Dynamical Theories of Gravitation. 147 



poraries (to whom they would not listen), and only a part of 

 the knowledge of Geometry which was then prevalent, they 

 would in all probability have discovered the laws of universal 

 gravitation and its mechanical cause. Laws, the discovery 

 and the demonstration of which constitute the fame of the 

 most powerful genius that has ever existed ; and Cause, which 

 after having been the ambition for a long time of the greatest 

 scientists, is at present the despair of their successors. So 

 that, for example, the celebrated laws of Kepler, discovered 

 somewhat less than 200 years ago, partly by gratuitous 

 conjectures, and partly by repeated trial and error, would 

 have been no more than inevitable corollaries which could 

 have been arrived at by these ancient philosophers by in- 

 vestigating the mechanism of nature. The same conclusion 

 applies also to the laws of Galileo upon the fall of bodies, the 

 discovery of which took place still later, and which have been 

 more contested, because the experiments upon which this dis- 

 covery was based permitted a latitude in their results 

 (necessarily rough), which would make them fit equally well 

 with other laws, so that one did not fail to contest them : 

 whereas the inferred consequences of the shock of atoms would 

 have been unmistakably in favour of the only true principle, 

 viz., equal accelerations in equal times." (Trans, of Royal 

 Berlin Academy, 1782.) 



On this paragraph the following opinion is emitted by 

 Lord Kelvin, viz. : — 



" If Le Sage had but excepted Kepler's third law, it must 

 be admitted that his case, as stated above, would have been 

 thoroughly established by the arguments of his " memoire" ; 

 for the Epicurean assumption of parallelism adopted to suit 

 the false idea of the earth being flat, prevented the discovery 

 of the law of the inverse square of the distance, which the 

 mathematicians of that day were quite competent to make, 

 if the hypothesis of atoms moving in all directions through 

 space, and rarely coming into collision with one another, had 

 been set before them, with the problem of determining the 

 force with which the impacts would press together two 

 spherical bodies, such as the earth and moon were held to be 

 by some of the contemporary philosophers to whom the 

 Epicureans "would not listen." But nothing less than 

 direct observation, proving Kepler's third law — Galileo's 

 experiment on bodies falling from the tower of Pisa, Boyle's 

 guinea-and-feather experiment, and Newton's experiment of 

 the vibrations of pendulums composed of different kinds of 

 substance — could either give the idea that gravity is pro- 

 portional to mass, or prove that it is so to a high degree of 



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