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. 152. =Compressibility of Air.=--In looking at the influence of gravitation upon air, it must be remembered that air is very compressible, while water is very nearly incompressible. While, therefore, in a body of water the particles are very little nearer together at the bottom than at the surface, the particles of the air are much nearer together close to the earth than they are far away from it. For as all the particles of the air are attracted or drawn toward the earth, those below are pressed together by the weight of those above. The air is therefore thinner as we go up from the surface of the earth, and in the outer regions of the sea of air it is too thin to support life. Even at the tops of very high mountains, or the heights sometimes reached by balloons, disagreeable effects are often experienced from the thinness of the air. The air has been compared, in regard to its varying density at different heights, to a heap of loose compressible substance; as, for example, cotton-wool, which is quite light at the top, but is pressed more and more together as you go toward the bottom. Hydrogen gas has only one fifteenth the weight of air at the surface of the earth; and therefore the hydrogen balloon rises till it reaches a height where the air is so thin that the balloon is of the same weight with an equal bulk of air, and there it stops. 153. =In what Aeriform Substances and Liquids are Alike.=--You have seen in § 36 and § 38 how the air and gases differ from liquids. But in one very important respect they are alike, viz., the movability of their particles. Hence pressure is in air, as well as in water, equal in all directions, so that in the experiment with the bladder, in § 126, it makes no difference in the result whether there be water or air in it. For the same reason pressure is as the depth in aeriform substances as in liquids, and the laws of specific gravity apply to the one as well as to the other. You are now prepared to understand the results of _the action of gravitation upon air and the gases_; or, in other words, the principal phenomenon of Pneumatics.