Page:Mars - Lowell.djvu/76

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fifteen miles a minute; and those of hydrogen, which are the fastest known, at the enormous speed of more than a mile a second. But this average velocity may, for any particular molecule, be increased by collisions with its neighbors. The maximum speed it may thus attain Clerk-Maxwell deduced from the doctrine of chances to be sevenfold the average. What may thus happen to one, must eventually happen to all. Sooner or later, on the doctrine of chances, each molecule of the gas is bound to attain this maximum velocity of its kind. When it is attained, the molecule of oxygen travels at the rate of one and eight tenths miles a second, the molecule of water vapor at the rate of two and one half miles a second, and the molecule of hydrogen at over seven miles a second, or four hundred and fifty times as fast as our fastest express train.

Now, if a body, whether it be a molecule or a cannon-ball, be projected away from the Earth’s surface, the Earth will at once try to pull it down again: this instinctive holding on of Mother Earth to what she has we call gravity. In the cases with which we are personally familiar, her endeavor is eminently successful, what goes up coming down again. But even the Earth is not omnipotent. As the velocity with which the body is projected increases, longer and longer time is needed for