478 POPULAR SCIENCE MONTHLY. 



heralded and complete. The ordinary method of transition from wide- 

 spread and plausible error to the truth is by slow and gradual progress, 

 and the discovery of the law of gravitation, so far from being an excep- 

 tion to this rule, is but one of its most striking illustrations. Such 

 an accident as that wliich the discovery of the law of gravitation is 

 supposed to have been is of the kind which only happens to men of large 

 knowledge, profound thought, and after intense and protracted mental 

 effort. Simple as this law is known to be, and easily apprehended 

 and even demonstrated by ordinary minds, it needed one endowed 

 v/ith the most gigantic intellect probably ever given to mortal — avail- 

 ing himself of the suggestions and the results of the labors of those 

 who had preceded him in the same field of inquiry — to make the dis- 

 covery. 



In tracing the history of this discovery, from the epoch when by 

 the previous discovery of all the necessary data it for the first time 

 became possible, the first place in the order of time, and next to 

 Newton in the order of merit, is undoubtedly due to Kepler. Possessing 

 a singularly lively imagination — we might say, volatile fancy — com- 

 bined with a love for the truth that amounted to a ruling passion, and 

 a breadth of knowledge in his favorite science far in advance of any 

 other man of his age, he was eminently fitted for the work which he so 

 successfully performed of scientific discovery. Fertile in hypotheses — 

 sometimes the most extravagant — he was indefatigable in his labors to 

 test his hypotheses by the facts. Without the slightest pride of opinion, 

 he seemed to take a satisfaction in exploding his own theories when 

 they were false, that was only exceeded by his delight when successful 

 in demonstrating their truth. Of the men who have contributed to the 

 advancement of science, there are few to whom we are under greater 

 obligation, or whose character as an investigator of nature is more 

 worthy of admiration, than 'The Legislator of the Heavens' — the father 

 of modern astronomy. 



In the introduction to his memorable work on 'The Motions of 

 Mars,' referred to above, he opposed the doctrine of Aristotle on the 

 subject of terrestrial gravity, and in the course of the discussion uses 

 the following remarkable language : 



A mathematical point, whetlier the center of tlie universe, or not, has no 

 power to move heavy bodies to approach it. Let philosophers prove, if they 

 can, that natural things have any sympathy with that which is nothing. 



The true theory of gravity is founded on the following axioms. Oravity is 

 a mutual affection between cognate bodies toicard union or conjunction, similar 

 to the magnetic virtue. If we assume the earth to be the center of the world, 

 heavy bodies are not carried toward its center in virtue of its quality of center 

 of the world, but in virtue of its quality of center of a cognate round body; 

 BO that wheresoever the earth may be placed, or whithersoever it may be car- 

 ried by its animal faculty (alluding to a fanciful theory which we shall have 

 occasion presently to notice) heavy bodies will always be carried toward it. 



