The story of the earth's atmosphere

Front matter

Cbe Library of Useful $torie$.

A series of little books dealing with various branches of useful knowledge, and treating each subject in clear, concise language, as free as possible from technical words and phrases, by writers of authority in their various spheres. Each book complete in itself. i6mo. Cloth, 40 cents per volume.

NOW READY.

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 w^th which to stimulate scientific research. It-s study has consequently 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 application of the knowm laws of physics.

I have therefore written, not for the minority, \vho vaguely wonder at the relation of extraordinary facts and pass on, but for what I believe to be that much more numerous section w^ho are not content w4th 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 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 endeavoured 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 PAGE

I. The Origin and Height of the Atmosphere. 9 II. The Nature and Composition of the Atmosphere ....... 17

III. The Pressure and Weight of the Atmos-

II

of the Component Gases

Fig. 2—

Strato-Cumulus

of the Atmosph<

;re

lOI

(high) .

.

Fig. 25 — Cirrus Cloud {var

Fig. 3 — Cirro-Cumulus

Tracto Cirrus,

Fig. 4 .

.

Fig. 5 .

.

.

Fig. 6

Fig. 28 — Festooned Cumu

-

Fig. 7 lus

Distribution 0

Atmospheric Temperature in

Latitude foi

January, July, and the year

.

Fig. 9

Fig. lo

Fig. 11

Fig. 36 — Tornado Funnel

Fig. 12

Cloud .

Fig. 13

Fig. 37 — Thunderstorm

Fig. 14 in Section .

.

Fig. 15

Fig. 38— Kestrel

Hawk

Fig. i6

. 73

Hovering .

.

Fig. 17

Fig. i8

Fig. 19

Fig. 20

.

Fig. 21

Fig. 43 — Yachting

in Syd-

Fig. 22

.

. 86

ney Harbour

.

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 w^hich 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 inhaling 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 certain 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 language 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 history of the universe.

If we are to believe the " nebular theory," propounded years ago by the great French astronomer, La Place, and which, far from being upset, has rather been confirmed by recent discovery, lO THE STORY OF THE EARTH'S ATMOSPHERE.

all existing suns and planets have been simply condensed from clouds or nebulae of matter originally scattered through space.

By the mutual attraction of their matter (which force we now term gravitation), these separate aggregations became highly heated globular masses, every element of which was at first in a state of fiery gaseous incandescence. As they gradually cooled and threw off planetary excrescences, these masses became condensed at first into liquid spheres or suns, surrounded by atmospheres 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, particularly 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 present condition of our own planet.

We must not, however, suppose that this state of things holds on every other planet. The rate at v/hich such changes progress is different for each planet.

The planet Jupiter is still so hot that it is believed to be partly self-luminous, and its atmosphere 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. Therefore, according to our ideas, no life would be possible on the moon. The liquid, which would 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 undoubtedly have possessed, have been either absorbed into its mass or else diffused into space beyond the power of recall by gravitation.

The condition of each globe at present depends 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 cooling. 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 commenced and the surface was cool enough to admit of animal and vegetable growth, the atmosphere 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 knowlledge, 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 vapour 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 irom the earth is estimated as being barely enough to raise it yJo"^hs 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 existed 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 at our ow^n, far beyond the orbit of Venus. This, however, is known to be contrary to fact.

Fig. 2. — Strato-cumulus (hig^h).

The sun's atmosphere is not more than about 500,000 miles deep, while that of the earth is certainly 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 suddenly 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 mountains is believed to be that of Zurbriggen on Aconcaqua, w^hen 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. 2

His thermometer went down to 54 degrees below zero Fahr., while the mercury in his barometer 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 observations 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 appearance depends upon their heating to incandescence by friction against a resisting medium, shew that some air exists at 100 miles, though at such great altitudes it is probably in a condition 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 believe, inhabit the celestial regions, it must be infinitely pathetic to see the poor little 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 elements, and it was not until the middle of the last century that Priestley discovered that air was a 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 w^e 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, without action or passion, lifeless — in fact, matter without motion. The existing proportion therefore is decidedly adapted to our present requirements. The average proportion in which the two principal components of the atmosphere are found to occur is 2 1 of oxygen to 79 of nitrogen by volume, and 23 of oxygen to 77 of nitrogen by weight.

The proportion in which the rem;^ining constituents enter is so small, that it may be practically neglected when we consider the physical properties of the atmosphere, though it cannot be neglected when we regard its vital and chemical functions. The other constituents are carbonic acid, which occupies y^f^-g^ths by volume, traces of ammonia, ozone, and the recently discovered argon.

NATURE AND COMPOSITION OF ATMOSPHERE. 19

Oxygen, which forms one-fifth of the atmosphere, 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 produced 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 manifested 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 elevations differ but little from those at sea-level.

Thus in a balloon ascent at Kew, the percentage 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 concerned, therefore, the atmosphere at great heights is just as suitable for man as it is at sea-level. The only practical drawbacks arise from its greater rarity and cold, as we ascend from the surface.

The Nitrogen, which forms three-fifths of the atmosphere, represents the inert^ negative element 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 would be used up too rapidly in pure oxygen. It would not be easy, in fact, to find any other diluent 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, nitrogen, being slightly denser than oxygen in the proportion of no to 97, renders the air a better vehicle for sound, support, and power than it would be otherwise.

Nitrogen is also absorbed from the atmosphere by plants, through the agency of those marvellous little bacilli parasites, the Nitragin, which have recently been shewn by Prof. Dobbe to nourish certain plants by abstracting the nitrogen 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 consumption 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. Especially during the carboniferous epoch of geology, when owing to special climatic conditions enormous 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 reduced 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 forest 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 by rain. Every year about 30 lbs. of ammonia are carried down to each acre of ground. Tiie above constituents are blended together like different brands of spirit, but are free to enter into combination 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 impossible 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 components, 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 temperature 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 particular temperature at which the air fails to support it in solution, when a layer of cloud forms

NATURE AND COMPOSITION OF ATMOSPHERE. 23

and perhaps rain falls. After this an interval occurs 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 condensation takes place, and a second stratum of cloud is formed and so on. Ultimately a point is reached at which the vapour-sphere nearly vanishes, 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 investigation, and has found that the atmosphere, especially in its lower parts over land, contains thousands 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 counter in various parts of the world, embracing the Atlantic and Pacific Oceans, New Zealand, California, 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 especially 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.

Number of dust particles per cubic centimetre.*

Atlantic Ocean . . . 2053

Pacific " ... 613

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 polluted 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 regularly with the ascent. From observations on the Bieshorn, Fridlander found the number gradually diminish in the following ratio.

Height above sea-level.

Numbe particles p

8,400 *' 10,665 "

11,000 "

13,200 "

13,600 "

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 l^ths of what it was at the lower level. The bearing 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 towards 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 barometer 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 discoveries, he cynically remarked that Nature evidently abhorred a vacuum up to thirty-three feet. His pupil, Torricelli, however, was not content with this perfunctory explanation, and applying his genius to the question, conjectured that the column of thirty-three feet of water exactly balanced 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 mercury 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, leaving 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 atmospheric pressure.

The reason the ttrm. 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 experiment 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 w^eight 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 lbs.

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 y^i/_^o^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 mountain 18,000 feet high it would only weigh half as much. The weight of a cubic foot of water vapour 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.