TO the casual observer the heavenly bodies may seem isolated; but careful study will show how closely connected they are. Not only are they much alike in composition, but across the vast gulfs separating them they hold constant intercourse. Before the time of Newton the only influence known to pass from one heavenly body to another was what we now call radiant energy. To this Newton added a second influence, the force of gravitation. To this list we may also add a third, electrical attraction. The one with which this paper deals is gravitational attraction.
To the student who has at least an elementary knowledge of physics or astronomy gravitational attraction is one of the best known properties of matter; and it can just as truly be said that it is one of the least understood properties of matter, not only by elementary students but by mature physicists and astronomers. So often has the problem of gravitation evaded solution that some in despair have doubted the ability of the human intellect to grapple successfully with it. This feeling may have been strengthened by Newton's references to himself.
At present we can't tell how one body attracts another; and the science-producing value of the efforts made to explain the mechanism of attraction is not to be judged by the prospects afforded of ultimately obtaining a solution but by the stimulus afforded in furthering thorough investigation. What we want is a hypothesis that will conform to well-known phenomena and that will stimulate further investigation. It is needless to say that final solution finds no place in our attempt.
Most of the conjectures concerning gravitation have not reached the dignity of a hypothesis. Some of them seem even too wild for a romance. I think it is Schuster who has referred to his conjectures regarding gravitation as a-holiday-dream. I like this word in this connection. However since they are the best we have let us make the most of them or supplant them with better ones.
Before reciting some of the attempts to explain gravitation it might be well to refer briefly to its discovery, magnitude, peculiarities, etc. Kepler had stated in his first law that the earth revolves in an elliptical orbit with the sun at one focus. With this knowledge at hand, by strictly dynamical reasoning Newton showed that bodies attracted