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<h2>CHAPTER XXX.</h2> 638. What is the atmosphere? 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. 639. To what height does the atmosphere extend? It is estimated to extend to from forty to fifty miles above the surface of the earth. 640. Why is it supposed that the atmosphere does not extend beyond that height? 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 [Pg 147] although the sun's rays might be pouring through the darkened space, to illuminate the atmosphere. Upon the summit of Mont Blanc, the report of a pistol at a short distance can scarcely be heard. When Gay Lussac reached the height of 23,000 feet, he breathed with great pain and difficulty, and felt distressing sensations in his ears, as though they were about to burst. Upon the high table-lands of Peru, the lips of Dr. Ischudi cracked and burst; and blood flowed from his eyelids. <hr class="bible-verse" /> "For he looketh to the ends of the earth, and seeth under the whole heaven; To make the weight for the winds."—Job xxviii. <hr class="bible-verse" /> 641. What is the amount of atmospheric pressure at the earth's surface? 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. 642. Is that the weight of dry or moist air? That is the weight of air at what is called the point of saturation, when it is fully charged with watery vapour. 643. What is the proportion of watery vapour in the atmosphere? 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.) 644. What is the total amount of atmospheric pressure on the earth's surface? 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. 645. What is the pressure of the atmosphere upon the human body? 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, [Pg 148] arising out of the changes in the state of the atmosphere, may amount to as much as a ton and a-half. <hr class="bible-verse" /> "I therefore so run, not as uncertainly; so fight I, not as one that beateth the air."—Corinth. ix. <hr class="bible-verse" /> 646. Why does not man feel this pressure? 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. 647. What is the weight of air relative to that of water? A cubic foot of air weighs only 523 grains, a little more than an ounce; a cubic foot of water weighs one thousand ounces. 648. What is the greatest height in the atmosphere which any human being has ever reached? M. Gay Lussac, in the year 1804, ascended to the height of 23,000 feet. 649. What is a vacuum? 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. 650. Is it possible to form a perfect vacuum? 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. 651. Why does the depression of a pump-handle cause the water to flow? 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 [Pg 149] downward movement, to prevent the water from rushing back again. <hr class="bible-verse" /> "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. <hr class="bible-verse" /> 652. How high will atmospheric pressure raise water in the bore of a pump? It will raise water to an elevation of thirty feet above its level. 653. Why will it raise water to an elevation of thirty-feet? 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. 654. How is water raised to a greater elevation when it is required? By mechanical contrivances, by which the water is forced to a greater elevation. 655. Why does water run through the bent tube called a syphon? 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. 656. Why does water run through the longer arm of the syphon? 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. 657. Why does water issue from the earth in springs? 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. [Pg 150] <hr class="bible-verse" /> "Ascribe ye strength unto God: his excellency is over Israel, and his strength is in the clouds."—Psalm lviii. <hr class="bible-verse" /> 658. Why, if a wine glass is filled with water, and a card laid upon it, and the whole inverted, will the water remain in the glass? Because the pressure of the atmosphere upon the surface of the card counteracts the weight of the water. 659. What has the card to do with the experiment? 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. 660. Why will not beer run out of the tap of a cask until a spile has been driven in at the top? 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. 661. Why does a cup in a pie become filled with juice? 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. 662. Does the cup prevent the juice from boiling over? 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. 663. Why can flies walk on the ceiling? 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. 664. How did Mr. Sands perform the feat of walking across the ceiling? By having large discs of wet leather attached to his feet, so that [Pg 151] when they were placed upon a smooth surface, the air was excluded, and when he allowed his weight to act upon one of the discs, it formed a hollow cup and a vacuum. By forming a vacuum of only twelve square inches he gained a pressure of 180 lbs.; this being more than his weight he could accomplish the feat with no other difficulty than that of remaining in an inverted position. The air was admitted underneath the discs by valves, which were closed by springs, which being pressed by the heels of the performer, let in the air, and set the feet free. <hr class="bible-verse" /> "And God made a wind to pass over the earth."—Genesis viii. <hr class="bible-verse" /> 665. Why is it difficult to strike limpets from rocks? 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. 666. Why can snails move over plants in an inverted position? Because they form a vacuum with the smooth and moist surfaces of their bodies, and are supported by atmospheric pressure. <hr class="chap" /> <div style="break-after:column;"></div>

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