  The atmosphere is the transparent and elastic body of mixed gases and vapours which envelopes our globe, and which derives its name from Greek words, signifying sphere of vapour. It is estimated to extend to from forty to fifty miles above the surface of the earth. Because it is found, by experiment and observation, that the air becomes less dense in proportion to its altitude from the earth's surface. The gradual decrease of atmospheric density observed in ascending a mountain, or in a balloon, supplies sufficient data to enable us to calculate the height at which the atmosphere would probably altogether cease. At an altitude of 18,000 feet the air is indicated by the barometer to be only half as dense as at the surface of the earth. And as the densities of the atmosphere decrease in a geometrical progression, the density will be reduced to one-fourth at the height of 36,000 feet; and to one-eighth at 54,000 feet. The effects of the decreasing density of the atmosphere are, that the intensity of light and sound are diminished, and the temperature is lowered. Persons who have reached a very high elevation, state that the sky above them began to assume the appearance of darkness; and there can be no doubt that, if it were possible to reach an altitude of some fifty to sixty miles, there would be perfect blackness "For he looketh to the ends of the earth, and seeth under the whole heaven; To make the weight for the winds."—Job xxviii. The pressure of the atmosphere at the earth's surface is fifteen pounds to every square inch of surface. That is to say, that the column of air, extending fifty miles over a square inch of the earth, presses upon that square inch with a weight equal to fifteen pounds. That is the weight of air at what is called the point of saturation, when it is fully charged with watery vapour. The proportion constantly varies. Evaporation is not a result of accident; it seems an established law that the air shall constantly absorb vapour until it has reached the maximum that it can hold. Experiments have been tried, in which dry air has been pressed upon the surface of water with great force, but no degree of pressure could prevent the formation of vapour. (See 431.) The total amount of atmospheric pressure on the earth's surface, at 15 lbs. to the square inch, amounts to 12,042,604,800,000,000,000 lbs. This pressure is equal to that of a globe of lead of sixty miles in diameter. Estimating the surface of man's body to be equal to fifteen square feet, he sustains an atmospheric pressure of 32,400 lbs., or nearly fourteen tons and a-half. The mere variation of weight, "I therefore so run, not as uncertainly; so fight I, not as one that beateth the air."—Corinth. ix. Because the diffusion of air which, surrounding him in every direction, and acting upon the internal as well as the external surfaces of his body, and probably surrounding every atom of his frame, establishes an equilibrium, in which every degree of pressure counteracts and sustains itself. A cubic foot of air weighs only 523 grains, a little more than an ounce; a cubic foot of water weighs one thousand ounces. M. Gay Lussac, in the year 1804, ascended to the height of 23,000 feet. A vacuum is a space devoid of matter. The term is generally applied to those instances in which air is drawn from within an air-tight vessel. It is probably impossible to do so, even with the most powerful instruments—some portion of air would remain, but in so thin a form that it would be imperceptible. Because the putting down of the handle lifts up the piston with its valve closed, thereby tending to produce a vacuum; but the pressure of the air upon the water not contained in the pump, forces more water up into the part where a vacuum would otherwise be formed. Then, when the handle is raised, and the piston forced downwards, the valve opens, and the water rushes through. There is a second valve, below the piston, which closes with the "The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit."—John ii., iii. It will raise water to an elevation of thirty feet above its level. Because a column of water of thirty feet high, nearly balances the weight of a column of air of equal surface, extending to the whole height of the atmosphere. When, therefore, water is elevated to the height of thirty feet, the power of the pump is enfeebled, as the air and the water balance each other. By mechanical contrivances, by which the water is forced to a greater elevation. Because the atmospheric pressure upon the water on the outside of the syphon forces it into the tube as fast as the syphon empties itself through its longer arm. Because the weight of the water in the longer arm of the syphon is greater than that in the shorter; therefore it runs out by its own gravity. And, as in running out, it creates a tendency towards a vacuum, the pressure of the outer air comes into operation, and forces the water through the tube. Some springs are caused by natural syphons formed in the fissures of rocks, which, communicating with bodies of water, are continually filled by atmospheric pressure, and therefore convey streams of water to the point where they are set free. "Ascribe ye strength unto God: his excellency is over Israel, and his strength is in the clouds."—Psalm lviii. Because the pressure of the atmosphere upon the surface of the card counteracts the weight of the water. It forms a base upon which the water may rest, while the glass is being inverted; and it prevents the air from acting upon the fluidity of the water, and forcing it out of the glass. Because the pressure of the air upon the opening of the tap counteracts the weight of the beer. But when the spile is driven in, the air enters at the top, and counteracts its own pressure at the bottom. Because the heat expands the air, and drives nearly all of it out of the cup. When the pie is taken out of the oven, and begins to cool, air cannot get into the cup again, because its edges are surrounded by juice. A partial vacuum, therefore, exists within the cup, and the pressure of the external air forces the juice into it. No. So long as the heat exists, the cup remains empty; and as it occupies space, the air is driven out of it, into the pie, it rather tends to force the juice over the sides of the dish. It is only when cooling that the juice enters the cup. Because their feet are so formed that they can form a vacuum, under them; their bodies are therefore sustained in opposition to gravitation by atmospheric pressure. By having large discs of wet leather attached to his feet, so that "And God made a wind to pass over the earth."—Genesis viii. Because they have the means of forming a vacuum under their shells, and are pressed on to the rocks by the weight of the atmosphere. Because they form a vacuum with the smooth and moist surfaces of their bodies, and are supported by atmospheric pressure. |
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