146. What Pneumatics Teaches.—As Hydrostatics treats of the pressure and equilibrium of liquids, Pneumatics treats of the same in air and the gases, or aeriform substances. The name comes from the Greek word πνευμα, meaning air, breath, spirit.
Fig. 92.
147. Air Material and has Weight.—That air is a material substance has been already proved to you, for it was shown in § 46 that it has impenetrability, one of the essential properties of matter. It has extension also, for bodies of air can be obtained in various shapes confined in vessels, so that we can speak of cubes and spheres of air; and besides, the ultimate atoms (§ 15) of air must have shape or extension. That air has weight can be proved by weighing it as you would any other substance. Let a hollow globe, A, Fig. 92, having a neck with a stop-cock, B, be emptied of air and weighed. If now you open the stop-cock, and so let in the air, the other beam of the scale will rise, because the globe is heavier than it was before. The additional weight required to make the scales balance will indicate the weight of the air which the globe contains. It is one eight hundredth ( 1 800 ) of the weight of the same volume of water. How the globe can be emptied of the air will be shown in another part of this chapter.
148. Air Attracted by the Earth.—The weight of the air is simply the result of the attraction of the earth (§ 52). Air is attracted by the earth just as water is; and the water takes its place below air because it is attracted more strongly than the air. It is from the attraction of the earth that air descends into any hollow spot in the earth when water is removed from it. It takes the place of the removed water because from the influence of attraction it gets as near to the earth as possible. If you put into a vial mercury, water, and oil, the mercury will be at the bottom, because it is more strongly attracted by the earth than the other fluids. The water will be next, then the oil, and lastly, over all, there is air, that being less attracted than any of the other substances. It is this attraction of the air by the earth that gives us the chief phenomena of Pneumatics.
149. Why Some Things Fall and Others Rise in Air.—Most substances fall in air for the same reason that very heavy substances sink in water. They fall because the earth attracts them more strongly than it does the air. The reason that some substances rise in air is precisely the same as that given in § 136 for the rising of substances in water. The air being attracted more strongly than they are pushes them up to get below them, as cork or wood is pushed up by water. Thus a balloon filled with hydrogen gas rises in air for the same reason that a bladder filled with air rises in water. So, also, smoke rises in air, just as oil rises in water.
Fig. 93.
150. Thickness of the Earth's Air-Covering.—The air makes a covering for the earth about fifty miles deep. If the earth were represented by a globe a foot in diameter, the air might be represented by a covering a tenth of an inch in thickness. The line a, Fig. 93, gives us the curve of the surface of such a globe, and the space between a and b represents the comparative thickness of the covering of air. This is ascertained by calculation from the pressure of the air upon the earth. It is just as the depth of water may be calculated from the pressure which it makes. We do not take this mode of ascertaining the depth of water, because we can measure it from the surface by sounding. But we should be obliged to adopt it if we lived at the bottom of water, as we do at the bottom of the sea of air.
151. How the Air-Covering Adheres to the Earth.—The earth flies on in its yearly journey around the sun at the rate of 1100 miles per minute, and yet it holds on to this loose airy robe by its attractive force, so that not an atom of it escapes into the surrounding ether. Of itself it is disposed to escape; and it would do so, and be diffused through space, if the attraction of the earth for it were suspended. For, unlike liquids, the air has no disposition to keep together; that is, there is no attraction between its particles. On the other hand, there is a repulsion, so that they are disposed to keep far apart, and are kept together only by pressure. It is the pressure of the earth's attraction that keeps them together to the extent of fifty miles all around it.