All over the earth's surface is a l

All over the earth's surface is a layer of air which extends upwards for many miles. This air contains the oxygen without which neither plants nor animals could live. Its movements, temperature and pressure determine the weather, and it is a vehicle for the clouds of water vapor which condense and fall as rain. It forms a blanket which protects us from the extreme heat of the sun during the day and from the extreme cold when the sun has setIt is chiefly through air that sound travels, so that if there were no air we should hear practically nothing. The atmosphere is held to the earth's surface by the gravitational pull of the earth—that is, it has weight. High up it is thin, but near the surface it is compressed by the weight of air above, and is more dense. The weight of air pressing on each square inch of surface at sea-level is nearly pounds which means that the total force on the skin of an average man is about 30,000 pounds. He is not aware of this because the pressure is equal in all directions and the pressure inside him is equal to that outside. But should he go up in a balloon to a height at which the outside pressure is much less would suffer acutely. It is for this reason that the cabins of aeroplanes are 'pressurized'.