THE LA^Y OF GEAVITATION. 475 



THE DISCOVERY OF THE LAW OF GRAVITATION. 



By the late Professor JOHN T. DUFFIELD, 



PRINCETON UNIVERSITY. 



rr^HE law of gravitation, that all matter attracts all matter, directly 

 -'- as the mass and inversely as the square of the distance, whether 

 we consider the extent of its reach, or the nnmher and variety and 

 peculiar interest of the problems of which it furnishes the solution, or 

 the grandeur of many of those problems by reason of the magnitude of 

 the elements involved ; whether we consider the power which it gives us 

 to anticipate nature, and predict with the minutest accuracy andcertainty 

 of a mathematical demonstration celestial phenomena for ages to come ; 

 we cannot but regard it as the most important truth in the whole 

 book of nature and its discovery as the most interesting event in the 

 history of physical science. As there is but one material universe and 

 the law of gravitation solves the enigma of its structure, no other 

 problem of equal interest and importance can ever occupy the attention 

 of the student of nature. 



Kepler has remarked that: "The occasions by which men have 

 acquired a knowledge of celestial phenomena, are not less admirable 

 than the discoveries themselves." If this be so, the history of the dis- 

 covery of that great law of nature by which all celestial phenomena 

 are determined can never cease to be a matter of peculiar interest. 



In the account which we propose to give of the discovery we shall 

 select as our chronological starting point the beginning of the seven- 

 teenth century. At that period the theory in regard to the structure 

 of the material universe which, with few exceptions had been held from 

 time immemorial, still prevailed. The earth was regarded as the center 

 of the universe, about which the sun, moon, planets and stars per- 

 formed their ceaseless revolutions. More than half a century before 

 (in 1543) Copernicus, in his memorable work, 'De Orbium Coelestium 

 Revolutionibus,' had, indeed, announced the true system of the uni- 

 verse, yet as he was led to the adoption of the theory he proposed, not 

 so much by positive evidence in its favor as by the difficulty of recon- 

 ciling certain phenomena with the Ptolemaic theory; moreover, as the 

 objections to this theory were from their very nature such that few 

 could appreciate their force, whilst in the apparent motions of the 

 heavenly bodies every one could see what seemed to be an ocular dem- 

 onstration of its truth, it is not strange that the doctrine of Coper- 

 nicus should have been for so long a time generally regarded as an 



