lifornia] 
 
 tonal 
 
 lity
 
 THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 LOS ANGELES 

 
 is "" r " last rf'te . 
 
 ^ >J'-
 
 THE STORY 
 
 OF THE EARTH'S 
 
 ATMOSPHERE 
 
 BY 
 DOUGLAS ARCHIBALD, M.A. 
 
 FELLOW AND SOMETIME VICE-PRESIDENT 
 OF THE ROYAL METEOROLOGICAL SOCIETY, LONDON 
 
 WITH FORTY-FOUR ILLUSTRATIONS 
 
 NEW YORK 
 MCMXII
 
 COPYRIGHT, 1897, 1902, 
 BY D. APPLETON AND COMPANY.
 
 tngmeering & 
 
 Mathematical 
 
 Sciences 
 
 Library 
 
 IK* 
 
 A 47 
 
 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 
 5
 
 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 OF 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 ng 
 
 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 PAGE 
 
 Cumulus Cloud 
 
 
 Fig. 23 " After the 
 
 
 Frontispiece 
 
 
 Storm "... 
 
 97 
 
 Fig. i Strato- Cumulus 
 
 
 Fig. 24 Diffusive Limits 
 
 
 (low) .... 
 
 ii 
 
 of the Component Gases 
 
 
 Fig. 2 Strato-Cumulus 
 
 
 of the Atmosphere 
 
 10 1 
 
 (high) .... 
 
 14 
 
 Fig. 25 CirrusCloud (var 
 
 
 Fig. 3 Cirro-Cumulus . 
 
 17 
 
 Tracto Cirrus, 1889) . 
 
 H3 
 
 Fig. 4 .... 
 
 33 
 
 Fig. 26 .... 
 
 US 
 
 Fig. 5 .... 
 
 35 
 
 Fig. 27 .... 
 
 116 
 
 Fie 6 
 
 A.1 
 
 Fig. 28 Festooned Cumu- 
 
 
 Fig 7 
 
 *TJ 
 
 AC 
 
 lus .... 
 
 nS 
 
 Fig. 8 Distribution of 
 
 T-D 
 
 Fig. 29 . 
 
 121 
 
 Atmospheric Tempera- 
 ture in Latitude for 
 
 
 Fig. 30 . 
 Fig. 31 . 
 
 I2 3 
 124 
 
 January, July, and the 
 
 
 Fig. 32 . 
 
 12 9 
 
 year .... 
 
 47 
 
 Fig. 33 . 
 
 I 3 2 
 
 Fig. 9 .... 
 
 53 
 
 Fig. 34 . 
 
 133 
 
 Fig. 10 .... 
 
 55 
 
 Fig. 35 
 
 J 33 
 
 Fig. n . 
 
 56 
 
 Fig. 36 Tornado Funnel 
 
 
 Fig. 12 . 
 
 
 . 
 
 57 
 
 cioud : . 
 
 J 55 
 
 Fig- 13 . 
 
 
 
 67 
 
 Fig. 37 Thunderstorm 
 
 
 Fig. 14 . 
 
 
 
 69 
 
 in Section . 
 
 157 
 
 Fig. 15 . 
 
 
 . 
 
 72 
 
 Fig. 38 Kestrel Hawk 
 
 
 Fig. 16 . 
 
 
 
 73 
 
 Hovering . 
 
 168 
 
 Fig. 17 . 
 
 
 . 
 
 74 
 
 Fig. 39 ..-. 
 
 171 
 
 Fig. 18 . 
 
 
 . 
 
 75 
 
 Fig. 40 .... 
 
 175 
 
 Fig. 19 . 
 
 
 
 78 
 
 Fig. 41 . 
 
 176 
 
 Fig. 20 . 
 
 
 
 81 
 
 Fig. 42 . 
 
 173 
 
 Fig. 21 . 
 Fig. 22 . 
 
 
 , 
 
 83 
 86 
 
 Fig. 43 Yachting in Syd- 
 ney Harbour 
 
 181
 
 THE STORY OF THE EARTH'S 
 ATMOSPHERE. 
 
 CHAPTER I. 
 
 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, 
 9
 
 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 which 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 
 
 
 FIG. i. Strato-cumulus (low). 
 
 special study, to be irrigated thence by the 
 system of so-called canals which intersect its 
 surface. 
 
 Finally, our 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 /. <?., 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 -j-J^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 we 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 Coxwell 
 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 ATMOSPHERE. 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 i o jnr tns 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. / v 
 
 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. Dobb6 
 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 arc 
 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 |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 we 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 
 less numerous. He has also found that the presence 
 of this dust is necessary to the formation of rain. 
 
 A recent series of observations by Mr. E. D. 
 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 " . . . 5 12 
 
 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 
 
 8,20O 
 8,400 
 10,665 
 II.OOO 
 13,200 
 13,600 
 
 4 80 
 513 
 406 
 257 
 2I 9 
 157 
 
 The general rule for the diminution in the 
 number of dust particles may be simply expressed 
 thus: For every rise of 3000 feet the amount is 
 $ths 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 up to thirty-three feet. 
 His pupil, Torricelli, however, was not content 
 with this perfunctory explanation, and applying
 
 Z 6 THE STORY OK 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 itvm 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 xoV^ths 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 uf 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 montagne 
 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 
 
 910 
 
 1,850 
 2,820 
 
 3,820 
 
 4,850 
 
 5,910 
 
 7,oio 
 
 8,150 
 
 9,330 
 
 10,550 
 
 18 " 13,170 
 
 16 " 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
 
 PRESSURE A^D 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 : 
 
 " The difference of level in feet between two alti- 
 tudes is equal to the difference of the barometric pres- 
 sures observed at each in inches divided by their sum 
 and multiplied by the number 55,76*, when the ai>er- 
 age of the temperatures at the two places is 60 f."
 
 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 temperature 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 
 3
 
 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^ 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 7th to 
 August yth, the zone of greatest heat reception 
 lies in 41 N., while from May 3ist 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., B.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-g^th 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. o^f 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 dratfn 
 upon when our coal supply shows symptoms of 
 giving out. 
 
 One effect has not been alluded to viz., 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 1611, 
 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 of 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-
 
 40 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 um, 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- 
 
 Centre of 
 Earffi 
 
 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 i'n 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 five 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 
 
 IATN 70 60 50 40 30 20 '0 10 20 30 40 50 60 s 
 
 c6 
 
 
 
 
 
 ^** 
 
 ( 
 
 
 
 
 
 
 p. 
 
 
 
 
 
 / 
 
 /\ 
 
 S^. 
 
 
 
 ^< 
 
 % 
 
 O- 
 
 
 
 
 
 
 / 
 
 / 
 
 / 
 
 / 
 
 
 
 
 ^ 
 
 ^\ 
 
 s- 
 
 
 
 
 / 
 
 
 / 
 
 / 
 
 Mi-a 
 
 n o, 
 
 ' Ke 
 
 ar J 
 
 or C 
 
 lobe 
 
 N 
 
 \ 
 
 
 j 
 
 $* 
 
 
 / 
 
 / 
 
 
 
 
 
 
 
 
 \ 
 
 ^. JU 
 
 / 
 
 
 / 
 
 / 
 
 
 
 
 
 
 
 
 
 
 ^, u 
 
 
 / 
 
 
 I 
 
 FIG. 
 t 
 \ 
 
 eart 
 whi 
 and 
 lar 
 
 
 Distribution of atmospheric 
 erature in latitude, for Jan- 
 July, and the year. 
 
 axis to the plane i 
 the centres of the su 
 anets lie, causes a sim 
 gration in the area c 
 
 0-^ 
 
 4? 
 
 / 
 
 
 emp 
 ary 
 
 h's 
 ch 
 
 Pi 
 
 m 
 
 / 
 
 
 / 
 
 
 
 A 
 
 
 
 , 
 
 j? 
 
 
 
 / 
 
 
 
 
 
 greatest heat north and south of the geographi- 
 cal equator. While the sun shifts from 23^ N. to 
 23^ 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 
 4
 
 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, Gobi desert, Rajputana, Colo- 
 rado, etc., where it often amounts to 40 F., 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
 
 5 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-breeze 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 1811, -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. 10. 
 
 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. (10) represent a vertical section of the 
 atmosphere and let the dotted lines represent
 
 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 
 
 denoted by the varying size of the arrows). 
 Meanwhile the loss of air above will lessen the
 
 THE TEMPERATURE OF THE ATMOSPHERE. 57 
 
 pressure on the earth's surface near the centre of 
 the area. Consequently the surrounding air will 
 flow 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 flows 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. 
 
 FIG. 12. 
 
 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 NN 1 represents where the tendency
 
 5 8 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 " courant 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. 6l 
 
 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 Bourke and Denili- 
 quin. In February, 1896, a temperature of 108 
 degrees was recorded at Sydney, due to a remark- 
 able prevalence of dry N. W. 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. Bezold of Berlin, 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 b y 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 Weather 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 fig. (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 skater 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 (curved 
 
 norm i ^f in which, air 
 
 area. I 
 
 Direction, 
 in which - 
 atr starts to 
 move 
 
 jTMtmiiimuiiiinnnin 7 ~~&* 
 
 Inertia curve 
 into which 
 
 iff** .....-.,.,-. .,..-.--- - cu'r tends to 
 
 Cold area & deflected 
 
 force 
 exerted by 
 pressure qraaienC 
 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 right be- 
 comes left and the high pressure would be to
 
 70 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, 29.8 etc.. 
 represent sections of the actual atmospheric iso- 
 bars or pressure contours. B, C, D, points where
 
 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 B 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 
 
 296 
 
 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 Maury'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. 
 
 FIG. 18. 
 
 Ferrel'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, /". e. 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 things 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 B 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 
 
 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 
 6
 
 8o THE STORY Ot 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 y^ths 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 en 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 south, 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 
 
 ,/ 
 
 FIG. 
 
 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, we 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 
 so 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 during 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 under ly 
 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' Extreme Southern, 
 position ofrainbelt position of -ainbelt 
 
 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 N. 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.
 
 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: 
 
 Towns Velocity of wind in 
 
 miles per diem. 
 
 ~ ) Bombay 408 
 
 On coast Uurrachee 497 
 
 Sf * 
 
 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 diminished 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 , 
 
 Height A 
 in feet. r 
 , i 676 
 
 verage speed in 
 niles per hour. 
 
 IQ 
 
 
 5,326 
 
 24 
 
 
 12 724 
 
 34 
 
 
 21 888 
 
 
 Cirrus . . . 
 
 . . 20.117 
 
 78 
 
 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, 
 
 FIG. 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 move. 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. e,, the pressure increases. That is 
 Boyle's law. 
 
 In like manner, if, without compression, the 
 temperature of the gas is raised, the speed of 
 the movements is increased. (Molecular speed is 
 heat.) Therefore, the effect in this case is just the 
 same as when the gas was compressed and the 
 paths were shortened viz., an increased number 
 of impacts or kicks, and therefore increased pres- 
 sure. That is Charles's law. 
 
 When the gas is suddenly compressed, the 
 atoms have been pushed towards each other, and 
 their speed has therefore been increased by this 
 push. Consequently, a rise of temperature takes 
 place. This is Poisson's law. 
 
 In all these cases, no energy is lost or created. 
 It is simply a transformation of motion into dif- 
 ferent " modes," from which it can be retrans- 
 formed without sensible loss, though it is easier 
 to transform motion into heat than heat into mo- 
 tion. Dr. Joule, of Manchester, was the first to 
 determine the exact equivalence between what we 
 term motion and heat. His corrected law may be 
 stated thus : 
 
 When a pound weight falling through 783 feet is
 
 THE LAWS WHICH RULE THE ATMOSPHERE. 99 
 
 arrested, as much heat will be developed as will raise 
 one pound of water one degree above absolute zero* 
 
 The converse is equally true. Here we see 
 the true cause of the marvellous manifestations 
 of energy in the movements of the atmosphere, 
 the devastating hurricanes which overturn build- 
 ings and destroy ships, and the terrible tornadoes 
 of America which have been known to carry solid 
 objects like wooden church spires a distance of 
 15 miles and kill hundreds of people in the space 
 of a few minutes. They are all due to the heat 
 of the sun, which is converted into motion on the 
 above scale. 
 
 Charles's law, by which air expands by heat, 
 and Poisson's, by which it cools by expansion 
 under diminished pressure, have a very important 
 bearing on the formation of clouds, rain, thunder- 
 storms, tornadoes and tropical cyclones. 
 
 When a mass of dry air, or air only containing a 
 small proportion of vapour, rises, it cools at the rate 
 of i F. for every 183 feet it ascends. This would 
 be about 1.6 in 300 feet, or about 5.2 in 1000 feet. 
 
 The rate, however, at which the air is found 
 by observation to remain cooler as we ascend in 
 the atmosphere is much slower than this. Con- 
 sequently, if dry air is warmed up until it expands 
 and gets lighter than the surrounding air, it can- 
 not rise very far before it is cooled by ascent, down 
 to the temperature and density of the air around 
 it at the same level, when it is bound to stop. 
 
 When, however, air contains as much vapour 
 as it can hold invisibly, or is said to be saturated, 
 the case is different. As it ascends and cools, the 
 vapour condenses into cloud and finally falls out 
 
 1 (*. e., 461 below zero Fahr.)
 
 100 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 as rain and at the same time gives out the heat 
 which is absorbed in the act of conversion from 
 water into vapour. This heat, which is latent so 
 long as it is vapour, becomes patent when it con- 
 denses and retards the cooling of such a mass of 
 air so much that under ordinary conditions of 
 temperature andpressure in these latitudes(barom- 
 eter 30 inches, thermometer 62), it only cools 
 -fths of a degree Fahr. in ascending 300 feet. An 
 ascending current of air saturated with vapour 
 once started, therefore, could go on ascending 
 until nearly all the vapour had fallen out of it, or 
 until it had risen into very lofty and cold regions 
 of the atmosphere. 
 
 Air saturated with vapour is thus essential to 
 the formation of clouds, especially cumulus clouds, 
 like that in our frontispiece, and of upward cur- 
 rents generally, which are the chief cause of local 
 storms. Diffusion by which one gas tends to 
 work its way through another plays an impor- 
 tant role in atmospheric economy. Were it not 
 for diffusion the heavier gases would all lie near 
 the surface and the lighter near the top, and we 
 should all be poisoned off by even the small 
 amount of carbonic acid there is. 
 
 Some years ago the great chemist Dalton 
 founded the law of gaseous pressure, and deduced 
 that of diffusion from it, but it has since been 
 found that though his conclusions were fairly cor- 
 rect, they are not due to the causes he alleged. 
 The rule is that the lighter the gas the more rap- 
 idly it tends to wander through its neighbours, 
 and the tendency is for each gas to behave as 
 though its neighbours were not in existence. 
 Thus, the tendency is for the component gases of 
 the atmosphere to take up positions in which they
 
 THE LAWS WHICH RULE .THE ATMOSPHERE. IOI 
 
 would exist as separate atmospheres up to the 
 limits in miles indicated opposite each in the ad- 
 joining diagram. 
 
 This ideal final arrangement never takes place 
 
 80 
 
 
 
 70 
 
 
 
 60 
 
 62 
 
 -Hydrogen. 
 
 50 
 
 
 
 4-0 
 
 37 
 
 -Water Vapour* 
 
 30 
 
 32 
 31 
 
 . tfitroyen/. 
 
 20 
 
 
 
 10 
 c\ 
 
 9 
 
 - Carbonic acid 
 
 FIG. 24. Diffusive limits of the component gases of the 
 atmosphere. 
 
 owing to the constant motion, but since all the 
 gases diffuse upwards as well as downwards it is 
 quite possible that some of the hydrogen and 
 lighter gases have diffused upwards until they have 
 got beyond the power of recall by gravitation. 
 
 Long before the water vapour has reached 37 
 miles, a great deal is lost by being condensed 
 into rain. At a height of 9 miles above the sur- 
 face, for example, the actual amount of vapour
 
 102 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 present is only g^th of what would exist if it 
 were incondensibie. 
 
 The laws which determine the passage of the 
 sun's heat and light through the atmosphere 
 present problems which are even yet only par- 
 tially solved, partly because light and heat are 
 made up of a variety of wave movements in that 
 wonderful medium which pervades all space 
 termed aether, and partly because the conditions 
 in the atmosphere can never be exactly imitated 
 in the laboratory. 
 
 However, this much is known. Ordinary 
 white light from the sun when passed through a 
 prism is found to be made up of a variety of 
 coloured rays ranging from violet to red. The 
 visible violet rays are made up of the smallest 
 waves, about .00001 in. in length, and the visible 
 red rays of waves about .00003 i- Beyond 
 these visible rays are invisible ones which affect 
 the atmosphere and earth, and whose existence 
 can be proved by photography. The rays to- 
 wards the red end produce more heat than light, 
 and those towards the violet end more light than 
 heat. The general action of the atmosphere on 
 these rays from the sun is twofold. In passing 
 through it they are partly absorbed and partly 
 scattered. The blue and violet rays are most 
 affected, and the red least. In fact, as Prof. 
 Langley has pointed out, so much of the blue 
 rays are filtered out by the atmosphere that if we 
 could see the sun as he appears at the outside 
 of our atmosphere he would be blue instead of 
 white. 
 
 The absorption particularly of the heat rays 
 has been shown by the late Prof. Tyndall to 
 be mainly effected by the water-vapour present
 
 THE LAWS WHICH RULE THE ATMOSPHERE. 103 
 
 in the atmosphere, while the scattering is effected 
 by the fine particles of dust and frozen vapour. 
 
 The red colours at sunset are due to the fact 
 that the sun's rays, before they reach our eyes 
 in this position, pass through a thickness of at- 
 mosphere about 900 miles instead of 50 miles when 
 overhead. The blue rays, in consequence, are 
 nearly all filtered out, and nothing is left but the 
 longer waves at the red end of the prismatic spec- 
 trum. The red colour of the sun when seen 
 through a fog is due to a like excess of absorption 
 and scattering of blue rays by the particles of fog. 
 
 Similarly, the blue colour of the sky when the 
 sun is high up in the heavens is explained by 
 Lord Rayleigh to be due to these missing blue 
 rays. Scattered mostly at right angles to the di- 
 rection of the sun's rays, these spurned rays reach 
 us from all parts of the sky, and shew up blue 
 against what would, in their absence, be the black 
 background of space. 
 
 When the luminous rays from the sun pass 
 through the air, they heat the dry part of it very 
 little. The absorption is mainly effected by the 
 small but valuable vapour constituent, which 
 Prof. Hill estimates as being 764 times as effect- 
 ive as dry air. The remaining rays on reaching 
 the earth are changed into invisible heat rays, 
 which possess the peculiar property of being un- 
 able to repenetrate the atmosphere. Trapped 
 thus like lobsters in a basket, they expend their 
 energy, first upon the earth's surface and then 
 upon the adjacent air. The atmosphere, in fact, 
 acts like the glass in a greenhouse, which lets the 
 luminous rays in and prevents their escape when 
 they have become converted into dark heat. 
 
 The upper parts of the atmosphere would thus
 
 104 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 remain colder than they are, were it not for the 
 conveyance (convection is the technical word) of 
 the heat from the lower strata to the upper re- 
 gions in other words, from the ground floor to 
 the attics. This convection plays such an impor- 
 tant part in the atmospheric economy, and in the 
 formation of clouds, rain, and storms of every 
 kind, that it demands a brief consideration. 
 
 We have seen that in general, ascending air 
 cools at the rate of i F. for every 183 feet. Con- 
 sequently, so long as the rate of decrease of tem- 
 perature with the height is equal to or slower than 
 this, the atmosphere tends to remain in vertical 
 equilibrium that is to say, vertical motions will 
 not arise spontaneously. The only interchange 
 under such circumstances would be due to expan- 
 sion and overflow, such as that described in 
 Chap. IV., and which gives rise to the general cir- 
 culation between the equator and the poles des- 
 cribed in Chap. V. When, however, the atmos- 
 phere near the surface is not only heated by 
 radiation of the changed solar rays by the earth, 
 but is surcharged with vapour, it cools even at 
 62 F., our summer temperature in England, only 
 i F. in every 400 feet of ascent, while at 92 F., 
 as often occurs within the tropics, it cools only 
 i F. in 500 feet. An upward movement once 
 started, therefore, is able to continue, since the 
 air is always warmer, and therefore lighter than 
 that which it reaches above. Similar downward 
 motions of the cool air above take place until a 
 large proportion of the heat received near the 
 surface is carried up aloft. The process is pre- 
 cisely analogous to that by which the hot water 
 from the kitchen boiler is conveyed through the 
 pipes to the cisterns at the top of a house.
 
 THE LAWS WHICH RULE THE ATMOSPHERE. 105 
 
 These upward convection currents carry the 
 life-giving heat to the cold regions above us, just 
 as the arterial blood conveys warmth to our ex- 
 tremities, and are quite as necessary to the life of 
 the atmosphere as the circulation of blood heat is 
 to that of the animal. Were it not for this safe 
 conveyance, moreover, of the suplus heat away 
 from our midst, there would often be a dangerous 
 accumulation which would render life more insup- 
 portable than it is, especially in the tropics. When 
 the existing rate of decrease becomes anything 
 like i F. in 100 feet, or greater, rapid convection 
 sets in, even if the air is comparatively dry, and 
 clouds are formed with rain, and often lightning. 
 If the air over a large district is affected, as some- 
 times occurs in the tropics, a cyclone is formed 
 by the inrush of surrounding air, or, if the action 
 is very intense and quite local, a tornado or whirl- 
 wind may result. This may be termed convection 
 run riot. 
 
 Prof. Abbe", of the U. S. Weather Bureau, con- 
 siders the limit of convection currents to be 
 about 30,000 feet, or about the height of Mount 
 Everest. Above this the temperature diminishes 
 very rapidly, as indeed we find from the observa- 
 tions recorded on the free balloons, L'Aerophile 
 in France and the Cirrus in Germany, which on 
 March 2ist, 1892, and July yth, 1894, reached the 
 same height of 10 miles. In the case of the 
 French balloon, the temperature descended to 
 104 F. below zero, and at the same rate the cold 
 of space viz., minus 461, would be reached at a 
 height of about 30 miles. 
 
 The ordinary rate of decrease is in general 
 about i in 320 feet after we rise above the first 
 100 feet.
 
 106 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 From what has been said about the slower 
 rate at which air saturated with vapour cools by 
 ascent when the temperature is high near the sur- 
 face than when it is low, we can readily under- 
 stand why cumulus clouds and rain showers occur 
 in the daytime and in warm latitudes more readi- 
 ly than at night and near the poles. In fact, 
 since at freezing-point and at sea-level even sat- 
 urated air would cool as rapidly as i F. in every 
 277 feet; if it were not for imported convection 
 systems and clouds there would be very little as- 
 cent and precipitation of condensed vapour at all 
 in the arctic zones. 
 
 CHAPTER VII. 
 
 THE DEW, FOG, AND CLOUDS OF THE ATMOSPHERE. 
 
 WHEN we gaze skywards and see the filmy 
 wisps of high cirrus cloud, touching as it were 
 the very vault of heaven, or when we notice the 
 ragged scud of the approaching storm, half cov- 
 ering the low hills, we are witnessing one of the 
 first stages by which the water of our atmosphere 
 becomes visibly separated from its gaseous com- 
 panions. 
 
 Another stage is manifested when the pearly 
 drops of dew gather on the blushing petals of our 
 roses, or the rain drops from the frowning storm 
 clouds. Still another transformation scene, and 
 the beautiful six-rayed flakes of snow fall like 
 flowers, scattered by an angel hand, and cover up 
 the gloomy earth with a mantle of dazzling white. 
 
 Yet one more strange scene, and from the
 
 DEW, FOG, AND CLOUDS OF THE ATMOSPHERE. 107 
 
 fiery thunder-cloud white balls of ice rattle down 
 as though from some aerial glacier. 
 
 The chameleon character of this same water 
 element is indeed a most fortunate circumstance. 
 Imagine what a dull world it would be without 
 our gaudy sunset cloud tints. What a desert if 
 it never rained. 
 
 ^"It happens, however, that, unlike the other 
 gases, water-vapour can undergo all its changes 
 within the gamut of the temperatures we experi- 
 ence on this planet. 
 
 Solid at 32 F., liquid thence to 212 F., after 
 which it becomes gas. 
 
 Moreover, the air is thirsty as it were, and so, 
 from the liquid water, at all temperatures, and 
 even the solid ice, vapour is ever ascending by 
 evaporation and rendered invisible as it passes 
 through the other gases. 
 
 There is a limit, however, to the capacity of 
 air for such a temperance beverage, which, like 
 the thirst of men, depends on the temperature. 
 Thus, while a cubic foot of air at zero F. can hold 
 but a grain of vapour, at 60 F. it can soak up 
 5 1 grains. At 80 F. as much as n grains can 
 remain invisible in the same space. 
 
 To give a larger example. Suppose a room, 
 20 feet square by 10 feet high at 60 F., to be 
 supplied with vapour until it could hold no more, 
 then the air in such a room would weigh 304 
 Ibs., while the vapour, if it were condensed to 
 water, would weigh but 3 Ibs., and fill three pint 
 measures. 
 
 When air can hold no more vapour it is said to 
 be saturated, and since, when it is cooled, it is able 
 to hold less and less water, it can, even when un- 
 saturated, be made saturated by being cooled
 
 108 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 down to a point of temperature some few degrees 
 below. This point is called its dew point, and 
 depends partly on how damp, partly on how 
 warm, it was at first. 
 
 When very warm and moist, a very slight 
 lowering of temperature produces condensation 
 into cloud and finally rain. Hence clouds and 
 rain will form easier in warm countries, though 
 other conditions may make them more constant 
 in cold countries. 
 
 What we ordinarily term the dampness of the 
 air, is not simply a question of how much vapour 
 is present, since warm air may hold more than 
 cold air and yet feel drier. 
 
 It is determined by the nearness of the dew- 
 point to the existing temperature, and this de- 
 pends on both the amount of vapour present and 
 the temperature of the air in which it is dis- 
 solved. 
 
 Ordinarily the dampness in England is about 
 60 per cent, of what could be, but in very wet 
 weather it rises to 90 per cent. ' 
 
 Over the ocean it is generally high both in 
 warm and cold latitudes, while in the interior of 
 continents and deserts it is occasionally as low as 
 15 per cent. As we rise above the earth towards 
 the level of the lower clouds the dampness in- 
 creases, until, at the cloud level, we reach dew- or 
 cloud-point, where the air is saturated. 
 
 Dew itself is the moisture deposited on the 
 surface of bodies near the earth's surface, which 
 have cooled down by radiation below the dew- 
 point of the surrounding air. 
 
 Dr. Wells, in 1783, was the first to offer this 
 explanation, and thought the moisture came en- 
 tirely from the air around. Of late, however, Mr.
 
 DEW, FOG, AND CLOUDS OF THE ATMOSPHERE. 109 
 
 John Aitken, of Edinburgh, and others, have 
 shown that a large part of the moisture comes 
 from the ground and the plants on which it is 
 deposited. They are, in fact, constantly perspir- 
 ing like human beings. In the day-time this 
 perspiration is evaporated by the warmth and 
 carried off by the winds. Only in the cool and 
 calm of the night and early morning does it be- 
 come deposited in drops of water. 
 
 Hoar Frost is simply dew formed when the 
 dew-point happens to be below the freezing point 
 of water. 
 
 Fog, or mist, may be termed a cloud of vapour, 
 formed near the ground or water. 
 
 Sometimes, like dew, it is occasioned by the 
 earth parting with its heat so rapidly as to cool 
 down a stratum of air, just above it, below its 
 dew-point, as occurs in the quiet, anti-cyclonic 
 weather, as it is termed, occasionally experienced 
 in these latitudes in winter. Such fogs are usu- 
 ally fairly dry. Sometimes it occurs with a wind, 
 when it is wetter and warmer. In such cases it usu- 
 ally comes off the sea, and is due to a warm moist 
 air from the sea passing over a cool ground surface. 
 
 Locally, mists usually form in low river val- 
 leys, where the air is nearly saturated with 
 vapour, rising from the water or moist land. 
 
 It seems strange, but it is nevertheless true, 
 that the pretty white valley mist of the country 
 is a near relative of the ugly nauseous fogs of our 
 large cities. London fog is simply Thames river 
 valley mist, mixed with the smoke poured forth 
 by innumerable chimneys, which is unable to be 
 carried off, owing to an absence of wind above. 
 When we can consume our own smoke, London 
 fog, as we know it, will disappear.
 
 110 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 Fogs at sea occur most frequently in summer, 
 and especially near cold currents, as off New- 
 foundland, where the warm moist air off the Gulf 
 Stream passes over the cold Labrador current ; 
 in the Behring Sea, where the Japan current meets 
 the Arctic ice; off Cape Horn, &c. 
 
 At San Francisco, the Pacific Ocean breeds a 
 chilly fog, which rolls up the streets, and obliter- 
 ates the sun, even in May, but it is fortunately 
 white. 
 
 The clouds of heaven have ever been an ob- 
 ject of wonder and admiration to the sons of 
 men. Long before Aristophanes wrote his im- 
 mortal comedy, "The Clouds," we have numerous 
 references to the clouds in the ancient Scriptures. 
 Job says: " He bindeth up the waters in his thick 
 clouds." "For he maketh small the drops of 
 water ; they pour down rain according to the 
 vapour thereof." "Also can any understand the 
 spreadings of the clouds"; while in Ecclesiastes 
 we have a wonderful insight into the whole 
 scheme of water circulation in the verse which 
 says, " All the rivers run into the sea, yet the 
 sea is not full. Unto the place from whence the 
 rivers come thither they return again." 
 f"* The first person who seriously observed and 
 / described the clouds was Luke Howard, the 
 Quaker, who was first attracted to the subject in 
 
 ^ 
 t? 
 
 . _ 
 
 Howard roughly divided clouds into three j 
 primary forms stratus, cumulus, and cirrus 1 
 and the same division also roughly applies to I 
 their height low, intermediate, high. J 
 
 Of late years, as our knowledge of their vari- 
 ous forms and mode of origin has increased, this 
 simple division has been found inadequate. The
 
 DEW, FOG, AND CLOUDS OF THE ATMOSPHERE. 1 1 1 
 
 late Rev. Clement Ley, Mr. Ralph Abercromby, 
 Dr. Hildebrandsson, Dr. Vettin, and Messrs. 
 Ekholm and Hagstrom have observed their 
 forms, behaviour, and heights, and developed 
 quite a science of clouds. 
 
 The outcome of their researches is briefly sum- 
 marised in the International Cloud Atlas, which 
 has just been published, as a result of the Interna- 
 tional Committee held at Upsala, Sweden, in 1894. 
 
 Without going into details, the following gives 
 a general idea of the varieties and corresponding 
 heights, beginning with those near the surface; 
 the character of the cloud being indicated as wet 
 or dry according to the weather by which it is 
 usually accompanied : 
 
 VARIETIES OF CLOUDS. 
 
 Height in feet. 
 
 Name. 
 
 Description. 
 
 Char- 
 acter. 
 
 ; I. Sea-level 
 
 Stratus. 
 
 Elevated fog, so-called. 
 
 Dry and 
 
 up to 3000. 
 
 
 
 wet. 
 
 2. 4500 to") 
 6000. 1 
 3. 4500 to f 
 
 Cumulus. 1 
 Cumulo- y 
 nimbus. 
 
 Rounded heap. 
 Tower-like clouds with 
 round tops and flat 
 
 Dry. 
 Wet. 
 
 24,000. J 
 
 J 
 
 bases. 
 
 
 4. 6400. 
 
 Strato- 
 
 Rolls of dark cloud. 
 
 Dry. 
 
 
 cumulus. 
 
 
 
 3 5- 6400. 
 
 Nimbus. 
 
 Masses of dark formless 
 
 Wet 
 
 
 
 cloud. 
 
 
 6. 10,000 to 
 
 Cirro- 
 
 Fleecy cloud, mackerel 
 
 Dry. 
 
 21,000. 
 
 cumulus. 
 
 sky. 
 
 
 Average height. 
 
 
 
 
 1 7. 27,000. 
 
 Cirro-stratus. 
 
 Fine whitish veil giving 
 halos round sun and 
 
 Wet. 
 
 
 
 moon. 
 
 
 ' 8. 27,000. 
 
 Cirrus. 
 
 Isolated feathery white 
 
 Dry. 
 
 
 
 clouds. 

 
 112 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 I have left out one or two purposely for sim- 
 plicity's sake. 
 
 Pictures of i, 2, 4, 6, 8 are given in figs. (23) ; 
 frontispiece, (i) and (2), (3) and (25), respect- 
 ively. ^. 
 
 It used to be thought that clouds were simply 
 produced by the mixture of cold and warm air. 
 In 1788 Dr. Hutton of Edinburgh propounded 
 his celebrated law of mixtures, which briefly 
 asserted that, when two masses of air at different 
 temperatures mingled, the colder air caused the 
 vapour in the warmer air to condense, owing to 
 the lower temperature of the mixture not being 
 able to support the same amount of vapour in 
 solution. ^__ 
 
 Dr. Von Bezold has recently (1890) investi- 
 gated the question, and has shewn that when 
 saturated warm air mixes with unsaturated _cold 
 air, more cloud will be found than when >atu- 
 rated cold air penetrates unsaturated warm air. 
 These two cases are successively illustrated by 
 opening the door of a laundry on a cold day and 
 the door to an ice-house on a hot day. In the 
 former case fog is at once formed, but not in the ] 
 latter. s 
 
 On the whole, however, he finds the effect of 
 mixture to be very small and ineffective. In the 
 case of Nature it usually occurs when a layer of 
 warm air overlies a layer of colder air. ^ 
 
 The shallow stratus, cirro-cumulus, and cirro- A 
 stratus clouds are partly due to this action. 
 
 Where one current crosses another at a differ- / 
 ent speed it raises waves or billows in it, just as 
 when wind passes over the sea. In such waves I 
 there will be cloud at the crests and clear spaces 
 in the troughs. , -
 
 DEW, FOG, AND CLOUDS OF THE ATMOSPHERE. 113 
 
 Mackerel sky, or cirro-cumulus, and the long 
 rolls of strato cumulus which follow one another 
 at the rear of a storm, with showers and clear in- 
 tervals of several hundred feet, are examples on 
 a small and large scale of such aerial billows. 
 
 FIG. 25. Cirrus Cloud (var Tracto Cirrus, 1889). 
 P. Gamier, Boulogne Observatory. 
 
 \ The most frequent cause of clouds, however, 
 / is the cooling, due to expansion of air, which 
 ' ascends either freely, or by being forced up a 
 jountain-slope, or drawn up an aerial eddy. 
 
 When the existing rate of decrease of tempera- 
 
 I ture with the height is greater than i-| degrees 
 
 per 300 feet, air locally warmed will ascend, and 
 / cool at this rate until the dew-point is reached, 
 
 when vapour will be condensed and cloud formed. 
 l After this the air, since it is now saturated, will 
 I cool at a much slower rate viz., about F. in 
 
 300 feet.
 
 114 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 Ascent after cloud level is reached is easy, 
 therefore, so long as sufficient moist air is sup- 
 plied. 
 
 ; Clouds are often found hugging mountains 
 / when the surrounding plains are clear. 
 
 J- In popular language mountains are said toj 
 " attract clouds." This, of course, is literally in- / 
 correct. The clouds are due in this case chiefly/ 
 to the lower air being forced up the mountain' 
 side, and cooled by expansion down to dew-point. 
 r~~ Clouds also occur in connection with cyclones 
 
 I or large storms, both in the tropics and high lati- 
 tudes. 
 
 This is because the air in the centres of such 
 movements is damp to start with, and is continu- 
 ally rising and flowing out over the surrounding 
 drier air. 
 
 The approach of such a storm when miles 
 away is frequently heralded by the appearance 
 of tangled masses of the lofty cirrus cloud, which 
 appear to converge to a point below the horizon,, 
 as in fig. 26, and which represent the overflow of 
 the ascending damp air from the centre of the 
 storm. These clouds are called the " warning 
 cirrus," because their appearance and motion 
 (from the storm-centre, unlike the lower clouds 
 which move towards it) indicate the position and 
 character of the approaching disturbance. Soon 
 after this storm signal has been hung out by the 
 Celestial weather-bureau, sheets of cirro-stratus 
 appear below these cirrus wisps, p. in, No. 7, 
 which hide the sun and often cause a large halo, 
 by refracting its rays through the prisms of ice 
 of which they are composed. The final act in 
 the aerial drama is ushered in by the appearance 
 of high stratus and nimbus, No. 5, with ragged
 
 DEW, FOG, AND CLOUDS OF THE ATMOSPHERE. 115 
 
 scud of stratus, No. i, quite low down. These 
 lower clouds move round and in towards the 
 storm-centre, and thus make a considerable angle 
 with that of the cirrus flowing out from it. 
 
 FIG. 26. 
 
 If a storm-centre, for example, bears S. W., the 
 cirrus will also bear S. W, the cirro-stratus S., the 
 low clouds S. E., and the surface wind E. S. E. 
 
 The storm culminates with the arrival of the 
 nimbus, from which rain or snow falls, after 
 which the clouds disappear in the reverse order 
 of their arrival, except that, owing to the more 
 rapid motion above, in the direction in which the 
 storm is moving, the upper clouds are blown 
 towards the front, and are less prevalent in the 
 clearing up showery weather in the rear. Fig. 
 (23) represents the upper sky as seen after a 
 storm, from a height of 7000 feet, in the Alps. A 
 sheet of lower cloud or fog is below the spectator.
 
 Il6 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 Fig. (27) shows the position of the clouds 
 round a European cyclone. The continuous 
 lines are the isobars, and the dotted lines iso- 
 therms. 
 
 Almost the entire cloud mass, it will be no- 
 ticed, lies to the front of the central ring of low 
 pressure. 
 
 Amongst special forms of clouds may be men- 
 tioned the table-cloth of Table Mountain at the 
 
 FIG. 27. 
 
 Cape, and similar table-cloths on Mount Pilatus, 
 the Rock of Gibraltar, Atlas, &c. These are all 
 formed by the passage of a warm moist current 
 over a cold mountain which condenses its mois- 
 ture while it is moving across the summit. When,
 
 DEW, FOG, AND CLOUDS OF THE ATMOSPHERE. 117 
 
 however, it has passed beyond the mountain, the 
 vapour cloud mixes with the warmer air around, 
 and, the vapour becoming reabsorbed, the cloud 
 gradually tails off into invisibility. 
 
 In New Zealand, when a wet north-wester is 
 blowing against the Southern Alps, and heavy 
 rain is falling on their western sides, a long bar 
 of cloud, due to similar causes, may be observed 
 stretching along their summit ridge, extending 
 some little distance from it above the eastern 
 plains. 
 
 The eastern edge of this cloud roll remains 
 fixed, though the wind may be blowing through 
 it, as it often is in these cases, with hurricane 
 violence. 
 
 Cumulus clouds indicate upward convection 
 movements, and are more frequent in summer and 
 warm countries. 
 
 They are frequently as tall and thick as they 
 are broad, and often pierce up right into the 
 cirrus. 
 
 Clouds of the stratus form, on the other hand, 
 are usually very shallow compared with their 
 area, which may extend for hundreds of miles. 
 They indicate horizontal motions, and prevail 
 mostly in winter and cold latitudes. 
 
 Clouds generally increase during the day-time, 
 and reach their greatest height in the afternoon 
 and evening. 
 
 The average rate at which clouds move in- 
 creases with their altitude. This accords with the 
 theory of the general circulation in Chapter V. 
 
 Cirrus clouds have occasionally been observed 
 to move as fast as 250 miles per hour ; but their 
 average pace is more nearly 80 miles in America 
 and 40 in Europe.
 
 Il8 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 The average velocities for the different cloud 
 levels observed at Blue Hill, near Boston, have 
 already been given on p. 92. 
 
 The average heights of the clouds are greatest 
 in summer and least in winter. Their average 
 speed is least in the former and greatest in the 
 latter season. 
 
 In fig. (25) a cirrus cloud is shewn, in which 
 the delicate fibrous nature of this cloud is plain- 
 ly seen. The contrast between this ice crystal 
 cloud, like a puff of white tobacco smoke, and 
 the cumulus in the frontispiece, composed of 
 heavy water drops, is very striking. 
 
 The Festooned cumulus in fig. (28) is a special 
 form of cloud associated with thunder and hail- 
 
 FIG. 28. Festooned Cumulus. 
 Sydney, N. S. Wales. Jan. 18, 1893. 
 
 storms. It is a kind of inverted cumulus, in 
 which a cold, very moist air is moving over a hot
 
 RAIN, SNOW, AND HAIL OF THE ATMOSPHERE. 119 
 
 and very dry air such as frequently occurs in Col- 
 orado, Australia, and India. Under these circum- 
 stances all the condensation occurs in the cold 
 layer, and none in the lower, hot one. Portions 
 of the upper layer drop down in large bulbous 
 masses like water balloons, which burst like soap 
 bubbles and drop their moisture like water run- 
 ning out of a cask. Sometimes this water never 
 reaches the ground, being re-evaporated while 
 passing through the hot air. 
 
 CHAPTER VIII. 
 
 THE RAIN, SNOW, AND HAIL OF THE 
 ATMOSPHERE. 
 
 RAIN is the final stage of condensation of va- 
 pour back into water, of which cloud is a half- 
 way stage. The mist which composes a cloud is 
 formed of tiny drops of water about ^-g^-inch in 
 diameter. It used to be a puzzle to explain how 
 these water particles were sustained, and it was 
 at one time supposed that cloud particles were 
 hollow. We know now that this is neither neces- 
 sary nor true, since very small particles even of 
 gold will remain suspended for a long time in air; 
 the finer the particles the longer they take to fall. 
 A slight upward motion of the air is therefore 
 enough to keep them balanced. As condensation 
 proceeds these particles grow larger by fresh coat- 
 ings of water, and the larger ones fall down 
 against the smaller and mingle with them until 
 large drops from -fa to -^ inch thick form, which 
 are no longer capable of being suspended and fall
 
 120 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 to the earth. Snow forms when the temperature 
 at which this further stage of condensation oc- 
 curs is below freezing point. Every snow crys- 
 tal is a variety of a six-rayed cluster, and is sim- 
 ilar to the crystals of salts which are precipitated 
 from a chemical solution. No one has watched 
 the formation of snow, but it must be very simi- 
 lar to that of crystallisation out of a solution 
 which is saturated with a chemical salt. 
 
 Hail, unlike the delicate snow crystals, is 
 frozen water-drops. Its frequent association with 
 thunder-storms led to the belief that it was 
 caused in some way by electricity. This is, how- 
 ever, found to be untenable in the searchlight of 
 modern science, which shews that electricity is 
 mostly an effect, not a cause of such mechanical 
 disturbances. It is believed, that in such storms 
 the rain-drops formed in one part of a storm are 
 carried upwards by powerful ascending currents 
 (twenty-five miles an hour is enough to sustain 
 large drops) into higher regions of the atmosphere 
 where they are solidified by the excessive cold, 
 and being carried over with the overflow which 
 takes place near the top, fall down until they are 
 redrawn into the interior of the storm and again 
 whirled up aloft. Receiving alternate meltings 
 and freezings, and growing larger with each cir- 
 cuit they make in the atmospheric churn, they are 
 finally thrown out on either side of the storm cen- 
 tre. This explains the fact that in a travelling 
 hailstorm there are two bands where hail falls on 
 either side, while, under the centre, it is often 
 found that only rain has fallen. 
 
 Hailstones have often fallen of enormous sizes. 
 In 1697, Robert Taylor found hailstones in Hert- 
 fordshire 14 inches in circumference.
 
 RAIN, SNOW, AND HAIL OF THE ATMOSPHERE. 121 
 
 In India, the writer remembers a hailstorm on 
 the great Brahmaputra river when the hailstones 
 cut holes through the tarpaulin cover of the 
 steamer, which were so large that each one had 
 
 IAPHICAI, MAP 
 
 FIG. 29. 
 
 to be mended with a separate patch. Hailstorms 
 are intimately connected with tornadoes, and, like 
 these phenomena, are more frequent over flat 
 plains and in very hot and moist summers and 
 countries. 
 
 The destruction dealt by hail on standing 
 crops, vineyards, and orchards has led to means
 
 122 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 being proposed for its prevention. Under the old 
 idea of its connection with electricity, lightning 
 rods were erected, but without avail. In France 
 and other countries shooting with cannon into the 
 forming clouds has been tried, but with little suc- 
 cess. 
 
 Planting of trees would be m:re effective, 
 since this would tend to check the rapid heating 
 up of the lowest stratum of air, which is one of the 
 chief causes of tornado and hailstorm action. 
 
 The general distribution of rain in belts over 
 different areas of the earth's surface has already 
 been alluded to. Rainfall, like clouds, is more 
 prevalent in mountainous than over flat countries, 
 and for similar reasons, especially cooling by 
 forced ascent of air. 
 
 In the accompanying mean annual rainfall 
 maps of England and India, this will be readily 
 seen. In England the heaviest falls will be ob- 
 served to occur in the mountains of Cumberland 
 and Wales, and generally along the hilly country 
 of the West and North. In Scotland and Ireland 
 it is the same. The lowest rainfalls under 20 
 inches all occur on the eastern sides of the coun- 
 try. This difference is partly due to the fact that 
 the prevailing and most rainy winds are south- 
 west and drop a good deal of their moisture be- 
 fore reaching the eastern parts, but even were 
 these barriers absent, the rainfall over the flatter 
 country on the eastern sides would not be very 
 much increased. In India, in like manner the 
 dark shading along the Western Ghats down the 
 Bombay coast and along the Himalaya shews the 
 influence of the mountains, the heaviest fall oc- 
 curring near the north-east corner of the Bay of 
 Bengal in the Khasia hills, which offer an abrupt
 
 RAIN, SNOW, AND HAIL OF THE ATMOSPHERE. 123 
 
 wall 4000 feet high up which the southerly mon- 
 soon winds, see fig (20), are forced. 
 
 Chirapunji, at the edge of these hills, has the 
 largest rainfall in the world (about 500 inches), 
 
 FIG. 30. 
 
 half of which falls in June and July. On the 
 western side of the Ghats rain falls heavily up 
 to 250 inches at Mahalleshwar on their summits, 
 while the tableland of the Deccan on their east- 
 ern lee side has a scanty supply and is one of the 
 areas liable to drought. 
 
 The greatest amount of rain in a vertical 
 direction occurs at altitudes where the lowest
 
 124 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 cloud is thickest, that is, at about 3000 feet in 
 Europe and 4000 feet in India above sea-level. 
 Jn the interior of large continents where moun- 
 tain ranges are absent, especially when they lie 
 
 FIG. 31. 
 
 like Australia in the zone of perpetual high 
 pressure dividing the tropical from the polar 
 wind systems, the rainfall tails off to a very few 
 inches as we go inland. 
 
 The accompanying rain map of Australia 
 shews this very plainly. 
 
 Over the whole of the lightly shaded area of 
 central and west Australia there are less than 
 10 inches per annum. 
 
 This district can never support a large popu- 
 lation.
 
 THE CYCLONES OF THE ATMOSPHERE. 125 
 
 CHAPTER IX. 
 
 THE CYCLONES OF THE ATMOSPHERE. 
 
 EVERY large storm of the atmosphere is now 
 called a cyclone, because it is found that the air 
 moves round and in towards a central area. 
 
 Formerly the word cyclone was only applied 
 to the rare but violent storms of the tropics, 
 while the words hurricane and tornado are still 
 popularly used to signify small and large storms, 
 indifferently. 
 
 The term tornado is now applied by meteor- 
 ologists entirely to certain storms of quite a 
 special class, differing from cyclones both in size, 
 mode of origin, and effects, and it is to be hoped 
 that the newspapers will learn eventually to give 
 up a habit which only leads to confusion. 
 
 A cyclone is a large disc of nearly horizontally 
 moving air circulating spirally round a central 
 area over which the barometric pressure varies 
 from one-fifth to as much as three inches below 
 that at its border. 
 
 The direction in which the wind circulates is 
 the same as that in which the earth's surface 
 would appear to rotate in each hemisphere, if we 
 stood several miles directly above the pole and 
 looked downwards. 
 
 Cyclonic storms range in diameter from 20 to 
 as much as 3000 miles. 
 
 A tornado, on the other hand, consists of a 
 narrow column of air varying in width from 20 
 feet to 1400 feet which is rotating with immense 
 velocity (up to 500 miles an hour) round a 
 central shaft up which it is also ascending with
 
 126 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 a speed in some cases amounting to 100 miles 
 an hour. 
 
 A cyclone is an elephant, while a tornado is 
 a mouse, and they differ just as much in other 
 respects as these two animals. 
 
 Tornadoes will therefore be specially referred 
 to in the next chapter, together with whirlwinds, 
 waterspouts, and thunderstorms, which belong to 
 the same family. 
 
 Many poetic and graphic descriptions of the 
 awful grandeur of a real tropical cyclone have 
 been given. All descriptions, however, pale 
 before the real thing. 
 
 The writer once experienced in Eastern Bengal 
 the full violence of perhaps the most disastrous 
 cyclone in regard to destruction of human life 
 on record viz., what is known as the Backer- 
 gunge cyclone of November ist, 1876.* 
 
 After several days of unusually quiet, muggy 
 warmth and murky skies, lurid sunsets, and a 
 general sense of impending doom, the rain began 
 to fall in torrents and the wind to rise as the 
 night came on, until at last I had to pile up the 
 furniture against the windows to prevent their 
 being burst inwards. The lightning flashed un- 
 ceasingly, the thunder crashed, the wind tore 
 past like a raging fiend. All the elements 
 seemed to have broken loose, and one could 
 almost fancy that the sober laws of physics were 
 having "a night out." The very rarity of such 
 hurricane violence made it all the more alarming. 
 After a night of Tartarean gloom, mingled with 
 
 * June I2th of this same year witnessed the heaviest fall of 
 rain ever measured in the world viz., 40 inches in 24 hours, 
 at Chirapunji, Assam.
 
 THE CYCLONES OF THE ATMOSPHERE. 127 
 
 truly horrible noise, morning broke sadly through 
 gaps in the rampart of furniture, and I awoke to 
 a knowledge that the plaster coating of my house 
 lay strewn all over the compound. Later on I 
 learnt to be thankful nothing worse had occurred, 
 when I heard the awful news that 100,000 
 natives in the adjoining province had been 
 drowned by the storm wave which was forced 
 up the Bay on to the low-lying islands of Dakhin, 
 Shabaspore, and Noakolly at the mouth of the 
 giant Brahmaputra. 
 
 While cyclones are comparatively rare in the 
 tropics, they are very prevalent, though fortu- 
 nately as a rule in a milder form in higher 
 latitudes. North and south of latitude 35 con- 
 tinual streams of small cyclones travel along the 
 borders of the large permanent areas of low 
 pressure or polar cyclones which surround either 
 pole. 
 
 Sometimes one of these streams passes over 
 us, in which case we experience wet and stormy 
 weather. Sometimes they take a more northerly 
 or southerly track. 
 
 Their place is frequently occupied by large 
 areas of high pressure from which the air flows 
 quietly outwards to feed the cyclones. These 
 areas are termed anti-cyclones. Modern observa- 
 tions shew that the air which flows in towards 
 and up the centres of the cyclone hollows flows 
 out above and pours down the centres of these 
 anti-cyclone heaps. While the weather in the 
 cyclones, owing to the ascending damp air, is 
 cloudy and rainy, the weather in the anti-cyclones, 
 where it is descending, is dry and clear. Years 
 ago, until about 1830, there was little known 
 about the course of the winds in cyclones, 
 9
 
 128 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 and ships which mostly experienced their full 
 fury were at their mercy or the individual 
 caprice of their commanders. 
 
 About the beginning of the century, Capper, 
 of the East India Company's Service, announced 
 that the storms of the Bay of Bengal were vast 
 whirlwinds. 
 
 In 1828 Professor Dove of Berlin, and soon 
 after Redfield of America, Reid of England, and 
 Piddington of Bengal, developed, though with 
 much diversity of opinion, the memorable " Law 
 of Storms." 
 
 The chief point of this law was the fact that 
 the wind always circulated round the area of 
 lowest barometer in a nearly circular spiral (there 
 was much unnecessary dispute on this point) 
 against watch hands in the northern hemisphere. 
 
 They also ascertained that tropical cyclones 
 originated in a belt about 10 on either side of 
 the equator and travelled thence polewards along 
 parabolic paths, occasionally crossing the tropical 
 belts of high pressure, where they were broken 
 on their western sides and into the north and 
 south temperate zones. 
 
 The accompanying fig. (32) shows their gen- 
 eral course under these circumstances. 
 
 An immediate consequence of these rough 
 rules was to enable the mariner to avoid what 
 was termed the "dangerous semicircle " (/. e., the 
 front half in the direction of travel), and to tell 
 the direction of the storm-centre by noting the 
 direction of the wind. A rule by which this is 
 simply remembered was afterwards enunciated 
 by Dr. Buys Ballot, the eminent Dutch meteor- 
 ologist, thus : 
 
 " Stand with your hands stretched out on
 
 THE CYCLONES OF THE ATMOSPHERE. 129 
 
 either side and your back to the wind, then in 
 the northern hemisphere the centre of the cyclone 
 will be to your left hand." In the Southern 
 hemisphere substitute " right " for " left." 
 
 FIG. 32. 
 
 Stated in this form the incurvature of the 
 wind or its inclination to the isobars which con- 
 tour round the central area is completely over- 
 looked. 
 
 This inclination is found to increase with the 
 distance from the centre, and the distance of the 
 storm from the equator, quite apart from the 
 fact that on land it is always greater than at sea.
 
 130 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 Thus in the Philippine Islands (latitude 14) 
 it was found to be 62, in the Bay of Bengal (lati- 
 tude 20) to be 57, in the United States 40, over 
 the Atlantic 30, and in England the late Rev. 
 Clement Ley found it to be about 20, while Cap- 
 tain Toynbee found it to be as much as 30 for 
 the Atlantic in latitude 50 N. 
 
 Near the equator, therefore, it would be mani- 
 festly unsafe for a mariner to trust to the famous 
 old "circular theory," which made the winds 
 blow directly along the isobars, since there the 
 centre of the storm, instead of being directly to 
 his left if he stood until the wind blew directly 
 upon his back, would actually be nearly in front 
 of him. 
 
 Dr. Meldrum of Mauritius, who was one of the 
 most indefatigable reformers of the old circular 
 law, mentions a case where as recently as January 
 24, 1883, the captain of the ship Caledonien delib- 
 erately ran his ship straight into the centre of a 
 storm by following the old rule. The modern 
 rules now advise the mariner (i) to avoid running 
 before the wind, (2) to lie to on the starboard 
 tack in the northern hemisphere or the port tack 
 in the southern. By this means the vessel may 
 be safely guided out of the dangerous vortex. 
 
 Tropical cyclones occur most frequently on 
 the western sides of the N. Atlantic, the N. and S. 
 Pacific oceans, and the S. Indian Ocean, also in the 
 Bay of Bengal and the China Sea, where they are 
 termed Taifuns. The months in which they occur, 
 September and October in the N. hemisphere, and 
 February and March in the Southern, are soon 
 after the periods when the equatorial calms or 
 doldrums which lag behind the sun have reached 
 their extreme northerly and southerly positions.
 
 THE CYCLONES OF THE ATMOSPHERE. 
 
 '3* 
 
 The air is calm and full of moisture, and this, 
 combined with the fact that they are preceded 
 and accompanied by torrential rain, has led to the 
 conclusion that they are due to the upward con- 
 vection of damp air which causes an indraft 
 towards some central area. The heavy clouds 
 and thunder and lightning which accompany 
 them, fully bear out the same view. 
 
 Moreover, the energy supplied by the con- 
 densation of the vapour which allows the air to 
 recoup itself for the loss due to expansion has 
 been calculated to be sufficient to account for the 
 immense wind energies they exhibit. Professor 
 Reye of America calculated that the Cuban cyclone 
 of October 5, 1844, used up in three days 473 mil- 
 lion horse-power. Indeed, when we consider that 
 the air in a cyclone 100 miles in diameter and a 
 mile high weighs as much as half a million ocean 
 steamers of 6000 tons a-piece, we can hardly 
 wonder at the enormous amount of energy re- 
 quired to kieep this in motion, at, say, 40 miles an 
 hour. 
 
 On reaching land, tropical cyclones frequently 
 break up. They are nearly unknown on the 
 equator itself. 
 
 Ferrel .again proved to be the Newton, who 
 was able to weave all the disconnected facts relat- 
 ing to cyclones into a reasonable theory of cause 
 and effect. Assuming an inflow towards, and an 
 upflow over, a given area, he was able to shew 
 that at some little distance from the equator the 
 spiral rotation of the winds and all the other 
 phenomena of a cyclone would follow from the 
 law of inertia on a rotating sphere explained in 
 Chap. V. In an ideal case, where friction was 
 unconsidered, the air would tend to rotate round
 
 132 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 .a central area, as in fig. (33). At the centre the 
 pressure would be very low, gradually rising to a 
 maximum on the line separating the interior from 
 the exterior gyrations. Outside this line the 
 
 gyrations would 
 be reversed. The 
 interior region 
 would be the 
 true cyclone and 
 the exterior a 
 kind of anti-cy- 
 clone, usually 
 termed a peri- 
 cyclone. Where, 
 as in nature, the 
 air experiences 
 friction, the 
 
 pressure near 
 the centre would 
 
 FIG. 33. be moderately 
 
 low, the interior 
 
 arrows would point inward towards the centre, 
 and the exterior arrows outwards. The vertical 
 circulation of the interior zone is simply shewn in 
 
 fig. (34). 
 
 All the results from theory agree with those 
 observed. For example, an increase of the vio- 
 lence of the wind until it suddenly drops near the 
 centre, where in tropical storms the clouds also 
 disappear and the air becomes clear and calm for 
 a space of occasionally 20 miles. This is called 
 the eye of the storm, and the course of the air is 
 believed to be that in the accompanying diagram, 
 fig. (35), where the violent rotation produces such 
 a centrifugal force as to cause some of the upper 
 air to descend to fill the vacuum. Though the
 
 THE CYCLONES OF THE ATMOSPHERE. 
 
 133. 
 
 FIG. 34. 
 
 air is calm, the sea is here of that confused char- 
 acter most apt to make a vessel founder. 
 
 The weather of the tropical regions is con- 
 trolled almost entirely by the regular daily and 
 seasonal changes 
 produced by the 
 path of the sun in 
 the sky between 
 sunrise and sun- 
 set, together with 
 that in its average 
 altitude between summer and winter, and is scarce- 
 ly affected except temporarily by the rare passage 
 of a cyclone. 
 
 In the extra tropics, that is to say from about 
 latitude 35 to the Pole, the weather on the 
 contrary is almost entirely made up of a succes- 
 sion of cyclones and anti-cyclones. The changes- 
 introduced by these, completely dominate those 
 brought about by the daily and seasonal causes. 
 
 Extra-tropical cyclones are moreover believed 
 to be due to different causes to those of the 
 
 tropics. Unlike the latter, they are most fre- 
 quent in winter when the lower air is in a stable 
 condition, and they sometimes occur without 
 rain. They are supposed by modern specialists 
 to be for the most part eddies in the large upper 
 return currents, as they flow over from the Equa- 
 tor, and crowd into the narrowing space towards 
 the poles. The effects at the earth's surface are
 
 134 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 the same as though the air rose spontaneously, 
 since an eddy in mid-air causes the lower air to 
 ascend just as a whirlpool sucks down water that 
 is drawn into its vortex. The air which is thus 
 forced to rise, forms clouds and usually rain, but 
 the storm generally in such cases belongs more 
 to the upper regions of the atmosphere, and is 
 less violent near the earth. 
 
 The movement of cyclones is quite different 
 from that of the winds that blow round their 
 centres. The latter may vary from a gentle 
 breeze to a hurricane of 100 miles, and the centre 
 may remain stationary, but usually the cyclone 
 itself moves over the earth at a speed which 
 varies in different localities and for each disturb- 
 ance. 
 
 We have already noticed the general move- 
 ment of cyclones in fig. (32). In all cases when 
 once they are formed they appear to be guided 
 chiefly by the upper currents. In the tropics the 
 west and poleward motion results from a combina- 
 tion of the lower westward moving trades and the 
 upper poleward moving return (or anti-trade) 
 currents from the equator, but they often move 
 here as elsewhere in very different paths, though 
 generally north or south-westward. 
 
 Beyond the tropics they are driven eastward 
 by the prevalent west to east winds, both above 
 and below, and like eddies on a river are carried 
 along by the stream. 
 
 They also exhibit a tendency to move round 
 the anti-cyclone heaps which are here chiefly 
 forced down-flows (just as the cyclones are here 
 forced up-flows) so as to keep the anti-cyclones 
 on their right in the northern hemisphere. This 
 principle is made use of in forecasting their
 
 THE CYCLONES OF THE ATMOSPHERE. 135 
 
 probable motions. There appears to be little 
 known about the movements of detached anti- 
 cyclones or areas of fine weather, but they fre- 
 quently shew a tendency to move from E. to W. 
 as well as from W. to E. 
 
 The average speed of cyclones in different 
 parts of the world has been determined by the 
 late Professor Loomis from a very large number 
 of cases, thus : 
 
 Average Speed of Cyclones over the Earth. 
 
 United States 28 miles per hour. 
 
 North Atlantic 18 
 
 Europe 16 
 
 West Indies 14 
 
 Bay of Bengal and China Sea 8 
 
 The greatest amount of cloud and rain and 
 the highest temperature occurs in their front 
 halves, and their vitality, especially in the 
 tropics, appears to depend on the continuance 
 of condensation and rainfall, which allows the 
 air to flow readily up their centres. 
 
 The vitality and longevity of some of these 
 storms is wonderful. Thus a few years ago Mr. 
 H. Harries, of the English weather bureau, traced 
 a storm which started in the Japan seas right 
 across the Pacific, America, the Atlantic, and 
 Europe, until it was lost sight of in the Baltic. 
 
 The forecasting of daily weather in high lati- 
 tudes requires a knowledge of the birth and 
 subsequent movements of particular storms. In 
 the tropics, where the daily weather changes are 
 small, the most important problem is the pre- 
 vision of average seasonal weather. This re- 
 quires something more than a mere knowledge 
 of travelling cyclones, and observes large waves 
 of pressure which take months to travel from
 
 136 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 equator to pole, or from west to east. By this 
 means, seasonal forecasting six months ahead 
 is carried on in India, and a similar method 
 could be applied to the average weather of other 
 countries. The waves of pressure caused by the 
 passage of cyclonic storms compared with these 
 large waves are like the ripples on an ocean 
 billow. 
 
 The passage of cyclones and anti-cyclones 
 introduce special winds which possess peculiar 
 characteristics, due to origin and direction. The 
 sirocco, of Italy and Greece, the leveche and 
 solano of Spain, the leste of Madeira, the Kham- 
 sin of Egypt, the Kona of Hawaii, and the brick- 
 fielder pf Southern Australia are all examples of 
 the wind in the front half of a cyclone which, 
 coming from regions nearer the equator, is in- 
 variably warm, and dry and exhilarating, or damp 
 and muggy, according as they have travelled 
 over sea or land. 
 
 The lassitude and irritability produced by the 
 solano has given rise to the Spanish proverb, 
 " Ask no favour during the Solano." The 
 Italian sirocco induces similar weariness. 
 
 At the rear of the cyclones of the temperate 
 zone which travel from W. to E. in both hemi- 
 spheres, the wind blows from some polar direc- 
 tion. 
 
 Locally are thus produced the cold " Nortes " 
 of Mexico, the "blizzard" of the States, which 
 is accompanied by blinding snow, the "Mistral" 
 of the Rhone Valley and the Gulf of Lyons, 
 the " pampero " of Mexico, and the " southerly 
 bursters " of Australia. 
 
 The " bora " of the Adriatic is of the same 
 species, but possesses a peculiar, penetrating cold
 
 THE CYCLONES OF THE ATMOSPHERE. 137 
 
 by being drawn down towards the cyclonic 
 depression from a lofty plateau where it has 
 acquired great cold by radiation. 
 
 A peculiar wind arises in connection with the 
 motion of cyclones over mountain ranges, called 
 in Switzerland the " foehn" zx\& in America the 
 " chinook." In New Zealand it is locally known 
 as a " hot north-wester," and it is found to occur 
 everywhere on the lee side of mountain ranges 
 running athwart the paths in which the cyclones 
 travel. This wind is uncommonly hot and dry, 
 and melts the snow on the Alps in one night 
 more than the sun shining for several weeks. 
 
 In America the chinook blows on the eastern 
 side of the Rockies and raises the temperature 
 of a long belt of that part of the country per- 
 manently above what it would otherwise experi- 
 ence. 
 
 The way in which this heat is derived has 
 been revealed by a knowledge of atmospheric 
 physics, and is really quite simple when, as the 
 professor of legerdemain is fond of saying, "you 
 know how it's done." 
 
 On the windward side, the air after it has- 
 reached cloud level, loses only about 4 / s every 
 300 feet it ascends. When it has reached the top- 
 of the range it has lost a great deal of moisture 
 in the shape of rain, and as it descends on the lee 
 side as dry air, it gains heat at the rate of i 3 / s 
 per 300 feet, or, in other words, it gains an extra 
 4 / 5 for every 300 feet it descends. Consequently, 
 by the time it reaches the lee valleys it appears 
 as a hot, dry wind. The higher the mountain 
 chain the hotter the wind. It used to be thought 
 that the famous hot nor'-wester of the Canterbury 
 Plains in New Zealand derived its heat from.
 
 138 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 Australia, but it is found that when it is blow- 
 ing hot and dry in Christchurch it is rainy and 
 cool on the western side of the southern Alps. 
 
 CHAPTER X. 
 
 THE SOUNDS OF THE ATMOSPHERE. 
 
 As inhabitants of this earth planet we are far 
 more dependent for our happiness on sound, even 
 inharmonious noises, than we are inclined to ad- 
 mit. A world without the voices of men and 
 animals, without music and song, wrapped in 
 profound silence would be insupportable. And 
 of all phenomena, sound is one which pecul- 
 iarly belongs to the atmosphere, since while light 
 can travel wherever aether exists, even through 
 vacuum, sound cannot exist apart from air. Every 
 sudden movement of the air propagates a series of 
 waves from the point of origin of the motion sim- 
 ilar to what occurs in water when a boy throws a 
 stone into a pond. 
 
 A sudden meeting of two solid objects gives 
 a blow to the adjoining air which suffices to 
 originate a series of such air waves. These 
 waves differ from those on the surface of water 
 in one essential point viz., that whereas in 
 water waves, the water moves up and down, 
 while the wave motion is propagated horizon- 
 tally ; in the case of air waves, the air moves 
 backwards and forwards in the same direction as 
 that in which the wave is transmitted. The air 
 is thus alternately compressed and dilated, and 
 as such conditions travel forward in all directions
 
 THE SOUND OF THE ATMOSPHERE. 139 
 
 from the origin of the disturbance, the sensation 
 of sound which is produced when such waves 
 meet the ear is propagated through considerable 
 distances. When these waves enter the human ear 
 they beat up against a delicate plate, or tympanum 
 as it is termed, of hard skin, and cause it to shake 
 backwards and forwards. The movements of 
 the tympanum are passed on by a series of bones 
 in loose contact, which filter out irregularities 
 and pass the waves into a kind of aural piano 
 fitted with a number of delicate filaments instead 
 of keys, each of which is attached to a separate 
 nerve. Upon this piano tunes are played, as 
 in the case of an ordinary piano, while from our 
 brains we experience the sensation of high and 
 low notes, harmonies and discords, just as simi- 
 lar effects can be produced on the artificial in- 
 strument. We can only distinguish such waves 
 as sound, when they follow one another more 
 rapidly than 16 times per second, or less rap- 
 idly than 38,000. Waves exist beyond these 
 limits, but to us they are inaudible. A deep 
 bass voice causes about 100, and the highest 
 soprano has reached about 2000 waves per sec- 
 ond. 
 
 All sounds travel at about the same rate 
 1 1 20 feet every second in air of ordinary tem- 
 perature. Consequently when we hear thunder 
 follow about five seconds after a flash of light- 
 ning, we know it is a mile distant. 
 
 The dense air near sea-level is a better me- 
 dium for transmission of sound than the more 
 rare air at great elevations. Sound rises upwards 
 easier than it descends, and travels better through 
 damp than dry air. In a balloon Mr. Glaisher 
 heard the noise of a railway train at four miles
 
 140 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 high when in the clouds. When clouds were far 
 below him no sound was heard. 
 
 Echoes or reflections of sound are often a 
 very curious atmospheric phenomenon. To echo 
 the last word spoken distinctly, the reflecting 
 surface must be at least no feet away. A full 
 sentence requires a much greater distance. A 
 dome-shaped roof often produces a multiple echo, 
 the reflected waves undergoing continual reflec- 
 tions between the floor and the roof, until the 
 wave motion is finally converted into heat. 
 
 In the Taj Mahal, at Agra, the incomparable 
 marble mausoleum, erected by Shah Jehan to the 
 memory of his wife, the central dome gives a 
 beautiful multiple echo. 
 
 In buildings of a paraboloid form, such as the 
 Mormon tabernacle at Salt Lake City, Utah, the 
 slightest sound, such as a pin dropped at one end 
 of the building, can be heard near a certain point 
 at the other end. Yet this building can seat 
 11,000 people. The Whispering Gallery at St. 
 Paul's is another example of the same kind. In 
 the first case the sound waves are all reflected 
 toward the same point, and therefore reinforce 
 each other enough to render their continued 
 effect audible. In the latter, their continued 
 reflection between the walls of the gallery 
 prevents the loss they would usually experience 
 by spreading out in all directions. 
 
 Thunder can be heard at 30 miles, explosions 
 at 100 miles or over. Thus the firing at Water- 
 loo was heard at Dover. The sounds of volcanic 
 eruptions, however, have been heard at immense 
 distances. In 1883 the eruption of Krakatoa, a 
 volcanic island in the Sunda Straits, was heard 
 over an area equal to one-thirteenth of the entire
 
 OPTICAL PHENOMENA OF THE ATMOSPHERE. 141 
 
 globe. In one direction the sounds were heard 
 at Rodriguez in the Indian Ocean, 3000 miles 
 away (they took four hours to reach it), and in 
 another, at Alice Springs, in the very centre of 
 Australia. At intermediate points every place 
 thought a vessel was firing distress guns, and 
 search was made for the supposed vessel over an 
 area as large as Europe. Besides sounds, large 
 air waves were propagated, which expanded in 
 circles until they girdled the earth and then con- 
 verged upon the Antipodes of Krakatoa, whence 
 they were reflected back again to Krakatoa, and 
 so on no less than seven times. Every recording 
 barometer in the world shews little notches in its 
 record for August 27 and following days. Each 
 notch shews the passage of the wave backwards 
 and forwards from Krakatoa. These waves 
 travelled with the same velocity as the sound 
 waves, and took thirty-six hours to perform 
 each circuit of the globe. 
 
 CHAPTER XI. 
 
 THE COLOURS AND OPTICAL PHENOMENA OF 
 THE ATMOSPHERE. 
 
 THE story of our advance in the knowledge 
 of Light, like that in most other branches of 
 physical knowledge, is one of gradual dispersal 
 of error and perplexity, and the dawning of 
 truth and harmony. 
 
 Even the great Newton's emission theory, by 
 which light was supposed to be due to a kind of 
 bombardment of minute corpuscles, broke down
 
 142 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 when subjected to the keen analysis of modern 
 science, and another generation, led by Huyghens, 
 Euler, Young, and Fresnel, was required to formu- 
 late and develop the modern theory of wave 
 motion of the invisible aether which surrounds 
 and penetrates all matter. This theory of aether- 
 wave motion accords with all the observed facts, 
 and enables discovery to march forwards with 
 certainty and power. 
 
 Light and heat are simply effects of the same 
 wave motion. When the waves of aether are 
 between e^oo-th and ^-^nyth of an inch in length, 
 they produce the effect of light upon our eyes, 
 and at the same time heat upon our faces. 
 
 When the rays of other lengths between these 
 extreme limits reach us, they appear of certain 
 colours corresponding to their wave-length or 
 position in the so-called spectrum which is pro- 
 duced when white light is passed through a glass 
 prism. The longest waves produce red light, and 
 the shortest blue or violet, the order of colours 
 corresponding to decreasing wave length, and 
 therefore greater rapidity of wave succession, 
 being red, orange, yellow, green, blue, violet. 
 White light is made up of all these rays mixed. 
 When these rays either singly or mixed, as gener- 
 ally happens, come in contact with air, or matter 
 floating in it, they set up small oscillatory mo- 
 tions in the tiny molecules of which it is com- 
 posed. The effect of these motions constitutes a 
 condition which we term heat or light, according 
 as it affects certain nerves. A portion of the ra- 
 diant energy is used up in this generous perform- 
 ance, or in other words is said to be absorbed. 
 Relatively, more heat can be derived from the 
 long wave rays of red colour, and more light
 
 OPTICAL PHENOMENA OF THE ATMOSPHERE. 143 
 
 from the short wave rays of violet colour, but 
 both are produced all through the spectrum. 
 Heat and light, therefore, are simply effects of 
 the same wave motion according as it specially 
 affects our senses of sight or feeling, and they both 
 inseparably belong to the same radiant energy 
 of wave motion of the aether, started by a body 
 already in a state of incandescence like our sun. 
 Rays near the violet end of the spectrum pro- 
 duce the chemical action noticed in photography 
 besides being converted into heat and light. Dawn 
 and twilight have ever formed expansive themes 
 to the poet. " Rosy-fingered dawn " is a familiar 
 metaphor of the immortal Homer. These half 
 lights are the result of reflection of the sun's rays 
 when below the horizon, chiefly by the small dust 
 and water particles at great heights in the atmos- 
 phere. The fingered appearance alluded to by 
 Homer is due to the light passing between clouds 
 or mountains below the horizon. The reddish 
 colours of the clouds at both times are chiefly 
 due to the selective scattering which is exerted 
 by the dust and vapour suspended in the air. 
 The smaller waves corresponding to the blue 
 rays, as we have already remarked in Chap. VI., 
 are more easily turned aside than the larger ones 
 corresponding to the red rays, and this dispersion 
 reaches its greatest effect when the sun is shining 
 through a great thickness of air at its rising and 
 setting. Consequently red rays predominate at 
 these times and tint the clouds as they succes- 
 sively receive its parting or coming rays. Occa- 
 sionally when a sunset has disappeared below the 
 western horizon it is brilliant enough to cause a 
 second sunset on clouds near the western horizon 
 by reflection, just as though it were the sun itself.
 
 144 THE STORY OF THE EARTH'S ATMOSPHERE. 
 
 When the dust ejected by the volcano of Kra- 
 katoa Island in 1883 had spread in a layer above 
 50,000 feet all over the world we had such bril- 
 liant primary and reflected sunsets, which often 
 lasted i hours after the sun had disappeared. 
 
 The reflection was assisted by a peculiar ac- 
 tion called diffraction, by which white light meet- 
 ing fine dust is split up into its coloured elements, 
 just as though it passed through a glass prism. 
 
 In this way a huge coloured ring called a corona 
 was produced round the sun. This ring is blue 
 inside, and exhibits thence all the spectral colours 
 in turn, ending with red at its border. 
 
 Inside this ring a white central glow was pro- 
 duced even when the sun was high up in the sky. 
 When it was setting this diffraction glow became 
 pink and finally red through the extinction of all 
 other colours except the reds owing to the great 
 length of air traversed by the rays. Such glows 
 are always present to some extent, due to dif- 
 fraction by suspended water particles, but when 
 the Krakatoa dust was still in the upper atmos- 
 phere they were intensified, and the ordinary re- 
 flections prolonged far beyond their .usual limits. 
 
 Similar small coronse are produced when small 
 clouds pass over the sun and moon. On a small 
 scale they can be frequently observed when we 
 look through our eyelashes at the flame of a 
 candle or gas lamp. 
 
 The smaller the particles of cloud the larger 
 the corona. Hence the large corona seen round 
 the sun after Krakatoa, called Bishop's ring from 
 its discoverer in Honolulu, showed that the ma- 
 terial was composed of very small particles. 
 
 A halo is a large ring seen when the sun and 
 moon shine through a thin sheet of cirrus or
 
 OPTICAL PHENOMENA OF THE ATMOSPHERE. 145 
 
 cirro-stratus, and can only be produced by re- 
 fraction through ice-prisms. Consequently its 
 presence is one indication of the ascent of vapour 
 into very lofty regions, such as occurs in cyclones. 
 It is thus a signal of the approach of rainy and 
 stormy weather. 
 
 A primary halo is always the same size 45 
 diameter. Sometimes, however, secondary halos 
 are formed by more complicated refractions and 
 reflections of light through the ice prisms. 
 
 For example, outside the ordinary halo, and 
 concentric with it, an extraordinary halo 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 paraselenae. 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 nothing 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 where 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 aurorse 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 loo 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, for 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 par excellence over the 
 flat river basins of the Mississippi, Missouri, 
 
 Fio. 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 (b] of similar cloud is seen, underneath which 
 is a rain curtain (r). 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 ^V-h f 
 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 1000 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
 
 l6o 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
 
 162 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 THE 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 Rozier 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
 
 1 64 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
 
 bUSPENSION AND FLIGHT IN ATMOSPHERE. 165 
 
 the air towards some definite point. If he com- 
 mits himself to them he must needs go nolens 
 volens 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 
 fly 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 wing 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 flown for a considerable distance while the 
 motive power held out. 
 
 Mr. Hargrave 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 (Dwnudea 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 construe- 
 
 FIG. 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 n 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 
 fans'crew 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. (3g). 
 
 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 IDS. 
 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 and 7333 feet 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. 4 i. 
 
 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 ATMOSPHERE. 177 
 
 Since then Capt. Baden Powell, in May, 1895, 
 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 Capt. 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 size of kite required 
 for such a purpose, Capt. 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. Hargrave, at Stanwell Park, N. S. 
 Wales, 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 by Hargrave is fol- 
 lowed, this could be readily attained by the ad- 
 dition of more kites. Under these circumstances, 
 by two or more Hargrave 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. 
 
 '79 
 
 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 Hargraves, 
 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 
 horizon. 
 
 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 trn boat, 
 turn continually towards the wind. 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 FLIGHT 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
 
 1 82 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 ATMOSPHERE. 
 
 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 the 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-
 
 1 84 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 EARTHS 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. 
 
 Type. 
 
 CLIMATES. 
 
 Examples. 
 
 10 
 
 Cumana, 
 
 (2) Tropical, lat. 10 to 23 
 
 Description. 
 
 Hot, moist, equable, sa- 
 lubrious. 
 
 (a) Coastal, 
 
 r , . ( Similar to (i) but less 
 Htg'K'ong, j ^able aU salubri- 
 
 
 Jfhore. } Hot , dry, and extreme, 
 
 (J) Inland, 1 M^ndalay, j" p**' exce P l in win ' 
 
 1 Timbuktu, J ten 
 
 f Riviera, f Temperate and dry, 
 (3) Sub - Trop - S. California, owing to position be- 
 ical, lat. 23 to -i Cape Colony, -^ tween tropical and 
 
 lat. 35, j Southern Aus- polar rain-belts, very 
 
 (_ tralia, [ salubrious. 
 
 (4) Temperate 
 
 f England, 
 
 
 1 Europe, 
 
 Cool, moist, and equable 
 
 (a) North, lat. I United States, 
 35 to lat. 6o,1 Central Siberi ' a> 
 
 > near sea, dry and ex- 
 treme inland. 
 
 L and China, 
 
 
 (6) South, lat. 
 35 to lat. 50, 
 
 New Zealand, 
 Tasmania, 
 
 Cool, moist, and equa- 
 ble, most salubrious 
 in the world. 
 
 (5) Polar, lat. 
 60 to poles, 
 
 N. Siberia, j Cold and fairly dry, ex 
 Greenland, ( treme inland. 
 
 Judged by averages alone, a climate with an 
 annual average temperature between 
 
 75 and 85 is hot. 
 65 " 75 is warm. 
 55 " 65 is mild. 
 
 50 " 55 is temperate. 
 40 " 50 is cold. 
 Below 40 is arctic.
 
 I po 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. 
 
 Bora, 136. 
 
 Abbe, Professor, 105. 
 Abercromby, Ralph, in. 
 Actinometer, 36. 
 Albatross, flight of, 168, 170. 
 Ammonia in atmosphere, 18, 21. 
 
 Boyle's law, 95, 98. 
 Brickfielder, 136. 
 Brocken, Spectre of the, 148. 
 Bull's-eye squall, 150. 
 Buys Ballot's law, 128. 
 
 Anticyclones, 29, 127, 132, 134. 
 
 
 Anti-trades, 84. 
 
 C. 
 
 Arched squall, 150, 162. 
 Argon in atmosphere, 18. 
 
 Capper, 128. 
 Carbonic acid, 18, 20. 
 
 Atmosphere, composition, 17; 
 electricity, 159; height, 9, 14; 
 history, 9; laws, 94; life in, 
 
 Cavallo, Thomas, 163. 
 Celsius, 39. 
 Chanute, Octave, 166. 
 
 183; movements (winds), 64; 
 
 Charles, law of, 96, 98. 
 
 optics of, 141; origin, 9; pres- 
 sure, 15, 16, 25; sounds of, 141; 
 temperature, 31 ; weight, 15, 
 
 Chinook, 137. 
 Cirrus, high. See Clouds. 
 Clayton, 92. 
 
 16, 25. 
 
 Climate, 183. 
 
 Aurora borealis, 148. 
 
 Clouds, 99, 106, 119; cirro-cu- 
 
 
 mulus, in; cirro-stratus, in. 
 
 B. 
 
 114, 145, 156; cirrus, no, in, 
 
 
 114, 117, 118, 144; cumulus, 100, 
 
 Bacilli, nitrogen abstracting, 20. 
 
 no, in, 117, 118; cumulo-nim- 
 
 Back, Admiral, 173. 
 
 bus, in; formation of, 22; 
 
 Baden Powell, 177. 
 
 forms of, no: height of, 111, 
 
 Baldwin, 164. 
 
 118; high-cirrus, 106; nimbus, 
 
 Balloons, 15, 88, 93, 163. 
 
 in, 114; stratus, no, ill, 114, 
 
 Ballot, Dr. Buys, 128. 
 
 117; strato-nimbus, in; veloc- 
 
 Barometer, 16, 25. 
 Barometric pressure, variations 
 
 ity of movement, 117. 
 Coal, 21. 
 
 in, 27, 58, 68, 81. 
 Berson, Dr., balloon ascension, 
 
 Colour of the sky, 25, 102, 141. 
 Colours, sunset, 103. 
 
 15- 
 
 Convection currents, 104, 158. 
 
 Birds, flight of, 166. 
 
 Conservatism of energy, 97. 
 
 Birt, 173. 
 
 Corona, 144. 
 
 Blanchard, 163. 
 Blizzards, 136. 
 
 " Corpse candle," 148. 
 Courant, ascendant, 59. 
 
 Blue Hill Observatory, 92, 118, 
 
 Coxwell, balloon ascension, 15, 
 
 1 60, 174. 
 
 164. 
 
 Blueness of sky, 102. 
 
 Cumulus. See Clouds. 
 
 13 IQI
 
 192 
 
 Cumulus, festooned, 156. 
 
 Halo, 144. 
 
 Currents, ocean, 46, 60, no. 
 
 Hann, 63. 
 
 Cyclones, 29, 59, 79, 84, 99, 125. 
 
 Hargrave, 166, 170, 177. 
 
 
 Hargrave kite, 170. 
 
 D. 
 
 Harries, 162. 
 
 Davis, Professor, 153. 
 
 Haughton, Dr., 60. 
 Hawk, flight of, 167. 
 
 Dawn, 143. 
 
 Heat, 31, 97, 142. 
 
 Dew, 106. 
 Diffusion of gases, too. 
 Disease, 184. 
 Doldrums, 82, 130. 
 Dove, Professor, 128. 
 Dust in atmosphere, 23, 143, 184. 
 
 Hildebrandsson, Dr., in. 
 Hot north-wester, 137. 
 Howard, Luke, no. 
 Hurricanes, 99, 125. 
 Huyghens, 142. 
 Hypsometry, 30. 
 
 E. 
 
 I. 
 
 
 
 Ekholm, 92, in. 
 Electricity of the atmosphere, 
 
 Ignis fatuus, 148. 
 Inclined plane, 167. 
 Inertia, curve of, 67. 
 
 Emission theory of light, 141. 
 Energy, conservation of, 97. 
 
 isobars, 30, 71. 
 Isothermals, 44. 
 
 Euler, 142. 
 
 
 Explosions, 140. 
 
 J. 
 
 F. 
 
 Fahrenheit, 39. 
 
 Japan current, 60, no. 
 Joule, 98. 
 Jupiter, atmosphere of, 10. 
 
 Fata Morgana, 146. 
 
 
 Ferrell, 48, 60, 63, 66, 74, 131. 
 
 K. 
 
 Finley, Lieutenant, 152. 
 
 
 Flying machines, 88, 93, 163, 165. 
 
 Kelvin, Lord, 182. 
 
 Foehn, 137. 
 
 Khamsin, 136. 
 
 Fog, 109. 
 
 Kinetic theory of gases, 90. 
 
 Fonvielle, 164. 
 Forecasting weather, 30, 135, 157. 
 
 Kites, 167, 173, 179, 180; Har- 
 grave, 170, 174, 177; observar 
 
 Franklin, Benjamin, 159. 
 Fresnel, 142. 
 
 tion, 173; photography, 174; 
 war, 173, 176. 
 
 Frost, 109. 
 
 Kona, 136 
 
 
 Krakatoa, 140, 146. 
 
 G. 
 
 
 Galileo, 25, 39. 
 
 L. 
 
 Garnerin, 164. 
 Gases, kinetic theory of, 90. 
 Gay-Lussac, 96, 164. 
 Glaisher, balloon ascension, 15, 
 41, 139, 164. 
 Gravitation, 10, 12. 
 
 Labrador current, no. 
 Langley, Prof. S. P.. 36, 166, 
 172, 180. 
 La Place, nebular theory, 9. 
 Laws, atmospheric, 94. 
 
 Green, 164. 
 Gulf Stream, 46, 60, no. 
 
 Lemstrom, 149. 
 Leste, 136. 
 Leveche, 136. 
 
 H. 
 
 Ley, Rev. Clement, in, 157. 
 
 Hadley, theory of the winds, 6-(, 
 
 Lichtenberg, 163. 
 Light, 102, 141. 
 
 74- 
 
 Lightning. 157, 159. 
 
 Hagstrom, in. 
 
 Lightning rods, 122, 160. 
 
 Hail, 106, 119, 120, 157. 
 
 Lilienthal, 166.
 
 INDEX. 
 
 Looming, 146. 
 
 Poisson's law, 96, 98. 
 Priestley, Dr. Joseph, 17. 
 
 Mackerel sky, 113. 
 
 R. 
 
 Mat dc ntontagne, 27. 
 
 Radiation, 31. 
 
 Marriotte's law, 95. 
 Mars, atmosphere, n. 
 
 Rain 22, 84, 99, 106, 119, 157. 
 Rainbow, 147. 
 
 Marvin, Professor, 179. 
 
 Rainfall, 122. 
 
 Maury, theory of the wind, 64, 
 
 Redneld, 128. 
 
 74- 
 
 Reid, 128. 
 
 Maxim, Hiram, 166, 172. 
 Meldrum, Dr., 130. 
 
 Reye, Professor, 131. 
 Rotch, 174. 
 
 " Merry Dancers," 148. 
 
 Rozier, Pilatre de, 163. 
 
 Meteorites, 15, 16. 
 
 
 Mirage, 145. 
 
 
 Mist, 109. 
 Mistral, i;l 
 Mock-moons, 145. 
 Mock-suns, 145. 
 
 Sailing, 180. 
 St. Elmo's fire, 161. 
 Seasonal changes, 33. 
 
 Mollcr, 93. 
 
 Simoom, 149. 
 
 Monsoons, 58, 80. 
 Montgolfiers, the, 163. 
 Moon, atmosphere of the, it. 
 Mormon tabernacle, 140. 
 
 Sirocco, 136. 
 Smyth, Prof. Piazzi, 84. 
 Snow, 106, 119, 120. 
 Snow line, perpetual, 41, 42. 
 Solano, 136. 
 
 
 Sound, atmosphere as the con- 
 
 N. 
 
 veyer of, 138. 
 
 Nadar, 164. 
 Nebular theory, 9. 
 
 Southerly bursters, 136. 
 Space, temperature of, 13, 32, 62, 
 
 
 105. 
 
 Nimbus clouds. See Clouds. 
 
 Spectre of the Brocken, 148. 
 
 Nitragin, bacilli, 20. 
 
 Spectrum, 102, 142. 
 
 Nitrogen in atmosphere, 18, 19. 
 Nortes, 136. 
 Northern lights, 148. 
 Nor'-wester, 150. 
 
 Squalls, 150, 157. 
 Stevenson, T., 91. 
 Storms, 99, 104, 106, 125. 
 Storms, law of, 128. 
 Stratus clouds. See Clouds. 
 
 Q 
 
 Sun, atmosphere, 9, 12, 14; heat 
 
 
 of, 32. 
 
 Ocean currents, 46, 60, no. 
 Oxygen in atmosphere, 18, 19. 
 
 Sunset, colours at, 103, 143. 
 Sun spots, 54, 149. 
 
 Ozone in atmosphere, 18, 21. 
 
 
 
 T. 
 
 p. 
 
 Taifuns, 130. 
 
 
 Taj Mahal, 140. 
 
 Pampero, 136, 150. 
 
 Temperature, 31; variations in. 
 
 Parachute, 163. 
 Paraselenae, 145. 
 
 Thermometers, 39; self-record- 
 
 Parhelia, 145. 
 
 ing, 40. 
 
 Peri-cyclone, 132. 
 
 Thunder, 140, 160. 
 
 Photography, 174. 
 
 Thunder-storms, 59, 99, 120, 149, 
 
 Piddington, 128. 
 
 156. 
 
 Planets, origin and atmosphere 
 
 Tissandier, 164. 
 
 of, 10. 
 
 Tornadoes, 59, 99, 121, 125, 149, 
 
 Pocock, 173. 
 
 150. 
 
 Poisson, 66. 
 
 Torricelli, 25; vacuum of, 26.
 
 I 9 4 
 
 Trade winds, 64, 79. 
 Trilobites, 13. 
 Twilight, 15, 16, 143. 
 
 Vettin, Dr., 92, in. 
 
 Viscosity, 90. 
 
 Von Bezold, Dr., 63, 112. 
 
 Water in atmosphere, 22, 27, 
 
 1 06. 
 Waterspouts, 149, 152. 
 
 Weather forecasting. 30, 135, 157. 
 
 Wenham, 166. 
 
 Whirlwinds, 59, 149. 
 
 Whispering Gallery, 140. 
 
 Windmills, 181. 
 
 Winds, 64; effect of tempera- 
 ture, 60; trade, 64, 79; velocity 
 of, 88. 
 
 Y. 
 
 Young, 142. 
 
 Zero, determination of, 39. 
 Zurbriggen, 15. 
 
 THE END.
 
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