. Handbook of nature-study for teachers and parents, based on the Cornell nature-study leaflets. Nature study. 864 Handbook of Nature-Study There are two forces in constant operation on each gas that surroimds the earth, viz., expansive force and gravity. Expansive force pushes the gas up and gravity pulls it down, but the force of gravity decreases as the distance from the center of the earth increases, so there is a point at a cer- tain distance above the earth where the two forces just balance each other, and each gas will expand upward to that point but will not rise beyond it. Therefore,
. Handbook of nature-study for teachers and parents, based on the Cornell nature-study leaflets. Nature study. 864 Handbook of Nature-Study There are two forces in constant operation on each gas that surroimds the earth, viz., expansive force and gravity. Expansive force pushes the gas up and gravity pulls it down, but the force of gravity decreases as the distance from the center of the earth increases, so there is a point at a cer- tain distance above the earth where the two forces just balance each other, and each gas will expand upward to that point but will not rise beyond it. Therefore, if we know the expansive force of a gas and the rate at which gravity decreases, it is possible to calculate the height to which the differ- ent gases that compose the air will rise. In this way it has been determined that carbon dioxide, which is one of the heavier gases, extends upward about ten miles, water vapor about 12 miles, oxygen about 30 miles and nitrogen about 35 miles while hydrogen and helium, the lightest gases known, do not appear at the surface at all, but probably exist at a height of from 30 miles to possibly 200 £ The zone of twilight in midwinter. From Todd's New Astronomy. There are other ways in which we are able to gain some idea of the approximate height at which there is an appreciable atmosphere. When the rays of light from the sun enter our atmosphere they are broken up or scattered—diffracted—so that the atmosphere is partially lighted for some time before sunrise and after sunset. This is called twilight. If there were no atmosphere, there would be no twilight, and darkness would fall the instant the sun passed below the horizon. Twihght, which is caused by the sun shining on the upper atmosphere, is perceptible until the sun is about 16° below the horizon.' ' From this it is calculated that the atmosphere has sufficient density at a height of 40 miles to scatter, or diffract, sunlight. Observations of meteors, commonly called shootin
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