CHAPTER XXX.
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 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.
[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.]
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_, arising out of the changes in the state of the atmosphere, may amount to as much as a _ton and a-half_.
[Verse: "I therefore so run, not as uncertainly; so fight I, not as one that beateth the air."--CORINTH. IX.]
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 downward movement, to prevent the water from _rushing back again._
[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.]
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.
[Verse: "Ascribe ye strength unto God: his excellency is over Israel, and his strength is in the clouds."--PSALM LVIII.]
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 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_.
[Verse: "And God made a wind to pass over the earth."--GENESIS VIII.]
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.