Part 18

A philosopher of the East, with a richness of imagery truly oriental, describes the Atmosphere as “a spherical shell which surrounds our planet to a depth which is unknown to us, by reason of its growing tenuity, as it is released from the pressure of its own superincumbent mass. Its upper surface cannot be nearer to us than 50, and can scarcely be more remote than 500, miles. It surrounds us on all sides, yet we see it not; it presses on us with a load of fifteen pounds on every square inch of surface of our bodies, or from seventy to one hundred tons on us in all, yet we do not so much as feel its weight. Softer than the softest down, more impalpable than the finest gossamer, it leaves the cobweb undisturbed, and scarcely stirs the lightest flower that feeds on the dew it supplies; yet it bears the fleets of nations on its wings around the world, and crushes the most refractory substances with its weight. When in motion, its force is sufficient to level the most stately forests and stable buildings with the earth--to raise the waters of the ocean into ridges like mountains, and dash the strongest ships to pieces like toys. It warms and cools by turns the earth and the living creatures that inhabit it. It draws up vapours from the sea and land, retains them dissolved in itself or suspended in cisterns of clouds, and throws them down again as rain or dew when they are required. It bends the rays of the sun from their path to give us the twilight of evening and of dawn; it disperses and refracts their various tints to beautify the approach and the retreat of the orb of day. But for the atmosphere sunshine would burst on us and fail us at once, and at once remove us from midnight darkness to the blaze of noon. We should have no twilight to soften and beautify the landscape; no clouds to shade us from the searching heat; but the bald earth, as it revolved on its axis, would turn its tanned and weakened front to the full and unmitigated rays of the lord of day. It affords the gas which vivifies and warms our frames, and receives into itself that which has been polluted by use and is thrown off as noxious. It feeds the flames of life exactly as it does that of the fire--it is in both cases consumed and affords the food of consumption--in both cases it becomes combined with charcoal, which requires it for combustion and is removed by it when this is over.”

UNIVERSALITY OF THE ATMOSPHERE.

It is only the girdling, encircling air that flows above and around all that makes the whole world kin. The carbonic acid with which to-day our breathing fills the air, to-morrow makes its way round the world. The date-trees that grow round the falls of the Nile will drink it in by their leaves; the cedars of Lebanon will take of it to add to their stature; the cocoa-nuts of Tahiti will grow rapidly upon it; and the palms and bananas of Japan will change it into flowers. The oxygen we are breathing was distilled for us some short time ago by the magnolias of the Susquehanna; the great trees that skirt the Orinoco and the Amazon, the giant rhododendrons of the Himalayas, contributed to it, and the roses and myrtles of Cashmere, the cinnamon-tree of Ceylon, and the forest, older than the Flood, buried deep in the heart of Africa, far behind the Mountains of the Moon. The rain we see descending was thawed for us out of the icebergs which have watched the polar star for ages; and the lotus-lilies have soaked up from the Nile, and exhaled as vapour, snows that rested on the summits of the Alps.--_North-British Review._

THE HEIGHT OF THE ATMOSPHERE.

The differences existing between that which appertains to the air of heaven (the realms of universal space) and that which belongs to the strata of our terrestrial atmosphere are very striking. It is not possible, as well-attested facts prove, perfectly to explain the operations at work in the much-contested upper boundaries of our atmosphere. The extraordinary lightness of whole nights in the year 1831, during which small print might be read at midnight in the latitudes of Italy and the north of Germany, is a fact directly at variance with all we know according to the researches on the crepuscular theory and the height of the atmosphere. The phenomena of light depend upon conditions still less understood; and their variability at twilight, as well as in the zodiacal light, excite our astonishment. Yet the atmosphere which surrounds the earth is not thicker in proportion to the bulk of our globe than the line of a circle two inches in diameter when compared with the space which it encloses, or the down on the skin of a peach in comparison with the fruit inside.

COLOURS OF THE ATMOSPHERE.

Pure air is blue, because, according to Newton, the molecules of the air have the thickness necessary to reflect blue rays. When the sky is not perfectly pure, and the atmosphere is blended with perceptible vapours, the diffused light is mixed with a large proportion of white. As the moon is yellow, the blue of the air assumes somewhat of a greenish tinge, or, in other words, becomes blended with yellow.--_Letter from Arago to Humboldt_; _Cosmos_, vol. iii.

BEAUTY OF TWILIGHT.

This phenomenon is caused by the refraction of solar light enabling it to diffuse itself gradually over our hemisphere, obscured by the shades of night, long before the sun appears, even when that luminary is eighteen degrees below our horizon. It is towards the poles that this reflected splendour of the great luminary is longest visible, often changing the whole of the night into a magic day, of which the inhabitants of southern Europe can form no adequate conception.

HOW PASCAL WEIGHED THE ATMOSPHERE.

Pascal’s treatise on the weight of the whole mass of air forms the basis of the modern science of Pneumatics. In order to prove that the mass of air presses by its weight on all the bodies which it surrounds, and also that it is elastic and compressible, he carried a balloon, half-filled with air, to the top of the Puy de Dome, a mountain about 500 toises above Clermont, in Auvergne. It gradually inflated itself as it ascended, and when it reached the summit it was quite full, and swollen as if fresh air had been blown into it; or, what is the same thing, it swelled in proportion as the weight of the column of air which pressed upon it was diminished. When again brought down it became more and more flaccid, and when it reached the bottom it resumed its original condition. In the nine chapters of which the treatise consists, Pascal shows that all the phenomena and effects hitherto ascribed to the horror of a vacuum arise from the weight of the mass of air; and after explaining the variable pressure of the atmosphere in different localities and in its different states, and the rise of water in pumps, he calculates that the whole mass of air round our globe weighs 8,983,889,440,000,000,000 French pounds.--_North-British Review_, No. 2.

It seems probable, from many indications, that the greatest height at which visible clouds _ever exist_ does not exceed ten miles; at which height the density of the air is about an eighth part of what it is at the level of the sea.--_Sir John Herschel._

VARIATIONS OF CLIMATE.

History informs us that many of the countries of Europe which now possess very mild winters, at one time experienced severe cold during this season of the year. The Tiber, at Rome, was often frozen over, and snow at one time lay for forty days in that city. The Euxine Sea was frozen over every winter during the time of Ovid, and the rivers Rhine and Rhone used to be frozen over so deep that the ice sustained loaded wagons. The waters of the Tiber, Rhine, and Rhone, now flow freely every winter; ice is unknown in Rome, and the waves of the Euxine dash their wintry foam uncrystallised upon the rocks. Some have ascribed these climate changes to agriculture--the cutting down of dense forests, the exposing of the unturned soil to the summer’s sun, and the draining of great marshes. We do not believe that such great changes could be produced on the climate of any country by agriculture; and we are certain that no such theory can account for the contrary change of climate--from warm to cold winters--which history tells us has taken place in other countries than those named. Greenland received its name from the emerald herbage which once clothed its valleys and mountains; and its east coast, which is now inaccessible on account of perpetual ice heaped upon its shores, was in the eleventh century the seat of flourishing Scandinavian colonies, all trace of which is now lost. Cold Labrador was named Vinland by the Northmen, who visited it A.D. 1000, and were charmed with its then mild climate. The cause of these changes is an important inquiry.--_Scientific American._

AVERAGE CLIMATES.

When we consider the numerous and rapid changes which take place in our climate, it is a remarkable fact, that _the mean temperature of a place remains nearly the same_. The winter may be unusually cold, or the summer unusually hot, while the mean temperature has varied even less than a degree. A very warm summer is therefore likely to be accompanied with a cold winter; and in general, if we have any long period of cold weather, we may expect a similar period at a higher temperature. In general, however, in the same locality the relative distribution over summer and winter undergoes comparatively small variations; therefore every point of the globe has an average climate, though it is occasionally disturbed by different atmospheric changes.--_North-British Review_, No. 49.

THE FINEST CLIMATE IN THE WORLD.

Humboldt regards the climate of the Caspian Sea as the most salubrious in the world: here he found the most delicious fruits that he saw during his travels; and such was the purity of the air, that polished steel would not tarnish even by night exposure.

THE PUREST ATMOSPHERES.

The cloudless purity and transparency of the atmosphere, which last for eight months at Santiago, in Chili, are so great, that Lieutenant Gilliss, with the first telescope ever constructed in America, having a diameter of seven inches, was clearly able to recognise the sixth star in the trapezium of Orion. If we are to rely upon the statements of the Rev. Mr. Stoddart, an American missionary, Oroomiah, in Persia, seems to be, in so far as regards the transparency of the atmosphere, the most suitable place in the world for an astronomical observatory. Writing to Sir John Herschel from that country, he mentions that he has been enabled to distinguish with the naked eye the satellites of Jupiter, the crescent of Venus, the rings of Saturn, and the constituent members of several double stars.

SEA-BREEZES AND LAND-BREEZES ILLUSTRATED.

When a fire is kindled on the hearth, we may, if we will observe the motes floating in the room, see that those nearest the chimney are the first to feel the draught and to obey it,--they are drawn into the blaze. The circle of inflowing air is gradually enlarged, until it is scarcely perceived in the remote parts of the room. Now the land is the hearth, the rays of the sun the fire, and the sea, with its cool and calm air, the room; and thus we have at our firesides the sea-breeze in miniature.

When the sun goes down, the fire ceases; then the dry land commences to give off its surplus heat by radiation, so that by nine or ten o’clock it and the air above it are cooled below the sea temperature. The atmosphere on the land thus becomes heavier than that on the sea, and consequently there is a wind seaward, which we call the land-breeze.--_Maury._

SUPERIOR SALUBRITY OF THE WEST.

All large cities and towns have their best districts in the West;[38] which choice the French _savans_, Pelouze, Pouillet, Boussingault, and Elie de Beaumont, attribute to the law of atmospheric pressure. “When,” say they, “the barometric column rises, smoke and pernicious emanations rapidly evaporate in space.” On the contrary, smoke and noxious vapours remain in apartments, and on the surface of the soil. Now, of all winds, that which causes the greatest ascension of the barometric column is the east; and that which lowers it most is the west. When the latter blows, it carries with it to the eastern parts of the town all the deleterious gases from the west; and thus the inhabitants of the east have to support their own smoke and miasma, and those brought by western winds. When, on the contrary, the east wind blows, it purifies the air by causing to ascend the pernicious emanations which it cannot drive to the west. Consequently, the inhabitants of the west receive pure air, from whatever part of the horizon it may arrive; and as the west winds are most prevalent, they are the first to receive the air pure, and as it arrives from the country.

FERTILISATION OF CLOUDS.

As the navigator cruises in the Pacific Ocean among the islands of the trade-wind region, he sees gorgeous piles of cumuli, heaped up in fleecy masses, not only capping the island hills, but often overhanging the lowest islet of the tropics, and even standing above coral patches and hidden reefs; “a cloud by day.” to serve as a beacon to the lonely mariner out there at sea, and to warn him of shoals and dangers which no lead nor seaman’s eye has ever seen or sounded. These clouds, under favourable circumstances, may be seen gathering above the low coral island, preparing it for vegetation and fruitfulness in a very striking manner. As they are condensed into showers, one fancies that they are a sponge of the most exquisite and delicately elaborated material, and that he can see, as they “drop down their fatness,” the invisible but bountiful hand aloft that is pressing and squeezing it out.--_Maury._

BAROMETRIC MEASUREMENT.

We must not place too implicit a dependence on Barometrical Measurements. Ermann in Siberia, and Ross in the Antarctic Seas, have demonstrated the existence of localities on the earth’s surface where a permanent depression of the barometer prevails to the astonishing extent of nearly an inch.

GIGANTIC BAROMETER.

In the Great Exhibition Building of 1851 was a colossal Barometer, the tube and scale reaching from the floor of the gallery nearly to the top of the building, and the rise and fall of the indicating fluid being marked by feet instead of by tenths of inches. The column of mercury, supported by the pressure of the atmosphere, communicated with a perpendicular tube of smaller bore, which contained a coloured fluid much lighter than mercury. When a diminution of atmospheric pressure occurred, the mercury in the large tube descended, and by its fall forced up the coloured fluid in the smaller tube; the fall of the one being indicated in a magnified ratio by the rise in the other.

THE ATMOSPHERE COMPARED TO A STEAM-ENGINE.

In this comparison, by Lieut. Maury, the South Seas themselves, in all their vast intertropical extent, are the boiler for the engine, and the northern hemisphere is its condenser. The mechanical power exerted by the air and the sun in lifting water from the earth, in transporting it from one place to another, and in letting it down again, is inconceivably great. The utilitarian who compares the water-power that the Falls of Niagara would afford if applied to machinery is astonished at the number of figures which are required to express its equivalent in horse-power. Yet what is the horse-power of the Niagara, falling a few steps, in comparison with the horse-power that is required to lift up as high as the clouds and let down again all the water that is discharged into the sea, not only by this river, but by all the other rivers in the world? The calculation has been made by engineers; and according to it, the force of making and lifting vapour from each area of one acre that is included on the surface of the earth, is equal to the power of thirty horses; and for the whole of the earth, it is 800 times greater than all the water-power in Europe.

HOW DOES THE RAIN-MAKING VAPOUR GET FROM THE SOUTHERN INTO THE NORTHERN HEMISPHERE?

This comes with such regularity, that our rivers never go dry, and our springs fail not, because of the exact _compensation_ of the grand machine of _the atmosphere_. It is exquisitely and wonderfully counterpoised. Late in the autumn of the north, throughout its winter, and in early spring, the sun is pouring his rays with the greatest intensity down upon the seas of the southern hemisphere; and this powerful engine, which we are contemplating, is pumping up the water there with the greatest activity; at the same time, the mean temperature of the entire southern hemisphere is about 10° higher than the northern. The heat which this heavy evaporation absorbs becomes latent, and with the moisture is carried through the upper regions of the atmosphere until it reaches our climates. Here the vapour is formed into clouds, condensed and precipitated; the heat which held their water in the state of vapour is set free, and becomes sensible heat; and it is that which contributes so much to temper our winter climate. It clouds up in winter, turns warm, and we say we are going to have falling weather: that is because the process of condensation has already commenced, though no rain or snow may have fallen. Thus we feel this southern heat, that has been collected by the rays of the sun by the sea, been bottled away by the winds in the clouds of a southern summer, and set free in the process of condensation in our northern winter.

Thus the South Seas should supply mainly the water for the engine just described, while the northern hemisphere condenses it; we should, therefore, have more rain in the northern hemisphere. The rivers tell us that we have, at least on the land; for the great water-courses of the globe, and half the fresh water in the world, are found on the north side of the equator. This fact is strongly corroborative of this hypothesis. To evaporate water enough annually from the ocean to cover the earth, on the average, five feet deep with rain; to transport it from one zone to another; and to precipitate it in the right places at suitable times and in the proportions due,--is one of the offices of the grand atmospherical machine. This water is evaporated principally from the torrid zone. Supposing it all to come thence, we shall have encircling the earth a belt of ocean 3000 miles in breadth, from which this atmosphere evaporates a layer of water annually sixteen feet in depth. And to hoist up as high as the clouds, and lower down again, all the water, in a lake sixteen feet deep and 3000 miles broad and 24,000 long, is the yearly business of this invisible machinery. What a powerful engine is the atmosphere! and how nicely adjusted must be all the cogs and wheels and springs and _compensations_ of this exquisite piece of machinery, that it never wears out nor breaks down, nor fails to do its work at the right time and in the right way!--_Maury._

THE PHILOSOPHY OF RAIN.

To understand the philosophy of this beautiful and often sublime phenomenon, a few facts derived from observation and a long train of experiments must be remembered.

1. Were the atmosphere every where at all times at a uniform temperature, we should never have rain, or hail, or snow. The water absorbed by it in evaporation from the sea and the earth’s surface would descend in an imperceptible vapour, or cease to be absorbed by the air when it was once fully saturated.

2. The absorbing power of the atmosphere, and consequently its capability to retain humidity, is proportionally greater in warm than in cold air.

3. The air near the surface of the earth is warmer than it is in the region of the clouds. The higher we ascend from the earth, the colder do we find the atmosphere. Hence the perpetual snow on very high mountains in the hottest climate.

Now when, from continued evaporation, the air is highly saturated with vapour, though it be invisible and the sky cloudless, if its temperature is suddenly reduced by cold currents descending from above or rushing from a higher to a lower latitude, its capacity to retain moisture is diminished, clouds are formed, and the result is rain. Air condenses as it cools, and, like a sponge filled with water and compressed, pours out the water which its diminished capacity cannot hold. What but Omniscience could have devised such an admirable arrangement for watering the earth?

INORDINATE RAINY CLIMATE.

The climate of the Khasia mountains, which lie north-east from Calcutta, and are separated by the valley of the Burrampooter River from the Himalaya range, is remarkable for the inordinate fall of rain--the greatest, it is said, which has ever been recorded. Mr. Yule, an English gentleman, established that in the single month of August 1841 there fell 264 inches of rain, or 22 feet, of which 12½ feet fell in the space of five consecutive days. This astonishing fact is confirmed by two other English travellers, who measured 30 inches of rain in twenty-four hours, and during seven months above 500 inches. This great rain-fall is attributed to the abruptness of the mountains which face the Bay of Bengal, and the intervening flat swamps 200 miles in extent. The district of the excessive rain is extremely limited; and but a few degrees farther west, rain is said to be almost unknown, and the winter falls of snow to seldom exceed two inches.

HOW DOES THE NORTH WIND DRIVE AWAY RAIN?

We may liken it to a wet sponge, and the decrease of temperature to the hand that squeezes that sponge. Finally, reaching the cold latitudes, all the moisture that a dew-point of zero, and even far below, can extract, is wrung from it; and this air then commences “to return according to his circuits” as dry atmosphere. And here we can quote Scripture again: “The north wind driveth away rain.” This is a meteorological fact of high authority and great importance in the study of the circulation of the atmosphere.--_Maury._

SIZE OF RAIN-DROPS.

The Drops of Rain vary in their size, perhaps from the 25th to the ¼ of an inch in diameter. In parting from the clouds, they precipitate their descent till the increasing resistance opposed by the air becomes equal to their weight, when they continue to fall with uniform velocity. This velocity is, therefore, in a certain ratio to the diameter of the drops; hence thunder and other showers in which the drops are large pour down faster than a drizzling rain. A drop of the 25th part of an inch, in falling through the air, would, when it had arrived at its uniform velocity, only acquire a celerity of 11½ feet per second; while one of ¼ of an inch would equal a velocity of 33½ feet.--_Leslie._

RAINLESS DISTRICTS.

In several parts of the world there is no rain at all. In the Old World there are two districts of this kind: the desert of Sahara in Africa, and in Asia part of Arabia, Syria, and Persia; the other district lies between north latitude 30° and 50°, and between 75° and 118° of east longitude, including Thibet, Gobiar Shama, and Mongolia. In the New World the rainless districts are of much less magnitude, occupying two narrow strips on the shores of Peru and Bolivia, and on the coast of Mexico and Guatemala, with a small district between Trinidad and Panama on the coast of Venezuela.

ALL THE RAIN IN THE WORLD.