Imágenes de páginas
PDF
EPUB

COMPOSITION OF THE ATMOSPHERE.

53

second. In 1000 parts, moreover, one part of carbonic acid gas may be reckoned. Each of these constituents has specific duties to perform, towards animate and inanimate nature. Thus the seemingly inactive azote, dilutes the oxygen to a proper pitch for respiration and combustion. Did the latter element act alone, it would make animal existence a short-lived fever, and every fire a furnace, in which not merely common fuel, but even metals would burn. The active uses of azote, in this scene of being, are as yet but imperfectly explored. The substance of all animal bodies is replete with it, in a concrete state; and yet we cannot tell by what channel, or from what source, it is introduced. In the food of the elephant, the horse, and the ox, science has not hitherto discovered the origin of that vast quantity of the azotic element, which is hourly assimilated to the solid and liquid texture of their bodies, to repair what is daily wasted and thrown off in excrementitious matter.

In viewing the atmosphere, as consisting of oxygen and azote, we cannot help remarking the delicate equilibrium of chemical proportions on which the well-being of organic life, and even the whole aspect of Nature, depend. Were the proportion of the oxygen or vital air diminished, breathing would be laborious, every warm-blooded animal would become asthmatic, and coal would not cheer the domestic hearth. On the other hand, were the proportion of the vital ingredient doubled, that is, instead of 1 of it to 4 of azote, as at present, were there 2 to 4, the temperate breath of heaven might

suddenly change into an atmosphere of intoxicating gas; for these are the chemical proportions, and sole constituents of this curious air. Were the bulk of oxygen quadrupled, so that its quantity should equal that of the azote, a most noxious air called nitrous gas (deutoxide of azote) might result; a gas which, with an additional charge of oxygen, would condense into an ocean of Aqua Fortis, or Nitric Acid. A slight modification of chemical affinity, would convert even our existing atmosphere into the most corrosive of liquids; a result which the Hon. Mr. Cavendish many years ago produced, by merely transmitting electric explosions through a small portion of common air. But science shows that the chemical equilibrium of the atmospheric elements, is fixed by the same Beneficent Wisdom, which confines the turbulent ocean, by an apparently slender barrier of sand.

The atmosphere contains another constituent of great importance, aqueous vapour, on which many of its most valuable properties depend. In raising this, it becomes the purifier of the ocean waves, and their distributor over the dry land, thence generating in the sky, clouds, rain, snow, hail, and dew, which by their deposition on the earth, give origin to fresh water springs, rivers, and lakes, all indispensable to the sustenance of animal and vegetable life. The contemplative author of the Book of Job was deeply affected with these meteoric wonders. "He bindeth up the waters in his thick clouds, and the cloud is not rent under them." Moses has accurately described this grand function of the atmosphere: "And GoD

ITS FIRST FUNCTION WELL DESCRIBED BY MOSES. 55

said, Let there be a firmament in the midst of the waters, and let it divide the waters from the waters. And God made the firmament, and divided the waters which were under the firmament, from the waters which were above the firmament; and it was so. And GoD called the firmament Heaven. And the evening and the morning were the second day."-Genesis, i. 6, 7, 8.

How perfectly does his order of elementary development accord with every principle of Natural Science! The most elastic medium, (called sometimes in popular language, the ethereal and imponderable element,) in which light, heat, and electricity seem to reside, first sprung forth; and next the weighty and less elastic fluids or gases were evolved. The Mosaic arrangement is in fact that now adopted by chemical philosophers.

Were the surface of the earth of uniform temperature throughout, and not chequered with water, the Atmosphere would every where be equal in height, density, and elasticity; but this density would necessarily decrease with the decreasing pressure, in proportion as the weight of the superior columns were removed, in a geometrical ratio to arithmetical heights, as already described. The average weight of the atmosphere is measured by a column of mercury 30 inches high; which column, an inch in area, weighs 14 pounds and seven-tenths. At the temperature of 32o F., mercury is 10500 times denser than air; therefore if 10500 be multiplied by 30 inches or 2 feet, the product 26250 feet will represent the height of a homogeneous

atmosphere, supposed of equal density above and below. But since the atmosphere is progressively attenuated, as it recedes from the earth, its temperature progressively sinks by its enlarged capacity for heat. The amount of this refrigeration is about one degree Fahr. for every hundred yards of ascent. And as the air contracts in volume, by decrease of temperature, the length of the superior columns will thus be diminished.

On the supposition of an unchanging temperature, the mercury of a barometer would fall from 30 inches to 20, by carrying it to the top of Ætna, or 10000 feet high; but in the actual predicament of an atmosphere condensed in the upper regions by cold, the equipoising mercurial column would fall to about 19 inches at the same elevation. If the surface of the earth acquire a higher temperature than that stated above, suppose 48°, the atmosphere will expand and be elongated throughout, so that on the top of Ætna, a greater weight of air will rest, so as to sustain the mercury there, at 19 inches and. If the terrestrial temperature increase progressively from the poles to the equator, then the atmospheric altitude will also increase from the first to the second position, forming an inclined plane on its summit, instead of a curve concentric with our spheroid. Calling the thermometric degree 80 at the equator, and 0 at the poles, the barometer raised 10000 feet in the former region, will still be pressed by a residuary weight equal to 19 inches and fully of mercury; while 34 in the latter, it will sustain only 18 inches and

CONSTITUTION OF A DRY ATMOSPHERE.

57

about a half. In the circumpolar latitudes, the portion of the aerial columns near the surface of the earth, having a greater specific gravity, than the lower strata towards the tropics, will displace the latter, and cause a current towards the equator; but at greater elevations, the circumpolar atmosphere will be more attenuated, and will give way to the other, whence a counter-current towards the poles will ensue. Hence a perpetual circulation is maintained; the colder air in our hemisphere flowing southward below, and the warmer air northward above, thus tending to equalise the aerial temperature over the globe.

=

At the

24896

24

[ocr errors]

By the revolution of the earth on its axis, the various points of its surface move eastwards with a velocity proportional to their respective distances from the poles, where the motion is = 0. equator the velocity is 1037 miles hour per and at every degree distant from this circle, it may be found very nearly by multiplying the number 1037 by the cosine of the latitude. The atmosphere incumbent on any region, participates (when apparently at rest) in this eastward motion; and therefore the air over a northern circle of latitude, in passing towards a southern, retaining by inertia, its smaller velocity of rotation, will seem to linger, as it were, behind the aerial particles, among which it enters, and have the effect of a current from the west. Thus, also, by composition of motion, an impulsion towards the south-west will result, constituting what is called the trade wind. In the southern hemisphere, this wind will, for a

« AnteriorContinuar »