38o NATURE AND MAN. 



ktiow (as I have already pointed out) is that which we learn from 

 our own experience as to the attraction of the earth for bodies 

 near its surface. And although Newton is commonly credited 

 with having " demonstrated " the identity between terrestrial 

 gravity and the force which deflects the moon out of its straight 

 course, and with thus having "proved" the universality of the 

 mutual attraction of masses of matter, I speak with the authority 

 to which I consider myself entitled, not by my own study of this 

 subject, but by the answers of the greatest masters of it to questions 

 I have put to them, — that what Newton really did was to show 

 that such an exact numerical confoi'mity exists between the rate of 

 fall of the moon towards the earth (that is to say, her deflection 

 from her onward rectilineal path) in any given time, and the rate 

 of a body actually falling to the earth's surface (according to 

 Galileo's law), as justifies the assumption of the identity of the 

 force which causes the former, with that of which we have 

 experience in the production of the latter. 



Now, in regard to the sun's attraction for the earth and planets, 

 we have no certain experience at all. Unless we could be trans- 

 ported to his surface, we should have no means of experimentally 

 comparing solar gravity with terrestrial gravity ; and if we could 

 ascertain this, we should be no nearer the determination of his 

 attraction for bodies at a distance. The doctrine of universal 

 gravitation, then, is a pure assumption ; and, as a highly com- 

 petent writer,* who obviously takes my own view of the matter, 

 has lately said with reference to Descartes' theory of "vortices" 

 (which, essentially the same with Kepler's, for some time disputed 

 the field with Newton's theory) : — " Had Descartes been able to 

 " show that the parts of his vortex must move in ellipses having 

 " the sun in one focus, that they must describe equal areas in equal 

 " times, and that their velocity must diminish as we recede from 

 "the sun, according to Kepler's third law, his theory would have 

 "so far been satisfactory." But while "all three of Kepler's laws 

 " were expressed in the single law of gravitation towards the sun, 

 " with a force acting inversely as the square of the distance," 

 Descartes' theory entirely failed to grasp them, and therefore fell 



♦ Professor Simon Newcomb, of the United States Naval Observatory, in 

 his admirable " Popular Astronomy." 



