and more of the positive electricity residing in it, and thus occasion a current. As the cloud gradually recedes, the repulsion it exerts is diminished, and a current then arises in the opposite direction. 392. Fourthly. Every flash of lightning which oc- curs within many miles of the line, produces powerful electrical currents in the telegraphic wires. To this influence Prof. Henry attributes the phenom- ena witnessed by himself, on the 19th of June, 1846, in the telegraph office, at Philadelphia, and which he thus describes. "In the midst of the hurry of the transmis- sion of the congressional intelligence from Washington to Philadelphia, and thence to New York, the apparatus began to work irregularly. The operator at each end of the line announced at the same time a storm at Washington, and another at Jersey City. The portion of the telegraphic wire which entered the building, and was connected with one pole of the galvanic battery, happened to pass within the distance of less than an inch of the wire, which served to form the connection of the other pole with the earth. Across this space, at intervals of every few minutes, a series of sparks in rapid succession was observed to pass ; and when one of the storms arrived so near Philadelphia that the lightning could be seen, each series of sparks was found to be simultaneous with a flash in the heavens. Now we cannot suppose, for a moment, that the wire was actu- ally struck at the time each flash took place, and indeed it was observed that the 'sparks were produced, when the cloud a.n& flash wer,e at the distance of several miles to the east of the line of the wire. The inevitable conclusion is, that all the exhibition of electrical phe- nomena witnessed during the afternoon, was purely the effect of induction, or the mere disturbance of the natu- ral electricity of the wire at a distance, without any transfer of the fluid from the cloud to the apparatus. What the fourth ? Describe the phenomena witnessed by Professor Henry. ACTION UPON TELEGRAPHIC WIRES. 161 " The discharge between the two portions of the wire continued for more than an hour, when the effect be- came so powerful, that the superintendent, alarmed for the safety of the building, connected the long wire with the city gas pipes, and thus transmitted the current silently to the ground." 393. By a simple apparatus, Professor Henry rendered manifest the inductive action of the lightning-flash. The arrangement consisted of a copper wire, fastened at one end to a building, and extended to another, 400 feet distant. Here it entered the Professor's study, and thence, passed through a cellar window into an adjoining well. With every flash of lightning that occurred within a cir- cle of twenty miles, needles were magnetized in the study by the. induced current of electricity developed in the wire. (0. 1033.) What apparatus was constructed by this gentleman for exhibiting '.he inductive action of the lightning 1 What effect was produced ? PART V. OPTICAL PHENOMENA. CHAPTEK I. OF THE COLOR OP THE ATMOSPHERE AND CLOUDS. 394. Color op the Atmosphere. This is caused by the decomposition of the solar light. It is well known, from the experiment of the prism (C. 788), that the white light of the sun consists of seven colors ; and, that of all these, the violet and blue rays have the least power to overcome any resistance they meet with ; and consequently deviate most from their original course in passing through the prism. 395. The action of the atmosphere upon the sun- beams in their passage to the earth, is precisely similar to that of a prism. After entering the atmosphere they are constantly passing in their onward progress, from rarer into den- ser media, and are therefore decomposed. A portion of the blue rays, unable to overcome the resistance of the air are scattered throughout its extent ; and being reflected from its particles, tinge the sky with an azure hue ; for it is to be remarked, that a body appears of the same color as the light it reflects and by which it is seen. 396. Cyanometer. In order to determine the inten- sity of the blue, a cyanometer is employed, an instru- What is the subject of part fifth 7 Of what does chapter first treat 1 What is the cause of the color of the atmosphere 7 For what purpose is the cyanometer employed 1 COLOR OP THE ATMOSPHERE. 163 ment which derives its name from the Greek words, kuanos, azure, and mefron, measure. That of Saussure is made in the following manner : A circular card is divided into fifty-one parts, and each is painted of a different shade of blue increasing from the palest tint, formed by a union of blue and white, to the deepest produced by a mixture of blue and black. The colored card being held in the hand, the observer marks the particular tint corresponding to the color of the sky, and its number, counting from the palest shade, denotes the intensity of the azure. 397. Effect of Latitude. The brilliancy of the sky decreases with the latitude. Humboldt discovered that at corresponding heights above the horizon, the blue in 19° N. Lat. was two shades below that in 16° N. Lat. The intensity also at Cumana, 10° N. Lat., is twenty- four, while the average tint for Europe is only fourteen. 398. This diminution in brilliancy is caused by the less perfect absorption of the atmospheric humidity in the temperate and arctic regions, than in the equatorial clime^ — a circumstance arising from their comparative low mean temperature, and consequent decrease in the capacity of the air for moisture. 399. In the same place, the color increases in bright- ness from the horizon to the zenith — the point in the heavens directly over-head. Baron Humboldt found in 16° N. Lat., that his cyanometer indicated the 3d shade at the horizon, but at an altitude of 60°, or two-thirds of the distance to the zenith, the 22d tint. The blue of the sky is palest at the horizon, in consequence of being mixed with and diluted by the thin vapors of the air, which settle down towards the earth. Within the torrid zone, the sky undimmed by vapors glows with the purest azure ; and,- under like circum- stances, in regions beyond the tropics, the same bright Describe this instrument. How is the brilliancy of the sky affected by latitude 1 State the result of observations. How is this difference accounted lor? What is the law in respect to altitude at the same place? Give the observations. What is said of the torrid zone t 164 OPTICAL PHENOMENA. skies are seen. Over Italy, California, and the Canary Isles, hangs a canopy of the deepest blue, and even on the western coast of Spitzbergen, the rich azure of the heavens has equaled at times the splendid hue of the tropic skies. 400. Effect of Altitude. In ascending from the plains to the mountains, the vapors are left below, the purity of the atmosphere increases, and the pale tint of the sky changes to a vivid blue. This fact, long known to the chamois hunters of Switzerland, was verified by the observations of Saussure upon the Alps, and those of Humboldt on the Cordilleras. ., 401. Capt. Mundy thus speaks of the color and pure- ness of the air at Simla, which is the most northern European settlement in India, and possesses an altitude of 7,800 feet. " To the north of Simla, the mountains rise gradually one above another, until the panorama is majestically terminated by the snowy crescent of the great Himalaya belt, fading, on either hand, into indis- tinct distance. In fine weather, these stupendous icy peaks cut the dark blue sky with such sharp distinct- ness of outline, that their real distance of sixty or seven- ty miles is, to the eye of the gazer, diminished to one- tenth part." 402. Brantz Mayer, in his interesting work upon Mexico, thus alludes to the same facts. " The moonlight of Mexico is marvelously beautiful. That city is 7,500 feet above the level of the sea. The light comes pure and pellucid from heaven. You seem able to touch the stars, so brilliantly near do they stand out, relieved against the back-ground of an intensely blue sky. Strolling on such a night in Mexico, I saw the sharp lines of tower and temple come boldly out with shape and even color almost as bright as, yet softer than at noon-day." ■403. At Mussoori, a village situated upon the first What is said of Italy, California, the Canary Isles and Spitzbergen'} What is the law in regard to altitude at different stations 1 What is said of the blueness of the sky and the purity of the atmosphere" en the Alps and the Cordilleras, on the Himalayas and in Mexico 1 COLORS OF CLOUDS. 165 range of the Himalayas, 7,500 feet above the sea, so remarkably clear is the air in the month of November, according to Lieut, Bacon, that the white houses at Moozaffirnuggur, a distance of eighty-two miles, have been distinctly seen with the aid of a spy-glass. 404. When, however, very lofty elevations are at- tained, the heavens assume & blackish hue ; for a great portion of the atmosphere is then beneath the observer, and but little blue light is reflected from the compara- tively small number of particles. composing the attenu- ated air above. The celestial orbs there shine with a singular brilliancy, since their light reaches the eye be- fore its lustre has been dimmed, in consequence of pass- ing through the dense strata of the atmosphere near the surface ofrhe earth. 405. Captain Hodgson remarked, near the sources of the Ganges, that the tint of the sky was a dark blue, approaching to blackness ; and that the stars in their rising emerged with a sudden flash from behind the snowy peaks of the Himalayas. At Zinchin, sixteen thousand feet above the sea level, the heavens appeared of a dark black color, the sun shining without, the least haze. • At night, that part of the horizon, where the moon was expected to rise, could scarcely be distin- guished by the irradiation of her beams before the orb touched it, and the stars and planets shone with a daz- zling light. 406. Colors of Clouds. These are attributed to the power which the atmosphere possesses of absorbing light (C. 802), in common with other transparent media. When a sun-beam falls upon the ocean, the more refran- gible rays are successively absorbed as the light contin- ues to pierce the translucent water ; until, at last, far beneath the surface, nothing but red light is perceived, according to the statements of divers, and of those who What is the color of the sky when very lofty heights are attained 1 Relate the remarks of Capt. Hodgson. Explain the cause of the colors of clouds, and the manner in which they arise. io6 OPTICAL PHENOMENA. have been engaged in submarine researches. The ac- tion of the atmosphere is precisely the same. In the morning and evening, the sun-light traverses the densest portions of the air, and passes through a onger track than at any other time. So much thicker and more dense is the stratum of air upon the horizon than the stratum over-hea'd, that the sun-light is dimin- ished 1300 times in traversing the former ; and of 10,000 rays falling towards the surface of the earth, 8,123 arrive at a given point if they pass perpendicu- larly through the air, but only jive if they come through a horizontal stratum. From these causes, the more re- frangible rays, and especially the violet and blue, are un- able to struggle through and are absorbed v while the rest emerge, and being reflected from the light masses of vapor floating in the sky, clothe them with their own bright hues. 407. The three most powerful rays of the solar spec- trum are red, orange, and yellow ; and these colors are the common tints assumed by clouds. At times, how- ever, they glow with the richest variety of hues, partic- ularly beneath the tropic skies. In those regions, green, violet and purple clouds are not of unfrequent occurrence. Bishop Heber, on his passage to India, beheld, one evening at sunset, when near the equator, large tracts of cloud of a pale, translucent green, surpassing in beauty every effect of paint, glass, or gem. The sunsets of California are among the most beauti- ful in the world, and the clouds that rise from the Pacific are bathed in exquisite tints of green, purple, and violet. 408. Clouds, possessing these singular colors, are rarely seen in the higher latitudes ; they are however not entirely unknown. Violet clouds have been wit- nessed at Avignon, in France, and also in a most gorge- ous sunset that occurred at Hartford, Ct., on the 3d of Ju- ly, 1844, and which presented the following phenomena. 409. The* day had been showery, but towards its What is said as to the diminution of light 1 In what regions are the richest tints beheld % Give instances. Where are violet and green clouds most frequently seen 1 COLORS OF CLOUDS. 16 1 " close, the dense canopy of clouds was broken up, and the eastern sky filled with light and floating masses of vapor. Soon after sunset, the stratum of clouds which rested upon the western horizon, rose throughout its whole length, revealing between the mountains and its lower edge a belt of sky of the purest azure. Above this, the whole field of vapor was gleaming with a rich amber light, which, as it streamed through rarer or denser portions of the mass, presented every phase of brilliancy and depth ; at the same time displaying the curiously wrought structure of the airy fabric. When tjje rays of the sun fell upon the fragments of vapor floating in the eastern quarter of the heavens, their jut- ting heads and broken edges gleamed with a flame-like hue ; while, between the masses, the sky appeared of the dee-pest indigo. As the evening advanced, portions of the western stratum assumed the tints of lead, lake, pink, green, purple, violet, orange and crimson. About eight o'clock, the vapor in the south-west presented a singularly beautiful appearance ; the heavens seemed as if covered with a delicate lace-work woven of pris- matic rays, and this phenomenon was succeeded by green, purple, and violet clouds in the west. The last hue of this brilliant pageant was an intensely vivid crimson, which was gradually lost in the shades of night. 410. At the same city, in May, 1845, a cloud of un- common beauty was seen by the writer resembling marble paper, the intermingling colors consisting of bronze, orange, a, vivid grass green, and a golden yellow. 411. Green clouds occur, when the vapor is illumin- ated at the same time by the deep blue light, reflected from a distant quarter of the sky, and the yellow rays of the sun ; green being produced by the union of blue and yellow. In the same manner, purple and violet clouds appear, when they glow at once with the red rays of the sun and the azure tint reflected from the heavens ; since purple and violet arise from a mixture of red and blue in due proportions. Describe the sunset at Hartford. Explain the origin of green and violet clouds. 168 OPTICAL PHENOMENA. CHAPTER II. OF THE RAINBOW. 412. The rainbow is that beautifully colored arch which, at times, is seen during a shower and in the re- gion of the sky opposite to that where the sun is shin- ing. 413—When perfect, the rainbow consists of two arches, the inner, called the primary bow, and the outer the secondary, each composed of seven colored bows, formed of the prismatic hues, viz., violet, indigo, blue, green} yellow, orange, and red. In the primary bow the red ring occupies the highest place, the. orange comes next, and so on, the violet assuming the lowest position: but in the secondary the order of colors is reversed. 414. The cause of the rainbow, with the exception of its colors, was first unfolded by Descartes ; but the dis- covery by Newton, of the different refrangibility of the sun's rays, enabled this great philosopher to explain, with the utmost completeness, all the laws of this brill- iant phenomenon. 415. In order to understand the theory of the rain- bow, we must have recourse to diagrams. Imagine, in the first place, that PQ,L, figure 20., is a section of a globe of water, and that S P is a ray of light, which, passing through the hole of a window- shutter in a darkened room, falls upon the surface of the globe at P. Here a portion of the light is reflected, and the rest, entering section of a globe op water. the globe is refracted (C. 702) and decomposed into the seven primary colors ; one of which, the red ray, we will What is the subject of chapter second 7 Define the rainbow. Of what does it consist when perfect ? Who first explained the cause of the rainbow t Trace the course of a ray of light in figure 20. RAINBOW. 169 now alone consider. This ray, traversing the water, strikes the interior surface of the globe at Q, ; where a part of its light is lost by transmission, and escapes into the air, while the remainder is reflected to the point L ; here the light is once more subdivided, one portion being refracted to the eye situated in the direction L M, and the other reflected into the globe. That the results are such as have been described may be ascertained by placing the eye at the point Q, of the globe, and observing likewise the course of the ray through the air and water. •416. These successive transmissions and reflections are unlimited in number, and, since light is lost at each impact, the intensity of the ray, after a few reflections, will become so much diminished, that, upon its emer- gence from the globe, it ceases to make 'any impression upon the eye. 417. In order to apply these remarks to the subject before us, we have only to suppose, that figure 20. is the section of a rain-drop, instead of the section of a globe of water ; for all the changes which light undergoes in one case, will take place in the other. Thus the ray of light S P, falling upon the drop at P, is refracted towards the perpendicular P D, reflected at Q,, and refracted from the perpendicular L D as it passes into the air at L, to meet the eye in the direction L M : all these results occurring, in accordance with well-known optical laws. (C. 706, 709.) 418. In the case supposed, the ray S P suffers two refractions and 'one reflection, and if it strikes the drop more or less obliquely, different quantities of light will be brought to the eye at M. Now it is only when the greatest amount is conveyed to the eye that the light is sufficiently interise to produce any impression upon the sight, and this is found to occur, in respect to .the red Apply the illustration to the subject. In the case supposed, how many refractions and reflections doos the ray undergo 1 Under what circumstances only does the emergent ray make anv im- 1 pression upon the sight ? 8 170 OPTICAL PHENOMENA. ray, when the angle SBM, figure 21., made by pro-, longing the lines of the incident and emergent rays S P and M Q, till they meet at B, and called the angle of deviation, is equal to 42° 2'. * This angle of greatest intensity varies however for each prismatic color, being 40° 17' in the case of the violet, and increasing, lor each hue, from the violet to the red. 419. Primary Bow. If we consi- der PQ.LD, figure 21., to be a section of a rain-drop ; of all the rays that fall upon it from any one point in the sun, ■ some, asSP, will so strike it, as to meet the eye of the observ- er (supposed to be at -, T ^ , r T , SECTION OP A RAIN-DUOP. 1V1) With the greatest One Reflection— two Refractions. * An angle is the opening between two straight lines that meet each other. Thus the opening between the straight lines, A B and C B, which meet at B, is called the angle B, or the angle ABC; the letter at the point of meeting always being placed in the middle. The size of an angle is computed as follows. The circumference of any circle being divided into 360 equal parts, each part is called a degree; a degree being divided into 60 equal parts, each part is called a minute; and a minute being divided into 60 equal parts, each part is called a second. If now we take B as the centre of a circle, and describe the circumference, G E F, cutting the two lines, A B and C B, in any two points, as E and F, the number of degrees, minutes and seconds contained. in the part of the circumference, E F, included between the two lines, A B and C B., gives the vnlue of the angle, ABC. For example, if the length of the circumference, G E F, was so great that it measured 360 inches, and the part, E F, contained 40 inches and nine-sixtieths of an inch, ABC would be angle of forty degrees and nine minutes (40° 9'). Degrees are des s- nated by the following character, ° ; minutes thM, ' ; and seconds thus, ". What is the angle of deviation for the red ray, when the greatest amount of light comes to the eye 1 What is the angle of greatest intensity for the violet 1 Explain figure 21. •RAINBOW. 171 possible brilliancy : making the angle S B M equal to 42° 2', in the case of red light. If the line M O is now drawn parallel to S B, it may be regarded as a ray of the sun passing through the eye of the spectdtor ; and since alL the rays of the sun are parallel to each other at the earth, the angle B M O will also be equal to 42° 2'. 420. The observer then being placed with his back to the sun, and his eye at M, will receive the impression of red light from the- drop P L & D, in the direction B M; and not only from this drop, but also from every other drop,, whose angular distance from the line M isj at that moment, the same. It is therefore evident, if we suppose the line M B to turn about M O, like the legs of a pair of compasses, that all the points at which red light is seen lie in the cir- cumference of a circle whose centre is O ; and that aroUnd this centre an arch of red light will appear in the heavens. 421. The breadth of this arch will be equal to the apparent diameter of the sun, or about 32'; for what has been said in regard to rays proceeding from any one point in the sun, viz., that some of them will reach the eye under the angle of greatest brilliancy, is equally true of those which'emanate from every point of his disk. 422. The explanation of the origin of the red arch is equally applicable to the rest of the colored arches. The latter will be found, however, below the former ; for, since their angles of greatest brilliancy are each less than that of the red, they must consist of portions of smaller, concentric circles. Thus, the violet arch can only be seen from drops below and within B, when the light that meets the eye coming in the direction B s M, makes the angle B'MO equal to 40° 17'. Between the violet and red arches the other colored bows will be seen arranged in the order of the spectrum ; the whole forming, by their uniou,the pri- mary bow. Explain the manner in which the red arch is caused. What is its breadth 1 Apply the same mode of reasoning to the other colored arches. How is the primary bow formed 1 172 OPTICAL PHENOMENA. 423. Secondary Bow. The secondary bow is form- ed when the sun's rays, entering the bottom of the drop, suffer two reflections from the interior surface, and emerging at the top, reach the eye of the spectator after two refractions. The course of the ray is Fig. 22. seen in figure 22., where S E A is the incident ray, B and C the two points of reflection^ and D E H the emergent ray, supposed to meet the eye of the ob- server at H. 424. So much light is lost, by these successive changes in direction, that only at certain inclinations section of a rain-dhop. asufficietlt quantity reaches Tm > Reflections-two Refractions. the eye, from each of the prismatic colors, to produce (.he secondary bow. Its tints, after all, are faint com- pared with those of the primary. 425. The violet light can only be seen when the angle of deviation SEHis 54° 9', and the red when it is 50° 59'. Suppose, as in the case of the primary bow, that H L is the direction of a ray from the sun passing through the eye of the observer, and making with H E the angle LHE equal to the angle of devia- tion. If then, the line H E revolves about H L, the spectator, with his back to the sun and his eye at H, will behold in the heavens, between the limits of 54° 9' and 50° 59' a prismatic bow consisting of similar por- tions of seven concentric circles ; the violet arch assum- ing the highest position and the red the lowest. The other colors occup}^ intermediate places; the greater their refrangibility the greater their elevation. Under what circumstances does the secondary bow occur 1 Trace the course of the ray in figure 22. What is said of the brilliancy of the secondary bow 1 What must be the size of the angle of deviation that the violet light can tie seen 1 What the size that the red ray may be visible 1 How is the secondary bow formed 1 RAINBOW. 173 Fig. 23. RAINBOW. 426. The subject is further illustrated by the following figure, where the four parallel lines represent rays of the sun falling upon four drops of water, and O P the direction of an- other ray imagined to pass through the eye of the spectator, RO and V O are the red and vio- let rays of the primary bow; R' O and V O the red and violet rays of the seeondary ; and the positions of the red and violet arches of the two bows are indicated by the doited lines. The other colored arches are found between the red and violet, following the order of colors in the prismatic spectrum! P is the centre of the rainbow. 427. In the explanation just given, we have reasoned as if the rain-drops were stationary, which of course is not the Case ; but this supposition leads to no error, inasmuch as the air is filled with rain-drops during the prevalence of a shower, and before one set of drops, by sinking too low, ceases to present to the eye the colors of the bow, another set has descended, taken their place, and is performing their office. While the observer is stationary the rainbow is fixed in position, but the drops.th.aX give rise to its glowing tints are continually changing. 428. Breadth of the Bows. The angular distance from the middle of the red arch to the middle of the violet in the inner bow, is the difference between 42° Illustrate farther from figure 23. Why is the bow stationary although the drops are in motion'? 174 OPTICAL PHENOMENA. 2' and 40° 17' or 1° 45' ; a quantity nearly equal to three and a half times the apparent breadth of the sun. This space is occupied by the remaining five colored arches, and, as each is 32' in width, (Art. 421,) they ne- cessarily overlap one another, and cause, by their mutual blending, an indistinctness in the boundary of the several hues. The two half-breadths of the red and violet arches added to 1° 45' give the whole width of the bow, which is equal to 2° 17', or about four and a half times the apparent diameter of the sun. 429. The breadth of the exterior bow, from the mid- dle of the red to that of the violet, is found in like man- ner to be 3° 10'— the difference between 54° 9' and 50° 59'. To this quantity 32' must be added to obtain the entire breadth. The interval between the bows, computing from the red of the primary to that of the secondary, is 8° 57'. All these results, deduced theoretically, precisely agree with those obtained by actual measurement. 430. Position and Size of the Rainbow. Since the centre of the rainbow is in the direction of the line imagined to be drawn from the sun through the eye of the spectator, its position will evidently vary with that of the spectator, and its size with the altitude of the sun. If this luminary is 42° 2 above the horizon, the top of the inner bow will be just visible ; but if upon the hori- zon, the bow will be a semicircle, having an elevation of 42° 2'. If the observer, in the latter case, were upon the summit of a mountain, the arch would be somewhat greater than a semicircle ; since the line of direction from the sun through his eye, would strike the sky opposite, at a point above the horizon. Should a person happen to be upon a mountain, when the sun is high in the heavens, and a shower at the same time occur in the vale below, he will some- times perceive a rainbow forming a complete circle. State what is said in regard to the breadth of the bows. What in respect to the position and size of the rainbow. When are entire circles beheld ? EXTRAORDINARY BOWS. 175 Such are said by Ulloa to be frequently seen on the mountains of Peru above Quito. The foaming waters of cataracts are often spanned by richly tinted bows, caused 'by the rising spray. They are regularly seen at the falls of Schaffhatrsen, on the Rhine, and at the cataract of Niagara. At Ternj, in Italy, where the river Velino rushes over a precipice 200 feet high, a bow of rare beauty is beheld. It appears, to a spectator below, arching the, falls with its glowing tints, while two other bows are reflected on the right and left. 431. Rainbows in the North. Rainbows are sometimes seen at mid-day. On the 13th of Dec. 1847, at one o'clock, P. M., Prof. Oimstead beheld at Yale College an entire bow in the north. During the same week, the writer observed at Hartford a similar bow at nearly the same hour of the day. Such a phenom- enon can never arise, in the case of the primary bow, unless the sun's altitude at the time is considerably less than 42°, "which only happens in the winter. 432. Extraordi- nary Bows. When the light of the sun is reflected from the sur- face of tranquil wa- ter, rainbows of sin- gular form are ' at times observed. On the 6th of August, 1698, Dr. Halley be- extraordinary bow. held, while walking on the walls of Chester, by the river Dee, a rainbow of the form represented in figure 24., where A B C is the primary bow, D E F the secondary, and A H G C the extraordinary bow, cutting the secondary at H and G. Its colors were arranged like those of the primary. Give the instances of rainbows over cataracts. When can rainbows appear in the north 1 Explain from figure 24. the extraordinary bow seen by Halley. % 176 OPTICAL PHENOMENA. 433. The sun was shining c.early upon the calm sur- face of the river, and Dr. Halley discovered that the extraordinary bow was nothing more than the rest of the circle of which the primary was a part, bent upwards by reflection from the water. A similar rainbow, formed by reflection from the river Eure, was beheld at Chartres, in 1665 ; when a faint arch was seen crossing the primary at its summit. 434. Supernumerary Bows. Arches of prismatic colors are sometimes seen, both within the primary, and without the secondary bows, to which the name of supernumerary or supplementary bows is given. 435. On the 5th of July, 1828, Dr. Brewster saw three supernumerary bows within the primary, each composed of green and red arches. Outside of the secondary a red arch was clearly seen, and beyond this a faint green one. At Montreal, in September, 1823, three supplementary bows were noticed by Prof. Twining, within the prima- ry ; exhibiting however, only a single color, which was violet or rather a dull red. At Hartford, Ct., on the 5th of August, 1847, at sun- set, two supernumerary bows were seen by the writer, within the primary, extending throughout the whole semicircle. The first, in contact with the primary, con- sisted of green and red arches, and the second of a sin- gle band of pale red light. The most remarkable phenomenon of this kind, was that observed by the Rev. Mr. Fisher, in Dumfrieshire, and related by Dr. Brewster, at a meeting of the Brit- ish Association, in 1840. In this case the primary was attended by Jive supernumerary bows, and the secondary by three. Kaemtz remarks, that it is not easy to account for these supplementary bows in a satisfactory manner; hut according to Young, Arago, and others they arise -om the action of the rays of light upon each other : the vxplanation however, is too abstruse to be here introduced. U36. Lunar Bows. Rainbows are sometimes pro- Relate the cases given of supernumerary bows. MIRAGE. , 177 duced by the light of the moon ; their occurrence, how- ever, is - extremely rare, and their tints so very faint as to be scarcely perceptible. One of the most brilliant ever beheld, was seen by Mr. Tunstall, at Gretna Bridge, in Yorkshire, on the night of the 18th of Octo- ber, 1782. It became visible about nine o'clock, and continued, with varying degrees of brightness, till past two. At first it appeared as a distinct bow without colors,' but afterwards the tints were very conspicuous and vivid, preserving the same order as in the solar bow, though paler ; the red, violet, and green being the brightest. At twelve o'clock it attained its greatest splendor. This phenomenon occurred three days before the moon was full ; during its continuance, the wind was very high, and a drizzling rain fell for most of the time. Another bow was seen by the same observer, on the 27th of February, in the same year. The colors were tolerably distinct, but the orange appeared to predom- inate. A lunar bow with colors, was also noticed near Chesterfield, about Christmas, in the year 1710, and is described by Thoresby in the Philosophical Transac- tions. ' CHAPTER III. OF MIRAGE. 437. When a ray of light, proceeding from any ob- ject, passes obliquely out of one medium into another of a different density, it is refracted, or bent from* its course, (C. 704,) and when it reaches the eye, the object is seen in the direction of the last refracted ray. Relate the several instances of lunar bows. What is the subject of chapter third ■? In what direction is an object seen, when the rays that come from it t» the eye first pass through media of different densities ? 8* 178 OPTICAL PHENOMENA. ATMOSPHERIC REFRACTION. 438. Thus, if E "«-^_ represents the earth, and 1-2, 2-3, 3-4, dif- ferent strata of the atmosphere, decreas- ing in density from 1 to 4, a ray of light proceeding from the star S, and meeting the exterior stratum of the atmosphere at 4, will be successively refracted in the directions 4-3, 3-2, and 2-1 ; so that a spectator at 1 will not see the star S in its real position, but in the direction of 1-2 S'. For this reason all celes- tial objects, (unless in the zenith, where there is no re- fraction,) appear above their true position. (C. 703.) Thus, the sun and moon, for instance, at their apparent vising and setting are actually below the horizon. 439. The variations in the density of the atmosphere near the earth, produced by local changes in tempera- ture, occasion a similar displacement of terrestrial ob- jects ; this is ordinarily seen in the slight elevation of coasts and ships, when viewed across the sea, and is then called looming ; but to the more extraordinary phenomenon of this nature, the name of mirage has been given. When this phenomenon occurs, images of ships erect and inverted are seen in the air, delightful visions of tranquil lakes and verdant fields delude the fainting traveler of the desert, and sometimes, as in the case of Reggio, a noble city with all its splendid panorama of towers and arches, stately palaces and terraced heights, appears like a fairy scene upon the slumbering waters of the sea. Explain the effect of atmo8pheric refraction from figure 25. What is looming ? What is mirage ? MIRAGE. 179 440. Instances. On the first, of v 's- ss - August, 1798, Dr. Yince observed, at Ramsgate, a vessel in the dis- tance, the topmast only being visi- ble above the horizon, as at A, fig. 26. Two complete images of the vessel were seen at the same time in the air, the one at C erect, and the other below at B inverted: between them a distinct image of the sea appeared at D E. The two images were still visible when the real ship had passed entirely out of sight. 441. Similar phenomena were noticed by Capt. Scoresby in 1820, while navigating the arctic seas. In one instance he beheld from the mast-head eighteen sail of ships, at mirage the distance of twelve miles ; one appeared taller than its actual height, another shorter ; and above several of the rest, inverted images were seen. In 1822, he recognized his father's ship, the Fame, by an inverted image of the vessel in the air. though it was subsequently found to have been at that time thirty miles distant, and seventeen .miles beyond the horizon. 442. During the late Exploring Expedition, a singu- lar instance of mirage was seen off Terra del Fuego, from the decks of the Vincennes and Peacock, and which is thus related. "On the 17th of February, 1839, we had an extraordinary degree of mirage or refraction of the Peacock, exhibiting three images, two of which were upright and one inverted. They were all extremely well defined. The temperature on deck was 54° Fah., that at the mast-head 62° Fah. A vessel, that was not in sight from the Vincennes' deck, became visible, and the land was much distorted, both vertically and horizontally. Relate the several cases of mirage, 1 440 — 446. ISO OPTICAL PHENOMENA. On board the Peacock, similar appearances were observ- ed of the Vincennes and Porpoise. There was, however, a greater difference between the mast-head temperature and that on deck, the thermometer standing at 62° Fah. at the mast-head, while on the deck it was but 50° Fah., being a difference of 12° ; that on board the Vincennes differed only 8°." 443. Simpson, while exploring the coasts of the north polar seas, in the summer of 1837, beheld a remarkable display of the mirage. As he rowed over the tranquil ocean, he seemed to be traversing a valley ; the waters apparently, rising on either hand, like the sides of a mountain, and the huge icebergs upon theii surface ap- pearing ready to topple down upon him. 444. During the march of 'the French army over the sandy plains of Egypt, many instances of the mirage occurred. The villages, situated upon small eminences, were successively seen like so many islands in the midst of an extensive lake, and beneath each village appeared its inverted image. In the same direction, an image of the blue sky was seen, clothing the sand with its own bright hues, and causing the wilderness to ap- pear like a rich and luxuriant country. So complete was the deception, that the troops hastened forward to refresh themselves amid these cool retreats ; but, as they advanced, the illusion vanished, only to re-appear at the villages beyond. 445. This phenomenon is so common on the deserts of Asia and Africa, that the Koran calls every thing de- ceitful by the word serab, which signifies mirage. It Re- marks, for example, that " the actions of the incredulous are like the serab of the plain ; he who is thirsty takes it for water, and finds it to be nothing." 446. While Baron Humboldt was at Cumana, he frer quently saw the islands of Picuita and Boracha, appa- rently hanging in the air, and sometimes with inverted images ; and at Mesa de Pavona, cows were beheld Where does this phenomenon frequently occur 1 What instances are given by Humboldt and Tachudi 1 FATA MORGANA. 181 seemingly suspended in the air, at the distance of 2jl32 yards. When Dr. Tschudi and his party were traversing a deep sandy plain, near the river Pasamayo in Peru, they beheld the figures of themselves, riding over their own heads, magnified to gigantic proportions. 447. Fata Morgana. This name is given to an extraordinary optical phenomenon, which has been often seen in the straits of» Messina, between the island of ' Sicily and the Italian coast. It has been described by many writers, and, though known for centuries, has but lately been considered as the effect of mirage. The following is the description by Antonio Minasi, which is regarded as the most correct. " When the rising sun shines from a point, whence its incident ray forms an angle of about 45° on the sea of ' Reggio, and the bright surface of the water in the bay is not disturbed either by the wind or the current, a spec- tator placed on an eminence in the city of Reggio, with his back to the sun, and his face to the sea, suddenly beholds in the water numberless series of pilasters, arches, castles well delineated, regular columns, lofty towers, superb palaces, with balconies and windows, ex- tended valleys of trees, delightful plains with herds and flocks, armies of men on foot and horsebaek, all passing rapidly in succession along the surface of the sea." In a peculiar state of the atmosphere, when its dense vapors extend like a curtain over the waters, the same objects are not only reflected from the surface of the sea, but are likewise seen in the air, though not so distinct or well defined, and if the atmosphere is slightly hazy, the images seen upon the surface of the water are vivid- ly colored or fringed with all the prismatic hues. 448. But a most extraordinary instance of the mirage occurred at Hastings, on the coast of Sussex, on the 26th of July, 1798. The cliffs of the French coast are fifty miles distant from this town, and in the usual state of the atmosphere, are below the horizon and completely Describe the Pata Morgana. 182 OPTICAL PHENOMENA. kid from view ; but on the day mentioned, at five o'clock P. M., they were seen extending to the right and left for several leagues, and apparently only a few miles off. As the narrator, Mr. Latham, walked along the shore, the sailors, who accompanied him, pointed out and named the different places on the opposite coast, which they were accustomed to visit. By the aid of a telescope, held simultaneously at these cities, the difference in the reckoning of time will be nearly /jw hours and a half (C. 939). 578. There is much reason for believing that the aurora sometimes occurs during the day, though rendered invis- ible by the presence of the sun. Richardson perceived at Bear Lake, the motion of the aurora before the entire disappearance of twilight, and even during the day he discerned clouds, arranged in columns and arches, resem- bling those of the northern lights. Besides, as we shall show hereafter, a brilliant display of this phenomenon is always accompanied by a greater or less disturbance of the magnetic-needle, (C. 997,) and as these disturb- ances take place in the day as well as in the night, it is reasonable to infer that they are caused by the presence of an invisible aurora. 579. Fbeq.uency. This phenomenon is more fre- quently seen in winter than in summer ; we must not, however, hastily conclude from this circumstance, that the number of auroras during the former season is actu- ally greater, for the increased length of the nights du- ring the winter would enable us then to see more dis- plays of the northern light, even if the times of its occur- rence were equally distributed throughout the year. About the period of the equinoxes they also appear to be more frequent. These facts are shown from the follow- ing table of Kaemtz, which gives the number of auroras that have been seen in each month. NUMBER OF AURORA BOREALES IN EACH MONTH. January, 229. July, 87. February, 307. August, 217. March, 440. September, 405. April, 312. October, 497. May, 184. November, 285. June, 65. December, 225. Why i9 it supposed sometimes to occur \a the day 1 What is said respecting the frequency of its appearance in winter and summer ? Recite the table. FREQUENCY. 237 580. In addition to this annual variation, there ap- pears to be another which extends through a consider- able number of years, but of which very little is known. Thus, from 1707 to 1752, the northern lights became more and more frequent ; but after the latter date, a period of twenty years occurred, in which they dimin- ished in number. An increase in their frequency began in 1820, and since that period many magnificent displays have been witnessed. The number observed for the last ten years, at New Haven, Ct., by Mr. E. 0. Herrick, is shown in the fol- lowing table. 1838, to May. ,1839, Number of Auroras 35. 1839, 1! 1840, 36. 1840, 11 1841, 36. 1841, U 1842, 21. 1842, ii 1843, 7. 1843, a 1844, 7. 1844, a 1845, 12. 1845, a 1846, 19. 1846, u 1847, 20. 1847, u 1848, 28. Between the 12th of September, 1838, and the 18th of April, 1839, no less than one hundred and forty-three distinct auroras were seen by the French observers at Bossekop. They were most frequent at the period when the sun was below the horizon, viz. : from the 17th of November to the 25th of January. During this night of ten weeks, sixty-four auroras were visible. 581. Disturbance of the Magnetic-Needle. During the prevalence of the aurora, the compass-needle, instead of remaining motionless, in the magnetic meridi- an, is often much disturbed. Sometimes it is deflected toward the east several minutes and even degrees ; then Is there any other probable variation 7 Recite the table. * What is said respecting the disturbance of the compass-needle J 238 LUMINOUS PHENOMENA. it is agitated, and returns either slowly or rapidly, to the meridian, which it passes at times and moves toward the west. These deviations are as changeable as the phenomenon itself. When the arch is motionless the needle is quiet ; its disturbance commences when the streamers begin to play. 582. Franklin observed at Fort Enterprise, that the disturbance of the needle was simultaneous with some change in the form or action of the northern lights, and that after being deflected it returned to its former posi- tion very gradually, not resuming it before the follow- ing morning, and sometimes even not before noon. Moreover when the auroral arch was either at right angles to the meridian, or its western extremity north of west, the needle was deflected toward the west ; but if its western extremity was south of west, the needle moved toward the east. During the aurora of November 14th, 1837, the en tire range of the needle at New Haven, was observed by Messrs. Herrick and Haile to be nearly six degrees. It was not until the morning of the next day, between seven and nine o'clock, that the needle was at rest in its usual position. 583. This effect upon the magnetic needle during the prevalence of the northern lights, was noticed for the first time by Celsius and Hiorter, at Upsal, on the 1st of March, 1741. 584. It is asserted by Wilke, that when the aurora appears in great splendor, the position of the dipping- needle is as variable as that of the compass-needle ; the former rising and falling with the northern crown. Hansteen has also observed, that the dipping-needle descends very much below its usual position before the aurora is visible ; but that after the display commences it begins to rise : and more rapidly in proportion to its brightness. The needle then slowly resumes its origi- nal position, which it frequently does not attain until How great was its range at New Haven, November 14th, 18371 What has been observed respecting the dipping-needle 1 cause. 239 twenty-four hours have elapsed. From numerous ob- servations at Bossekop, M. Bravais has likewise obtained the same results. 585. Cause. No satisfactory explanation has ever been given of this singular phenomenon : that a connec- tion exists between the aurora and the magnetism of the earth, is evident from the preceding facts ; but the nature of that connection is still unknown. To trace all the hypotheses which have been started would be an unprofitable task ; but a glance at some of the most prominent may be given. Canton supposes the aurora to be caused by the passage of electricity from positive to negative clouds, in the upper and rarefied regions of the atmosphere. He adduces in support of this view the fact, that when the air within a long, glass tube is rarefied, and electricity passed through it, the whole tube is illumined by flashes of light traversing its entire length. It may, however, be stated in reply, that the general height of the northern lights far exceeds that of the highest clouds. 586. Beccaria supposes, that there is a constant cir- culation of the electric fluid from north to south, and that the aurora is seen, Avhenever the electrical current passes nearer than usual to the earth, or the state of the atmosphere is such as tn render it luminous. Faraday has demonstrated, that the electricity of the earth neces- sarily tends from' the equator towards the poles ; and has suggested, that the aurora may possibly arise from an upward current in the atmosphere flowing back from the poles towards the equator. Kaemtz conjectures, that since a spark is perceived every time an electric current produced by a magnet is broken, the northern lights may perhaps be caused by a rupture in the magnetic equilibrium of the globe. At the same time, however, he utterly disclaims the idea of ac- counting for all the circumstances of this wonderful phe- nomenon, in our present imperfect state of knowledge. What is known of the origin of the northern lights t State the hypotheses given. 240 LUMINOUS PHENOMENA. 587. Utility. The light of the aurora, from its fre- quency and splendor, serves materially to relieve the darkness and enliven the gloom of the long polar night. During this period, its play is almost, incessant, (Art. 580,) and its coruscations exceedingly vivid and beautiful. So brilliant is the aurora in these regions, that Mau- pertius and others, who were sent to Lapland in 1735, by the Academy of Sciences of Paris, for the purpose of measuring an arc of the meridian, were enabled to pur- sue their difficult work by the light it afforded, long after the sun had ceased to be visible. And Maupertius remarks, that its light, together with that of the moon and stars, is sufficient, during this season, for most of the occasions of life. What useful purpose does the aurora subserve in the polar regions 1 K PART VII. MISCELLANEOUS PHENOMENA. CHAPTER I. OF THE FALL OF TERRESTRIAL SUBSTANCES FOREIGN TO THE ATMOSPHERE. 588. In addition to storms of rain, hail, and snow, •which are products peculiar to the atmosphere, and are the results of the operations of well-known agencies and laws, showers of matter of a terrestrial nature have not unfrequently occurred, which have been traced, upon close examination, to a mineral, vegetable, and even animal origin. The most remarkable of these singular phenomena are dust-storms and Hood-rains, which will now be de- scribed. DUST-STORMS AND BLOOD-RAINS. 589. From time to time, and in regions of the globe widely separated from each other, dust in large quantities has descended from the heights of the atmosphere, not only upon the land, but also far out on the ocean, hundreds of miles from the shore. It is entirely distinct from that which is disseminated through the air by the winds, during the eruption of volcanoes, and for many years has been described, by observers and writers, under the various names of dust-storms, dust-ram, red fogs, Sirocco dust, What is the subject of part seventh 1 Of what does this chapter treat 1 la addition to storms of rain, hail, and snow, what other kinds of showers have not unfrequently happened 1 What are the most remarkable of these phenomena 1 What is said respecting the fall of dust from the heights of the atmo- sphere 1 What are the various names under which this phenomenon has been described 1 11 242 MISCELLANEOUS PHENOMENA. African dust, sea-dust, Atlantic dust, and tradewind- dust. 590. This dust not only falls dry, in the form of a fine, impalpable powder, but is occasionally mingled with rain, hail, and snow, which it dyes with its own hue. As it is usually of a reddish color, these showers of rain and storms of" hail and snow have received the appellation of Uood-radns. DUST-STORMS. 591. Instances. On the 20th of October, 1755, a black dust, like lamp-black, fell in Shetland, between 3 and 4 o'clock in the afternoon. The sky at the time was hazy, and the dust fell in such quantities as to cover the hands and faces of persons exposed to it, and to black- en their linen. 592. During the 5th and 6th of March, 1803, while the wind was blowing from the south-east, a shower of red dust fell in Italy. Ten years afterwards, on the 14th of March, 1813, a similar storm occurred at the town of Gerace, in Calabria. According to Prof. Sementini, of Naples, the wind, in the early part of the day, blew from a western quarter, bringing up dark, heavy clouds from the sea over the land. At about 2 o'clock in the afternoon the wind sub- sided, while a deep gloom pervaded the air, and the clouds grew red and threatening. Thunder followed, and soon after red dust, mingled with red rain and snow, descend- ed upon the town. This dust had the appearance of a fine powder. 593. A shower of dust fell at Malta on the 15th of May, 1830, and at the same time a similar fall occurred in the bay of Palmas, in Sardinia, while a Sirocco wind was blowing from a south-easterly quarter. The Maltese dust was of a brownish-red hue. Some of it was collected by Mr. R. G. Didman, of the ship Revenge, and for- What are blood-rains, and why are they so called ? Relate the various instances given of dust-storms in Shetland, Italy, fierace, Malta, and Genoa. DUST-STORMS. 243 warded to Mr. Charles Darwin, an eminent English nat- uralist, for examination. 594. On the 16th of May, 1846, a shower of Sirocco- dust occurred at Genoa, having the same brownish-red hue as the dust which fell at Malta in 1830. Six months afterwards a remarkable storm of this nature swept over Lyons, in France, and so thickly did the dust descend, that the amount which fell at this time was com- puted to weigh no less than thirty-six tons. 595. In the year 1831, the ship Beagle, under the command of Captain Fitzroy, was dispatched by the British government on a" voyage of scientific discovery around the world. Mr. Darwin, the naturalist just men- tioned, accompanied the expedition, and during the voyage observed a dust-shower, near St. Jago, the chief of the Cape de Verd isles. The morning before the Beagle anchored at Port Praya, in St. Jago, Mr. Darwin collected a little package of im- palpable brown-colored dust, which appeared to have been filtered from the wind by the gauze of the vane at the mast-head. In speaking of this phenomenon, he re- marks, that the atmosphere in this region is usually filled with a haze, caused by 'the falling of this fine, brown- colored dust. By the kindness of a friend, Mr. Darwin received four parcels of dust which fell upon the deck of a vessel, a few hundred miles north of the Cape de Verd isles. 596. Much valuable information respecting dust-show- ers on the ocean has been gathered by this gentleman, who has found fifteen different accounts of the descent of dust upon ships when far out on the Atlantic. It has often fallen upon them when they were several hundred, and even a thousand miles from the coast of Africa, and at points sixteen hundred miles distant in a north and south direction. 597. In some of the dust collected upon a vessel three hundred miles from land, particles of stone were discov- S tate what is said respecting the fall of dust on the ship Beagle. What is known, from the researches of Mr. Darwin, in regard to the Atlantic dust ? 244 MISCELLANEOUS PHENOMENA. ered, more than the thousandth of an inch square, mixed with finer matter. It falls in such quantities as to soil every thing upon which it descends, and to irritate the eyes of persons exposed to it. Ships have even been known to run ashore, owing to the obscurity of the atmo- sphere resulting from the presence of this dust. 598. The occurrence of dust-showers in the vicinity of the Cape de Verd isles has been noticed, at intervals, from the year 1579 to the present time. The extent of the region over which they here prevail varies, according to Darwin, from 960,000 to 1,280,000 square miles ; but a greater estimate is given by Captain Tuckey, who sup- poses that it ranges from 1,648,000 to 1,854,000 square miles. The Atlantic dust is believed by Mr. Darwin to come from Africa, since not only does wind blow from that quarter whenever it falls, but the showers also occur during those months when the harmattan is known to raise clouds of dust high into the atmosphere. 599. During a voyage from Richmond, Va., to Rio Janeiro, in the winter of 1845-6, Mr. Thomas Ewbank, of the U. S. Patent Office, met with many instances of the falling of sea-dust, and traced the rich and peculiar hues, that at times adorned the clouds and sky, to the diffusion of this fine powder throughout the intermediate atmosphere. 600. On the 10th of January, 1846, in 23° 33' N. Lat., and 34° 37' W. Long., he observed a narrow belt of slate- colored sky skirting the horizon, while upon this rested a broad band of vermilion, interspersed with soft dashes of Indian ink, shaded with umber. These hues changed, by insensible degrees, into a bright cream-color, and this again into a pale, delicate green, which deepened in tint as it approached the zenith, while over all floated amber- colored clouds, growing richer in hue and smaller in size as they sunk towards the horizon. What is the extent of the region over which the Cape de Verd and Atlantic dust-storms prevail 1 What is the opinion of Mr. Darwin as to the origin of this dust? Relate in full the account given by Mr. Ewbank of the dust-storma that he observed on a voyage from Richmond to Rio Janeiro. BLOOD-RAINS. _ 245 601. Three days afterwards, in 16° 0T N. Lat., and 31° 13' W. Long.,, the wind blew strongly from the east, bearing along with it a red, impalpable powder. This minute dust was seen on the windward side of the sails, where it was supposed to have been collecting during the two previous days. It was extremely fine, and could only be seen by bringing the loose fibres of a rope, upon which it had settled, between the eye and the sun, when its presence and color were readily discerned. 602. The sun throughout the day, as well as the moon at night, was enveloped in a haze, which was supposed to be caused, in some measure, by the dust that floated in the air. The captain of the vessel, who had noticed this phenomena before, called the red powder African sand. 603. During the two following days the heavens pre- sented scenes of gorgeous and surpassing beauty, the colors of the sky and clouds ranging through emerald green, pink, purple, crimson, yellow, chocolate, umber, and slate/ while beneath this rich and varied combination a groundwork of the purest cream-color extended, giving tone to the whole, and changing in tint from a. fawn-color to a pale white. 604. On the 16th of January, in 7° 44' N. Lat., and 28° 31' W. Long., the red dust was observed to accu- mulate upon the vessel — an old sail, looking as if it had been painted of a light brick color. The ship at this time was opposite Soudan and Senegambia, which border on the great African desert, whence the captain supposed the shower to come. A portion of the dust was collected by rubbing a piece of foolscap paper over the colored sail. 605. A fall of dust, accompanied by snow, occurred in the month of February, 1850, at Olsterholz, near Det- mold, in Westphalia. The wind, during this phenomena, blew from the south-west. The dust fell so thickly as to cover the earth to the depth of one eighteenth of an inch. BLOOD-RAINS. 606. Instances. On the 12th of August, in the year What is said of the dust-shower that occurred at Olsterholz 1 246 MISCELLANEOUS PHENOMENA. 1222, a red rain fell at Rome for the space of a day and a night ; and a similar event occurred at Cremona on the 3d of July, 1529. In 1608 a red rain descended for several miles around Aix, in France ; and in 1623 an- other blood-rain happened at Strasburg, between 4 and 5 o'clock in the afternoon. On the 5th and 6th of May, 1711, red rain fell at Orsio, in Sweden ; and a shower of this nature also occurred near Genoa in the year 1744. 607. A very remarkable rain of this character fell at Locarno, in Switzerland, on the 14th of October, 1755. A warm Sirocco wind was here blowing at 8 o'clock on the •morning of this day, and two hours afterwards the air was filled with a red mist. At 4 o 'clock in the afternoon a blood-rain descended, which left on the ground a reddish deposit. Nine inches of this colored rain fell, in the course of one night, over a region forty square German leagues in extent. It even reached Suabia, on the northern side of the Alps ; while amid the cold heights of these lofty mountains it changed into a reddish snow, which fell to the depth of nine feet. 608. The red matter that was deposited during this shower was found, by actual measurement, to be in some places an inch deej>, or one-ninth part of the quantity of rain. Upon the supposition that it fell, on an average, to the depth of only one-sixth of an inch, twenty-seven hundred cubic feet of this red substance must have cov- ered every English square mile. 609. On the 13th of November, 1755, a red rain fell in Russia, Sweden, Ulm, and on the Lake of Constance ; and on the 9th of October, 1763, a similar shower de- scended at Cleves, Utrecht, and many other places in Europe. 610. During the remarkable phenomenon that occurred, at Gerace, on the 14th of March, 1813, the red rain pre- vailed over a great extent of country, falling throughout the two Calabrias, and on the opposite side of the province of Abruzzo, in the kingdom of Naples. Relate, in detail, the several instances given of the fall of blood- rain. BLACK RAIN. 247 611. A red rain likewise fell at Sienna, and upon the adjacent country, on the 15th of May, 1830, at 7 o'clock in the evening, and 'also at midnight. The weather for two days previously had been calm, but the sky was over- roast with dense, reddish clouds. \ 612. Black Rain. The material that mingles with these extraordinary rains is not always of a red hue, but is sometimes of a dark color, and imparts an inky black- ness to the shower. A rain of this kind occurred at Montreal, in Lower Canada, on two several days during the month of November, 1819, under the following cir- cumstances : On the morning of the 21st of this month a dense gloom enveloped the city, while the whole atmosphere was obscured by a thick haze, of a dusky orange color, and at this time rain descended of a dark inky hue. The weather soon after became pleasant, and continued so until the following Tuesday, when at noon the whole city was again shrouded in a heavy, damp vapor, so dense that it became necessary to light candles in all the houses. At about 3 o'clock in the afternoon a slight shock of an earthquake was felt, attended by a noise like the discharge of distant artillery. Soon after, when the darkness was the deepest, the gloom was dispelled by a vivid flash of lightning, which was followed at once by a crashing peal of thunder ; and this was succeeded by a heavy shower of thick, Hack rain. 613. On the 22d of April, 1846, a copious black ram fell also in England, in the towns of Dudley, Stourport, Abberly, and Bewdley, which are situated in the northern part of Worcestershire. This shower lasted from 11 o'clock in the morning till 1 o'clock in the afternoon, the rain descending so abund- antly as to blacken the waters of the places where it fell, and darken the river Severn. Give an account of the Hack rain of Montreal. Of that which happened in Worcestershire. 248 MISCELLANEOUS PHENOMENA. 614. Red Hail. A storm of red hail is stated by Baron Humboldt to have once occurred at Paramo, in South America, between Bogota and Popayan. There likewise fell over all Tuscany, on the 14th of March, 1813, a shower of hail of an orange hue. 615. Black Hail. A hail-storm happened in Ireland on the 14th of April, 1849, which deposited upon the ground a Hack, inky substance. Some of this dark mat- ter was collected and examined, and found to be of the same nature as the coloring material of red rains. STORMS OF COLORED SNOW. 616. Red Snow. One of the most remarkable falls of red snow on record is that which has already been mentioned (Art. 607), as occurring simultaneously with the blood-rain of Locarno, in Switzerland, when snow of a reddish hue covered the neighboring Alps to the depth of nine feet. 617. On the 5th and 6th of March, 1808, red snow fell for the space of three nights in Carniola, a province of Germany, and throughout Carnia, Cadore, Belluno, and Feltri, to the depth of five feet and ten inches. The earth had been previously covered with white snow, and the storm of colored snow was succeeded by another, the flakes of which were as usual, of a pure and brilliant white. The two kinds were perfectly distinct. When a portion of the red snow was melted in a vessel, and the water evaporated, a fine rose-colored, earthy sediment remained at the bottom. Red snow, likewise, fell at this time on the mountains of the Valtelline, in Switzerland, at Brescia, and on the Tyrol. 618. During the dust-slwwer and Hood-rain, at Ge- race, red snow descended over a wide extent of country, embracing the two Calabrias, Tolmezzo, and the Carnian Alps. In Tuscany it fell of an orange hue, while at Bologna its tint was a brownish yellow. What instance is given of the occurrence of red hail ? What of black hail ! Where, when, and under what circumstances have storms of colored snow occurred 1 NATURE OF THE DUST. 249 619. On the 15th of April, 1816, colored snow fell in Italy, upon Tonal, and on other mountains. It was of a brick-red hue, and, when melted and evaporated, a light and impalpable earthy powder remained. 620. A storm of reddish snow took place on the 31st of March, 1847, in . Puster Valley, in the Tyrol. It de- rived its tint, which was a brownish red, from a fine colored dust, resembling that of the Atlantic showers. 621. Black Snow. A few years ago a fall of black snow occurred in New Hampshire, at Walpole, and the adjoining towns. A person writing to the Boston Journal from Walpole, remarks, in relating this extraordinary phenomenon : " I send you some writing, written with the snow as it fell, and with a clean pen?? This writing, according to the editor of the Journal, -was perfectly leg- ible, and appeared as if having been written with pale black ink. 622. These colored snows must not be confounded with those already described in Arts. 286, 287, and 288. The snows there mentioned are white before their fall, and acquire their red and green tints, after their descent, from the presence of a microscopic plant whose cells are filled with animalcules, and which, even in Arctic climes, spreads itself with extraordinary vigor over fields of snow. On the contrary, in storms of colored snow, the coloring matter is in the atmosphere, and the snow is dyed before itsfall. NATURE OF THE DUST. 623. It appears from the microscopic investigations of eminent observers, and especially from those of Ehrenberg, that the dust which causes dust-storms, and produces the phenomenon of blood-rains, is composed both of organised and unorganised matter : the latter being portions of various minerals, while the former consists principally of the shells of infusoria, mingled with fragments of pet- rified plants and parts of insects. Are these colored snows the same in character as those already de- scribed in Arts. 286, 287, and 288 ? Why not ' Of what is the dust of dust-storms and blood-rains composed 1 Of what does the organic matter consist 1 11* 250 MISCELLANEOUS PHENOMENA. It may not be amiss to explain to the student in this place the meaning of the term infusoria. 624. Infusoria. The general name of Infusoria has been given to those minute living beings which can only be seen by the aid of the microscope. On account of their being first detected in vegetable infusions, they are termed infusoria ; and since they are exceedingly small, they have also received the appellation of animalcules, or little animals. They are found in countless myriads in all waters, and in the fluids that circulate in animal and vegetable bodies, while their shells and eggs are dissem- inated by the winds over every part of the world. 625. More than eight hundred distinct species have been discovered, possessing the most grotesque and sin- gular forms. Some resemble globes, trumpets, stars, boats, and coins; others assume the forms of eels and serpents, and many appear in the shape of fruits, neck- laces, pitchers, wheels, flasks, cups, funnels, and fans. Their minuteness is almost incredible, for the monad, the smallest of all living beings, never exceeds in length the twelve thousandth part of an inch. A single shot, one-tenth of am inch %n diameter, occupies more space than seventeen hundred millions of these atoms — each in itself a perfect being, amply endowed with vital powers adapted to the mode and range of its existence. 626. Structure. The outer covering of the infuso- ria is of two kinds ; the first is soft and yielding, resem- bling the skin of the leech and slug ; but the second is a fine, transparent shell, possessing a flexibility like horn. Those animalcules that are protected by the latter integ- ument are termed loricated, from the Latin word lorica, a shell / while the name illoricated, or shelless, is assigned to those which are invested with the softer covering. The material that composes the shells varies in different species. In many kinds it consists entirely of fiint, and Describe the infusoria — the number of their species — their minute- ness. What is said respecting their structure i ITALIAN AND CALABRIAN DUST-SHOWERS. 251 in others of lime, united with oxide of won. In some cases it is combustible. 627. When the loricated infusoria die, their shells re- main undecayed for ages, often congregated in such countless myriads as to form large portions of the earth's surface. The city of Richmond, in Virginia, is built upon an extensive bed of JUnty marl, from twelve to twenty feet in thickness, filled with fossil, infusorial shells ; and it is stated by geologists, that nearly half of the bulk of all the chalk of Northern Europe is composed of the fossil remains of animalcules, and other minute shells. They are mingled with the mud that forms the bed of the Arctic Ocean ; they float with the iceberg in all' its wanderings, and lie loosely scattered over the surface of every land. These hieroglyphics of nature are interpreted by the aid of the microscope.* 628. The Italian Dust-shower of 1803, and the Calabrian of 1813. In the dust which fell in Italy during the month of March, 1803, forty-nine species of organic structures were discovered, and sixty-four in that which descended at Gerace, in Calabria, in 1813. Thirty- nine species in the Italian dust-shower, and fifty-one in the Calabrian, are identical with those discovered in more recent dust-storms. It is worthy of remark, that these two storms, though ten years apart, have no less than twenty-eight species in common, and in both nearly all the species are of fresh-water origin. Among the numerous infusorial shells, four South American forms were discovered ; of these, one occurs in Peru, another in Surinam, and the remaining two belong to Chili. No animalcular structures were found exclusively African. 629. Atlantic and Cape de Verd Dust. The dust ■When the lorioated infusoria die, what becomes of their shells 1 Of what did the dust consist which fell in the Italian and Calabrian dust-storms 1 * For farther information on the subject of Living and Fossil Infusoria, see " Views of the Microscopio World," by the author ; published by Farmer, Brace & Co., New York. 252 MISCELLANEOUS PHENOMENA. that was collected by Mr. Darwin on the Atlantic, in N. Lat. 17° 43', W. Long. 26°, and at the distance of about five hundred miles from the African coast, was submitted to the examination of Ehrenberg, who discovered that one-sixth part of it was composed of the flinty shells of fresh water and land infusoria, and of silicious fossil plants. There were eighteen species of the former, and as many of the latter. Of the animalcular remains, the greater part were European ; one species was decidedly of South American origin, and another probably ; but there were none that belonged exclusively to Africa. In the opinion of Ehrenberg, the two South American spe- cies were either brought from that country by the upper winds of the atmosphere, or from some other locality which is yet unknown. 630. In the dust of several other showers, which occur- red between the years 1834 and 1838, some at St. Jago, and some on the neighboring ocean, numerous organized structures were discovered, thirty of which were different from those detected in the dust just described. Among these were the shells of a few South American infusoria, and one beautiful microscopic shell, termed the Polytha- lamia* or many -chambered shell. A single species was observed that occurs in the Isle of France ; but none of the forms were recognized as peculiarly African. 631. Some of the dust collected by Mr. Ewbank, on his voyage to Rio Janeiro, was examined by Professor Bailey, of West Point ; but he was unable to discover in it any thing besides irregular, inorganic, mineral frag- ments. He believes, however, that more interesting results would have been obtained if the dust had been gathered with greater care. The entire number of distinct organic Relate, in full, what is said respecting the composition of the Atlan- tic and Cape de Verd dust. What iB said respecting the dust of several other Bhowers 1 Were any of the forms distinctively African'! Were any organisms discovered in the dust collected by Mr. Ew- bank ? What is, however, the opinion of Prof Bailey 1 * From polus, (Greek,) many, and thalamus, (Latin,) a chamber. SIROCCO DUST. 253 forms hitherto discovered in the Cape de Verd and Atlan- tic dust-storms is sixty-seven. X 632. Sirocco Dost. The dust that fell at Malta on the 15th of May, 1830, afforded forty-three distinct organ- ized forms ; of these there were fifteen infusorial struc- tures, twenty-one kinds of minute, petrified plants, and seven of Polythalamia. The species of animalcules were, for the most part, identical with those discovered in the Cape de Verd and Atlantic dust-showers. rig. 46. MIOROBCDPIO ORGANISMS OP THE LYONS DU8T-8H0WBB. (Fossil Infusoria.) One form was noticed belonging peculiarly to Chili, but none were found distinctively African. State the entire number of organic forma hitherto detected in the Cape de Verd and Atlantic dust-storms. 254 MISCELLANEOUS PHENOMENA. 633. The Sirocco dust that fell at Lyons, on the 17th of October, 1846, was so rich in organic remains that they constituted one-eighth part of its mass. They consisted of numerous species of infusoria and of petrified plants, mingled with a few kinds of Polythalatma, and minute, vegetable fragments. The species were nearly all of fresh-water origin, one-seventh only being marme. In figures 45 and 46 are delineated the various microscopic Fig. 46. MICEOSOOFIO OBQAN18M8 OP TUB LYONS DU8T-6HOWEK. (Fossil Plants.) organisms which were discovered in this dust. The most remarkable circumstance respecting it is the fact, that, notwithstanding its general resemblance to the dust of the Atlantic showers, which has always exhibited nothing but dead and empty infusorial shells, this, on the con- trary, was found, in many cases, to contain a species of Describe the nature of the Siroeeo dust that fell at Malta and at Lyons. What is remarkable respecting tho Lyons shower % ORGANISMS OF DUST-SHOWERS. 255 infusoria which was distinctly seen to be filled with green ovaries, or egg-sacks, and consequently was capable of life. 634. The dust collected, in the preceding instances, from the Cape de Verd, Atlantic, and Sirocco showers, being nine in all, afforded 119 distinct organisms. Of these there were fifty-seven species of infusoria, and eight of Polythalamia; forty-six kinds of fossil plants, together with particles of seven kinds of plants, and one fragment of an msect. Only seventeen of these organ- isms were marine ; while 102, six-sevenths of the whole, consisted of fresh-water species. In all these showers the dust exhibited no indications whatever of volcanic origin. 635. In three dust-showers which occurred in the years 1847 and 1848 — thejvrst in Salzburg, the second in Ara- bia, and the third in Silesia and Lower Austria — similar fresh-water organisms were detected. The same South American species were here found, as in other showers, without any characteristic African forms. 636. The red snow that fell in the Tyrol on the 31st of March, 1847, afforded sixty-six different organic forms. Of these, twenty-two were infusorial structures, twenty- eight fossil plants, two polythalamia, and thirteen par- ticles of plants. There was also one fragment of an insect. The greater part, by far, of all these species, were of land origin, two only being marine. A remarkable resemblance exists between the coloring matter of this shower and the dust of the Atlantic, Geno- ese, and Lyons storms, not only in its hue, but in its composition / for out of these sixty-six structures, forty- How many distinct organisms were discovered in the dust of nine showers 1 Describe the eeveral kinds. Was there any trace of volcanic dust in these showers t What is observed respecting the dust-storms which happened in the years 1847 and 1848 ! What organisms were detected in the red snow of the Tyrolese storm 1 What resemblance was observed between the coloring matter of this shower and the dust that fell on the Atlantic, at Genoa, and at Lyons 1 256 MISCELLANEOUS PHKNOMENA. six are found in the Atlantic and Sirocco dust ; and twelve species of infusoria and twenty of fossil plants are com- mon to all. 637. In the dust that fell, mingled with snow, at Olsterholz, in the year 1850, Ehrenberg detected fifty organic forms, forty of which he had previously observed in the dust of other showers. The remaining ten species had never been before discovered in atmospheric dust. 638. Number of Distinct Organisms Discovered. In the dust of the various showers examined by this dis- tinguished naturalist, no less than 320 distinct species of organisms were discovered. Of these, five only were of marine origin, and fourteen were forms peculiar to America. 639. Number and Extent of Dust-storms and Blood-rains. According to the researches of Ehren- berg, 340 instances of dust-storms and blood-rains are mentioned in history and in the annals of science, of which 81 took place before the Christian era, and 259 after it. These remarkable phenomena extend through- out the world, occurring on the ocean, on all the conti- nents, and even in Australia. They appear, however, to prevail most within a zone, extending from that part of the Atlantic off the west coast of Middle and North Africa, along in the direction of the Mediterranean Sea, reaching a short distance north of this sea, and continuing into Asia between the Caspian Sea and the Persian Gulf, perhaps to Turkistan, Kaschgar, and even China : they seldom happen as far north as Sweden and Russia. This zone, according to the observations of Captain Tuckey, has a breadth of 1800 miles. What organic forms were discovered in the dust that fell at Olster- ho'z? How many distinct organisms have been detected by Ehrenberg in the dust of numerous dust-storms I "What is said respecting their oriein 1 What, is the number and extent of dust-storms and blood-rains, accord- ing to Ehrenberg'! Where do they appear to prevail most ? What is the breadth of this zone 1 ORIGIN OF THE DUST. 257 640. Their Periodicity. These phenomena occur most frequently during the first half of the year ; for out of 199 showers, whose dates are ascertained, 118 happened between January and July, and 81 between July and December. The distribution of the showers through the several months is as follows : January, 27. July, 9. February, 14. August, 17. March, 23. September, 7. April, 18. October, 18. May, 18. November, 16. June, 18. December, 14. 641. Origin of the Dust. The color and natwre of this dust ; the circumstance that a great quantity of earthy matter sometimes falls in a single shower, as in that of Lyons ; and the fact that dust-storms and blood- rains have occasionally happened from the time of Homer (900 B.C.) to the present day, have led Ehrenberg to advance a most extraordinary hypothesis. He believes that these phenomena are not to be traced to mineral mat- ter belonging to the earth's surface ; neither to masses of dust revolving in space, like the meteoric matter of Chaldni (Art. 555) ; nor yet to the influence of atmospheric cur- rents, sucli as the trade-winds and harmattan, carrying the dust of the earth aloft into the air ; but to some gen- eral law, as yet unknown, according to which infusoria, and other Iwiing organisms, exist and are propagated in the upper regions of the atmosphere. The locality which constitutes the dwelling-place of these organisms he imagines to be of vast extent, and to be sit- uated at the height of about 14,000 feet above the sea- level. In what parts of the year do these phenomena most frequently occur 1 How are they distributed through the months 1 What are the views of Ehrenberg respecting the origin of the dust that falls in these singular storms 1 Where does he suppose the abode of these organisms to be sit- uated' 258 MISCELLANEOUS PHENOMENA. 642. The apparent periodicity of the showers he ac- counts for by supposing that this cloud of organisms lies in the region of the trade-winds, and suffers partial and pe- riodical deviations. 643. In the present imperfect state of our knowledge in regard to these phenomena, it would be highly unsafe to adopt this singular hypothesis. Both the organic and inorganic matter contained in these storms are terrestrial in their nature, and the at- mospheric currents are most probably the agents, which elevate this dust from the surface of the globe, and bear it along to distant regions. 644. The opinion, that the Atlantic, Cape de Verd, and Sirocco dust comes from the deserts of Africa, is incon- sistent with certain known facts respecting it, and has therefore not been universally adopted. For instance, the color of thia dust is red, while the sand of the African Saharas is white and gray ; and we have also seen that none of the organized forms which it contains are peculiar to Africa ; while many of them are distinctively South American. 645. It is the belief of Lieutenant Maury, that the red powder, which falls in these dust-storms, is brought by an upper wind from South America to Africa ; where it de- scends and becomes the lower trade-wind, which dissem- inates the dust throughout the regions where it blows. It is not improbable that a portion of this dust, carried on- wards by the higher current, falls within the sweep of the Sirocco — a circumstance which will fully explain the sim- ilarity that exists between the Sirocco and Atlantic dust. How does he account for the apparent periodicity of these showers and storms'! Are there, at present, sufficient reasons for adopting this hypothesis 1 What is the nature, both of the organic and inorganic bodies, which constitute this dust t How are they probably raised into the atmosphere 1 What reasons exist for believing that the Atlantic, Cape de Verd, and Sirocco dust does not come from Africa 1 What is the opinion of Lieutenant Maury upon this point 1 How can the similarity in the nature of the Sirocco and Atlantic dust be explained 1 VOLCANIC SHOWERS. 259 VOLCANIC SHOWEES. 646. The fall of ashes and dust soon after the eruption of volcanoes, is a phenomenon entirely different from dust-storms and blood-rains y for the materials which are precipitated in volcanic showers contain no organic forms, and are easily traced to their source. 647. Cause. The mighty energies that are at work, when a volcano is in full action, carry up the lighter por- tion of the ejected matter high into the air; it is then borne along by the upper winds, and at length falls, in showers, in regions often far remote from the burning crater. 648. Instances — Jortjllo. During the eruption of Jorullo,' in Mexico, which began on the 28th of Septem- ber, 1759, the sky was darkened with clouds of dust that afterwards fell at Queretaro, 100 miles distant ; and during another eruption of the same volcano in 1819, dust, to the depth, of six inches, descended in the streets of Guanaxuato, at the distance of 160 miles. 649. Souffriere. One of the most remarkable vol- canic dust-showers on record is that connected with the eruption of the Souffriere mountain, in the island of St. Vincent, which occurred on the 30th of April, 1812. 650. On the 27th of this month the volcano, which had been slumbering for a hundred years, again burst forth, showering down sand, mixed with ashes and gritty, cal- cined particles of earth. This dust, driven before the wind, darkened the air like a cataract of rain, and covered the ridges, woods, and cane-lands with light grey-colored ashes, resembling snow. As the activity of the volcano increased, this continual shower extended farther and farther, destroying every trace of vegetation. 651. For three days the appearance of the burning mountain grew more awful and portentous, when at length, on the night of the 30th, a most terrific eruption State what is said respecting volcanic showers — their cause. Give an account of the showers attending the eruptions of Jorullo. What remarkable shower of this kind is next mentioned \ Da- scribe it." 260 MISCELLANEOUS PHENOMENA. took place. From the midst of a lofty pyramid of flame issued streams of glowing lava, which, pouring down the sides of the mountain, flowed in torrents to the sea ; while the sullen roar of these burning rivers was swelled by the thunderings and loud explosions of the crater. Stones, fire, ashes, and calcined masses rained down for hours, and earthquake following earthquake, almost incessantly, the whole island undulated like water shaken in a bowl. 652. On the next day, the air was so filled with vol- canic dust, that it was dark at 8 o'clock in the morn- ing ; a dense haze shrouding sea and land. Most of the plantations in the vicinity of the Souffriere mountain were covered ten or twelve inches deep with dust and stones. 653. But the eifects of this eruption were not confined to this island. During the night of the 30th the' terrific explosions of the volcano were heard as far as Barbadoes, which is situated seventy miles due east from St. Vin- . cent. On the next morning, at 4 o'clock, the atmosphere at Barbadoes was liright and clear, but at 6 o'clock the sky was obscured by thick clouds, from which issued in torrents, like rain, particles of volcanic matter finer than sand. At 8 o'clock, an appalling darkness, as intense as that which prevails in the depth of a stormy night,' over- spread the island, and continued till noon, but the showers of- dust still descended at intervals until 7 o'clock in the evening. 654. This dust descended to the depth of two inches, and, according to the computation of observers, an average weight of 40,000 lbs. rested upon every acre on which it fell. Vessels at sea, some 300 miles, and others 500 to the windward of St. Vincent, had their decks covered with this volcanic dust. (Art. 103.) X 655. Tomboro. Still more surprising was the dust- shower, caused by an eruption of Tomboro, a volcano sit- To what other island did this dust extend % How far is it from St. Vincent ? How was the atmosphere of Barbadoes affected by this volcanic, dustl At what distance from St Vincent were vessels covered with this dust? ERUPTION OF COSIGUINA. 261 uated in the island of Sumbawa, which lies east of Java, and south of Borneo. 656. The eruption occurred on the 12th of April, 1815. According to Sir Stamford Raffles, who was then governor of Java, the roar of the volcano was distinctly heard, in one direction, at Ternate, 720 miles distant from Tomboro, and in another, as far as Sumatra, at the distance of 970 miles. 657. Such vast clouds of ashes and dust were ejected, that the day at Sumbawa was as dark as the blackest night ; these, rising within the sweep of the higher winds, were carried in immense quantities to Java, 300 miles distant, and hung like a pall over the island. At Macas- sar, 250 miles from Sumbawa, a total darkness prevailed long, after the sun had risen, and volcanic dust fell an inch and a lialf deep. Some of the ashes were carried even as far as the island of Amboyna, which is situated 800 miles from Tomboro. 658. Near Sumbawa, such quantities of lava, cinders, and ashes fell into the sea, that they formed a cake on the surface two feet in thiekness, and, for miles around the island, .the ocean was so completely covered with this floating matter, that the progress of ships was materially impeded. 659. Cosiguina. During the eruption of the volcano of Cosiguina, in Nicaragua, on the 20th of January, 1835, immense clouds of dust darkened the sky, and were borne by the winds to a great distance. 660. At Union, a sea-port on the western shore of the bay of Conchagua, and the nearest place to the volcano of any importance, s/wwers of dust fell at intervals from the 20th to the 27th of January. It descended in the form of a fine powder like flour, and in such quantities as Describe the eruption of Tomboro. What is said respecting the ejected ashes and dust 1 How did they affect the atmosphere, and how far were they car- ried 1 "What is said of the condition of the sea around Sumbawa? Give an account of the showers of ashes and dust caused by the eruption of the volcano of Cosiguina. 262 MISCELLANEOUS PHENOMENA. to cover the earth to the depth of jwe inches / causing, for the space of forty-three hoivrs, so intense a darkness that lights and torches were needed, and even these were insufficient to render objects clearly visible. 661. At Leon, the capital of Nicaragua, showers of ashes and dust descended on the 23d of January to the depth of nine inches ; and at Nacaome the falling dust was mingled with coarse sand, which, together, formed a layer upon the surface of the ground seven or eight inches deep. Some of the ashes ejected during this eruption were carried even as far as Kingston, Jamaica, seven hundred and thirty miles distant from Cosiguina. (Art. 103.) 662. Yellow Rains — Pollen-Rains. Showers of rain, mingled with recent vegetable matter, consisting of the pollen of various plants, have been noticed for a con- siderable period of time. A shower of this kind once fell at Lund, in the south of Sweden, the pollen having been borne by the wind, from a forest of fir, thirty-five miles distant. A similar rain fell on the lake of Zurich, in the year 16*1*1, and another of the same nature, at Bordeaux, in 1761. During a thunder-storm at Banff, in Scotland, on the 9th of June, 1835, a shower of yellow rain de- scended, which tinged the waters of the river Devern and of the neighboring pools with the same color. The hue in this case, as in the preceding instances, was derived from the pollen that was mingled with the rain. 663. A few years ago, a rain of this color extended over the western and south-western regions of the United States. At Carrollton, Ohio, the ground, after the rain, was covered with a yellow substance; and the same phe- nomenon was likewise noticed at this time at Zanesville, Cincinnati, Louisville, St. Louis, Natchez, and New Or- leans. The hue of this rain is undoubtedly to be attrib- uted to the presence of pollen. 664. Gossamer-Shower. A phenomenon of a very extraordinary nature was observed by the Rev. Gilbert What is a pollen-rain 1 Give the several instances. GOSSAMER-SHOWER. 263 White, on the 21st of September, 1741, and of which an account is given in his charming " Natural History of Selborne." It appears from the statement of this gentle- man, that on the day just mentioned, at about 9 o'clock in the morning, a shower of cobwebs, falling from very ele- vated regions, was observed, and which continued to descend, without any interruption, till the close of the day. 665. These webs were not single, filmy ■ threads, but perfect flakes or rags — some being nearly an inch broad and 'five or six long, which fell with a degree of velocity that- showed they were considerably heavier than the at- mosphere. On every side, as the observer turned his eyes, he might behold a continual succession of fresh flakes falling into view, and twinkling like stars as they reflected the rays of the sun. This singular shower was noticed at Bradley, Selborne, and Alresford, three places which lie in a kind of triangle, the shortest of whose sides is about eight miles long. Whether it extended farther, is not certainly known. 666. At Selborne, a gentleman, who observed this phe- nomenon while taking his morning ride, supposed, at first, the falling flakes to have been blown, like thistle-down, from the fields above, and imagined that he would be free from the shower when he had gained the summit of a hill that rose near his. house. But upon reaching this point, 300 feet higher than his residence, he found the webs, in appearance, still as much above as before — still descending into sight in a constant succession, and twinkling brightly in the sun. Neither before nor after was any such fall observed in these places, but on this day the gossamer flakes hung so thickly in the trees and hedges, that a person might have gathered them by basketfuls. 667. In explanation of this curious phenomenon, Mr. Give an account of the gossamer-shower described by Rev. Gilbert White. How does he explain this phenomenon 1 2(34 MISCELLANEOUS PHENOMENA. White observes, that the gossamer threads, which float in the air, are the production of small spiders, that swarm in the fields in fine weather in autumn, and have the power of shooting out webs so as to render themselves buoyant and lighter than the air. If taken in the hand, they will run along the fingers, throw out a web, and sail aloft. He supposes, that, possibly, these spiders, with their webs, are carried up into the higher regions of the atmosphere by the warm and light currents of air which ascend from the earth ; and that while thus elevated they have the power, perhaps, of thickening their webs — as some naturalists sup- pose — thus rendering them heavier than the atmosphere, when of course they must fall, and will thereby occasion, if they descend simultaneously in large flakes and in great abundance, a gossamer -slwwer. 668. Dr. Lister ascended one day, when the air was very full of gossamer, to the highest part of York Minster, and still found these filmy threads floating far above him. CHAPTER II. DRY FOG AND INDIAN-SUMMER HAZE. 669. Dry Fog. A peculiar haze sometimes pervades the atmosphere, which has received from meteorologists the name of dry fog. It is different from humid mist, for it not unfrequently prevails when no visible vapor exists in the air, and during seasons of great heat. 670. When this phenomenon occurs, the sky, although it may be perfectly free from clouds, has lost its fine azure tint, and is dull and discolored. Terrestrial ob- jects at a distance, and of a deep color, are lost to view, and appear as if covered with a blue veil. The sun loses What did Br. Lister observe 1 What is dry fog 7 How does this phenomena affect the appearance of celestial ac i terrestrial objects 1 DRY FOGS. 265 its brilliancy, even when high in the heavens, and its light is of a reddish hue. As it approaches the horizon it as- sumes a blood-red color, and may be gazed at without dazzling the eyes. At times, the haze is even so thick that the solar orb ceases to be visible before it has descended below the horizon. 671. Instances. In the year 1782, a remarkable fog of this kind occurred, extending over Europe from Lapland to the Mediterranean. It was succeeded the next year by another still more extraordinary. This fog, known as the dry fog of 1783, produced a great sensation throughout Europe. According to Kaemtz, its intensity was such, that in some places objects at the distance of three rwiles could not be distinguished. Sometimes they appeared blue, or else surrounded with vapor. The sun, shorn of its beams, appeared of a fiery red, and at noon could be looked at without injury to the naked eye. At its rising and setting it was completely obscured by the dense haze. This dry fog first appeared at Copenhagen on the 26th of May. It reached Rochelle on the 6th of June, . and was noticed, almost everywhere throughout Germany, France, and Italy, from the 16th to the 18th of this month. It was seen at Spydberg, in Norway, on the 22d of June, at Stockholm two days after, and on the 25th it appeared at Moscow. In Syria it was observed towards the close of June, and on the 1st of July it shrouded the Altai mountains. In England it continued from the 23d of June until the 20th of July. During the prevalence of this phenomenon the heat was intense. 672. In the summer of the year 1834, dry fogs wera noticed in various localities in Germany. Kaemtz ob- served one on the 29th of May, enveloping one of the peaks of the Hartz mountains. Give the instances stated. 12 266 MISCELLANEOUS PHENOMENA. For three days, during the latter part of this month, a haze of this kind prevailed at Munster, and the phenom- enon was seen at Halle, Freiberg, and Altenberg, in Sax- ony, on the 28th and 29th of July. In the northern and western parts of Germany, as well as in Holland, dry fogs very frequently occur. 673. Cause. The origin of this phenomenon is not yet satisfactorily explained. Many philosophers suppose it to arise, either partially or wholly, from the influence of elec- tricity, without being able to show very clearly in what manner it is possible for this agent to produce such an effect. Others believe it to result from smoke caused by the conflagration of forests, the burning of peat-bogs, and the eruption of volcanoes. Thus Lalande attributed the dry fog of 1783 to electricity, Cotte to the union of metallic emanations with electricity, while other philos- ophers traced it to a volcanic source. 674. In the opinion of Kaemtz, the dense, dry fog of 1834 arose, partly from the combustion of peat, and partly from the unusual number of extensive fires that occurred in this year. While the fog was among the Hartz mount- ains and in the vicinity of Orleans and Basle, many peat- bogs were reduced to ashes, the fire penetrating deeply beneath the surface. One bog in particular, that of Dachau, in Bavaria, was burned to the depth of more than eight feet, the fire running even beneath ditches filled with water. In July there were vast conflagra- tions of forests and peat-bogs in Prussia, Silesia, Sweden, and Russia. The drought, which then prevailed, favored the propagation of these fires and the diffusion of the smoke. 675. The dry fogs, that occur in Holland, and in the north and west of Germany, are attributed by Finki to the combustion of peat. 676. Indian-Summer Haze. Throughout the conti- What is the cause of dry fogs \ What views are held respecting them 1 How is the dry fog of 1834 accounted for by Kaemtz 1 What is Finki's opinion regarding the origin of the dry fogs of Holland and Germany 1 INDIAN-SUMMER HAZE. 267 nent of North America, there occurs, about the close of October or the beginning of November, a warm and pleasant interval, termed the Indian summer, which lasts for the space of two or three weeks, and agreeably retards the approach of winter. During this season the air is soft and bland, and a mild temperature prevails, while the atmosphere is filled with a dense, dry haze, that causes the distant objects of the landscape to appear as if veiled in a cloud of smoke. 677. Cause. This obscurity has been supposed by some writers to originate in the same way as aqueous mists / while others imagine it to be due to the presence of smoke, borne by the wind from the distant conflagra- tions of vast prairies and forests". In respect to the first view it may be remarked, that the Indian-summer haze bears little resemblance to an aqueous mist. It does not change into rain, and during its continuance the hygro- metric state of the atmosphere is different from that which exists when moist fogs occur. The second hypoth- esis fails, inasmuch as it assigns a local cause for the solution of a general phenomenon — not to mention other objections which might justly be urged against it. 678. No sufficient explanation of this singular phenom- enon has yet been found, but there is one circumstance connected with it which may possibly give a clue to its cause. The Indian summer, with its genial warmth and misty veil, occurs at that period of the year when the leaves of the forest are falling, and the vegetation that covers the surface of the earth is beginning to decay. In view of this fact, the author was led to think, some years ago, that the decomposition of the decaying vegetation, which Liebig What is the Indian summer"! What opinions are entertained in regard to its haze ? Has any adequate explanation been yet given ? What circumstance is worthy of notice in connection with this phe- omenon % In view of this fact, what has been supposed ? 268 MISCELLANEOUS PHENOMENA. terms a slow corhbustion, (eremacausis), might impart that peculiar haziness to the atmosphere which is seen during the Indian summer. It was afterwards ascertained that this phenomenon was ascribed to the same cause by another ob- server, Dr. E. B. Haskens, of Clarksville, Tenn., who also "suggests," that the Indian-summer haze consistsof carbon- aceous matter or smoke produced by the oxidation of the lifeless vegetation. The warmth of this season he attributes to the same cause. These views, however, are merely spec- ulative. FINIS. TEXT BOOKS FOR SCHOOLS AND COLLEGES, INCLUDING THE PBIMABY, ENGLISH, AND CLASSICAL DEPARTMENT, WA TUBAL SCIENCE, GEOGRAPHY, MATHEMATICS, BOOK-KEEPING, ETC. PUBLISHED BY PRATT, OAKLEY AND COMPANY, NO. 21 MURRAY STREET, NEW YORK. # * It will be noticed that most of these works were written by Teachers of too highest eminence. Elements of Astronomy ; with explanatory Notes and ele- gant Illustrations. By John Brocklesby, A. M., Professor in Trinity College $1 25. From the Connecticut Common School Journal. We take pleasure in calling the attention of teachers and students to this truly ex Eellent book. It is not a milk-and-water compilation, without principles and with *ut demonstration. It contains the elements of the science in their proper integrity ind proportions. Its author is a learned man and a practical instructor, as the tuthor of every school-book should be. The style is a model for a text-book, com- *ining in a high degree perspicuity, pfecision, and vivacity. In a word, it is the very >est elementary work on Astronomy with which we are acquainted. This notice is echoed by a large number of academies, who are promptly intro tucing the book. Elements of Meteorology; designed for Schools and Ac ademies. By John Brocklesby, A. M., Professor of Mathematics and Natural Philosophy in Trinity College, Hartford 84 ients. The subject of Meteorology is of the deepest interest to all. Its phenomena every wnere surround us, and ought to be as familiarly known to the scholar as his arith- metic or philosophy. This work treats of Win."* in general, Hurricanes, Tornadoes, VWer- spouts, Rain, Fogs, Clouds, Dew, Snow, Hail, Thunder- storms, Rainbows, Haloes, Meteorites, Northern Lights, Munich, now ot Union College, Schenectady. $1.00 From the New York Churchman. f\' all the German Grammars we have ever examined, this is the most modest and tnoretending, and yet it contains a system and a principle which is the life of it, as ;lear, as practical, as effective for lea'iiing grammar as any thing we have ever seen >>ut forth, with so much more prei nse of originality and show of philosophy. It nvUI be found, too, we th nk, that the author has not only presented a new idea ol much interest in itself, bu> has admirah'y carried it out in the practical lessons and xprcises of his work. From Prof J. 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