Page:Astronomy for Everybody.djvu/267

No work of the human intellect farther transcends what would seem possible to the ordinary thinker than the mathematical demonstrations of the motions of the heavenly bodies under the influence of their mutual gravitation. We have learned something of the orbits of the planets round the sun; but the following of the orbit is not the fundamental law of the planet's motion; the latter is determined by gravitation alone. This law, as stated by Newton, is so comprehensive that nothing can be added. The law that every particle of matter in the universe attracts every other particle, with a force which varies inversely as the square of the distance between them, is the only law of nature which, so far as we know, is absolutely universal and invariable in its action. All the other processes of nature are in some way varied or modified by heat and cold, by time or place, by the presence or absence of other bodies. But no operation that man has ever been able to perform on matter changes its gravitation in the slightest. Two bodies gravitate by exactly the same amount, no matter what we do with them, no matter what obstacles we interpose between them, no matter how fast they move. All other natural forces admit of being investigated, but gravitation does not. Philosophers have attempted to explain it, or to