376 NATURE AND MAN. 



the earth ; but found, time after time, that Mars " burst all the 

 "chains of the equations, and broke forth from the prisons of the 

 "tables." At last it occurred to him to try an ellipse; and on 

 projecting this as the path of the planet, he found, to his great joy, 

 that the observed places of Mars in the heavens corresponded so 

 exactly with what they should be on that assumption, as to afford 

 the strongest assurance of its truth. But this hypothesis of the 

 elliptical orbit of Mars did not " explain " anything ; it did no 

 more than state in general terms the course of that one planet's 

 motion. Why Mars should take that course, was a question on 

 which he threw no light. And, however probable he might think 

 it that the other planets also move in elliptic orbits, he neither 

 proved it as a fact by the like experiential investigation, nor could 

 adduce any other ground for such probability than that general idea 

 of uniformity and harmony which was the basis of his whole work. 

 It is clear, then, that Kepler's /r^/ " law of planetary motion " has 

 in itself no "governing" power whatever. 



While working out his conception of elliptical motion, Kepler 

 was baffled for a time by the discordance between the observed 

 places of Mars, and the places which would be predicted for him 

 on the assumption of "uniform " motion in an elliptic — instead of 

 in a circular — orbit. Finding that motion to be much more rapid 

 in the part of the orbit nearer the sun, than in the part more 

 remote from it, he again applied himself to his old work oi guessing ; 

 and it is singular that he was led to hit upon what is known as his 

 second law — the passage of the "radius vector" over equal areas 

 in equal times — by an erroneous physical conception of a driving 

 force emanating from the sun, and acting more powerfully on near 

 bodies than those at a distance. Now this second " law," like the 

 first, was simply nothing else than a theoretical generalization of a 

 class of facts ; its value lay entirely in the correctness with which 

 it expressed them ; and so far was Kepler from having attained to 

 any higher conception of its import, that what he regarded as a 

 triumphant confirmation of his doctrine came out of a merely 

 accidental relation between the ellipse and the circle.* 



* I do not know any more instructive or interesting scientific biography 

 than the " Life of Kepler," by Drinkwater, published by the long-since-defunct 

 "Society for the Diffusion of Useful Knowledge," which did much good work 

 of this kind half a century ago. 



