LIBRARY University of Californis IRVINE THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA IRVINE GIFT OF MRS. THOMAS A. ROCKWELL THE ST C) R Y OF THE EARTH'S ATMOSPHERE^ BY DOUGLAS ARCHIBALD, M.A. FELLOW AND SOMETIME VICE-PRESIDENT OF THE ROYAL METEOROLOGICAL SOCIETY, LONDON \V I T H F O R T Y - K O U R I L L U S T R A T I O N S NEW YORK McCIA'RK, I'HII.I.II'S CO. MCMIV Psu GC COPYRIGHT, 1807, 1902, 3v D. APPLETOX AND COMPANY. PREFACE. I HAVE desired in the present little work to put forward the main features of our knowledge of the conditions which prevail in our atmosphere as they are interpreted through the science of to- day. The Atmosphere, unlike its solid partner, contains no gold or coal mines with which to stimulate scientific research. Its study has con- sequently been somewhat neglected until of late years, and is even now only just emerging from the stage of myth and speculation into that of fact and certainty. This desirable result has been chiefly attained by the disuse of vague speculation and the appli- cation of the known laws of physics. I have therefore written, not for the minority, who vaguely wonder at the relation of extraordi- nary facts and pass on, but for what I believe to be that much more numerous section who are not content with a mere collection of facts, but want to know the reason why. I have levied largely upon the original works of the more modern school of meteorologists which is so ably represented in America, India, and Germany and am under especial obligations to those of Prof. Davis of Harvard, Prof. Loomis 6 THE STORY OF THE EARTH'S ATMOSPHERE. of Yale, Mr. Ferrel of Washington, Prof. Sprung, and Prof. Waldo. I have purposely omitted the subject of weather and descriptions of instruments, and only briefly touched upon climate, and have rather endeav- oured to show, especially in the chapter on Flight, that the Atmosphere possesses growing uses and interests quite apart from, and in addition to, its consideration as a vehicle of weather. DOUGLAS ARCHIBALD. CONTENTS. CHAPTER PACK I. THE ORIGIN AND HEIGHT OF THE ATMOSPHERE. 9 II. THE NATURE AND COMPOSITION OF THE AT- MOSPHERE 17 III. THE PRESSURE AND WEIGHT OK THE ATMOS- PHERE 25 IV. THE TEMPERATURE OF THE ATMOSPHERE . 31 V. THE GENERAL CIRCULATION OF THE ATMOS- PHERE 64 VI. THE LAWS WHICH RULE THE ATMOSPHERE . 94 VII. THE DEW, FOG, AND CLOUDS OF THE ATMOS- PHERE 106 VIII. THE RAIN, SNOW, AND HAIL OF THE ATMOS- PHERE 1 19 IX. THE CYCLONES OF THE ATMOSPHERE . . 125 X. THE SOUNDS OF THE ATMOSPHERE . . . 138 XI. THE COLOURS AND OPTICAL PHENOMENA OF THE ATMOSPHERE 141 XII. WHIRLWINDS, WATERSPOUTS, TORNADOES, AND THUNDERSTORMS OF THE ATMOSPHERE . 149 XIII. SUSPENSION AND FLIGHT IN THE ATMOSPHERE 163 XIV. LIFE IN THE ATMOSPHERE 183 LIST OF ILLUSTRATIONS. PAGE Cumulus Cloud Frontispiece Fig. i Strato- Cumulus (low) . . . .11 Fig. 2 Strato-Cumulus (high) .... 14 Fig. 3 Cirro-Cumulus . 17 Fig. 4 -33 Fig. 5 -35 Fig. 6 . . . .43 Fig. 7 . . _. .45 Fig. 8 Distribution of Atmospheric Tempera- ture in Latitude for January, July, and the year . . . .47 Fig. 9 . . . -53 Fig. 10 . . . .55 Fig. ii . . . .56 Fig. 12 . . . .57 Fig. 13 . . . .67 Fig. 14 . . . .69 Fig. 15 . . . 72 Fig. 16 . . . .73 Fig. 17 -74 Fig. 18 . . . .75 Fig. 19 . . . .78 Fig. 20 . . . .81 Fig. 21 . . . .83 P^i. 22 . . . .86 8. Fig. 23 " After the Storm " . . -Q7 Fig. 24 Diffusive Limits of the Component Gases of the Atmosphere . 101 Fig. 25 CirrusCloud (var 7'racto Cirrus, 1889) . 113 Fig. 26 . . . -US Fig. 27 . . . .116 Fig. 28 Festooned Cumu- lus .... 118 Fig. 29 . . . .121 Fig. 30 . . . .123 Fig. 31 . . . .124 Fig. 32 . . . .129 Fig. 33 . . . .132 Fig. 34 !33 Fig. 35 J 33 Fig. 36 Tornado Funnel Cloud . . . .155 Fig- 37 Thunderstorm in Section . . . 157 Fig. 38 Kestrel Hawk Hovering . . . 168 Fig. 39 . . . .171 Fig. 40 . . . .175 Fig. 41 . . . .176 Fig. 42 . . . .178 Fig. 43 Yachting in Syd- ney Harbour . . 181 THE STORY OF THE EARTH'S ATMOSPHERE. CHAPTER 1. THE ORIGIN AND HEIGHT OF THE ATMOSPHERE. THE atmosphere of air in which we live and breathe is really a part of the solid globe on which we stand. Until we think of it, we might be inclined to imagine we were surrounded by mere space, but when we place our heads under water we find we can not live more than a few seconds without in- haling the same air, and we have only to look at our ships sailing, our windmills rotating, and our slates blowing off our roofs in a storm, to be cer- tain that it is just as material as the solid earth to which it clings. Its past history, unlike that of its more solid partner, is not written in the unmistakable lan- guage of successive rock strata, or fossil remains, and we can only infer something of its ancient changes from analogy with what is now occurring in the sun, and a knowledge of the physical his- tory of the universe. If we are to believe the "nebular theory," propounded years ago by the great French astron- omer, La Place, and which, far from being upset, has rather been confirmed by recent discovery, y 10 THE STORY OF THE EARTH'S ATMOSPHERE. all existing suns and planets have been simply condensed from clouds or nebulae of matter origi- nally scattered through space. By the mutual attraction of their matter (which force we now term gravitation), these separate aggregations became highly heated glob- ular masses, every element of which was at first in a state of fiery gaseous incandescence. As they gradually cooled and threw off planetary ex- crescences, these masses became condensed at first into liquid spheres or suns, surrounded by atmos- pheres of the lighter and less condensible gases, still hot enough to be luminous. Of such a type is our own sun. A further stage of cooling took place, par- ticularly amongst the planetary offspring, during which the liquid cooled enough on its external surface to form a thin solid crust, beneath which it still remained more or less liquid, and above which enough gases still remained uncondensed to form a thin atmosphere, through which light and heat could penetrate, and yet substantial enough to support animal life. This is the pres- ent condition of our own planet. We must not, however, suppose that this state of things holds on every other planet. The rate at v/hich such changes progress is different for each planet. The planet Jupiter is still so hot that it is be- lieved to be partly self-luminous, and its atmos- phere probably contains clouds and vapours of substances which on our cooler earth have long since condensed into liquids or solids. Through the telescope it is seen to be covered with dense clouds, and most of its water probably still exists in the form of vapour (or water gas), and not in ORIGIN AND HEIGHT OF THE ATMOSPHERE. II liquid seas as on our own globe. The planet Mars, on the other hand, has so little water left in its atmosphere or on its surface that, while enough remains to supply its polar caps with snow during the winter, its parched equatorial deserts are believed by Mr. Lowell, of the Arizona Observatory, and others who have made it a Fin. i. Strato-cumulus (low). special study, to be irrigated thence by the system of so-called canals which intersect its surface. Finally, bur moon presents a picture of the condition eventually reached by a small globe viz., all solid, no liquid, and no gas left. There- fore, according to our ideas, no life would be possible on the moon. The liquid, which would 12 THE STORY OF THE EARTH'S ATMOSPHERE. be chiefly water, has been absorbed into the solid substance of the moon, while the last relics of the gaseous atmosphere, which it once must un- doubtedly have possessed, have been either ab- sorbed into its mass or else diffused into space beyond the power of recall by gravitation. The condition of each globe at present de- pends chiefly on the rate at which these changes from all gas, to gas and liquid, and thence to gas, liquid, and solid, occur /. e., on their rate of cool- ing. The larger the globe the longer it takes to cool. The final condition, however viz., whether a globe ultimately ceases to possess a liquid or gaseous covering, and becomes like our moon, or still retains an atmosphere and oceans like our earth, depends on the attraction (gravity, as we term it) by which it holds its gaseous portions to it. This, again, directly depends on the amount of matter it contains, and therefore again upon its size. Thus, our earth will probably never lose its atmosphere altogether, though considerable quantities of the lighter gases, such as hydrogen, have no doubt already escaped into space. The fact, therefore, that we possess at the present time a gaseous atmosphere of exactly that particular degree of tenuity that suits our breathing apparatus, remarkable though it may seem, is a direct consequence of the particular size of the globe on which we stand. Back through the corridors of time, before the earth had sufficiently cooled to acquire a solid crust, we were a little sun, with an atmosphere of hot, turbid, metallic vapours which poured down metallic rain, only to be boiled off again on approaching the heated surface. After a time, ORIGIN AND HEIGHT OF THE ATMOSPHERE. 13 however, such metallic rain would cease to rise again, and remain a part of the solidifying earth, and by the time that geologic history com- menced and the surface was cool enough to ad- mit of animal and vegetable growth, the atmos- phere must have been practically as clear as it is to-day. In proof of this we find that those remarkable trilobites or sea-lice of the Silurian period, which is nearly the oldest of which we have any knowl- ledge, were endowed with organs of vision, which shew that as much light penetrated the seas then as now. The atmosphere, therefore, must have been equally transparent. Doubtless, more va- pour and carbonic acid were present. Indeed, some of the latter has since been locked up in a solid form in the coal measures and limestone rocks of subsequent epochs. Continuing our globe history, there came a time when the atmosphere, after being heated mostly from the still warm earth, began to find its solid partner no longer the warm friend of its youth, and found itself compelled to depend on the solar beams, albeit after they had travelled through ninety-three million miles of space, to protect it from the terrible cold of space. By receiving and entrapping such rays, it is even now enabled to keep some 500 Fahr. warmer than outside space, while the heat which at present reaches it from the earth is estimated as being barely enough to raise it T ^ 7 ths of a degree in temperature. The atmosphere of our planet, therefore, is our own individual property, and in no sense part of a universal atmosphere spread all over space. In fact, if such a general atmosphere ex- 14 THE STORY OF THE EARTH'S ATMOSPHERE. isted at all, it has been calculated by Dr. Thiesen of Berlin that our sun would, by virtue of its enormous size a million times that of our earth and gravity, which is twenty-seven times greater, attach to itself a gaseous covering or atmosphere, which would be as dense as our own, far beyond the orbit of Venus. This, however, is known to be contrary to fact. FIG. 2. Strato-cumulus (high). The sun's atmosphere is not more than about 500,000 miles deep, while that of the earth is cer- tainly not more than 100 miles. The height of our atmosphere has never been ORIGIN AND HEIGHT OF THE ATMOSPHERE. 15 measured as \ve measure distances on the earth's surface, for the very simple reason that we can never hope to reach the top. Indeed, we should find it very difficult to know where the top was, even if we were able to approach it, since the air would shade off so gradually into where it sud- denly changed into the vacuum of space that we should with difficulty discover the place where we could say " thus far and no farther." We can, however, arrive at some knowledge of the probable height to which the air exists in such quantity as to possess weight and resistance by calculation of the rate at which the pressure of the atmosphere diminishes as we ascend, and also by observation of the duration of twilight and the heights at which meteorites (or, as they are still popularly termed, falling stars) are visible. Living as we do at the base of our ocean of air, like the flat-fish live at the bottom of the ocean of water, we are absurdly ignorant of the condition of the atmosphere a few miles overhead. The highest ascent made by man up moun- tains is believed to be that of Zurbriggen on Aconcaqua, when he reached about 24,000 feet, or a little over 4 miles, while the highest in a balloon was that made by Dr. Berson of Berlin, who in 1894 ascended to a height of 30,000 feet. Some years ago, in 1862, Glaisher and Cox well made a memorable ascent over Wolverhampton, when they became unconscious at 29,000 feet, after which they were supposed to have ascended for a short time, to nearly 36,000 feet, but in Dr. Berson's case, by inhaling oxygen he was able to observe his instruments and carefully note the conditions around him. 1 6 THE STORY OF THE EARTH'S ATMOSPHERE. His thermometer went down to 54 degrees below zero Fahr., while the mercury in his bar- ometer sank from 30 to 9 inches. Six miles is probably the limit to which man will ever care to ascend into the atmosphere, since above this height he can only survive by the aid of artificial assistance. For permanent habitation it is found to be prejudicial to live at greater heights than 15,000 feet, so that it is only within a thin slice of our atmospheric blanket that human life is lived. Actually, the marvellous complexity of human thought and action, and the development of modern civilisation on this earth, has taken place, and will probably always remain confined within the vertical distance of a London shilling cab fare above the surface. Apart from direct measurement, the pressure of the atmosphere gives us some clue to its height as well as to its weight. From the pressure obser- vations alone, it ought to disappear somewhere about 38 miles, since at that height the mercury column of the barometer, which measures the weight of air above, would tend to disappear. Observations of meteorites, however, whose ap- pearance depends upon their heating to incan- descence by friction against a resisting medium, shew that some air exists at 100 miles, though at such great altitudes it is probably in a con- dition of extreme rarity. Observations of the duration of twilight, which is due to reflection from particles of dust and air, gave about 50 miles as the limit. Practically, therefore, we may take 50 miles to be about the limit up to which the atmosphere exists in a coherent form as we know it near the earth's surface. NATURE AND COMPOSITION OF ATMOSI'HKRK. 17 CHAPTER II. THE NATURE AND COMPOSITION OF THE ATMOSPHERE. To one of those superior beings who, we be- lieve, inhabit the celestial regions, it must have been infinitely pathetic to see the poor human mites on this planet struggling for centuries through the mist of error and superstition, until they finally discovered one day the composition of the atmosphere in which they lived. By the Greeks FIG. 3. Cirro-cumulus. the air was considered to be one of the four ele- ments, and it was not until the middle of the last century that Priestley discovered that air was a 1 8 THE STORY OF THE EARTH'S ATMOSPHERE. mixture of oxygen and nitrogen, and that its neutral character was due to the blending of a most active element, oxygen, with a most inactive element, nitrogen. A slight difference in the proportion of either element would be fatal to life as we know it. With more oxygen in the air our lives, short enough as they are, would be still more brief, and though we might be more witty and brilliant, we should live in a state of such mental and physical intoxication that we should never be able to sit down quietly to do any solid work. In fact, the human race would be converted into a number of thoughtless, reckless, frivolous beings, who would probably end by destroying each other in a frenzy of over-excitement. On the other hand, too much nitrogen would reduce us to such a degree of dulness and inertia that our supposed national characteristics would be intensified and we should become like a row of statues or mummies, with- out action or passion, lifeless in fact, matter without motion. The existing proportion there- fore is decidedly adapted to our present require- ments. The average proportion in which the two principal components of the atmosphere are found to occur is 21 of oxygen to 79 of nitrogen by vol- ume, and 23 of oxygen to 77 of nitrogen by weight. The proportion in which the remaining con- stituents enter is so small that it may be practi- cally neglected when we consider the physical properties of the atmosphere, though it cannot be neglected when we regard its vital and chemi- cal functions. The other constituents are car- bonic acid, which occupies 7-5^0 -jj-ths by volume, traces of ammonia, ozone, and the recently dis- covered argon. NATURE AND COMPOSITION OF ATMOSPHERE. 19 Oxygen, which forms one-fifth of the atmos- phere, represents the active vitalising principle, a large proportion of which, by its former chemical union with certain terrestrial elements, such as silicon and aluminium, has solidified into large rock masses, by union with hydrogen, has pro- duced the liquid ocean, and the gaseous vapour of the atmosphere, and which, by its chemical union with carbon through the tissues of plants and animals, develops the energy which is mani- fested in their life and movements. Owing to the fact that the density of oxygen is very nearly the same as that of nitrogen, and to the constant mixture which takes place, the proportions in which they are found at high ele- vations differ but little from those at sea-level. Thus in a balloon ascent at Kew, the percent- age of oxygen present at a height of 18,630 feet was found to be 20.88, while it was 20.92 at the surface. Here it varies chiefly according to the lack of ventilation and the number of people who inhabit confined spaces. In the pit of a theatre the percentage is 20.7, in a law court 20.6, and in the gallery of a theatre about 20.5. So far as its chemical properties are con- cerned, therefore, the atmosphere at great heights is just as suitable for man as it is at sea-level. The only practical drawbacks arise from its greater rarity and cold, as we ascend from the surface. The Nitrogen, which forms three-fifths of the atmosphere, represents the inert, negative ele- ment which, though not actively hostile to life, by diluting the oxygen, lessens the activity and rapidity of the energy developed by the latter's combustion, and thus tends to prolong life, which 20 THE STORY OF THE EARTH'S ATMOSPHERE. would be used up too rapidly in pure oxygen. It would not be easy, in fact, to find any other dilu- ent of oxygen which could take the place of nitrogen without producing poisonous effects like those of carbonic acid. Regarded from a physical point of view, nitro- gen, being slightly less dense than oxygen in the proportion of 97 to no, renders the air a better vehicle for sound, support, and power than it would be otherwise. Nitrogen is also absorbed from the atmos- phere by plants, through the agency of those marvellous little bacilli parasites, the Nitragin, which have recently been shewn by Prof. Dobbe to nourish certain plants by abstracting the nitro- gen from the air and passing it into the substance of the plants. Each plant, moreover, appears to be fed by its own special bacillus, but starved by that of any other plant. The carbonic acid only forms a very small percentage of the air, but nevertheless plays an important part in the operations of nature. Animals consume oxygen and exhale carbonic acid as a product of their respiration. Plants, on the other hand, under the action of light on their green cells decompose the carbonic acid, absorb the carbon, and liberate the oxygen. By these means the balance between supply and consump- tion is about maintained. In former periods of the earth's history the amount of carbonic acid in the atmosphere was probably much greater than at present. Espe- cially during the carboniferous epoch of geology, when owing to special climatic conditions enor- mous quantities of trees and ferns grew which abstracted the carbon from the then existing at- NATURE AND COMPOSITION OF ATMOSPHERE. 21 mosphere, and by burying it for centuries in the solid form of coal all over the world materially re- duced the subsequent proportion of carbonic acid from what had previously existed. Though .03 per cent., the amount existing at present seems a small quantity, it is yet as we know, enough to supply all the vegetable world with its solid carbon. Huxley once calculated the amount of this gas which is contained in a section of the atmosphere resting on a square mile to be as much as 13,800 tons, while the amount of solid carbon which could be extracted from such a quantity of the gas would be about 3700 tons, enough to supply a small for- est of trees weighing 7400 tons. Ozone, of which traces exist in the atmosphere, is a peculiar form of oxygen, a molecule of which is composed of two atoms linked together, and a third which, on the principle of two is company and three is none, is inclined to walk off whenever it meets with a suitable companion. Fortunately for man the tastes of this third atom are distinctly low, since it has a partiality for sewers and places where matter is decomposing and which by its active oxidising power it renders neutral and harmless. Since towns usually contain more of such deleterious conditions than the country, more ozone is found on their windward than on their leeward sides. Ozone prevails most in the spring months and least in the autumn, and while it probably acts beneficially as a rule, by its active oxidation of poisonous gases, its excess is associated with the prevalence of certain forms of catarrhal disease. Traces of ammonia occur which help to supply nitrogen to the soil and plants when washed down 22 THE STORY OF THE EARTH'S ATMOSPHERE. by rain. Every year about 30 Ibs. of ammonia are carried down to each acre of ground. The above constituents are blended together like different brands of spirit, but are free to enter into com- bination with other substances. This freedom of contract is implied in the term mechanical union, which is employed to distinguish the mixture of oxygen and nitrogen forming atmospheric air from that of the chemical union between oxygen and hydrogen in the compound water. The vapour of water which as an invisible gas is generally more or less associated with dry air may be looked upon as a separate atmosphere of gaseous water. The fact, however, that it is im- possible to distinguish it from dry air by sight or smell, and that until it condenses out of the latter as rain or cloud it virtually forms one of its com- ponents, makes it desirable for us to regard it in this light, if we are careful to remember that its quantity (generally about i per cent, by weight) is ever varying, and that the volume of dry air it displaces and occupies itself, depends on the tem- perature as well as the mass of it present. When it occurs as an invisible gas it is -f ths as dense as dry air at the same temperature and pressure. The peculiarity of the position of aqueous vapour is, that if it existed alone on the earth, there would be only one temperature at which it would change from a gas into a liquid, and therefore only one level at which cloud would form and whence rain would descend altering with the time of day and season. Since, however, it exists in combination with air, it spreads upwards until it arrives at the par- ticular temperature at which the air fails to sup- port it in solution, when a layer of cloud forms NATURE AND COMPOSITION OF ATMOSPHERE. 23 and perhaps rain falls. After this an interval oc- curs in which the vapour is at first in defect, but as \ve ascend, its relative amount to that which is capable of being sustained increases until another level and temperature is reached at which con- densation takes place, and a second stratum of cloud is formed and so on. Ultimately a point is reached at which the vapour-sphere nearly van- ishes, but this must be very high, for although it is found that at a height of 23,000 feet in the Himalaya the amount of vapour in the air is only one-tenth of that which exists at sea-level, while at 46,000 feet it would only be one hundredth, cirrus clouds have occasionally been seen above the latter level. Dust is another constituent which plays an important role. Mr. John Aitken of Glasgow has made this question the subject of special investi- gation, and has found that the atmosphere, espe- cially in its lower parts over land, contains thou- sands of particles of the finest dust. Over the sea and in its loftier regions these particles are much lessnumerous. Hehas also found that the presence of this dust is necessary to the formation of rain. A recent series of observations by Mr. E. U. Fridlander, taken with Aitken's pocket dust coun- ter in various parts of the world, embracing the Atlantic and Pacific Oceans, New Zealand, Cali- fornia, the Indian Ocean, and Switzerland, shewed that these tiny dust particles are found in the lower atmospheric strata right out in the middle of the Pacific Ocean as well as on land, and espe- cially in towns. They are, however, less numerous at sea, especially in the Pacific and Indian Oceans. Thus comparing all three oceans we have at sea- level. 24 THE STORY OF THE EARTH'S ATMOSPHERE. Number of dust particles per cubic centimetre.* Atlantic Ocean . . . 2053 Pacific " ... 613 Indian " . . . 512 As low a value as 210 was found in the Indian Ocean after rain. On the other hand, over land areas the number frequently rises to 3000 or 4000 per cc. In large cities such as Edinburgh, Paris, and London, where the products of animal and fuel combustion enter the atmosphere in large quantities, the lower atmosphere is so pol- luted that in some cases as much as 150,000 dust particles in a single cubic centimetre have been counted. As we rise above the surface the number of dust particles is found to diminish pretty regu- larly with the ascent. From observations on the Bieshorn, Fridlander found the number gradually diminish in the following ratio. Height above Number of sea-level. particles per cc. 6,700 feet . . . 950 8,200 . . . 480 8,400 . 513 10,665 . . 406 II.OOO . . . 257 13,200 . . . 219 I3,600 . . 157 The general rule fur the diminution in the number of dust particles may be simply expressed thus: For every rise of 3000 feet the amount is iths of what it was at the lower level. The bear- ing of this fact on the question of the beneficial influence of high mountain resorts on pulmonary and other diseases is obvious. * About 15 cubic centimetres are equal to I cubic inch. PRESSURE AND WEIGHT OF ATMOSPHERE. 25 These same minute dust particles, by their scattering action on the small waves of light at the violet end of the spectrum, have been shewn by Lord Rayleigh to be the cause of blue sky, while its gradual deepening into black as we ascend is readily seen to be the result of their gradual diminution in number. CHAPTER III. THE PRESSURE AND WEIGHT OF THE ATMOSPHERE. ONE of the first facts which is brought to our notice in these days when those physical laws, which the ancient philosophers discovered to- wards the end of their lives, are taught us from childhood, is that the air has weight and exerts pressure. The story of the discovery of the bar- ometer or weight measurer is a romantic chapter in the history of science. About 1643, some Florentine gardeners found that they were unable to pump up water higher than thirty-three feet. Up to that time it was an accepted dogma that " Nature abhorred a vacuum," and this apparent lapse on the part of Nature was looked upon as inexplicable. When Galileo was informed of it, soured as he was with a world which had rejected some of his greatest discov- eries, he cynically remarked that Nature evi- dently abhorred a vacuum uf> to thirty-three feet. His pupil, Torricelli, however, was not content with this perfunctory explanation, and applying 26 THE STORY OF THE EARTH'S ATMOSPHERE. his genius to the question, conjectured that the column of thirty-three feet of water exactly bal- anced a similar column of air stretching to the limits of the atmosphere. Remembering that mercury was about thirteen times as heavy as water, he inferred that if this were true, a mer- cury pump would only raise mercury to a height of about 30 inches. He thereupon filled a long glass tube with mercury, and having stopped up one end, placed his thumb over the open end and inverted it over a basin of the liquid metal. The result proved his anticipations to have been well founded, since the mercury fell in the tube until it exactly reached this height of 30 inches, leav- ing what is known as the Torricellian vacuum in the upper part of the tube. This is substantially the mercurial barometer by which to-day we measure what we term atmos- pheric pressure. The reason the term pressure is employed and not weight is because air, in common with all fluids, not merely presses downwards, but equally in all other directions. This is readily shewn by the familiar experi- ment of placing a bit of paper over the mouth of a bottle full of water, and inverting it, when the water will be retained by the upward pressure of the air on the surface of the paper. When we want to measure the weight of air, we must remember that, since air is elastic, it is more compressed, and therefore weighs heavier near the surface than up above. At sea-level, where the barometer frequently registers a height of 30 inches, we shall find that at 32 Fahr. the column of mercury 30 inches high resting on one square inch weighs 14.7 Ibs, PRESSURE AND WEIGHT OF ATMOSPHERE. 27 It is easy from this, knowing that mercury is 13.6 times as dense as water, and air only yuWd^h 5 as dense, to measure the weight of a cubic foot of pure dry air, which under these conditions will be about 565 grains (troy). On the top of a moun- tain 18,000 feet high it would only weigh half as much. The weight of a cubic foot of water va- pour under the same conditions would be only 352 grains. From this it will be understood that, when vapour is mixed with dry air, the resulting compound is lighter that is, damp air is lighter than dry air. The weight of the atmosphere on the earth cannot be ignored. A flood of water 33 feet high over the globe would represent the same weight, and would evi- dently exercise a very considerable pressure on the surface. Westminster Hall alone contains 75 tons of air, while the entire weight of air resting on the earth has been estimated by Sir John Herschel to amount to nf trillions of pounds. Sudden alterations of this pressure, which are in- dicated by the rise and fall of the barometer, un- doubtedly affect some persons of a sensitive tem- perament, while the steady fall of pressure which occurs when we ascend a mountain or rise in a balloon occasions what is termed mal de montagnc in both men and animals. On the other hand, the excessive pressure ex- perienced in diving-bells or caissons, or in the digging of tunnels, where the men work under a pressure of two or more atmospheres, is found to bring on a species of paralysis. To give a general idea of the decrease of pres- sure with the height when the barometer marks 30 inches at sea-level, we find the following rela- 28 THE STORY OF THE EARTH'S ATMOSPHERE. tive scale for air of an average temperature and dampness. Pressure. Altitude. 30 inches ...... o 29 28 27 26 25 24 23 22 21 2O 18 16 910 1,850 2,820 3,820 4,850 5,910 7,010 8,150 9-330 10,550 13,170 16,000 At 18,000 feet the pressure is about half that at sea-level. It will be observed that at the lower eleva- tions the height in feet corresponding to one inch in the barometer is less than at the higher. The atmosphere is in fact more tightly packed near the earth, so that while i inch of mercury represents the weight of the first 900 feet of ascent, i inch at 16,000 feet represents the weight of about 1500 feet, and the proportion increases at greater heights. Were the scale i inch of mercury to 910 feet of atmospheric air preserved all the way up, we should reach the limit of the atmosphere at about 26,220 feet, or 5 miles, which is the height of what is termed a homogeneous atmosphere. Comparing the atmosphere with the ocean, we find that the volume of the former, assuming it to reach to a height of 100 miles, is as 65 to i, while its mass bears to ^that of the latter the ratio of only i to 300. The pressure at the average depth of the PRKSSURE AND WEIGHT OF ATMOSPHERE. 29 ocean viz., two miles, is as much as 320 atmos- pheres. The barometric pressure undergoes changes, some of which are irregular, and due to the pas- sage of what are termed cyclones and anticy- clones, in which the air is moving round moving centres, while others, such as those which complete their period in a year, are connected with seasonal transfers of air between sea and land and from hemisphere to hemisphere. Others, again, which run through their course in a day, are connected with the daily heating and cooling of the air by the sun, while certain short and nearly regular instantaneous changes over large areas, such as the five-day pressure oscillations recently noticed by Eliot in India, are still mysteries that require explanation. The seasonal changes and the gen- eral distribution of pressure will be alluded to in future chapters, where they are considered with reference to dependent phenomena. The diurnal variation of barometric pressure which is dependent on the daily rise and fall of sun-heat is largest, as we should expect, in the tropics, amounting to a range of as much as twelve hundredths of an inch at Calcutta, and diminishing thence as we travel polewards, until at Greenwich it is only about .02 inch, or one- sixth of its tropical value. Nearer the poles it vanishes altogether. Between the tropics, the irregular changes of pressure introduced by the passage of storms are so small and infrequent that the diurnal variation is noticeable above all other changes, and is so regular that the late Mr. Broun, of Trevandrum Observatory in India, used to declare he could tell the time of day by simply noting the height of the barometer. The rise and 30 THE STORY OF THE EARTH'S ATMOSPHERE. fall of the mercury column is a double one, reach- ing its greatest height at 10 A. M. and 10 p. M., and its least height at 4 A. M. and 4 P.M. The causes are not yet thoroughly worked out, since, al- though it undoubtedly depends on the action of the sun, the total effect is made up of a combina- tion of direct and indirect motions of the air. In temperate regions the diurnal change of pressure is so small that it is almost lost sight of in those much larger pressure changes introduced by the passage of cyclones, which frequently amount to i or 2 inches' rise and fall of the mercury. Barometric charts in which isobars, or lines of equal barometric pressure, are drawn over the representation of different parts of the earth, will be referred to in chap. V. These charts are simi- lar to those employed in weather bureaux in or- der to forecast the probable weather for the ensu- ing twenty-four hours. One practical use of the barometer is to deter- mine the altitude of a place above sea-level. The science of measuring heights by this means is termed hypsometry (from the Greek, hypsos, height, metron, measure). We have already seen that the pressure descends in a certain proportion as we ascend in the atmosphere, and formulae have been determined by which the height may be calculated under certain conditions of tempera- ture, humidity, etc. For rough and ready pur- poses, however, the following rule gives a very fair approximation : " 77/6' difference of level in feet between two alti- tudes is equal to the difference of the barometric pres- sures observed at cacJi in inches divided by their sum and multiplied by the number j5,"6j, when the aver- age of the temperatures at the two places is 60 J*\" THE TEMPERATURE OF THE ATMOSPHERE. 31 When the average temperature of the two sta- tions is above 60 the multiplier must be increased by 117 for every degree the average is above this temperature, and decreased in like manner for every degree it is below 60. Thus, if the values at the lower station are 30.15 inches pressure and 65 temperature, and those at the upper station are 28.67 inches and 59, a little household arith- metic will shew that the difference of their heights is 1409 feet. CHAPTER IV. THE TEMPERATURE OF THE ATMOSPHERE. THE temperatuie of the atmosphere, whether we are aware of it or not, is a condition in which we are more directly interested than any other. The most common form of salutation in the street involves a dictum or a query as to " how cold it is to-day," " much warmer than yesterday," " I do hope we are going to have some really warm weather now," or "some skating," as the case may be. In all this the temperature of the air is concerned, since it is the medium in contact with us, and from which, chiefly by conduction, we de- rive our sensation of heat or cold. When we talk of temperature we must take care to know what we mean by the term. Heat, as we know, is a " mode of motion," as Tyndall used to call it, a vibration of the small molecules of a body, and directly this mode of motion is communicated to it, by what is termed radiation, it tends to return the compliment to other bodies in its neighbour- hood, and set all their molecules in a similar state 32 THE STORY OF THE EARTH'S ATMOSPHERE. of oscillation. The process, however, is an ex- change all round, and the temperature of any body measures the rate at which it loses heat to or gains heat from surrounding bodies. This rate depends upon its capacity for heat, and its power of ab- sorbing and radiating heat rays, all of which vary in different bodies. In the case of the atmosphere, the radiating power exceeds the absorbing power for rays com- ing from the sun, but is considerably less for the heat radiated back again from the earth. So that, on the whole, the absorption power of the lower air for all kinds of rays is about 2 T 3 7 as great as its radiation power. It is this property of the atmosphere which al- lows us to keep decently warm. Otherwise, were we bereft of this valuable covering or envelope we should shiver in a temperature of 138 degrees below zero Fahrenheit, which is probably the mean temperature of the moon's surface. The only advantage that could be claimed for such a temperature is, that it would be 332 degrees higher than what would probably ensue in the event of the sun becoming cold. The temperature of the atmosphere is derived chiefly from the solar radiation which is arrested by the earth, and partly reflected, partly radiated back through the atmosphere towards space. Temperature is a result of radiation. Consequently before we speak of the tempera- ture it is necessary to see how radiation affects the atmosphere, since the conditions which regu- late radiation, affect the temperature of the at- mosphere in a somewhat similar manner. When the sun's radiations have reached the earth's surface from which the lowest stratum of THE TEMPERATURE OF THE ATMOSPHERE. 33 the atmosphere chiefly derives its temperature, their heating effect on a given area is modified by two circumstances, (i) their angle of incidence or the angle the direction of the sun makes with the horizon, and (2) the thickness of atmosphere they have traversed. When a certain width of the sun's rays is con- sidered it will be found to cover a smaller area in proportion as they fall more vertically or less in- clined. Thus in the accompanying diagram the Sun vertical ( at noon Equinox on Equator). same width of rays is concentrated upon A B in the one case, and spread over A C in the other, consequently the heat received by the earth is greatest when the sun is highest above the hori- zon, and shines most directly upon the ground. During a single day the heat received on the ground is greater at noon than at any other hour (about four times as great as at 10 A. M. or 2 p. M.). It is also greater in the summer when the sun is permanently at a higher angle all through the day after it has risen, than it is in the winter. These both operate together at any place on the earth. When we change our latitude we can, by travel- 34 THE STORY OF THE EARTH'S ATMOSPHERE. ling towards or from the equator at the rate of about 18 miles per day, obviate the seasonal change in the angle of the sun above the horizon and secure the same general amount of sun radia- tion. We should not, however, be able to secure the same average temperature since the direct ef- fects of the radiation on temperature are modified by what goes on over entire hemispheres. More- over the effect of changing our latitude introduces another consideration which has a potent influence upon the amount of heat falling in the 24 hours viz., the time during which the sun remains above the horizon. This time increases as we travel polewards in the hemisphere which is en- joying summer. There are thus two influences which work in opposite directions, one, the gen- eral angle of the sun above the horizon, which diminishes as we leave the equator, and the other, the length of the day, which increases under the same conditions. The conjoint effect must there- fore generally reach its maximum value at some intermediate latitude. As a matter of fact, this important problem has been worked out by several physicists, in- cluding Lambert, Poisson, and Meech. The last- named finds that on the average of the year, as we should expect, more heat falls on the equator than elsewhere. If we take the six months of the northern summer, more heat falls on latitude 25 degrees north (the latitude of northern India) than on the equator. If again we take the three months nearest midsummer, i.e. from May yth to August yth, the zone of greatest heat reception lies in 41 N., while from May jist to July i6th, more heat falls on the North Pole than on any other part of the earth. The temperature of the THE TEMPERATURE OF THE ATMOSPHERE. 35 Pole does not of course at once respond to this heating, since the average temperature effect lags about one month behind the solar radiation, and near the Pole the heat is mainly employed in melt- ing the Arctic ice floes, and in raising the temp- erature of the water. At the same time this beneficial arrangement obviously prevents the temperature there from becoming as low as it otherwise would. In addition to this, the amount of heat which is transmitted through the atmosphere so as to reach the surface at all, varies with the angle of FIG. 5. the sun for a different cause viz., the different thickness of the atmosphere traversed in each case. This is plain from the adjoining figure in which as the sun's rays fall vertically or inclined, we have the thicknesses A. P., 13. P., and C.P. This last circumstance exaggerates the differ- ence caused by the hourly and seasonal changes in the angle of the sun, especially as it approaches the horizon. 36 THE STORY OF THE EARTH'S ATMOSPHERE. Direct observations of the sun-heat by means of an instrument termed an actinometer, which has been employed with great success by Prof. S. P. Langley at Washington, have shown that of the heat which falls vertically on the upper sur- face of the atmosphere, 25 per cent is absorbed (Langley says 40 per cent, but this seems doubt- ful from other considerations) before it penetrates to the earth. When the rays are inclined, instead of 75 per cent being transmitted, only 64 per cent arrives at an angle of 50 degrees, and only 16 per cent at an angle of 10 degrees. The light varies in the same way. At sunrise and sunset the sun has only -j-^j-gth part of the brilliancy it possesses when vertical overhead. When we come to consider the actual quantity of heat that is received from the sun, we shall see how utterly it transcends all our means for deriv- ing warmth from (so-called) artificial sources. The intensity of solar heat may be measured by the temperature to which it would raise a certain quantity of water. If we suppose the rays which fall vertically on an area one square foot at the outside of the atmosphere, before any absorption has taken place to be applied to warming up 10 Ibs. of water, they would raise it i degree on a Fahrenheit thermometer in i minute. By the time these rays have reached the earth, as we have seen, about th of the original radiation has been absorbed or scattered by the atmosphere, and therefore only about 7 Ibs. of water could be raised i degree per minute. This however gives us some faint idea of the enormous quantity of heat which is continually falling on either the earth or the clouds. If we take the heat which falls on a square mile of the earth's surface per THE TEMPERATURE OF THE ATMOSPHERE. 37 minute, we shall find that it would be enough to raise 560 tons of water from the freezing to the boiling point. In a year, assuming that the sun's heat con- tinually penetrated to the ground, this heat would suffice to melt a layer of ice about 178 feet thick over the whole earth, or not much below the monument in London. The general effect has been popularly put by one writer in the following graphic manner, in which the different amount of heat received when the sun is inclined at different angles is properly considered. " Suppose the earth one vast stable covered with horses, and suppose that as the sun's angle varied according to season and latitude, the horses arranged themselves so that no horse's shadow fell upon or underneath his neighbour; then the solar heat falling upon the earth converted into horse power, would be always represented by all these horses working continuously at their utmost strength." Some of this heat energy is, no doubt, con- verted into mechanical energy in the winds, rivers, and rainfall, but a vast proportion of it is wasted so far as man is concerned, and it is plain, as both Lord Kelvin and Edison have recently pointed out, that we have still an immense source of power comparatively untouched, which can be drawn upon when our coal supply shows symptoms of giving out. One effect has not been alluded to vix., the change in the distance between the earth and the sun, which are nearer to one another in December than in July. Theoretically the effect would in any case be small. Practically it is counteracted 38 THE STORY OF THE EARTH'S ATMOSPHERE. by the large mass of water in the southern hemi- sphere, which responds more slowly to an increase of heat than the northern land, so that on latitude 20 degrees S., where it reaches its greatest effect, it only adds ^th to what would occur if the dis- tance were invariable. Since the temperature of the atmosphere re- sults from the accumulation of altered solar rays, in surrounding objects which radiate them to one another, instead of passing them back at once into space, the temperature epochs will always follow those of direct radiation. Thus the highest temperature of the day does not occur at noon, but an hour or two afterwards. Similarly the highest temperature of the year occurs on an average a month after midsummer day, and a like retardation occurs for the lowest temperatures. At the Pole, where one long day and night occurs in the year, the coldest month is delayed to Feb- ruary or March, in the northern hemisphere. When the sun's rays fall upon water, or where the locality is naturally moist, the heat is conducted through the top layer, and in any case takes longer to raise its temperature. Where, as always occurs, part of the water is evaporated, nearly 1000 times as much heat is needed to convert it into vapour as will raise its temperature i degree Fahr. Con- sequently, not only does the temperature of the air over oceans rise and fall less daily and sea- sonally than that over the continents, but the highest temperature of the year in maritime regions lags about 42 days behind midsummer day, while in the centre of the large continents, the lag is reduced to 25 days. This slowness to rise and fall in temperature on the part of large masses of water, accounts for THE TEMPERATURE OF THE ATMOSPHERE. 39 the equable temperature of the atmosphere of islands and coasts, compared with interiors of continents, and exerts an important influence in determining the changes in the general wind and weather system over oceans and continents in summer and winter. The measurement of atmospheric temperature dates back like that of the telescope to Galileo, who in 1597 devised the first liquid thermometer. This consisted of a glass bulb, containing air, terminating below in a long glass tube, which dipped into a vessel containing a coloured fluid. The variation of the volume of the enclosed air, caused the fluid to rise and fall in the tube to which an arbitrary scale was attached. Galileo further invented the alcohol thermometer in 161 1, which was adopted generally by the Florentines of that time. The determination of the zero and some fixed point above it, by which to graduate the scale, appears to have taken years to evolve. Newton suggested a scale in which the freezing point of water was o degrees, and the blood of a healthy man 12 degrees, and subsequently Fahrenheit, to whose scale with characteristic conservatism we still adhere in this country, in spite of the uni- versal use of that of Celsius on the continent and in physical investigation, in 1714 took blood heat and that of a freezing mixture of ice and salt as his fixed points. Since then the freezing and boil- ing points (jf water have been taken as the fixed points on the thermometric scale. Unlike the early Florentine thermometers, the modern alcohol and mercury thermometers con- sist of a bulb and tube, partially filled with the liquid, above which is a tolerably complete vacu- 4 40 THE STORY OF THE EARTH'S ATMOSPHERE. urn, allowing the liquid to move with perfect freedom up and down the tube. For measuring rapid variations in the temper- ature of the atmosphere, it is necessary to have the bulb small, since where the bulb is large, the effect of an exposure to heat is considerably delayed. Consequently, for determining the true shade temperature of the atmosphere at any moment where it is difficult to obtain proper shade conditions, a small sling thermometer is by far the most accurate. By tying any ther- mometer to a string, and whirling it round un- til the reading does not alter, a very fair notion of the true temperature of the air can be ob- tained. For standard purposes where momentary changes are not so important, thermometers with large bulbs are preferred, since by this plan the variations due to the expansibility of the glass bear a small ratio to the volume of mercury. We have now-a-days advanced so rapidly in our methods of investigation, that instead of be- ing content with two or three readings a day, we require to know the continuous changes in the temperature of the atmosphere in places where it is impossible to make eye observations. For this purpose the self-recording thermo- graph is employed, and by the use of such an in- strument the temperature of the atmosphere can be registered on the top of mountain peaks only occasionally accessible, and in the free atmosphere by the elevating power secured by kites and cap- tive or free balloons. When we examine the observed facts as they present themselves, we find in the first place a constant diminution of the temperature of the THE TEMPERATURE OF THE ATMOSPHERE. 41 atmosphere as we ascend from the earth's sur- face. This decrease of temperature with ascent varies somewhat in different latitudes, and is not the same in the free atmosphere as on mountain sides. From Glaisher's balloon ascents the rate be- gins quite near the surface at about 7 degrees in every 140 feet, and finally diminishes to i de- gree in every 400 feet at 10,000 feet, the entire diminution of temperature from sea-level up to 10,000 feet being 34 degrees, or i degree in 300 feet. If therefore we take the temperature of London to be about 50 degrees on the mean of the year, a temperature of freezing point or in other words the snow line would be reached at an elevation of about 4500 feet or a little above Ben Nevis in the free atmosphere (the mean tempera- ture at the top of Ben Nevis is about 30^ degrees F.). No higher mountains exist, consequently there are no perpetual snows in the British islands. In India the initial rate of decrease of tem- perature is very much more rapid, amounting to i degree in the first 33 feet, but it slackens down to about i degree in 330 feet at 15,000 feet. On the average it is about i degree in 270 feet in sta- tions away from the Himalaya where the moun- tain range appears to reduce the rate. If we take the region of the North West Himalaya we shall find that the mean tempera- ture of London would be reached at a height of 9600 feet, and the range of temperature through- out the year would not differ very much from that of England. Most of the Himalayan sanitaria lie between 6000 and 7000 feet where the temperature is 42 THE STORY OF THE EARTH'S ATMOSPHERE. about 60 degrees, and possess climates similar to those of the Riviera and the coast from Mar- seilles to Genoa. When therefore we wish to vary our climate as far as temperature is concerned, we can do so without changing our latitude by remembering that the temperature cools on an average about i degree for every 300 feet we ascend, or warms at the same rate as we descend the same distance. Since the mean temperature at the north pole is about o degree F. and at the equator between 80 and 90 degrees F., we can similarly alter our tem- perature i degree F. by travelling north or south about 70 to 80 English miles. As an illustration of a combination of these facts we can imagine a series of planes rising upwards from different points of the earth towards the equator along which the temperature would range on either side of a certain average throughout the year. These would rise to their highest level over the equator, and their height in any latitude would show us at what elevation we should experience some par- ticular temperature all the year round. The vertical scale above sea-level is of course immensely exaggerated. It will be seen that at an elevation of 27,000 feet over the equator, the temperature is about o degree F., and that the snow line or line of freez- ing point cuts the surface at sea-level about lati- tude 69 North. In the Himalaya this line throughout the year is about 15,400 feet above the sea, or about 17,850 feet in the summer months. Even this great alti- tude would still leave about 11,000 feet of the higher summits mantled with perpetual snow dur- ing the summer. THE TEMPERATURE OF THE ATMOSPHERE. 43 There is perhaps no point about which so much perplexity is generally felt and expressed as the reason for this decrease of temperature as we ascend. It is often popularly expressed as be- FEET 27000 18000 9000 Cent re of Eartft , V London^ Lot 69 l tf.Pole. FIG. 6. ing due to the greater rarity of the air above, but this simply leaves the matter as obscure as before. Like most other facts regarding the atmosphere, it results from the operation of a definite physical law. It is well known that the rapidity of the cool- ing of a body depends on the perfection of its en- closure, whether solid or gaseous. At the earth's surface the enclosure is nearly perfect, but as we ascend, the upper side of the enclosure weakens owing to the thinning of the air, until at the top of the atmosphere the enclosure is only half what 44 THE STORY OF THE EARTH'S ATMOSPHERE. it was at the earth's surface. The heat radiated from the earth is moreover intercepted to a large extent at the higher levels by the intervening lower air. Consequently on both accounts the temperature of the air remains cooler in propor- tion to the altitude. The distribution of the temperature of the atmosphere in a horizontal direction as ordinarily measured has reference merely to the tempera- ture of the lowest stratum. Unlike the barome- ter which gives us the sum total of the pressures of the superincumbent layers, a thermometer near sea-level simply gives us the temperature of the particular stratum in which it lies. The magni- tude of the daily and seasonal changes vary ac- cording as the locality is continental or maritime, and its soil is dry and rocky or damp and alluvial, and the average itself depends largely on these and other conditions besides mere latitude. The general distribution however shews de- cidedly that latitude is one of the principal causes which affect the mean annual tempera- ture. The map (fig. 7) shews the distribution of the heat in the lowest atmospheric stratum over the earth's surface on the average of the year, by lines of equal average temperature (isother- mals). The principal points to notice are the widening out of the area between the isothermals of 80 over the land areas and the contraction that takes place over the seas, particularly the Atlantic. Also that wherever a marked dip of the line, particularly that of 70, occurs toward the pole over the land, an equally marked dip of the line occurs in the opposite direction close alongside. This is specially visible in California, Peru, THE TEMPERATURE OF THE ATMOSPHERE. 45 46 THE STORY OF THE EARTH'S ATMOSPHERE. and in S. W. Africa, and is plainly due to the known existence of cold marine currents flowing along these several coasts towards the Equator. So far as the British Islands are concerned it is equally plain that the northward flow of the gulf-stream of the Atlantic raises the isotherm of 50 F. which normally belongs to latitude 40 and passes through Nippon (Japan) ten degrees further north so as to make it pass through Lon- don. We thus get in these islands an atmosphere artificially heated up about 10 degrees (the iso- thermal of 40 F. properly belongs to our lati- tude) more than we are entitled to by our latitude. In like manner Peru no doubt enjoys several degrees less heat than it would otherwise have owing to the cool Antarctic stream (Humboldt's current) which flows up its coast and cools the lower atmosphere. The area of greatest heat is where the largest land masses lie near the Equator, and of greatest cold where the largest land masses, such as N. Asia and N. America, lie nearest the Pole. The reason for this is too important to be omitted. If the same amount of sun heat falls upon an equal area of land and water it raises the temperature of the former four or jive times as much as that of the latter. Less heat energy is spent in agitating the molecules of dry earth than those of water. Consequently its effects are more patent. In the case of water the heat energy is not lost, no en- ergy ever is in this Universe- but it is more latent and the expressed temperature is less. The at- mosphere is more readily heated by the radiation from the hotter (as we say) earth than the cooler THE TEMPERATURE OF THE ATMOSPHERE. 47 water. Consequently the lowest stratum over the land areas under the more direct sun near the Equator exhibits a generally higher temperature than that which lies over oceans in the same latitude. Since heating and cooling are recipro- cal operations it is easy to see that the reverse applies to the temperature over polar seas and continents. The migration of the sun north and south of the equator by reason of the inclination of the iiN 70 60 50 40 3 20 _ iQ 1 3_?0_3 40 50 60 s F, \ F. S\ ^ ^T"""! ^ ^ / S^- / / // 'Mean 0} ' Ye ir } or C lobe N, y / N / / ^4 / i i F < 1 "IG. 8. Distribution of atmospheric temperature in latitude, for Jan- uary, July, and the year. earth's axis to the plane i vhich the centres of the si uul planets lie, causes a sim ar migration in the area ( / <*> V / 10 ' greatest heat north and south of the geographi- cal equator. While the sun shifts from 23!- N. to -,>^ S., however, the central line of greatest heat (or heat-equator) migrates to a less amount, par- ticularly over the oceans. On the Pacific it moves only 15 to 20 in latitude. On the Atlantic still less. On the land it shifts as much as 43 in Africa and 50 in America, while in India it runs 48 THE STORY OF THE EARTH'S ATMOSPHERE. up to the deserts of Persia in latitude 33 N. in the summer, and down to only 10 S. in the win- ter, because there are no southern lands to at- tract it further. The general distribution in latitude and mi- gration of the temperature may be best seen in the accompanying diagram (fig. 8) plotted out from the means given by the late Mr. Ferrel of the American Weather Department. The position of the thermal equator to the north of the line and the greater annual range of temperature in the northern hemisphere are plainly visible. If we were to undertake a balloon voyage round the world at an average altitude of about 5000 feet we should find very few signs of this pe- culiar distribution which prevails near the surface. The temperature over the equator would be about 64 F., an agreeable summer temperature in England, and though if we preserved the same elevation, the temperature would descend to about 34 over London, the seasonal and daily changes would be very much less conspicuous than near the surface. By means of thermometers and thermographs the temperature of the atmosphere near the surface is read at certain hours or recorded con- tinuously, and for various purposes particular attention is paid to its maximum, minimum, and average, either for a day of twenty-four hours, a month of 30 days, a year, or a series of years. Where it is difficult to have continuous hourly readings taken, three hours are chosen, which, when combined in a simple manner, give a value which is found by experience to closely approxi- mate to the average of the day. THE TEMPERATURE OF THE ATMOSPHERE. 49 Thus, the average of a single reading at 9 A. M. gives a very close approximation to the mean of the twenty-four hours. Or we may add the read- ings at six, fourteen, and twenty-two hours and divide by three, or take the lowest and highest readings and divide by two. Where the self-re- cording thermograph is employed, the mean can be found by measuring the area traced out by the recording pencil and bisecting it. The maximum and minimum in this case correspond to the highest and lowest points of the curve traced out, but usually they are meas- ured by separate maximum and minimum ther- mometers. Apart from the general distribution of its mean annual values shown in Buchan's isothermal chart, the temperature of the lowest air stratum and proportionally of those lying above it, is subject to regularly recurring daily and seasonal oscillations. These two series of changes are so important in their relation to our general comfort and welfare that it is of the highest in- terest for us to know whether they exhibit any signs of progressive change in obedience to law as we vary our position on the earth. As a general rule we find the greatest ranges of the temperature of the lowest atmospheric stratum between day and night occur in the driest parts of the earth, in the interior of continents, such as the Sahara, Arabia, (iobi desert, Rajputana, Colo- rado, etc., where it often amounts to 40 I'"., and the smallest ranges in small oceanic islands, such as Honolulu, Kerguelen, Madeira, Bermuda, where it is as small as 5 F. In India, which presents the greatest contrasts of dry interior and moist coast in the world, we 50 THE STORY OF THE EARTH'S ATMOSPHERE. have daily ranges of 30 to 40 in the Punjab, 20 to 30 in the Central Provinces, 16 at Calcutta, 8 at Bombay, and 6 at Galle in Ceylon. The daily range also decreases generally from the Equator to the poles when we take places at the same distance from the sea. Thus, while it is n degrees at Colombo, it sinks down to an average of only 3 degrees at Suchta Bay, in latitude 73 N. Even at St. Petersburg, surrounded by large continents, it is only 7. The reason for this is simple. The changes in the solar altitude between sunrise and sunset are manifestly more marked where the sun rises higher in the sky than where its path is at a small inclination to the horizon all day, while at the Poles, where it takes a year to rise and set once, the daily variation entirely vanishes. The diurnal range of the temperature of the air also diminishes with elevation above the sea level. Thus in the N. W. Himalaya, while the mean daily ranges at Mussourie and Ranikhet at 6000 feet above the sea are only 13 and 15 F., the ranges at Bareilly and Roorkee on the adjacent plains are 23 and 24 F. The reason is obvious if we remember that the heat received during the day is more absorbed by the denser air near sea- level than the rarer air on the mountains. Con- sequently, since the heat which falls on the moun- tain-top is more freely radiated back into space, the air over the mountains is less expanded than that over the adjacent plains. During the day since the air over the latter expands upwards about 13 feet for every degree F. the temperature of the entire mass up to 6000 rises. Meanwhile since the mountain cannot ex- pand the air over it remains sensibly stationary. THE TEMPERATURE OF THE ATMOSPHERE. 51 In consequence a downflow takes place towards the mountain somewhat like the sea-breexe towards a coast which brings with it the cooler tempera- ture in the free air at the same level and so cools that on the mountain. At night when the mountain which is a good radiator cools down rapidly and chills the air which lies on it, this air by reason of its increased density slides down the mountain side and its place being taken by the adjacent less cooled air, the temperature is again prevented from descend- ing too low. In valleys on the other hand even at high alti- tudes, the contrary conditions take place. By day, owing to the greater perfection of the atmospheric enclosure, the sun's heat is more effective in warming up the lower stratum of air, while at night the chilled air from the surround- ing mountain tops descends into the valley and increases the cold. Hence, at Leh in Thibet, which lies in a valley at 11,500 feet elevation the daily range is as high as 29 degrees. On a smaller scale it is practically recognised that frosts prevail more in valleys than on hill tops. The atmosphere and the ocean thus exert a similar tendency in reducing temperature ranges, and the man who builds his house on a hill and so rises into the atmosphere, enjoys similar ad- vantages to the one who takes up his residence on the sea-coast or an island. In both cases ex- tremes of temperature are avoided. The temperature is lowest as a rule on land shortly before sunrise. In tropical countries, such as India, where it occurs only just a few minutes before sunrise, it is often the only toler- able moment of the 24 hours. 52 THE STORY OF THE EARTH'S ATMOSPHERE. The highest temperature occurs nearly every- where on land between 2 and 3 P.M., but alters according to season. The greatest changes in the times at which the daily temperature variation reaches its high- est and lowest points are related to the position of the place as regards the ocean. Put briefly, the drier and more inland or con- tinentally the place is situated, the later will be the epochs, while out in the open ocean the mid- day maximum occurs soon after noon and the morning minimum as early as 4 A. M. The annual range of temperature, or in other words the difference between the average tem- peratures of the hottest and coldest months, in contrast to the diurnal range, increases from the equator, where it is least, to the poles. It also increases with the distance from the coast. Thus while it is only 3^- at Colombo, it is 11 at Bom- bay, 21 at Calcutta, and from 30 to 40 in N. W. India. The accompanying map, in which the lines of equal annual range of 5, 10, 20, 30, etc., are drawn, shews at a glance its general distribution over the earth, from which it is plain that while it is least over a broad belt surrounding the equator, it reaches its highest values in the poleward cen- tres of the continents. In England the range of temperature between summer and winter is about 20 degrees. In Honolulu it is only 5 degrees, as near the equator, while at Werkojansk in North Eastern Siberia it amounts to no less than 120 degrees. The man who boasts he can wear the same coat summer and winter through would have to change his habits in that district. There are several re- THE TEMPERATURE OF THE ATMOSPHERE. 53 markable features exhibited on this map. One is that in all the northern continents the position of the greatest annual temperature range is to the east of their geographical centres. This is chiefly owing to the influence of the warm cur- rents which bathe their western shores and the accompanying wind currents which carry the FIG. 9. moderating effect of the ocean over a large part of their western interiors. Western Europe is peculiarly favoured in this respect. Also the generally small range over the larger oceans which is due to the slow response of masses of water to the seasonal changes in the amount of solar heat falling on it, a point which has al- ready been attended to. As a result, the ranges over the southern hemisphere which is mostly water are uniformly small. In New Zealand, for example, December and June differ by only 10 degrees. Besides the diurnal and annual ranges of tern- 54 THE STORY OF THE EARTH'S ATMOSPHERE. perature it is found that there are slow periodic changes of a small amount in the mean tempera- tures of the year, in correspondence with the changes in the number and area of sunspots which recur about every eleven years. Whatever may be the exact cause, whether an increase or decrease of solar radiation corresponding to a great spot manifestation, the effects have been proved through the labours of Prof. Piazzi Smyth, Dr. Stone, Dr. Koppen of Hamburg, and Prof. Fritz of Zurich, to be visible in the temperature of the earth's atmosphere. In years grouped round those of fewest spots, such as i8n, -23, -34, -43, -56, -67, -77, -88, the temperature is highest, and in those similarly grouped round those of most spots such as 1860, -71, -83, -93, it is lower than the average. The effect is most noticeable in the tropics. For ex- ample, in India, the difference between the tem- perature at the two epochs varies from i to 2 degrees on the mean of the year. A similar periodic change is found to prevail in conditions which depend upon air temperature, such as fruit-harvests, vintages, rainfall, glacier extension, storms, cloud proportion, etc., while the late Professor Jevons endeavoured to shew that even commercial panics were brought about periodically through the medium of such indirect consequences. Though there is much scepticism as to the quantity of the temperature effect being of such importance as to bring about panics through bad harvests, there is no doubt that the condition of the sun affects our atmosphere in some peculiar way not only in regard to temperature, but also magnetically, since the appearance of the aurora THE TEMPERATURE OF THE ATMOSPHERE. 55 is admitted by those who dispute the heating ef- fect to be closely connected with the presence of sunspots and other forms of solar disturbance. This periodic oscillation of annual tempera- ture does not of course involve any steady pro- gressive change in the temperature of the atmos- phere. In fact, when some years ago the people of Paris were temporarily afraid that their climate was changing, the astronomer Arago proved to FIG. io. their satisfaction, by a recourse to statistics, that the temperature of Paris had not sensibly changed for 100 years, and within historical periods there does not seem to be any evidence that the tem- perature anywhere is sensibly changing perma- nently one way or another. We will now pass on to consider the circum- stances that attend a local accession of heat over land and water and the primary effects which it produces. Beginning with any area on a small scale. Let fig. (io) represent a vertical section of the atmosphere and let the dotted lines represent 5 56 THE STORY OF THE EARTH'S ATMOSPHERE. lines of equal barometric pressure beginning with 30 inches at the earth's surface, and let us sup- pose that the temperature of the central region is raised by a certain amount. All the air thus warmed will expand. The column H.A. will expand to height H.B., and as each layer will expand all the way up, the sur- face of the top layer will be most raised. Con- sequently there will be a flow outwards of the raised up air down the slopes marked by the thick lines toward the neighbouring air of the same pressure, which, not being expanded, lies at a lower level. The outflow will be greatest in the highest layer since it is the most raised (the increase is FIG. n. denoted by the varying size of the arrows). Meanwhile the loss of air above will lessen the Till-: TEMPERATURE OF THE ATMOSPHERE. 57 pressure on the earth's surface near the centre of the area. Consequently the surrounding air will How in towards this centre chiefly in the lowest layer, and the action having once started will continue so long as the central area is more heated than the neighbourhood. We have already noticed that where sun heat falls upon land it heats it up more readily than water. Therefore particularly in the case of an island lying in a tropical sea where the sun is powerful the above action takes place as in fig. (n) and we have the phenomenon known as the local sea breeze. When the sun disappears at night the action is precisely reversed, and the air near the surface tlows outwards as the land breeze, while above a certain height, which in local cases is often as low as 1000 feet, the air streams in over the rapidly cooling land. After the action has once started things ar- range themselves as in fig. (12) where the lower curved lines represent the depression caused by the loss of air which has flowed outwards above, and where 7\'.V l represents where the tendency 58 THE STORY OF THE EARTH'S ATMOSPHERE. to flow in and out neutralise each other and there is a plane of no motion called sometimes the neutral plane. The above action involves a certain amount of upward motion of the air over the central part of the heated area, and a corresponding down- ward motion over the surrounding cooler area, but these movements are evidently much smaller than the horizontal outflow and inflow. The same action also explains the origin of the manifest monsoons of Asia and Australia, where in the summer season the air blows more or less towards a heated land area, and in the winter from it towards the surrounding sea. It also accounts in part for the annual changes in the barometric pressure over large areas, espe- cially the low pressures in the middle of the larger continents like Asia and America during the sum- mer, and the corresponding very high pressures at the opposite season. Unless some such system of rise and overflow over the hotter areas and sinking and underflow over the cooler areas took place, the barometer would record a steady pressure over both areas, and if we ascended over the more heated area we should find the pressure greater than at the same level over the cooled area, because the air being more expanded vertically, there would be more top cover so to speak over our heads. As a matter of fact, notwithstanding the over- flow which relieves this state of things, the pressure at highly elevated stations like Leh (11,800 feet) north of Kashmir, rises until the beginning of May, and only falls very slightly in June and July. Consequently the lowering of pressure which appears so distinctly over Southern Asia in THE TEMPERATURE OF THE ATMOSPHERE. 59 the summer is confined to the lower half (by mass) of the atmosphere, that is to say below 18,000 feet, at which level the pressure is 15 inches. Above this level there is more or less an outflow in the summer and an inflow in the winter. A similar system of land and sea monsoonal circulation exists everywhere, only in high lati- tudes it is ordinarily masked by other motions of the air, introduced by the frequent passage of cyclones, and large travelling systems or waves of high and low pressure. Even along Western Europe the winds blow more towards the land in summer and from it in the winter. Where a small area on the land or sea is heated up above its neighbourhood we have the initial conditions for the formation of a disturb- ance of equilibrium. In hot countries where such a condition is more prevalent, there may arise a cyclone, tornado, whirlwind, or thunder-storm, under different conditions, which will be alluded to later on, but in order that there may be intense local action and a real " am rant ascendant," the air must not be merely gently lifted up and overflow, which is the only possible condition when it is dry, but it must be nearly saturated with vapour, in which case it will flow upwards so long as the lowest stratum continues to supply damp air. The part taken by temperature in causing these phenomena will be alluded to in a later chapter. The present account of the temperature of the atmosphere would be incomplete if it omitted to notice the transfer of heat from one part of the earth to another. So far we have merely examined the heat which falls locally or generally by means of the direct solar radiation. 60 THE STORY OF THE EARTH'S ATMOSPHERE. The temperature over any region is however largely dependent on the heat brought to it by winds. When they come from the sea their tem- perature is modified by the influence of the ocean currents, warm or cold, over which they have traveled. When they come from the interior of a continent, they are usually hotter in the summer and colder in the winter than the maritime re- gions towards which they advance. Thus in summer our hottest wind in England is the south-east, and the same wind often ac- companies our most severe frosts in the winter. The thermal effects of land winds therefore change with the season. Sea winds, especially where they are connected with ocean currents, and blow with some degree of constancy, exercise a permanent influence upon the temperature of countries over which they prevail. The most marked warm sea winds are felt on Western Europe, the Pacific slope north of lat. 40, and the eastern coast of South America. These winds are not merely warm because they have accompanied streams of warm water, such as the Gulf stream of the Atlantic and the Japan stream of the Pacific, but because their cooling is retarded by the latent heat set free in the condensation of the vapour they bring from the humid tropics. Several attempts have been made to measure the heat conveyed by both these streams. Dr. Haughton of Dublin some years ago estimated that these two streams together carried one-third of the total heat received by the northern tropi- cal zone towards the middle latitudes. Ferrel, however, has more recently shown that it is more THE TEMPERATURE OF THE ATMOSPHERE. 6 1 probably one-sixth. As we have already seen, the effect on England is to raise the mean tem- perature nearly 10 degrees above what it would otherwise be. Norway is raised as much as 16 degrees, and Spitzbergen 19 degrees. On the other hand compensating cold currents and the winds which blow off them depress the tempera- ture of Eastern Canada, northern China, western South America, and western South Africa. New- foundland is thus about 10 degrees colder than the normal for the latitude. The North China coast about 7 degrees colder, and even Honolulu, in the mid Pacific, has its temperature reduced 5 degrees by the return Japan stream cooled after losing its heat up north. The general influence of the ocean currents in reducing the difference which would exist be- tween the temperature at the equator and the poles, may be inferred from the fact, that accord- ing to Ferrel, if the surface of the earth were en- tirely dry land, and there were consequently no transfer of heat by oceanic or atmospheric cur- rents, theoretical considerations shew that the temperature at the equator would stand at about 131 F., while that at the Pole would be 108 be- low Zero. Observations, however, shew that the mean temperature day and night at the Equator is about 80 F., while that at the Pole is only o F. or Zero. Consequently the effect of the circula- tion of the ocean and the atmosphere together is to depress the temperature at the Equator about 50 degrees and raise that at the Pole no less than 100 degrees, and in this manner render the earth generally fit for human habitation, since, if such extremes as those mentioned prevailed, man 62 THE STORY OF THE EARTH'S ATMOSPHERE. would have been forced to inhabit a very con- stricted zone in middle latitudes. In like manner were the earth deprived of its atmosphere the mean temperature at the Equator would be 94 degrees below zero F., while that at the poles would be 328 degrees below zero F., and the mean temperature of the whole globe 138 degrees below zero F., a terrible frost. In fact, even if it were possible to do without air the human species as at present constituted would in such an event be quite unable to exist. With the protec- tion of an atmosphere the average temperature of the earth, or more correctly of the lowest stratum of the atmosphere is about 60 F. which is re- garded as the most delightful that can be enjoyed. So much do we owe to the invisible envelope of atmospheric air, which otherwise appears to con- stitute such a flimsy blanket between us and the terrible cold of stellar space. Extreme local temperatures are due to the concurrence of accidental causes tending to raise or lower the temperature, such as the passage of storms, prevalence of winds from north or south, long continued clear weather, combined with those of more regular incidence. Extremely high temperature will generally occur in these latitudes soon after noon in July and August, and extremely low ones early in the morning in January or February. Occasionally, however, the epochs are considerably displaced. The highest temperatures on the earth usually occur in India, the Red Sea, the Persian Gulf, and Australia. Thus in the centre of the Sahara, 130 degrees has been recorded. At Jacobabad in the Sind desert, the temperature frequently rises over 120 F. and even in New South Wales, 120 and THE TEMPERATURE OF THE ATMOSPHERE. 63 121 have been recorded at I5ourke and Denili- quin. In February, 1896, a temperature of 108 degrees was recorded at Sydney, due to a remark- able prevalence of dry N. \V. winds blowing over it from the interior. Paris has only once reached 106 degrees and London has seldom recorded anything over 96. The coldest temperatures are found not at the poles themselves, where the water circulation tends to bring heat from the equator but in the north-east of Siberia and north-east America. Werkojansk is the coldest place in the world. In January the mean temperature there is 55 F. below zero while all through the year the temper- ature is only 5 degrees above zero. During arctic expeditions, the Alert and Dis- covery experienced 73 below zero, while Capt. Nares once saw the thermometer descend to 84 F. below zero. Of recent years, a great extension of our knowledge of the phenomena of the atmosphere has been made by the application of what is known as thermo-dynamics. Prof. Hezold of I>erlin, the late Mr. Ferrel of Washington, Dr. Hann of Vienna, and others have cleared away much of the loose and misty reason- ings which characterize the work of their prede- cessors, but the subject is too difficult and tech- nical to be alluded to here. A few of the leading ideas however will be briefly touched upon when some of the particular atmospheric phenomena are being described further on. 64 THE STORY OF THE EARTH'S ATMOSPHERE. CHAPTER V. THE GENERAL CIRCULATION OF THE ATMOSPHERE. THE "Story of the Winds" is interesting and important enough to form the subject of a sepa- rate volume and within the compass of one which endeavours to cover the varied phenomena of the atmosphere generally, only the more salient points in connection with atmospheric motion can be reviewed. In these latter days, in spite of the old saying that " the wind bloweth where it list- eth " and the manifest and apparently capricious changes which characterize its behaviour in these midway latitudes, we know that there exists an independent dominating scheme of general circu- lation between the poles and the equator. This scheme results from the action of nearly perma- nent differences of temperature between these points in combination with certain mechanical laws resulting from the shape of the earth and its rotation on its axis. In former days many guesses were made more or less at variance with both facts and theory. Even Maury's fascinating attempt in 1855 to weave observation into a connected system, failed owing to the imperfect knowledge existing at that time of the winds of the entire globe as well as of the true laws which operated. The earliest attempt at any rational scheme of accounting for the more obvious features of the general circulation appears to have been made in 1735 by Hadley. The regularity of the " trade winds " was then GENERAL CIRCULATION OF THE ATMOSPHERE. 65 attracting the attention of scientists, and in a short paper in the Philosophical Transactions, Hadley advanced a theory to account for this which sounded so plausible, that for over a cen- tury it remained unquestioned. Hadley's theory in brief was, that owing to the general difference of temperatures between the polar and equatorial regions, a motion of the air took place similar to that just described in the last chapter, in the lower strata towards the zone of greatest heat, while the easterly * direction of the trades was attributed to the fact that as the air continually arrived at parallels where the earth's surface moved faster eastwards than the part it had just left, it tended continually to lag behind in a westward direction, and so appear to blow partly from the east. Hence it became the north-east trade on the northern and the south- east trade on the southern side of the equator. Carried to its logical conclusions Hadley's theory would require the trades to blow all the way from the poles to the equator, the return current being confined almost entirely to the upper air. Moreover the highest pressure as measured by the height of the mercury column in the barometer air balance, should be found at or near the poles. As a matter of fact, however, it was found that neither of these circumstances took place. The trades extended no further than latitudes 30 degrees N. and S. of the equator, the pressure at the poles, especially the south pole was per- manently lower than at the equator (about $ths of an inch of mercury) while the highest pressure * From the East. 66 THE STORY OF THE EARTH'S ATMOSPHERE was found to occupy two belts between 30 and 40 N. and S. of the equator. Obviously therefore there was something radi- cally wrong with Hadley's theory. In 1856 Mr. Ferrel, afterwards Professor in the United States '\Yeather bureau, tackled the subject and found out that Hadley had entirely overlooked the fact that the earth is a sphere. In consequence his theory contained two seri- ous errors, one of which was that only air moving north and south was deflected by the earth's rota- tion, while that moving in any other direction re- mained unchanged. The only circumstances to which Hadley's theory could possibly apply would involve the supposition that the earth was a perfectly flat plane composed of separate planks parallel to a straight line equator. Also that these planks moved along with different speeds beginning with 1000 miles at the equator and gradually decreasing to about 850 miles at latitude 30, manifestly a very different affair from a spherical surface like that of the earth. Some few years before Ferrel approached the question, the eminent French mathematician Poisson in 1837 read a paper before the Paris Academy, in which he demonstrated that when a freely moving body passes over the earth's surface in any direction, the effect of the earth's rotation is to cause it to deviate (not lag) to the right of its path in the northern hemisphere, and to the left in the southern. Employing the same reasoning as Poisson, but applying it to masses of air instead of solid bodies Ferrel gradually built up a satisfactory explanation of the general circulation, and with GENERAL CIRCULATION OF THE ATMOSPHERE. 67 the help of suitable modifications, applied the same principle to explain the leading features of cyclones and tornadoes. In general, if a mass of air initially tends to move on a rotating sphere toward a certain point, impelled in the first instance by a difference of density or pressure, it tends to move continually to the right when looked at from a point above the N. pole of rotation and unless prevented from do- ing so by any extra force resisting such motion, would continue to devi- ate until it had turned through a complete circle thus tig. (13). Suppose a particle of air at A starts to move FIG. 13. towards B. Instead of moving in the straight line AB, it will tend to move in the curve AC, and if it is very near the pole it will eventually complete a circle * as above in 12 hours, the size depending on its velocity. Thus for a speed of Radius of inertia circle in miles. 20 miles an hour ... 77 10 " " . . . .38 5 .... 19 We have the corresponding values of what is termed the curve of inertia. Prof. Davis of Harvard has suggested a very * In other latitudes the inertia curve as it is termed is more like the series of loops cut by a >kaU-r restricted be- tween certain limits of latitude. At the equator it vanishes. 68 THE STORY OF THE EARTH'S ATMOSPHERE. pretty experiment which can be performed by any one who wishes to have visual evidence of the existence of this inertia curve. Take a small circular table, and lay a sheet of white paper over it. Then take a marble and dip it in ink and lay it near the centre of the table. Next tilt the table slightly so as to give the marble a slight motion towards the edge, and at the same time rotate the table about its centre. Then the marble will be found to trace out in ink a curved line on the paper which will fairly rep- resent the inertia curve or the curve of successive deviations from a straight line by which the par- ticle through its inertia (or laziness) is unable to accommodate itself to the varying motion of the parts over which it rolls. In whatever direction the table is tilted the curve will still be traced out, the curvature sharp- ening with increased rotation of the table (analo- gous to increased latitude) and lessening with in- creased tilt by which the velocity of the particle is augmented. This tendency of air to move to the right of its original direction of motion, is what really ac- counts for the development of those permanent or rapidly changing differences of barometric pressure which accompany the large general or small particular air motions. A difference of temperature alone, for example, between the poles and equator, or between two neighbour- ing parts of the earth would cause a very slight alteration in the barometric pressures, but when the air begins to move in the direction of the lower pressure its tendency to push to the right, causes a squeezing and heaping up of air to the right of its path, and a corresponding stretching GENERAL CIRCULATION OF THE ATMOSPHERE. 69 apart or lowering of density and pressure to its left, until the difference of pressures becomes great enough to prevent its further movement to the right and it moves in a path regulated by these joint tendencies, thus Path (curred "Harm ^ inwfuck air area.) Direction. in which - air starts to move Cold area ultimately does more. Inertia curve into which car fencfj to be deflected Force exerted by pressure gradient FIG. 14. Amass of air at the cold area will tend initial- ly to move towards the less dense warm air, but once it starts it tends to move along the inertia curve. Eventually the high pressure (denoted by the shading) of the heaped up air on this side exerts a force indicated by the arrow directed towards the increased low pressure to the left, and finally the air making a compromise moves along a line between the two, indicated by the direction, labelled " Path," etc., so that instead of moving directly from high to low pressure it only partly moves towards the latter, keeping the high pressure to the right and the low pressure to the left of its path. In the southern hemisphere owing to the reversed point of view rig Jit be- comes left and the high pressure would be to yo THE STORY OF THE EARTH'S ATMOSPHERE. the left of the path and the low pressure to the right. This diagram will be found to supply the ex- planation of the general relations between pres- sure and wind, especially if it is remembered that where, as on land and near the surface, the air is prevented by friction from moving with freedom, the back thrust in the opposite direction tends to make the ultimate path point more towards the low pressure, while at sea and at great altitudes, where friction is small, it moves almost at right angles to the line joining the central areas of high and low pressures, or in the technical lan- guage borrowed from engineering, at right angles to the direction of the barometric gradient. Before alluding to Ferrel's explanation of the general circulation of air over the globe on these principles, let us see what this circulation really is like from observation. In the two plates adjoining, figs. (15) and (16), in which the actually observed barometric pres- sures and winds at two opposite seasons of the year are represented, it will be noticed that, over- looking minor features, there is a broad belt over the equator, over which the barometric pressure is about 29.80 inches, gradually rising on either side to two belts of high pressure, in latitude 30 in places reaching 30.2 inches, and generally about 0.2 inches higher than over the equator. Within this area, the trade winds blow through- out the year on each side of the equator, except over the North Indian Ocean, where in July they blow in towards an area of excessively low pres- sure and high temperature as the south-west monsoon of the Indian seas, which brings the rain, that has made India such a much more fer- GENERAL CIRCULATION OF THE ATMOSPHERE. 71 tile and populous country than the neighbouring peninsula of Arabia. In the map fig. (20), p. 81, the monsoon winds are represented blowing over India during July. In January, the south-west winds disappear, and in the general chart it will be seen that their place is taken by Northerly or North-easterly winds, blowing down towards the equator, from the large area of high pressure which at this season spreads over the whole of north-eastern Asia. On the polar sides of these bands or nuclei of high pressure, it will be observed that the winds blow more or less towards the poles, especially in the southern hemisphere. The lines (isobars) on these maps, by which the changes in the distribution of the mean monthly barometric pressure is indicated, are similar to the contour lines or lines of equal ele- vation employed to represent the contour of a hilly country. They do not necessarily represent real elevations or depressions of the atmosphere, because increased or decreased pressure is more due to a greater or less squeezing or density, than to a piling up of the atmosphere into absolute heaps and hollows, but since the effective results would be very much the same in either case, they may practically be considered as atmospheric contours. More correctly, they are the lines along which atmospheric contours intersect the earth's surface, the pressure over which at sea level, (about 30 inches), lies half way up the at- mospheric slope. The accompanying figure will render this clearer. The sloping lines marked 30.2, 30, 20. 8 etc., represent sections of the actual atmospheric iso- bars or pressure contours. 13, C, Z>, points where 6 72 THE STORY OF THE EARTH'S ATMOSPHERE. GENERAL CIRCULATION OF THE ATMOSPHERE. 73 74 THE STORY OF THE EARTH'S ATMOSPHERE. these lines cut the earth's surface. The dotted continuations represent how the lines would run if the atmosphere took the place of the solid earth. The curved lines starting from 13 and C to E and J?, denote the lines as they occur on the earth's surface or appear on a plane chart, when the contours curve round a central area A, where FIG. 17. the pressure in this case is about 29.7 inches. In considering' the general circulation, A may be taken to represent the North or South Pole, in which case the diagram represents something like what actually takes place. When we are dealing with particular motions, A would correspond with the centre of a cyclone or travelling disturbance. Returning to our story, it is plain from these maps, that the circulation of the atmosphere comprises a great deal more than a mere system of trade winds, blowing towards the equator. Any theory that pretended to explain the en- tire system, would have to account for the pre- vailing high pressures about latitude 30 N. and S., and the poleward trend of the wind on the polar sides of these atmospheric sierras. Ferrel, in his first paper in 1856, not only shewed that Hadley's theory was mathematically GENERAL CIRCULATION OF THE ATMOSPHERE. 75 incorrect, but that Matiry's fascinating scheme, put forward in his Physical Geography of the Sea, erred both in fact and the laws of physics. Reversing the usual procedure by which laws are induced from observations and starting with a few fundamental principles, such as the law of deflection already noticed, he shewed that the high pressure belt about 30 and the system of poleward winds on the polar sides of it were necessary consequences of these principles. Ferret's final ideal chart of atmospheric circu- lation on the earth is represented by fig. (18) 76 THE STORY OF THE EARTH'S ATMOSPHERE. where the average motions near the surface are represented in plan in the shaded circle, and in vertical section at the border, and though his ex- planation is a complicated piece of mathematical reasoning, the following represents the pith of it in simple language. Assuming that the air over the equatorial zone is heated above that near the pole, it expands near the equator and contracts over the pole. Consequently the uplifted air over the equator tends to flow downhill as it were towards the poles and a corresponding flow near the surface takes place towards the equator. If the earth did not rotate on its axis the upper air would flow direct to the pole then descend and return towards the equator along the surface. Since however the earth does rotate, the upper air is deflected in the northern hemisphere to the right (increasingly as it travels polewards) so much that were it not for the downward slope towards the pole, it would eventually deviate towards the equator. Consequently the pressure to the left of its path,/.,?, towards the pole, is decreased and the pressure to the right is increased. This increases what would otherwise be an insignificant slope to what is actually observed. By the time this upper air has reached latitude 30 which divides the hemisphere into two equal areas* (though it is only a third of the actual distance on a meridian) it has descended to the surface and overpowers any tendency towards contrary motion in the air there, and the entire atmos- * This most important fact is one of those tilings which is not as a rule taught at school, though it is of immense signifi- cance. GENERAL CIRCULATION OF THE ATMOSPHERE. 77 phere tends to move bodily eastwards from thence to the pole. Meanwhile the air near the surface between latitude 30 and the equator, moving towards the latter, deviates towards the west and heaps up pressure to its right and lowers the pressure to its left in the same way. Consequently on all accounts there is a tendency on the part of the air to heap up and increase in pressure about latitude 30 and to be reduced in density or pressure near the poles and the equator. Also since the air reaches a terminus at the poles and equator there will be calms at the surface at both these points. Moreover since on either side of latitude 30 or more correctly 35 the air moves along the surface in contrary directions, there will be an absence of prevailing winds over this region. These calms are known to exist, and owing to their proximity to the tropics used to be called the calms of Cancer and Capricorn. The preceding explanation may be better real- ised if we take a vertical section of the atmos- phere along a meridian as it actually exists, and draw sections of the general planes of equal baro- metrical pressure as they exist by observation on the average at different levels between the equator and the poles as in fig. (19). The poles are at N. and S. and the equator is at Q. The line A H C D E F represents a section of the general isobar of 30 inches at sea-level and shews two cols or hills at latitude 30. Then as we ascend these cols gradually disappear owing to the absence of the surface trades and therefore of the side pressure they create which keeps the opposite pressures exerted by the winds on their polar sides from pressing the air into one high 78 THE STORY OF THE EARTH'S ATMOSPHERE. pressure belt over the equator. As we ascend, therefore, these cols gradually coalesce into one FIG. 19. central hill from which the air descends as a westerly upper current (blowing partly from the south in the northern hemisphere and from the north in the southern), into the two polar valleys on either side. The depth of the atmospheric valleys of pres- sure below the top level of pressure at the surface of the earth converted into feet of air is found to be about 262 feet at the equator, about 320 feet at the north pole, and 640 feet at the south pole. At a height of 30,000 feet above the surface the north polar valley is 2,800 feet and the south polar valley 3,100 feet below the mean level. The height at which the equatorial valley dis- appears varies from 8,000 to 12,000 feet. Above this level there is a downward slope all the way to the arctic and antarctic circles and possibly to the poles themselves. GENERAL CIRCULATION OF THE ATMOSPHERE. 79 Viewed as a whole, the general circulation of the air according to Ferrel, may be considered as consisting of two huge atmospheric whirls, or, as they are technically termed cyclones, with the poles as centres, in which the air rotates in each hemisphere in the direction of axial rotation. On the equatorial side of each of these whirls, a belt occurs in which the motion of the air is con- trary to that of axial rotation. These are the trade wind belts. Between these two areas the air is heaped up into two zones of high pressure, reaching their highest values in the northern hemisphere about latitude 35 and in the south- ern about latitude 30. Since this system was established by Ferrel, Dr. Werner Siemens, von Helmholtz, Herr Moller, Professor Oberbeck, Dr. Sprung and others have developed the theory by the aid of more modern refined methods and closer reasoning, but their conclusions are substantially the same as those deduced by Ferrel, and the above sketch repre- sents as far as can be attempted in a work like the present the modern theory of the general cir- culation of the atmosphere. The most noticeable permanent modification from the ideal condition of things is afforded by the exceedingly low pressure round the south pole and the strong north-west winds which prevail south of the Atlantic, Indian, and Pacific Oceans, and which enabled Australian clippers in the old days to make passages of fabulous rapidity. This is due to the fact that the southern hemisphere is chiefly covered by water which, by exerting less friction on the air than land, allows its motion^ to occur with greater freedom. In consequence of this the Antarctic barometric depression is 8o THE STORY OF THE EARTH'S ATMOSPHERE. more developed and more symmetrical than the northern. For example the pressure on the lati- tude corresponding to our Antipodes is perma- nently yVths of an inch below what we experience, while the wind velocity is three or four times as great. The seasonal changes and migrations as the sun moves north and south are scarcely notice- able in the southern hemisphere for the same rea- son. In July the pressure over the tropical * belt as we may term it, is slightly increased, and the belt lies a little further south than in January. In the northern hemisphere on the other hand, the seasonal changes are far more conspicuous. The high pressure nuclei which in July lie on the eastern sides of the Pacific and Atlantic oceans have by January shifted on to the conti- nents of America and Asia, while the low pres- sures, which in July occupied the middle parts of North America and the centre of Africo-Asiatic continent, (the centre lying almost exactly over the Persian Gulf which is the geographical land centre) in January lie over the North Pacific and North Atlantic. Meanwhile the equatorial low pressure belt, or barometric equator as it may be termed, which in January is confined between its ideal equatorial limits, in July runs up into the northern continents and in Africo-Asia in particu- lar, may be said to lie entirely over the land sur- face, where it causes the Monsoon as in figure (20). The relation of these extensive migrations to the effect of seasonal changes in solar heat on * The term tropical is here used to signify on or near the tropics or turning points and not to the entire space between them as is usually the case. GENERAL CIRCULATION OF THE ATMOSPHERE. 8 1 the air lying over land and water surfaces is ob- vious. The result of these large transfers of air and air pressure north and suuth, and between the oceans and the continents, is to cause what is termed the annual variation of the barometric pressure at any single place on the earth. This annual variation reaching its extremes generally in January and July, will be found to be larg- est in the centres of continents such as Asia where the barometric shift is greatest and least on the coasts which are the axes of the annual 82 THE STORY OF THE EARTH'S ATMOSPHERE. pressure sea-saw between the oceans and the con- tinents. In England the change is small, amounting to about 0.12 inches between January and July. In India, it increases from 0.26 inches in Ben- gal to 0.62 inches in the Punjab, while over Siberia and central Asia it reaches about i inch. The mean pressure over the whole earth is 29.89 inches. In the northern hemisphere the mean pressure for January is 29.99, and for July 29.87. For the southern hemisphere the pressures in the same months are 29.79 and 29.91. From this it is evident that there is a much greater dif- ference between the quantity of air over the two hemispheres in the northern winter in January than in the southern winter in July. This difference in favour of the northern hemi- sphere really means that owing to the greater cooling and contraction over the northern land area in the winter 32,000,000 tons of air have shifted over to supply the defect. There is no protective tariff placed upon this valuable import from the southern hemisphere. A seasonal shift of the general wind system of the lower strata occurs like that in the barometric pressures, as the sun shifts north and south. The shift of the sun in latitude is 47, but the wind systems only shift from 5 to 8 on the north- ern, and from 3 to 4 on the southern side of the equator. The accompanying diagram fig. (21) gives an idea of the effect of the shift. The cen- tral belt (sub-equatorial) represents the district which is alternately subject to the doldrums or equatorial calms, formerly the bane of sailors, and the attendant bordering trades as they oscillate north and south. GENERAL CIRCULATION OF THE ATMOSPHERE. 83 The width of this belt varies from 350 miles in the Atlantic to 200 in the Pacific and lies on the north side of the equator all through the year, ow- ing to the fact that the system of circulation in the southern hemisphere and round the South FIG. 21. pole, owing to smaller friction, is so much more powerful than that in the northern that the pres- sure belts on that side are all pushed northwards. The sub-tropical belts, as the calms of Cancer and Capricorn are now termed, are similarly the alternate arena of westerlies, trades, and interven- ing calms. So far, w'e have mostly considered the general circulation with respect to the motion of the at- mosphere, near the surface. The Upper current 84 THE STORY OF THE EARTH'S ATMOSPHERE. which blows all the way from the equator to the poles, is usually termed the Anti-trade in the tropics, because it overlies and blows in the con- trary direction to the latter from the south-west on the northern, and from the north-west on the southern side of the wind equator. In the equa- torial zone its lowest limit is about 10,000 feet, and as we proceed polewards, this limit descends bo that along the range of the Himalaya in the winter season, this upper current descends to within 2000 or 3000 feet above the sea. On the Peak of Teneriffe, Prof. Piazzi Smyth, when he was conducting astronomical observa- tions there in 1860, was able to walk up through the north-east trade wind and find the south-west upper current blowing continuously at his station at Alta Vista, 10,000 feet up the mountain side. In like manner the smoke of lofty volcanoes such as Cotopaxi 18,000 feet, and Coseguina lying in the trade wind zone have been observed to blow from the west, contrary to the surface wind. Nearer the poles about latitude 35 to 40 the lower edge of this upper current touches the earth, and its lower half breaks up into what Prof. Helmholtz terms vortices. In plain lan- guage it separates into irregular currents which form the cyclonic storms which are so prevalent in high latitudes on either side of the equator. Meanwhile the upper part of the current, except where it is affected by local disturbances or whirls, continues to move generally from the south or north-west. Its motion is motion determined by observation of the high clouds which float at an average elevation, according to the most recent measurements, of about 27,000 feet. GENERAL CIRCULATION OF THE ATMOSPHERE. 85 The general circulation of the atmosphere and its seasonal shifts is intimately bound up with the general distribution of the rainfall of the world, and the permanent occurrence and seasonal shift- ing of zones of drought and rain. Locally, rain is due, especially in high latitudes, to the passage of cyclonic storms, but in the equatorial and trade wind zones, the rainy season is almost en- tirely determined by the shift of the doldrums. Without at present going into the question of how rain is produced in all cases, it is easy to see why the central belt of equatorial calms is an area of constant cloud and rain. For the air there, supplied with vapour by the inflowing trades on either side, is constantly rising up to higher and colder levels, where it cannot contain so much vapour as at sea-level. The surplus therefore condenses first into cloud, and then into rain- drops which fall back to the sea-level. On the equator, as at Singapore, it rains everyday. As the doldrum belt oscillates north or south, it may give rise to one rainy and dry season, or in some cases to two, thus. (See Fig. 22.) If a place is situated at a or b, just within the edge of the rainbelt at its extreme positions, it is within the belt during half of the year about, and without it the other half. Consequently, it has a rainy season for six months, and a dry season for about six months. This is the case at Panama, where it rains from May to November, and is comparatively dry (luring the remaining months. Similar equal periods occur in Bengal, the Nile Basin, and Northern Australia. When a place is situated at e or f, nearer the outer edge of the rainbelt in its extreme position, the rainy season is shorter, and the dry season longer. The Pun- 86 THE STORY OF THE EARTH'S ATMOSPHERE. jab, Upper Burmah, northern Mexico, and north- ern Central Australia are regions which underly the rain-belt for only three months of the year, and if, as in the year 1896 some irregularity oc- curs in the arrival of the belt, the rainy season may be so short as to cause drought and famine. Extreme Northern, position of rain belt Extreme Southern, position of "ainbelt te.---.-f. h FIG. 22. Places such as g and h, lying outside the influ- ence of the equatorial rainbelt altogether, would be rainless except for the extension equatorwards of the system of polar winds, which sometimes descends as far south as latitude 35 in the win- ter. Between latitude 35 and latitude 20 X. and S., except where as in India, monsoons blow from the equator, or along coast lines, no regular rain- fall belt arrives, and the dry desert zones of the earth tend to form. The dry regions of California, Arizona, and Colorado in North America, the great Sahara and GENERAL CIRCULATION OF THE ATMOSPHERE. 87 Nubian deserts of North Africa, and the Arabian and Persian dry areas all occur between these limits in the northern hemisphere. In the Southern hemisphere within the same parallels are the dry regions of the Argentine and Eastern Patagonia, a large dry region in South Africa and one comprising the whole interior of Australia. These areas coincide with the belts of high atmospheric pressure and may be said to suffer from permanent fine weather. Places at c between the two positions of the equatorial rainbelt, experience two short dry sea- sons alternated by two short seasons of rainfall. Such are Ceylon and southern India, Colombia in South America, parts of the Nile basin and Java. In the latitudes between 35 degrees and the poles, the seasonal rains are entirely regulated by the seasonal shifts in the polar system of gen- eral winds. Here again, owing to the shift in latitude of a principal single rain belt corresponding to the seasonal shift of the sun, some places in the mid- dle of the area between its extreme limits un- dergo two rainy and two dry periods. In South Europe, for example, the rain falls mostly in the winter, because the system of winds circling round the pole reaches its furthest extension towards the equator at that season. In middle Europe, the rains fall chiefly in spring and autumn as the belt moves north and returns, and in Northern Europe they fall chiefly in the summer. The general circulation of the atmosphere not only determines the prevalent direction of the winds on the surface and in the upper regions, but also exercises a very large influence upon their average velocity. 7 88 THE STORY OF THE EARTH'S ATMOSPHERE. The practical importance of possessing a knowledge of the general velocity of motion of the air above the earth's surface is evident when we touch upon the subject of ballooning or the coming flight of man. When man is able to circumnavigate the ocean of air with the same ease that he sails across the ocean of water he will require to possess as accu- rate a knowledge of atmography (to invent a title) as he does at present of hydrography. We have at present a hydrographer to the admiralty, and we shall then require the services of one who will tell us all about the movements and conditions of the air, not only at sea-level, but in the upper regions whither it will be necessary to ascend in order to cross mountain chains. From the theory of circulation as developed by Ferrel and Oberbeck, it appears that the surface wind ought to reach its greatest average velocity about latitude 50, and diminish thence both towards the poles on the one side and towards the equator on the other. As a matter of observation this appears to be the case. Taking an average of the winds throughout the year, the late Prof. Loomis found the follow- ing average values for the wind in typical lati- tudes: Mean velocity of wind in miles per hour. United States 9.5 Europe 10.3 Southern Asia 6.5 West Indies 6.2 In England the average surface wind is nearer 12 miles an hour. Like other elements the sur- face wind varies with distance from the sea, time of year, and time of day. GENERAL CIRCULATION OF THE ATMOSPHERE. 89 The movements of the air are much affected by the nature of the surface over which it passes. It moves faster over water than over land, and faster over flat bare land than where it is hilly or covered with forest. In the interior of continents it is much more sluggish than near the coasts or out at sea. Thus in India, the wind velocity diminishes as we leave the coast in the following manner: , Velocity of wind in miles per diem. ) Bombay.. . 408 On coast f Kurra / hce J^ Abou , t [Calcutta.. . 123 loo miles V D * from sea ) 500 miles/ . ,, , , , - Allahabad qi from sea \ 800 miles ) ,, , ,_ - Roorkee 65 from sea ] Again, while the velocity over Europe is 10 miles an hour, it is as much as 29 miles an hour over the North Atlantic. Everyone is aware of the great amount of wind experienced even in summer when crossing the Channel as compared with that felt on shore. A similar difference of velocity is observed as we ascend from the earth's surface. This is partly due to the decrease of friction and partly to the increased slope down which the upper air raised by the equatorial heat tends to flow towards the poles. Near the surface and for the first 50 or 100 feet the increased velocity with height is entirely due to the dimim>hed friction encountered by the air against the roughness of the surface, trees, 90 THE STORY OF THE EARTH'S ATMOSPHERE. houses, and other obstacles. After that the retardation of the lower layers is communicated to the upper ones in a gradually decreasing scale by means of the friction of the air molecules against each other, somewhat as a spoon passed through treacle or honey drags some of the sur- rounding mass along besides what it pushes directly in front of it. This property of the particles of a gas is termed viscosity, owing to its similarity to the visible behaviour of what is termed a viscous liquid like melted glass. Such resistance which neighbouring portions of gas offer to one another's motion is due, on the Kinetic theory of gases, to the collisions which are constantly taking place between the rapidly oscillating molecules. A good parallel is offered by a crowd of per- sons all moving along a road in the same general direction towards some common point of interest. If everyone moved at the same pace in parallel lines, the speed of the crowd would be the same as that of any individual person, but owing to the fact that some persons cannot walk so quickly as others, some stop to look at the shop windows, others walk crookedly and jostle their neighbours, while some walk back against the crowd because they have left something behind them, the average speed of the crowd is sensibly reduced below that of the quickest walkers, though it still remains above that of the slowest. In like manner if two streams of persons, one moving faster than the other, join together and personal interchanges take place between them, the persons who walk across from the slower stream into the quicker one tend to retard its GENERAL CIRCULATION OF THE ATMOSPHERE- 91 average pace. Those on the contrary who move across from the quicker stream into the slower one tend to accelerate its average pace. In a similar manner, adjacent strata of the atmosphere tend to equalise each other's speed on a small scale by interchange of molecules, and where large masses intermingle, as in the general circulation, by interchange of masses, moving with different average speeds. It is by this internal friction between inter- mingling air masses in addition to that experi- enced by the friction of the lowest layer against the earth, which is gradually communicated to those above, that the atmosphere does not assume unheard-of velocities and storms are not more violent than they are. The latter alone would not be enough. Helmholtz, for example, has calculated that if the atmosphere generally started moving over the earth with a certain average velocity, say of 20 miles an hour, it would take no less than 42,747 years to reduce this to 10 miles an hour by the action of friction with the ground. The first experiments to find the increase of velocity of the air with the height above the ground were undertaken by Mr. T. Stevenson of Edinburgh, who attached anemometers of the Robinson pattern at different heights on a 5o-foot pole. Here the retarding effect of the ground was found to rapidly diminish in the first 15 feet. Above this the rate decreased. For building and engineering purposes it is best to make experi- ments in the locality and trust to no formula, since the rule alters rapidly up to the first 100 feet. Beyond this and up to 1800 feet experiments conducted by the author in 1883-5, under a grant 92 THE STORY OF THE EARTH'S ATMOSPHERE. from the Royal Society, resulted in establishing the fact that the average velocity at 1600 feet is just double the velocity at 100 feet. Above the former height the rate appears to increase, if we are to judge from the observations of the clouds made at Blue Hill Observatory, near Boston, U. S. A. Mr. Clayton's recent observations there of the movements of the different cloud strata reduced to English measure give the following results throughout the year: Cloud level. Stratus in feet. n 1,676 verage speed in liles per hour. 19 24 34 7i 78 Cumulus ^,326 Alto-cumulus 12,724 Cirro-cumulus 21 888 Cirrus . . . 20,^17 The rule in this case may be simply remem- bered thus. For every 1000 feet of ascent add on about 2 miles an hour to the velocity of motion. In winter the speeds are twice as large at the upper levels as in summer. For the winter half year the speed of the cirrus is as much as 96 miles an hour, considerably faster than our ex- press trains travel at present. In Europe the velocities appear to increase less rapidly, but are still large when compared with those at the surface. An average of closely concordant results, obtained by Dr. Vettin of Berlin and Hagstrom and Dr. Ekholm of Upsala in Sweden, make the velocities at 4300 and 22,000 feet about 19 and 38 miles per hour respectively. Here the rule gives about i mile per hour for each 1000 feet of elevation, which is probably nearer the mark for Europe generally. GENERAL CIRCULATION OF THE ATMOSPHERE. 93 This great increase of velocity of the average motion of the aerial ocean as we rise above the surface is scarcely realised by us tiny mortals who dwell mostly at its base. The loftiest building is scarce 1000 feet above the ground, while the loftiest inhabited place is but half-way through the mass and probably a twentieth of the actual height of the atmosphere. The great velocity often attained by balloons is thus readily explained. At the same time it is equally plain that no navigable balloon will ever be able to stem the currents above 5000 feet, while flying machines would do well to travel with the wind above this elevation. In fact they may eventually utilise these rapid currents much in the same way as the Australian clippers formerly utilised the brave north-west wind which blows so powerfully round the watery expanse of the southern ocean. One very curious result of this great motion at high altitudes has been recently pointed out by Herr Moller, a German engineer. The energy of the motion of the air or the power it possesses of performing work is proportional to its speed and mass. Moller has thus calculated that the energy of the upper half of the atmosphere /. e., the half above 16,000 feet, is no less than six times the energy possessed by the lower half. Of this in- conceivably enormous store of energy we at present utilise a minute proportion in sailing ships and driving windmills. The rest is com- pletely wasted. 94 THE STORY OF THE EARTH'S ATMOSPHERE. CHAPTER VI. THE LAWS WHICH RULE THE ATMOSPHERE. THE story of the earth is for the most part a chapter of ancient history. The story of the at- mosphere is a tale of to-day, and even of to-mor- row. When we have opened up the earth's crust to view we can trace the operation of past changes in the physical and chemical nature and position of the different rocks. All the atmospheric mo- tions and changes, on the other hand, are going on before our eyes. Every action, moreover, is subject to the reign of law. The chaos which at first sight appears to surround the infinite com- plexity of atmospheric phenomena is reduced to harmony and order in proportion as we learn the true laws which operate in the grand laboratory of Nature. We have been a long time learning our lesson, and we are only now beginning to rise from superstitions and guesses to those intellectual " Delectable mountains " whence we may, even though it be "through a glass darkly," snatch a glimpse of the true character of the mysteries of our wonderful atmosphere. The earth is a symbol of rest, stability, and permanence. The atmosphere, on the other hand, is in ceaseless motion and constant ac- tivity, under the influence of the heat of the sun, the cold of space, the rotation of the earth, and the changes of the seasons, as it moves in its orbit round the sun. Physicists working in their laboratories have discovered that certain laws are obeyed by air in common with other gases, and it is only when we know these laws that we THE LAWS WHICH RULE THE ATMOSPHERE. 95 can interpret the phenomena which are daily and hourly observed in the sky and air around us. One of the first laws relating to the atmosphere was discovered by Dr. Boyle, the celebrated Glasgow chemist, and Marriotte of France, and is usually called Boyle and Marriotte's law. This law states that if a volume of gas (which is elastic and compressible), confined within certain limits, such as an elastic bag, is subjected to compression, its pressure increases in the same proportion as its volume decreases. Thus, if 6 cubic feet of air at the ordinary atmospheric pressure were squeezed together until they occu- pied only 3 feet, the pressure or resistance of the air would rise to that of two atmospheres; and if a mercury barometer, at first marking 30 inches, were placed within the containing vessel, the column of mercury would at the end of the experiment rise to a height of 60 inches. Human beings, when subjected to moral pressure, frequently exhibit similar characteris- tics, though their resistance cannot be measured on a moral barometer. In the free atmosphere such an ideal case seldom occurs, since the air gener- ally finds some escape from complete compression by expansion or motion in various directions. One immediate result of this law is the great density of the lower strata of the atmosphere, due to the compression to which they are sub- jected by the weight of the overlying layers. In like manner the rarity of the upper air is due to its smaller compression. Also, when any mass of air is forced upwards, it comes under gradually de- creasing pressure, and consequently by Boyle's law it expands. Conversely, if it is forced downwards, it contracts owing to the increasing pressure. 96 THE STORY OF THE EARTH'S ATMOSPHERE. It is important to notice the distinction be- tween cases where the air is forced upwards and where it ascends by reason of an expansion al- ready effected before it starts, by the action of heat. In the former case it stops when the forcing agency stops. In the latter it rises to the level where the air all round is equally ex- panded, and therefore of the same density. The former case occurs in Nature, where a wind blows athwart an abrupt chain of moun- tains, and forces the air up their sides. The lat- ter occurs wherever air is locally heated above or cooled below that surrounding it. When, instead of being compressed, the air is heated within a confined vessel, its pressure in- creases in direct proportion to its rise in tem- perature (when reckoned from an absolute zero 461 below zero Fahr.). If when heated it is allowed to expand freely (that is to say, still confined by ordinary atmospheric pressure) it ex- pands or increases in volume in like proportion. This is called the law of Charles or Gay-Lussac. A third law which is really the converse of this may, for convenience, be termed Poisson's law, and asserts that, if air is suddenly compressed it rises proportionately in temperature, and if suddenly allowed to expand, it falls in tempera- ture. The suddenness is only necessary in order that the heat engendered may not have time to escape before it can be detected. These three laws, in combination with a few special charac- teristics displayed by water vapour, explain all the varieties of atmospheric phenomena primarily due to the action of heat and cold, such as wind, storms, clouds, rain. These laws are really only variations of one THE LAWS WHICH RULE THE ATMOSPHERE. 97 grand principle which applies to everything in the Universe viz., what is termed the " conser- vation of energy." Thus, to take a single exam- ple, when a bullet hits a target it gets quite hot, Fir,. 23. "AFTER THE STORM." From the Croix de fer Switzerland, 7000 feet above sea-level. and the heat that is thus generated is the exact equivalent of the motion that is lost. Heat, as we have learnt, is " a mode of mo- tion." It is, in fact, a motion of the small atoms or molecules that make up a body instead of a motion of the body itself. According to the modern theory of gases, which applies equally to a mixture of gases such as air, the tiny atoms or molecules which com- 98 THE STORY OF THE EARTH'S ATMOSPHERE. pose them are in a state of constant motion back- wards and forwards, kicking, as it were, against each other, and against anything that obstructs their freedom to mo^e. The pressure exerted by a gas on its neighbourhood, where this is a solid liquid or gas, is measured by the number of kicks or impacts which its atoms perform in a given time. If the gas contained within a bag (say) is compressed, the paths of the atoms are shortened, and, in consequence, the number of impacts with each other and against the sides of the bag is increased -i. is occa- sionally seen of 90 diameter. Intersecting these halos, a huge circle passing though the sun and parallel to the horizon makes its appearance. At the points of intersection of these halos, the light is so reinforced that the patches look like sepa- rate suns, and form what are termed mock-suns or parhelia. Similar appearances round the moon or mock-moons are termed paraselenoe. At the oppo- site points of the sky similar mock-suns are occa- sionally formed. Some years back the author saw four mock-suns at the same time. Two in front where the primary halo intersected the large horizontal halo, 22^ on each side of the sun, and two behind him, making angles of 157! with the sun on each side. The mirage, or serab (illusion), as the Arabs term it, is a phenomenon which has often formed a subject for the poet as well as the artist. The thirsty traveller in the dreary and parch- ing wastes of the Sahara and Arabian deserts frequently sees looming up in the distance a beautiful lake dotted over apparently with islands and trees. This lake is an illusion produced by the bending or reflection of the light that occurs 146 THE STORY OF THE EARTH'S ATMOSPHERE. at the boundary of two strata of air of different temperatures. In this case a layer of cool air overlies one of very hot air just above the heated sand. Any object, such as a tree or mound above this layer, has its image inverted by reflection, while the light from the ground is thrown back by what is termed internal reflection. Conse- quently the effect is just the same as though a layer of water were really present. A special kind of mirage is termed " looming." In this case objects which are ordinarily below the horizon are seen raised above it, sometimes -inverted and sometimes erect. These effects are due to a great increase in the ordinary refraction which takes place near the horizon, due, probably, to a cold and dense layer of air over the sea, overlain by a warmer layer derived from the neighbouring land. The famous Fata Morgana or castles of the witch Morgana of Reggio are an instance of this kind of mirage. During certain conditions of the air the inhabitants of Reggio see castles and men and trees, etc., suspended above the sea in the direction of Messina, whose reflected image they really are. A southern imagination con- verts them into enchantments. A curious effect of looming occurred once at Malta, where the top of Etna appeared by refrac- tion like an island in the sea. Several ships sailed out to take possession of this supposed new island, but soon the image vanished and the quest was seen to be vain. This story was paralleled more recently when the gorgeous Krakatoa sunsets first made their appearance in America. A local fire brigade in a raw Western township, seeing the sky so red, with more zeal than wisdom harnessed up and OPTICAL PHENOMENA OF THE ATMOSPHERE. 147 set forth with all speed to put it out. When they ultimately found out their mistake they were not a little put out themselves. In the polar regions, where the sea is usually colder than the air, the images of objects below the horizon are frequently reflected to the ob- server from the top warm layer and appear in- verted. If the upper warm layer is of no great thickness, there is thus often both a direct and inverted image. Scoresby once recognised his father's ship, the Fame, by observing its inverted image through a telescope. The real ship was afterwards found to have been thirty-five miles distant. The rainbow has always been a majestic sym- bol of the union between earth and heaven. Iris, the goddess of the rainbow, was one of the most graceful of the Grecian deities. She was represented as the messenger between Olympus and his earthly subjects. According to the Teutonic mythology the rainbow was the bridge over which the heroes passed to the festive abode of Walhalla. Robbed of its fanciful mysticism, the rainbow loses nothmg of its beauty when we know that it is the result of the refraction of the white light from the sun as it enters the raindrop subse- quently reflected from the back of the drop to our eyes. The whole operation is so wonderful. The different coloured rays which make up the white ray when they meet the new surface, part company according to their wave frequency, and travelling along separate paths are reflected by the mirror back as though they were painted in the sky. The tiny violet waves being more bent inwards, appear inside the bow, while the longer 148 THE STORY OF THE EARTH'S ATMOSPHERE. red waves form the external boundary. Ordina- rily the earth cuts off the lower half of the bow, and when the sun is more than 40 above the horizon, the entire phenomenon disappears. Inside the bow the violet is occasionally seen repeated in what are termed supernumerary bows, while the external bow is often visible in which the colours are reversed. The explanation of these belongs rather to a book on optics. The " Spectre of the Brocken " is simply a shadow of the spectator projected on to a screen of vapour rising up from the surrounding valleys, and may be seen on any mountain w r here the conditions are favourable. The "ignis fatuus," or wandering flame occa- sionally seen in marshy land, or over church- yards, where it is called the " corpse candle," is believed to be merely a distillation from the soil of phosphoretted hydrogen gas which has the property of self-ignition on emerging into the atmosphere. The "aurora polaris " or "northern lights" are a manifestation of quiet electrical discharge round either pole, attaining its greatest brilliancy and frequency near the magnetic poles, which are at some distance from the true geographic poles. In the northern hemisphere the belt of greatest frequency (80 auroras per annum) occurs from latitude 50 to 62 in America, and from latitude 66 to 75 over Siberia. From thence they dimin- ish both north and south. The Aurora exhibits various forms. Stream- ers, curtains, bands, and rays, and it frequently coruscates, whence the name "Merry Dancers." It is believed that the Aurora is a sheet of rays which converge downwards toward the magnetic WHIRLWINDS, ETC., OF THE ATMOSPHERE. 149 axis of the earth, a kind of luminous collar, the top of whose arch is as much as 130 miles above the earth, though parts of it are believed to be quite near the earth. It is therefore an electrical discharge taking place in highly rarefied air or vacuum. Lemstrom of Finland recently suc- ceeded in causing an artificial aurora by suitably imitating what is believed to occur in Nature. The Aurora is certainly closely connected with the magnetic condition of the earth and also of the sun. When any great sun-spot appears on the latter orb, the magnetic balance of the earth is affected, as shewn by the irregular movements of the magnetic needles and the simultaneous appearance of auroroe at both poles. CHAPTER XII. WHIRLWINDS, WATERSPOUTS, TORNADOES, AND THUNDERSTORMS OF THE ATMOSPHERE. BESIDES the large cyclones, there is a peculiar group of local disturbances or storms of the at- mosphere which, according to their violence, occur in one or other of the above forms. The harm- less dust-whirl we see arise on a still day in early summer, and sweep across the young corn, is but the embryo of the terrible tornado of the Middle United States. The dreaded simoom of the Arabian Desert is simply a larger whirlwind laden with the dust of the desert. Where the whirl is broader and high- er, and the air is moist, we have the common thunderstorm of Europe with or without hail, the 150 THE STORY OF THE EARTH'S ATMOSPHERE. "nor'-wester" of India, the "pampero" of the Argentine, and the so-called " arched squall " and "bull's eye squall" of the tropical seas. When the action is very intense and concen- trated, we have the " tornado " which is common in the Mississippi Valley. The freaks of some of these tornadoes, while generally of the tragic or- der, occasionally border on the ridiculous. Thus even in India where they occasionally occur in a mild form it is stated that in the district of the Brahmaputra, on March 26, 1875, after a tornado had passed the village of Uladah a dead cow was found stuck in the branches of a tree some 30 feet from the ground. In America, in the tornado of June 4, 1877, at Mount Carmel, Illinois, the spire, vane, and gilded ball of the Methodist Church were carried fif- teen miles to the north-eastward. In other cases ploughshares and even houses (generally of wood) have been carried up into the air, and, so to speak, transplanted. In the recent terrible visitation at St. Louis, in June 1896, it was stated that a car- riage was lifted from the road up into the air and gently let down again 100 yards off without dam- age, while at the end of this remarkable perform- ance the coachman's hat was declared to have re- mained securely attached to his head. This last circumstance sounds extreme, but there is no ob- vious exaggeration in that given by one spec- tator who informed the writer that he looked up a street in St. Louis and saw everything horses, carriages, people, and furniture being whisked along in tumultuous chaos towards him as the centre of the tornado passed over it. When the centre of a tornado passes it seems to sweep everything movable along with it, often WHIRLWINDS, ETC., OF THE ATMOSPHERE. 151 destroys the most substantial buildings and cuts a clear lane through a forest. In all these cases, the prime cause appears to be a local instability of the air due to an aggregation of heat near the surface, combined with an incursion of cold air in the stratum above. These together cause a rapid fall of temperature in a vertical direction. In such a case even dry air may temporarily ascend in a narrow column and burst through the upper layers. When once this has taken place the surround- ing air rushes in to supply its place, and there ensues a whirling round just as in the case of water running down through a sink. In Tornadoes the whirling round of the air is not due as in that of the large cyclones to the deflection caused by the rotation of the earth, since this would be practically insensible for move- ments within such limited areas. It is due to the rapid development of gyration as the air is forced inwards towards the centre when once such gyration has started. The slightest devia- tion to one side of the direct path to the centre is enough to start a gyration, and any slight irregularity in the flow suffices to cause a devia- tion. After the whirling has once started, the gyra- tions near the centre become so rapid that ulti- mately a funnel shaped column of highly rarefied air is produced, which is marked by the appear- ance of a sheath of cloud or water within which, in extreme cases, is a nearly complete vacuum. Round and up the sides of this the air ascends, flows out above, and again quietly descends over a wider area. When the air is dry, the action, as we know, 152 THE STORY OF THE EARTH'S ATMOSPHERE. cannot continue very long, since the uprising air soon reduces the vertical temperature differences below i in 180 feet. Dust whirls and sand storms are consequently short-lived and never of destructive violence. When, however, the lower air is very damp as well as hot, the action can go on for a much longer time and with far greater energy. Lieut. Finley of the U. S. Navy, who has made a special study of American tornadoes, estimates that the velocity of the wind rotating near the centre of a tornado may reach as much as 500 miles an hour, and exert a pressure of 250 Ibs. to the square foot. Even the upward velocity near the vortex probably amounts, in many cases, to over ioo miles an hour, otherwise it could not sustain the objects it visibly does. The awful effects frequently produced by the arrival of such a piece of what may be termed meteorological dynamite can therefore be understood. The central column of rarefied air by reason of its expansion is cooled below dew point. Hence, whatever vapour exists there, becomes condensed into a visible sheath. This is the cause of what are termed waterspouts, which are only a mild form of tornado. In the real tornadoes, the black funnel shaped cloud, which forms one of their most marked features, is due to the same causes. The popular notion of a waterspout accounts for the water by imagining it to be drawn up from the sea. But this is erroneous. When water- spouts pass over the sea, they cause a disturbance and slight upward rise round their bases, but the long visible column, often half a mile in length, which dips down from the clouds, is entirely com- posed of vapour, condensed out of the inflowing WHIRLWINDS, ETC., OF THE ATMOSPHERE. 153 air. As Ferrel puts it " the cloud (or rather the conditions which favour the production of cloud) is here drawn down towards the earth by the re- duction of pressure produced by the rapid whirl- ing of the air." At the same time, the downward dip is only an apparent and not a real descent of water. As long ago as 1753, indeed, the great Franklin cor- rectly explained this where he says "The spout appears to drop or descend from the cloud though the materials of which it is com- posed are all the while ascending,/^/- the moisture is condensed faster in a right line downwards, than the vapours themselves can climb in a spiral line up- wards" The freshness of the water in a marine spout is clearly testified to in a story quoted by Prof. Davis in his Meteorology : A waterspout had fallen upon a vessel and poured its contents so freely over the captain, that he was nearly washed overboard. He was asked afterwards, rather jocularly, if he had tasted the water ? "Taste it," said he, " I could not help tasting it. It ran into my mouth, nose, eyes and ears." Was it then salt or fresh asked his querist ? " As fresh," said the captain, " as ever I tasted spring water in my life." Waterspouts occur mostly in the tropics, and during the day hours. They are children of the sunshine. The prevailing funnel shape, tapering down- wards, of the waterspout or tornado cloud, is a consequence of the increased pressure of the air near the surface. Above the surface the absence of friction and the lower pressure allows the cen- tral area of rarefaction, produced by the rapidly 154 THE STORY OF THE EARTH'S ATMOSPHERE. whirling air, to extend for some space laterally. Lower down the centrifugal tendency of the ro- tating air is met by increased inward pressure and is thus confined to a narrower space. Out- side the central core the air moves gently towards the centre. When water in a basin is descend- ing through a hole, a similar gentle flow may be observed, the rapid whirling only extending for a short distance immediately around the hole. Even in destructive tornadoes the area of dangerous damage and violent wind is confined to comparatively narrow limits. The width of the destructive path of the tornadoes in America has been found by Finley to vary from 20 feet to about 2 miles, the average being about 1369 feet. The length of their paths is usually not more than 20 miles, since the forces which give rise to them, unlike those of cyclones, depend entirely on specially marked vertical gradients of tem- perature which seldom prevail simultaneously over large areas. The mode in which the air travels up into and round these phenomena, may be gathered from the adjoining Fig. (36). Instead of rising up vertically it travels along the lines which are represented as winding spirally round the funnel until it becomes cooled partly by ascent and partly by expansion into the tornado-core and its vapour becomes visible at a point C considerably below the ordinary cloud level FH. Tornadoes may be regarded as a kind of at- mospheric eruption analogous to those by which the volcanic energy of the earth's interior is ex- pended in one spot. They prevail where the local conditions favour WHIRLWINDS, ETC., OF THE ATMOSPHERE. 155 the establishment of explosive heat conditions. For example, where the geographical conditions are favourable to the facile movement of cold air from the north alongside or above warm air from the south. Such an area exists far excellence over the flat river basins of the Mississippi, Missouri, FIG. 36. Tornado funnel cloud. and Ohio. The states lying in these basins are those in which tornadoes are found to be most prevalent. The general north and south trend of the mountains and hills in America favours the flow of air of such contrasted conditions, while the prevalent east and west ranges in the old world make them act as preventive barriers. The time of year most favourable to the pro- duction of tornadoes is Spring or early Summer, when the earth is heating up rapidly, and the air 156 THE STORY OF THE EARTH'S ATMOSPHERE. above it is still cold from the effects of the pre- ceding winter. Lieut. Finley found May to be the month of greatest frequency of tornadoes, while during autumn and winter they are almost absent. The time of day at which they mostly occur is in the afternoon when the accumulation of heat in the lower layers has reached its great- est amount. When the gun is loaded it only re- quires the slightest pull on the trigger to release an immense potential of energy. Half a degree more temperature and the tornado is born and starts off on its wayward journey. The destruction caused by these tornadoes in America is hardly realised in Europe which is so happily exempt from them. At the same time the deaths from this cause in the U. S. are esti- mated to be less than those caused by fire and flood. Thunderstorms, like tornadoes, originate from the uprise of a mass of warm moist air, but the width of the column of uprising air is much greater, and the whole action is much less con- centrated and violent. The vertical anatomy of a thunderstorm is shewn in Fig. (37) where the spectator is sup- posed to be standing to the right and viewing an advancing storm. First of all he sees a layer of cirro-stratus cloud (c) * commonly in the western sky in the afternoon. Gradually this grows thicker, and from its under surface festoons (/) similar to those in the " Festooned Cumulus," Fig. (28), appear. * This layer should extend as far again as the width of the figure to the right. The exigencies of space have necessitated its curtailment in the adjoining figure. WHIRLWINDS, ETC., OF THE ATMOSPHERE. 157 The cirro-stratus may extend from 10 to 50 miles in advance of the storm. In this way as soon as they are visible, thunderstorms may read- ily be forecasted within a few hours by experts such as the late Rev. Clement Ley. Then follow the thunderheads (/") of cumulo-nimbus (as in frontispiece) which represent the front portion of the uprising current. Below these, a low level FIG. 37. Thunderstorm in section. base (/>) of similar cloud is seen, underneath which is a rain curtain (/-). A ragged squall cloud (s) rolls beneath the dark cloud mass, a little behind its forward edge, and the whole structure moves over the land at the rate of from 20 to 50 miles an hour. As the squall cloud comes overhead the wind changes suddenly from an in-flow to an out-flow, represented in the figure by two arrows near the surface, with heads to the right. In con- trast with the hot muggy air preceding the storm, this squall is deliciously cool, especially in a Ben- gal north-wester. Simultaneously with the arrival of the squall, the barometer rises about ^th of an inch, the rain or hail begins to fall, the light- ning flashes and the thunder crashes right over- head, until the centre passes, and everything gradually resumes its former aspect, except the temperature which has been permanently low- 158 THE STORY OF THE EARTH'S ATMOSPHERE. ered. The outflowing squall is believed to be very similar to the recoil of a gun when it is dis- charged. The humid air in the centre of the storm expands so suddenly in rising, that it actu- ally kicks against the surface air, and drives it outwards in the direction where the pressure is least, that is towards the front of the storm. Thunderstorms travel along with the move- ment of the air near their tops, while the preced- ing inflow in front occurs as in the figure in the contrary direction. This has given rise to the saying that they travel against the wind. There are at least two kinds of thunderstorms. One is chiefly confined to the equatorial regions and the summer in high latitudes, and the other occurs in connection with cyclones in their south- east quadrants. They are both due to the convectional ascent of warm moist air, but in the former case it is locally manufactured during the daytime. In the latter, it is often imported from a distance. In the former storms, the cloud is isolated and continuous often from icoo feet up to the cirrus level at 30,000 feet. When it ceases to ascend it spreads out in a sheet in all directions, so that a thunderstorm cloud of this kind often pre- sents in the distance the appearance of a huge anvil. The cyclonic thunderstorms are not so de- pendent on local sun heat, and frequently occur at night, and in the winter season in Scotland, Norway and Iceland. In this case the cooling of the upper air produces the same effect as the heating of the lower. Like the tornadoes they travel mostly east- wards, and their occurrence generally betokens "WHIRLWINDS, ETC., OF THE ATMOSPHERE. 159 the existence of a cyclone centre to the N. W. in Europe, and to the S. W. in Australasia. As long ago as 1752, Franklin proved by his memorable kite experiment at Philadelphia, the identity of lightning with electricity artificially produced on the earth. There is, however, still very little known as to the exact cause of the accumulation of electrical potential which finds vent in the lightning discharge. The air is ordinarily found to be charged with a certain amount of positive electricity, while the earth is usually negative. The concentration ob- served in thunderstorms, is believed to be due to the increase in electrical quantity, and rapid increase in electric potential (or power of do- ing work) caused by the masses of damp air which rise up, form towering cumulus clouds, and discharge their vapour in drops, by conden- sation. As the tiny droplets of vapour in the cloud unite to form single large water drops, the elec- trical charges which always exist to some degree on their surfaces, become added together. Not so the surfaces; since the surface of a single globe is always smaller than that of two globes which unite together to form it. Consequently, as more and more droplets unite together, the electricity has less room over which to spread itself. It consequently increases in thickness, or in electrical language, density. It takes 300 trillions of droplets to form a single rain drop, and it thereupon results that the surface of the rain drop is one 8-millionth of the area made up of all the surfaces of its component droplets. Therefore the density of electricity on the re- sulting raindrop is 8 million times increased and 160 THE STORY OF THE EARTH'S ATMOSPHERE. by a simple electrical law its potential or power to discharge, is increased 50 billion times. We can thus understand how it is that so long as masses of damp air are ascending in sufficient quantity to cause the great condensation and rainfall which usually accompanies thunderstorms, the tremendous discharges of lightning may be produced and accounted for without recourse to any special theory of its origin. Lightning destroys about 250 persons per annum in America chiefly between April and Sep- tember. Lightning conductors act by equalising the flow of electricity between the air and earth and preventing a disruptive discharge. They are now generally made of iron and must always be in contact with damp earth since they act not by drawing the atmospheric elec- tricity down, but by allowing the earth electricity to flow upwards. Even in perfectly clear weather there is a con- stant difference of electrical condition between the air and earth. In flying kites at Blue Hill near Boston with steel wire, a conductor has to be attached to the earth, otherwise the observ- ers even on a cloudless day experience severe shocks. Lightning is of various kinds. Sometimes it branches out in all directions from cloud to cloud and is too far above the earth to strike through the intervening space. This frequently happens in the tropics where the author has often wit- nessed a beautiful electrical storm right over- head, the thunder of which was inaudible. At other times, especially in cyclonic thunderstorms, it occurs in lower clouds and strikes down to WHIRLWINDS, ETC., OF THE ATMOSPHERE. 161 earth in what is termed forked lightning accom- panied by loud thunder. Thunder is produced by the rapid heating and expansion of air by the discharge passing through it. The noise is occasioned precisely in the same way as the sudden generation and expansion of gas which ensues upon the ignition of gunpowder in a confined space such as a gun. The destruction of a tree or house is occa- sioned in like manner by the expansion of air or material which is unable to conduct the dis- charge. Upon a human being the effect is partly caused by heat and partly by shock to the nerv- ous system. A peculiar form of lightning is occasionally witnessed in which it descends from the clouds in a globular form. These isolated globes of electricity play pecul- iar pranks, meandering slowly along in the most wayward and capricious manner, and apparently doing little damage until they burst. They are believed to be somewhat of the nature of Leyden jars in which a layer of air takes the place of the glass. St. Elmo's fire is an appearance sometimes seen on the masts of ships in stormy weather. Each mast head is surrounded by a faint lumi- nous ball of electric light. It is really a brush discharge which takes place between the top of the mast and the highly charged atmosphere over- head. The most violent storms of lightning and thunder in the world are probably to be found in the north westers of Bengal where the lightning is continuous for more than an hour at a time. This is due to the enormous condensation caused 1 62 THE STORY OF THE EARTH'S ATMOSPHERE. by the upward convection of the very damp air of that region. The most awe-inspiring electri- cal manifestations, however, frequently occur when a thunderstorm occurs in a region like Col- orado where the air is usually dry. The author once experienced a storm at the Colorado Springs railway station in which every time a flash of lightning appeared, a miniature flash and loud re- port were simultaneously observed in the tele- graph office. The wire of the conductor outside was fused, and upon one of the party venturing out with an umbrella up he returned declaring it was raining lead. At the summit of Pike's Peak, 14,000 feet high in the same district, the observers in the now dis- continued observatory used occasionally to expe- rience most disagreeable shocks even in the sim- ple act of shutting the door, while after walking across the room they could light the gas with their fingers. In Canada, in winter when the air is very dry and frosty, the same phenomena are frequently observed. It was formerly supposed that thunder and hail were unknown in the Arctic regions, but Mr. Harries of the Meteorological Office has recently shewn that they both occur right up to Spitz- bergen and are fairly frequent in the Barents Sea. It seems possible that the warm ocean cur- rents bring enough warmth and moisture to these cold regions to cause the vertical instability of the atmosphere which originates them. The peculiar arched appearance of the clouds in norwesters, pamperos, and the arched squalls of tropical seas and higher latitudes is simply an effect of perspective caused by a long roll of cloud advancing athwart the spectator. SUSPENSION AND FLIGHT IN ATMOSPHERE. 163 CHAPTER XIII. SUSPENSION AND FLIGHT IN THK ATMOSPHERE. THE conquest of the earth by man may be looked upon as tolerably complete. The con- quest of the air has so far eluded all his efforts. Only for short periods and with great trouble and risk has he been able to mount into the air by the aid of balloons. The balloon itself, old though it may appear to most of us, dates back only 100 years. Lichtenberg of Gottingen, in 1781, was among the first to experiment, and made a small balloon of goat-skin, which ascended in the air when filled with hydrogen. Thomas Cavallo, an Italian ref- ugee, about the same time began by blowing soap bubbles filled with hydrogen, and watching them mount as the school-boy does to-day. Before he got much further, a step in advance was made in France by two brothers, Montgolfier, who curi- ously enough started by trying to make a cloud of steam ascend in a silk bag. On lighting a fire to increase the "cloud " they accidentally struck on the " hot-air balloon," which has rendered their names famous. The first human being to actually ascend in a balloon was Pilatre de Ro/ier on Nov. 21, 1783; but in this case ordinary coal gas was employed, and has ever since been generally adopted. Soon after this, in 1785, Blanchard safely crossed the English channel in a balloon, and thenceforward ballooning came into fashion, though at first it was frequently attended with mishaps and loss of life. The parachute, which 164 THE STORY OF THE EARTH'S ATMOSPHERE. is now so familiar to the world through the re- cent beautiful descents effected by Baldwin, was first employed by Garnerin on Oct. 21, 1797. He then descended safely from a balloon, but experi- enced violent oscillations. These are now obvi- ated by means of a central aperture through which the imprisoned air flows quietly upwards. The history of the balloon ascents of Lunardi, Tissandier, Fonvielle, Gay Lussac, Green, Nadar, Glaisher, and Coxwell is that of continual im- provement, success, and safety. Their voyages, particularly those of the two last, have added considerably to our knowledge of the conditions of the upper air. Within quite recent years great strides have been made in the construction of balloons, chiefly in relation to their use in opera- tions of war, by the English military balloon department at Chatham. The material employed is oxgut, which is ca- pable of holding pure hydrogen without leakage. Since pure hydrogen is nearly 2-| times as light as coal gas, balloons filled with it have greater buoy- ancy and are better fitted to withstand the de- pressing influence of the wind when captive. A balloon of this material, which contains 10,000 cubic feet of gas, weighs only 170 Ibs. The top valve is made of aluminium, and a telephone con- ductor is arranged for communication between the occupant of the car and those below. Men can readily be seen at a distance of two miles from the car, and general military reconnaissance, including photography can be conducted with considerable accuracy. By the aid of balloons man has certainly suc- ceeded in attaining suspension in mid air. They have not, however, aided him in travelling through SUSPENSION AND FLIGHT IN ATMOSPHERE. 165 the air towards some definite point. If he com- mits himself to them he must needs go nolens vole us whither the wind may carry him. Far from having conquered the air as he has conquered the earth and the sea, he has hardly more power to guide himself in a balloon than a piece of straw hurled along by a whirlwind. Some few years back Messrs. Krebs and Re- nard in France were supposed to have solved the problem of the dirigible balloon by means of a cigar-shaped balloon and a motor which drove a rotary fan screw at one end ; but though in calm weather progress at some few miles an hour was obtained, it was found to be useless against the wind which ordinarily prevails at any consider- able height above the earth's surface. The late Prof. Helmholtz dealt a death-blow to the practical realisation of the dirigible bal- loon by shewing on theoretical principles that a balloon could not be driven against the air at a rate of more than twenty miles an hour without destroying its framework. To accomplish aerial locomotion therefore, we must look elsewhere. From the earliest times the flight of birds has attracted the admiration and envy of mankind. The ancient legend of Icarus who made a pair of wings and singed them off by flying too near to the radiant Phoebus, was evidently based on the desire man has always shewn, to be able to fly like a bird. As long ago as 1470, that "preternatural gen- ius," Leonardo da Vinci, in the intervals of paint- ing the holy family, etc., amused himself by plan- ning amongst other things flying machines. More- over, he appears from his remarks, even then, to have realised that the main difficulty to be met 1 66 THE STORY OF THE EARTH'S ATMOSPHERE. with apart from elevating and motive power, was the question of balance. The recent accident by which that enthusiastic soarer Herr Lilienthal of Steglitz lost his life, oc- curred through his inability to accommodate his balance to a sudden gust of wind. The early history of the attempts of man to My is not calculated to inspire the human race with a belief in its intuitive sagacity. For the most part it is a history of miserable failures and fatuous inability to realise the feebleness of hu- man muscular power. The first serious attempt to grapple scientifically with the problem was inaugurated by Wenham in 1866 in a paper be- fore the Aeronautical Society, in which the prin- ciple of suspension by soaring as well as flapping was alluded to. Since that time great progress has been made in the development of what are termed flying machines by Prof. Langley of Pittsburgh, Hiram Maxim of England, Octave Chanute of Chicago, and Hargrave of Sydney. In these machines no attempt is made to imi- tate the flapping by which birds mount into the air, but only of those principles by which many of them are enabled to soar or sail with out- stretched wings when sufficient speed has been attained. Although it is a fairly safe rule to follow Nature, exact imitation is by no means in every case necessary or advisable. Thus, just as in travel on the earth's surface, it has been found more convenient to employ the wheel than rap- idly moving artificial legs, so in the atmosphere, it is better from an aerial engineering point of view to analyse the compound movement of a SUSPENSION AND FLIGHT IN ATMOSPHERE. 167 bird's \ving into the two distinct elements, sup- port and forward propulsion, and deal with them quite separately. In the case of the bird, the wing thrusts backwards, and also acts as an in- clined plane, which, when it is forced horizontally through the air, converts the pressure into sup- port. In the artificial flying machine, the back thrust is given by the fan screw or aerial wheel at the rear of the plane, and the plane itself re- mains fixed at a certain angle. The principle of the inclined plane is strictly analogous to that by which a kite is suspended when moored in a breeze. When the breeze fails, the boy converts his kite into a flying machine by running with it, and restoring support by the relative breeze thus created. If we cut the string of the kite and supply it with a motor and propelling fan, it will fly itself without the boy's aid, and become a veritable free flying machine. The kite, therefore, is the basis of the flying ma- chine. A flying machine is a self-propelled kite. There are two actions of the wind on a kite or inclined plane. Partly it tends to make it drift to leeward, and partly to lift it upward. Certain birds, such as the Kestrel hawk, shewn in fig. (38), the eagle, vulture, and albatross, (especially the two latter), possess the power of obviating the tendency to drift, and of keeping themselves poised, or of sailing for long periods without flapping by the action of the wind on their wing planes. The precise way in which this is accom- plished is not yet fully determined. Maxim regards it as effected by an intuitive utilisation on the part of the birds of local upward cur- rents which exist naturally, or else artificially up declivities. 1 68 THE STORY OF THE EARTH'S ATMOSPHERE. The albatross of the southern seas which the author has frequently watched for hours and days together, undoubtedly makes use of the wind blowing up a wave to restore its lift, after it has descended nearly to the surface of the water. Prof. Langley, on the other hand, attributes the suspension in both hovering and sailing, more FIG. 38. Kestrel hawk hovering. generally to a like intuitive adjustment on the part of the bird to certain rapid changes which are found to occur in the speed of the wind. When a strong gust comes, he slides down a little to meet it, and overcoming the back drift en- tirely by his forward momentum, is able to utilise it simply for lifting him vertically to the same height he was at before. When the lull occurs, by lying flatter, he is able in this way to derive a larger proportion of lift from the lighter wind, SUSPENSION AND FLIGHT IN ATMOSPHERE. 169 and therefore maintains nearly the same eleva- tion, and so on. In the circular sailing so commonly seen when vultures sight a piece of carrion, the inclination of the wing planes is similarly increased on the windward half and decreased on the leeward half of the circle. The soaring and sailing of birds is only pos- sible while the air is in motion. Directly there is a calm, even the Albatross is obliged to flap. It is therefore only when a wind is blowing, that soaring can be exactly imitated by an intel- ligently controlled flying-machine. In any other case an artificial wind must be created by means of the rotating fan-screw in order to ensure sup- port, and the plane must be kept constantly in- clined upwards. It will be long before man will be able to gain such a sense of flight as to be able to dispense with the motor of his flying machine and sail like the albatross without any apparent wing motion, but such a sense will doubtless gradually be de- veloped as soon as he is fairly launched into the air, on what is termed the motor aeroplane, and future generations will witness the ascent of man. The present position of human flight stands thus. Mr. Maxim has built a large machine on the aeroplane principle, which on being propelled forward, has lifted itself and several people a few feet from the ground. Professor Langley has made a small model machine actuated by a petroleum motor which has limvn for a considerable distance while the motive power held out. Mr. Ha r grave of Sydney is making a machine but no actual flight has yet been announced. 170 THE STORY OF THE EARTH'S ATMOSPHERE. The basis of this machine is the so-called cel- lular or double plane kite of which Mr. Hargrave is the inventor, and which has recently been shown to be the most efficient and stable kite yet made. Though a slavish imitation of bird architec- ture has never found favour with flying machin- ists, a study of birds, especially the large soaring and sailing birds, shows, what the Duke of Argyll in his " Reign of Law " has so lucidly demonstrated, that birds fly " not because they are lighter, but because they are immensely heavier than the air. If they were lighter than the air they might float, but they could not fly. This is the difference be- tween a bird and a balloon." Any machine to travel through the air can only do so in consequence of its superior momen- tum. Consequently a flying machine must be heavy in proportion to the resistance it offers to the air. Another important point is deduced from the circumstance that a bird's wing presents a great length (from tip to tip) and narrow width to the wind. For example, the wings of that king of flight the Albatross (JDiomedea exulans) measure 15 feet from tip to tip and only 8 inches across. There is a reason for this. When a plane sur- face is forced through the air, the upward pres- sure of the air is mostly concentrated near its front edge. If the surface extended far back from the edge, its weight would act at some dis- tance from the front edge. Consequently the unbalanced pressure of the air would tend to turn the plane over backwards. If, however, its width were small, the weight would act so close SUSPENSION AND FLIGHT IN ATMOSPHERE. 171 to where the resistance acts in the opposite direc- tion that the forces would neutralise each other and stability ensue. Mr. Hargrave has adopted this principle in his cellular or box kite in fig. (39), whose construc- FlO. 39. tion is sufficiently obvious from the figure to ren- der detailed description unnecessary. The dimensions in the figure are in inches. The length of each cell (from right to left in fig- ure) is 30 inches, and the width and height and opening between are about 11 inches; but these dimensions may vary, so long as the two cells to- gether form a nearly square area. An important feature of this peculiar tailless kite consists of the 172 THE STORY OF THE EARTH'S ATMOSPHERE. covered-in sides. These ensure stability even bet- ter than two planes, bent upwards in V shape, such as the wings of the kestrel when hovering, and they prevent the kite from upsetting, very much as the sides of a ship give it stability. Mr. Maxim once showed the advantage of such side planes by a simple experiment, in which a piece of paper, when held horizontally and let fall to the floor, is seen to execute a series of zig- zags in the air, frequently ending in its complete overthrow; whereas, when the same piece of paper is folded up round the edges like a boat, it sails to the floor quite evenly, and in a straight line. The flying machine of the future seems des- tined to be built somewhat after this pattern. The prime problem is to launch a stable aero- plane into the air, provided with an engine and screwfan powerful enough to drive it forward at the velocity required. Mr. Maxim places his planes at a slope of i in 13, and his practical ex- periments have shown that the support gained by the pressure of the air on such planes is more than twenty times, and the motive power of the fanscrew thirteen times what had formerly been supposed. The engine which drives the fan is a very light one, actuated by petroleum. Hargrave estimates the entire weight of an engine to gen- erate 3^- horse power at 30 Ibs. It is placed in the hollow between the two cells in fig. (3^). Prof. Langley's recent experiment with his model over the Potomac showed that the elevat- ing power derived from such an engine is suffi- cient. The main difficulty will be to ensure sta- bility under all conditions, and to accommodate the apparatus to the varying currents, by the aid of movable front and side wings. To essay a SUSPENSION AND FLIGHT IN ATMOSPHERE. 173 journey except in a dead calm, without consider- able practice, would at first probably end in mis- haps. An era of preliminary misadventure, in fact, appears to be almost a necessary corollary to the establishment of every new form of loco- motion. That success, however, will eventually be achieved is now the firm belief of all those who have studied the question. The development of the flying machine will also be much assisted by improvements in the kite. The most efficient kite will be the most suitable aeroplane basis for the flying machine. The kite was first invented by the Chinese gen- eral, Han Sin, in 206 B. c., for use in war, and was frequently employed after that date in China, by the inhabitants of a besieged town, to communi- cate with the outside world. After this kites ap- pear to have degenerated into mere toys. At the middle of the present century, how- ever, Pocock of Bristol employed them to draw carriages, and is said to have travelled from Bris- tol to London in a carriage drawn by kites. They were also occasionally employed to elevate thermometers to measure the temperature of the upper air, by Admiral Back on the Terror, and Mr. Birt at Kew in 1847. These observations had been quite forgotten when the author first suggested the employment of kites for systematic observations in 1883. It has since been discovered that Dr. Wilson of Glasgow, as long ago as 1749, resuscitated kites from their long burial with a similar idea of em- ploying them to measure temperature. In the author's experiments, steel wire was first employed to fly them with. Two kites of diamond pattern made of tussore silk and bam- 174 THE STORY OF THE EARTH'S ATMOSPHERE. boo frames were flown tandem, and four self- recording Biram anemometers weighing i Ibs. each were attached at various points up the wire. Heights from 200 to 1500 feet were reached by the instruments, and the. increase of the average motion of the atmosphere was measured on sev- eral occasions for three years. Kites were also employed, first by the author in 1887, to photo- graph objects below by means of a camera at- tached to the kite wire, the shutter being released by explosion. Since that time kite photography has leapt into popularity, and has been success- fully practised by M. Batut in France, Capt. Ba- den Powell in England, and Eddy in New jersey. The figure following represents a recent pho- tograph of Middleton Hall, Tamworth, taken by Capt. Powell with a kite-suspended camera at a height of about 400 feet above the ground. At the Blue Hill Meteorological Observatory, near Boston, Mass., which is carried on by Mr. A. L. Rotch, tandems of kites are used to elevate a box of self-recording instruments, cameras, etc. The adjoining fig. (41) shows the building, which is 630 feet above sea level, and a tandem of Hargrave kites supporting a camera with the adjustment involving the use of an extra cord for slipping the shutter, devised by Mr. W. A. Eddy. The height of the camera is determined by simul- taneous observations of theodolites at the end of a base line. By attaching several kites to the same main wire great altitudes have been reached at Blue Hill, and complete records of the pressure and temperature recorded on a revolving drum of a Richard's thermograph and barograph. The highest point attained so far was 9385 SUSPENSION AND FLIGHT IN ATMOSPHERE. 175 feet above sea level, in October, 1896. In order to accomplish this, nine kites (of moderate size) and three miles of steel wire were required. At FIG. 40. the highest point the temperature fell to 20, while at the observatory, 8755 feet below, it was 46. On other occasions when the author was present heights of 6079 an( l 7333 f cet were at- tained. For all such purposes, therefore, kites are able to do as much as free balloons up to about three miles. They are also cheaper and more portable 176 THE STORY OF THE EARTH'S ATMOSPHERE. than captive balloons, and possess far greater ele- vating power, especially in windy weather, when such balloons are nearly useless. FIG. 41. It was further suggested by the author in 1888,* that kites could be used for various pur- poses in war as well as science. * Les Cerf Volants Militaires. Bibliotheque des Con- naissances Militaires. Paris, 1888. SUSPENSION AND FLIGHT IN ATMOSPHKKF,. 177 Since then Capt. Baden Powell, in May, read a paper on " Kites, their uses in War." In both these publications it was pointed out that kites possessed several distinct advantages over balloons; next, that they could be applied to all the purposes for which balloons could be em- ployed, such as signalling, photography, torpedo projection, carrying despatches between vessels, and lastly, they could be employed to raise a man for purposes of reconnaissance. This question of " man raising " was long scouted as impossible, but both ('apt. Powell and Mr. Hargrave have practically proved its possi- bility by elevating themselves by kites, the former having reached a height of 100 feet. To give an idea of the si/e of kite required for such a purpose, Cupt. Powell was lifted by a single large kite spreading 500 square feet, weigh- ing 60 Ibs., and capable of folding into a package 12 feet long. Mr. Margrave, at Stanwell Park, N. S. U'ales, on Nov. 12, 1894, was raised 16 feet by four kites flown tandem which spread together an area of 232 square feet, the wind blowing about 21 miles an hour. The total weight sup- ported was 208 Ibs. An ounce of fact is said to be worth a ton of theory. Here we see that in an ordinary 20 mile an hour wind a kite area amounting to 250 square feet is ample to support a man. For a speed of only 10 miles an hour a larger surface would be required, but if the system of tandem kites recommended bv Margrave is fol- lowed, this could be readily attained by the ad- dition of more kites. I'nder the-e circumstances, by two or more Margrave kites a man could be raised, as in fig. 42, and effect a reconnaissance 178 THE STORY OF THE EARTH'S ATMOSPHERE. of an enemy's fortifications and dispositions, especially in mountainous country, with consider- able ease and far greater immunity than in a cap- tive balloon. The portability of such a series of kites even for man lifting may be guessed from the remark FIG. 42. by Mr. Hargrave, in a paper dated August 5, 1896, that "a nineteen square feet kite has been made, that weighs only 19 ounces, and folds to about the size of an umbrella. Ten of these could be tucked under one's arm, and with a coil of line and a decent breeze, an ascent could be made from the bridge of a torpedo boat or the top of an omnibus." The torpedo boat certainly sounds more he- roic, and probably less dangerous than the om- nibus. Numerous possibilities have been suggested by Capt. Baden-Powell, and there seems no rea- son why kites should not enter in as a regular SUSPENSION AND FLIGHT IN ATMOSPHERE. 179 part of the paraphernalia of naval and military operations. Some few years back, the author, with a kite of the ordinary diamond pattern, 18 feet by 14 feet, was able to carry up 600 feet of steel rope cable, by which Col. Templer tethered his large war balloon in Egypt. This weighed 50 Ibs., and as an additional test, a man's kit weighing 10 Ibs. was suspended to its tail. Two such kites could lift a man and pack away like fishing rods. Quite recently (July, 1896) a brochure by Prof. Marvin, dealing with the whole science of kites, has been published by the U. S. Weather Bureau. This represents the most complete discussion of kite-flying up to date, and one or two of the results are worthy of special record. The best kites are double plane Margraves, with certain improvements in details. Tandems of two kites only, with 9000 feet of wire out, have several times reached over 6000 feet in height. Kites can be made to fly at angles of 60 or more, and utilise most of the wind pressure in lifting. By adjusting the point of suspension or alter- ing the kite, we can make it fly in the ideal po- sition. This is found to occur when the direction of string or wire is inclined at an angle of 66 to the horizon, and cuts the kite plane at right angles, so that the latter is inclined at 24 to the hori/on. Also theory shews that, in order to gain the greatest effect when kites are flown tandem, the largest kite or a bunch of two ought to be placed at the top of the main wire. 180 THE STORY OF THE EARTH'S ATMOSPHERE. In conclusion. By balloons alone, man will never be able to complete the conquest of the air. For travel through the air, or as Prof. Langley terms it " aero dromics," steam propelled kites will be the future vehicle. For rest in the air, it is not impossible that kites will again be a serious rival of balloons. In fine, we may look upon kites as likely to take a very much more important place in the future than in the past story of our atmosphere. Before closing this chapter, it is worthy of notice that the principle of the inclined plane is made use of in two other important applications of the motion of the atmosphere besides that of supporting kites viz., in the sails of ships, and in windmills. In the former, the wind meets the sail at a certain angle, and produces effects analogous to those on a kite, especially when the latter forges overhead, under the influence of a freshening breeze. The water here acts like the controlling string, except that it allows the sail and boat to move through it, and, so to speak, form fresh attach- ments every instant. The slip to leeward is analogous to the lift in the kite, which is checked by the inextensibility of its string. The back drift is prevented by the pressure of the water, and the shape of the main-sail, which tends to make its forward part, and therefore the boat, turn continually towards the w T ind. The shape of the boat, the jib-sail, and the action of the rudder convert this turning-round force into con- tinuous motion ahead. As in the case of the kite, there is one posi- tion (different for each combination of sails and SUSPENSION AND I LIGHT IN ATMOSPHERE. 181 boat, and varying with the force of the wind) in which the greatest advantage or speed is attained for a given direction. To find this and maintain it is the object of the steersman. In practice it appears to be very similar to the best inclination for a kite, so that for any FIG. 43. Yachting in Sydney Harbour. wind between head and beam, the sail should not be inclined more than 24 to the keel. In the case of a windmill, " the angle of weather," as it is termed, or the angle which the sails make with the plane of rotation, answers to the angle between the keel and the boat-sail, and varies, according to circumstances, round an average of 24. Windmills are a means of converting the 182 THE STORY OF THE EARTH'S ATMOSPHERE. motion of the wind into mechanical energy, which may be employed either for pumping up water, grinding corn, or, as Lord Kelvin suggested in 1881, for generating electricity. Before the present coal-burning epoch, windmills used to be extensively employed for corn-grinding. To-day they are mostly employed in raising water for drainage, storage, or irrigation. Most railway stations, every farm-house, and almost every pri- vate country house in the Middle United States and Australia, have their windmill and tank. Labelled "cyclone" or "eclipse," according to their particular make, they form quite a feature of the landscape, and it is estimated that there are more than a million such mills in the United States alone. The " useful efficiency " of windmills, espe- cially in the modern geared form, is comparable with that of the best simple steam-engines. A geared modern wheel, 20. feet in diameter, will develop 5 horse-power in an 18 mile an hour breeze, and can be applied to work agricultural machinery and dynamos for electric lighting. With a single wheel of this size, Mr. M'Questen of Marblehead Neck, Mass., U. S. A., works an installation of 137 electric lights, for which he for- merly used a steam-engine. As a result, he finds that he effects a saving of more than 50 per cent. According to Lord Kelvin, wind still supplies a large part of the energy used by man. Out of 40,000 of the British shipping, 30,000 are sailing ships, and as coal gets scarcer, "wind will do man's work on land, at least in proportion com- parable to its present doing of work at sea, and windmills or wind motors will again be in the ascendant." LIFE IN THE ATMOSPHERE. 183 CHAPTER XIV. LIFE IN THE ATMOSl'HERE. THE limits of space warn us abruptly that we must bring our story to a close. And yet, facing us in the book of nature, there is a large unwrit- ten story of how the atmosphere affects the lives of men and plants, embracing questions connected with weather, climate, disease, hygiene, agricul- ture, sanitation. The chief elements of climate have already been dwelt upon in the chaper on temperature and rainfall. Hygiene and sanitation open out points in which other factors, such as soil enter as well as air. The relations of the atmosphere to agricul- ture, though a subject of immense interest to the agriculturist, is not a fascinating one to the gen- eral public. Prof. Hilgard, of the University of California, has exhaustively discussed this theme in a bulletin published by the U. S. Weather Bu- reau, 1892, and Sir J. B. Lawes and Professor Gilbert have carried out experiments in England, at Rothampsted, all of which show that in order to derive our maximum subsistence from the soil, we must have a thorough knowledge of ttie ac- tions which take place between it and our atmos- phere. The relation of climate to life, health, and disease is a very wide one, and though it has at- tracted man's attention for years, it has only re- cently been studied with anything like scientific accuracy. An excellent summary of the prin- J3 184 THE STORY OF THE EARTH'S ATMOSPHERE. cipal modern results will be found in Moore's Meteorology. As an example of how disease is dependent on season, the following table will suffice: Development measured by_ Mortality. Disease Maximum. Minimum. Enteric fever Oct., Nov. May, June. Smallpox Jan. to May. Sept., Oct. Measles June, Dec. Mar., Oct. Scarlet fever Oct., Nov. Mar. to May. The opposition between enteric and smallpox, in regard to season, shows clearly that seasonal conditions have a great deal to answer for in the development of disease. There is little doubt that besides the regular effects of seasonal changes, the quality of the air of a place is a potent factor in relation to health. We talk of going away for a change of air, and we know that beneficial effects usually follow if we choose our fresh locality aright. The air of cities, as we have seen, contains vastly more dust particles than that of the coun- try, and it is full of other impurities, thrown off by the multitudes of human beings crowded to- gether in a small space. The pallor of children in cities compared to the ruddy health of those who dwell in the com- paratively unpolluted country air is well known. Similarly the air on mountains and high plateaux is less dusty and vastly purer than that near sea- level. In certain parts where vegetation decays in presence of water, noxious exhalations arise called significantly mal-aria (bad air), and cause fevers not only in the Mangrove Swamps of the LIFE IN THE ATMOSPHERE. 185 tropics, but formerly even in the undrained fen- districts of England. This bad air usually remains quite close to the ground, and its effects can often be obviated in the tropics by sleeping on an upper floor. The atmosphere undoubtedly acts in many cases as a disease propagator by conveying germs from one place to another. For example the mysterious influenza, which has of late years so afflicted the whole world, is evidently propagated through the air. As a rule, however, water is a far more effective dissemi- nator of disease than air, and where a good water supply has been established, in many parts of India, where formerly cholera was rife, it now occurs very rarely and in a milder form. In general, the atmosphere acts as a health and life giver. The more fresh air we breathe, the more we dilute the poisons which would otherwise harm our systems. We are no doubt temporarily and permanently affected by the particular climate we live in, as well as by the air we breathe. Climate is an average of the general weather con- ditions, and is chiefly determined by the tempera- ture, rainfall, humidity, sunshine, and winds which prevail in a district. All the regular and irregular variations men- tioned in chapters (IV.) and (VIII.) are involved, particularly annual and daily temperature ranges. At some seasons a change to a drier and warmer climate such as that of Egypt or Colo- rado is desirable. Sometimes a mild one like that of Madeira or New Zealand is recommended, while a return to 1 86 THE STORY OF THE EARTH'S ATMOSPHERE. England or Europe is often indispensable to the Anglo-Indian who has endured years of Indian heat. Permanent residence in different climates tends to develop certain national characteristics. Thus the dry, rapidly changeable, continental climate of North America, accounts for the ac- tivity and impulsive go-aheadness by which the Americans are characterised. At the same time it accounts for their liability to neuralgia. The debilitating, nerveless lassitude of the natives of tropical coasts is directly due to the moisture and heat. The dry heat of central India and Arabia de- velopes the martial energy of the Sikh and the Bedouin, while the mild but cool and temperate climate of England and Western Europe is dis- tinctly accountable for the well-balanced mental and physical development of the races which have hitherto ruled the world. Climates may be hot or cold, moderate or ex- treme (i.e., of small or large range), dry or damp, calm or boisterous. It was formerly deemed sufficient to pay at- tention to the temperature alone, but it has now been found that the other factors are equally important. Even in regard to temperature, the average for the year is no safe criterion. The average is an artificial centre, round which the values oscillate, and may be very seldom experienced. The ranges are far more important. Thus Calcutta, in Bengal, has the same mean temperature of 77.7 F., as Agra, in the North- West Provinces, but their climates are very dif- ferent when the ranges of temperature are con- LIFE IN THE ATMOSPHERE. 187 sidered. The difference of average temperature between the hottest and coldest months at Agra is 34, at Calcutta only 20. The average daily range at Agra is about 30, at Calcutta only 16. When we touch rainfall and humidity we find Agra has only 29 inches to Calcutta 65 inches; while if 5 represents the humidity at Agra, 8 rep- resents the amount at Calcutta. Agra also has half the cloud, and therefore about double the bright sunshine of Calcutta. Such instances could be multiplied indefinitely. Here, therefore, we have two places situated in the same river valley, only 4 of latitude apart, and yet with totally different climates. To attempt to group climates together over large areas is therefore impossible, except very roughly. The old divisions of one torrid, two temperate and two arctic zones served as a rough outline. They are totally inadequate to explain the varia- tions found at places not far apart within the same zone. The only way to gain an idea of the climate of a place, apart from a study of actual figures, is to have a clear idea of the effects of all the different factors, such as (1) Latitude. (2) Hemisphere, north or south. This makes a great difference. The tempera- ture ranges are far smaller in the Southern hemisphere. (3) Situation with respect to large continents, particularly east or west. If on the east, as the U. S. or China, the temperature ranges, daily and seasonal, are much greater than on the west sides. 1 88 THE STORY OF THE EARTH'S ATMOSPHERE. (4) Position, oceanic, coastal, or continental. This affects both temperature range, and humid- ity very largely. (5) Elevation above the sea, and whether isolated or on a tableland. If the former, the climate is moderate; if the latter, extreme. In both cases the general temperature diminishes about i F. for every 300 feet of elevation above sea-level. (6) Situation with respect to neighbouring mountain ranges, especially leeward or wind- ward, with reference to prevailing winds. If on the windward side, such as Mull, Coimbra in Por- tugal, Vancouver, Bombay, Colombo, Valdivia in Chili, Brisbane, and Chirrapunji in Assam, the rainfall is often over 75 inches, while corre- spondingly on the lee sides of the adjacent ranges we find, Aberdeen, Salamanca (less than ten inches), Cariboo (east of the Coast range), Poona, Bandarawela, Bahia Blanca, Roma, and Shillong, with amounts varying from 20 to 30 inches only. (7) Situation with respect to prevalent winds, trades, anti-trades, monsoons. This determines the season of rain, such as the monsoon rains in the Indian summer, whereas the summer in Aus- tralia, exposed to the trades, is the dry season. The temperature conditions are thus consider- ably modified. (8) The neighbouring oceanic currents. The effects of these have already been alluded to on p. 60. (9) The nature and covering of the adjacent land. (10) Situation with respect to the tropical or circumpolar rain and wind-belts. LIFE IN THE ATMOSPHERE. 189 As types of various general climates at sea- level, the following may serve as illustrations. CLIMATES. Type. Examples. . , f Batavia, (1) Equatorial ! Colomb ^ at. o to lat. singapore> (^ Cumana, (2) Tropical, lat. 10 to 23 (a) Coastal, Inland, (3) Sub - Trop - ical, lat. 23" to lat. 35, (4) Temperate Calcutta, Hong Kong, Lahore, Delhi. Mandalay, Timbuktu, Riviera, S. California, Cape Colony, Southern Aus- tralia, 35 to lat. 60 , (6) South, lat. 35 to lat. 50, (5) Polar, lat. 60 to poles, f England, J g 1 Cenlra| sibcrb [ and China, New Zealand, Tasmania, N. Siberia, Greenland, Description. Hot, moist, equable, sa- lubrious. Similar to (i) but less equable and salubri- ous. Hot, dry, and extreme, trying, except in win- ter. Temperate and dry, owing to position be- tween tropical and polar rain-belts, very salubrious. Cool, moist, and equable near sea, dry and ex- treme inland. ( Cool, moist, and equa- ] ble, most salubrious ( in the world. (j Cold and fairly dry, ex. treme inland. Judged by averages alone, a climate with an annual average temperature between 75 and 85 is hot. 65 " 75 i warm. 55 (>5 i mild. 50 " 55 i temperate. 40 5 i cold. Below 40 i arctic. I QO THE STORY OF THE EARTH'S ATMOSPHERE. These adjectives are, however, only applica- ble when the range is small between summer and winter. Man can never hope to control or sensibly alter the climate of the countries in which he is placed. Nature works on too vast a scale. He can, however, by studying the different kinds of climate and their properties, discover which are suitable for certain diseases and ages, and by utilising this knowledge, to some extent shelter himself against influences which are recognised to be hostile, and which lead not merely to loss of individual life and health, but to degeneration of the human race. INDEX. A. Abbe. Professor, 105. Abercromby, Ralph, in. Actinometcr, 36. Albatross, flight of, i6S, 170. Ammonia in atmosphere, 18, si. Anticyclones, 29. 127, 13.% 134. Anti-trades, Sj. Arched squall, 150, 162. Argon in atmosphere, 18. Atmosphere, composition, 17; electricity, 159; height, Q, 14: history, 9: laws, 94; life :n. 183; movements (winds), (14 : optics of, 141; origin. 9; pres- sure, 15, 16, 25; sound' of. i)i; temperature, 31; weight, 15, i 6. as . Aurora borcalis, 148. I',. Bacilli. nitrogen abstracting Hack. Admiral, 173. Baden Powell. 177. Baldwin, 164. Balloons, 15. 88, 03. 163. Ballot, Dr. Buys. uS. Barometer, t6. 2;. Barometric pressure, variat in, 27. 58. 68, 81. Berson, Dr., balloon ascens 15- I'.irds. flight of, 166. I'.irt. 173. Blanch.-.rd, i^. Blizzards. 136. Blue Hill Observatory. 9-'. ifV). 171. Blueness j; forms of, i to; height of. ill, iiS; high-cirrus. ic/>; nimbus, in, 114: stratus, no, in. 114. 117; strato-nimbus, in; veloc- ity of movement. 117. Coal. 21. ( olour of the sky. 25, 10.:, 141. Colours, sunset, 1113. Convection currents, 104. 158. Conservatism of energy, 97. Corona, 144. " Corpse candle." 148. Conrant. ascendant, 59. Coxwell, balloon ascension, 15, 164. Cumulus. Sec Clouds. 191 192 INDEX. Cumulus, festooned, 156. Currents, ocean, 46, 60, 110. Cyclones, 29, 59, 79, 84, 99, 125. D. Davis, Professor, 153. Dawn, 143. Dew, 106. Diffusion of gases, 100. Disease, 184. Doldrums, 82, 130. Dove, Professor, 128. Dust in atmosphere, 23, 143, 184. E. Echo, 140. Ekholm, 92, in. Electricity of the atmosphere, iSp- Ernission theory of light, 141. Energy, conservation of, 97. Euler, 142. Explosions, 140. F. Fahrenheit, 39. Fata Morgana, 146. Ferrell, 48, 60, 63, 66, 74, 131. Finley, Lieutenant. 152. Flying machines, 88, 93. 163, 165. Foehn, 137. Fog, 109. Fonvielle, 164. Forecasting weather, 30, 735, 157. Franklin, Benjamin, 159. Fresnel, 142. Frost, 109. G. Galileo, 25, 39. Garnerin, 164. Gases, kinetic theory of, 90. Gay-Lussac. 96, 164. Glaisher, balloon ascension, 15. _4i, 139, 164. Gravitation, 10, 12. Green, 164. Gulf Stream, 46, 60, no. H. ITadley, theory of the winds, 64, 74- Hagstrom, in. Hail, 106, 119, 120, 157. Halo, 144. Hann, 63. Hargrave, 166, 170, 177. Hargrave kite, 170. Harries, 162. Haughton, Dr., 60. Hawk, flight of, 167. Heat, 31, 97, 142. Hildebrandsson, Dr., in. Hot north-wester, 137. Howard, Luke, no. Hurricanes, 99, 125. Huyghens, 142. Hypsometry, 30. I. Ignis fatuus, 148. Inclined plane, 167. Inertia, curve of, 67. isobars, 30, 71. Isothermals, 44. J. Japan current, 60, no. Joule, 98. Jupiter, atmosphere of, 10. K Kelvin, Lord, 182. Khamsin. 136. Kinetic theory of gases. 90. Kites, 167, 173, 179, 180; Har- grave, 170, 174, 177; observa- tion, 173; photography, 174; war, 173, 176. Kona, 136. Krakatoa, 140, 146. L. Labrador current, no. Langley, Prof. S. P., 36, t66, 172. 180. La Place, nebular theory, 9. Laws, atmospheric, 94. Lemstrom, 149. Leste, 136. Leveche, 136. Ley, Rev. Clement, m, 157. Lichtenberg, 163. Light, 102, 141. Lightning, 157, 159. Lightning rods, 122, 160. Lilienthal, 166. INDEX. '93 Looming, 146. Lunardi, 164. M. Mackerel sky, 113. Mai tic iiwiitiignc, 27. Marriotte's law, 95. Mars, atmosphere, n. Marvin, Professor, 1-9. Alaury, theory of the wind, 64, Maxim, Hiram, 166, 172. Mcklrum, Dr., 130. " Merry Dancers," 148. Meteorites, 15, 16. Mirage, 145. Mist, 109. Mistral, i;o. Mock-moons, 145. Mock-suns, 145. Mollcr, 93. Monsoons, 58, 80. Montgolfiers, the, 163. Moon, atmosphere of the, it. MorrrH :i tabernacle, 140. N. Xadar, 164. Nebular theory, 9. Newton, 141. Nimbus clouds. See Clouds. Nitragin, bacilli. 20. Nitrogen in atmosphere, 18, 19 Nortes. 136. Northern lights, 148. Nor'-wester, 150. O. Ocean current 1 ;. 46. 60, no. Oxygen in atmosphere. i3, 19. ( )zone in atmosphere, iS. 21. Pampero, 136, 150. Parachute. 163. I'arasclen.T, 145. P.'irheiia, 145. I'eri-cyclonc, 13-'. Photography, 174. I'iddington. i.'S. I'lanets, origin and atmosphere of, 10. I'ocock, 173. 1'oisson, 66. I I'oisson's law, 96, 98. Priestley, Dr. Joseph, 17. R. Radiation, 31. Rain, 22, 84, 99, 106, 119, 157. Rainbow, 147. Rainfall, 122. Kedfield. 128. Reid, 1 28. Reye, Professor, 131. Rotch, 174. Rozier, Pilatre de, 163. S. Sailing, 180. St. Elmo's fire, 161. Seasonal changes. 33. Simoom, i^g. Sirocco, 136. Smyth, Prof. Piazzi, 84. Snow, 106, 1 19, i_'o. Snow line, perpetual, 41. 42. Solano, 136. Sound, atmosphere as the con- veyer Hi, 138. Southerly bursters, 136. Space, temperature of, 13, 3.-, 6.', 105. Spectre of the Hrocken, 148. Spectrum. ioj, 14.-. Si|tialls, 150, 157. Stevenson, T., qi. Storms. 99, 104. 106, 125. Storms, law of, u8. Stratus clouds. Sec Clouds. Sun, atmosphere, <), u, 14; heat ot, 32. Sunset, colours at, 103, 1 13. Sun spots, 54, 140. T. Taifuns. no. Taj Mahal. 140. Temperature, 31; variations in. Thermometers, 30; self-record- _ ing. .10. Thunder. 140. 160. Thunder-storm-. 59. 99, i.-o, 140, 1 56. Tissandier, 164. Tornadoes, 5<>. o<), 121, 1.5. 149, 1 ^o. Torricelli. 25; vacuum of, 26. 194 INDEX. Trade winds, 64, 79. Trilqbites, 13. Twilight, 15, 16, 143. V. Vettin, Dr., 92, HI. Viscosity, 90. Von Bezold, Dr., 63, 112. W. Water in atmosphere, 22, 27, 106. Waterspouts, 149, 152. Weather forecasting, 30, 135, 157. Wenham, 166. Whirlwinds, 39. 149. Whispering Gallery, 140. Windmills, 181. Winds, 64; effect of tempera- ture, 60; trade, 64, 79; velocity of, 88. Young, 142. Y. Z. Zero, determination of, 39. Zurbriggen, 15. (5) THE END. DATE DUE 1979 f*\ r\~r OCT 8 ^002 UCLA COL DCTOPIW ro OCT ? 5 2005 ntL/uiv du _-. * TO GAYLORD PRINTED IN U A A 001 211 045