534 POPULAR LECTURES AND ADDRESSES. 



Those who know how much worse than " law's 

 delays " are the troubles, cares, and labour involved 

 in bringing through the press a book on any 

 scientific subject at the present day will admire 

 Halley's success in getting the Principia pub- 

 lished within about a year after the task was 

 committed to him by the Royal Society, two 

 hundred years ago. 



When Newton's theory of universal gravitation 

 was thus made known to the world Descartcs's 

 Vortices, an invention supposed to be a consider- 

 able improvement on the older invention of crystal 

 cycles and epi-cycles from which it was evolved, 

 was generally accepted, and seems to have been 

 regarded as quite satisfactory by nearly all the 

 philosophers of the day. 



The idea that the Sun pulls Jupiter, and Jupiter 

 pulls back against the Sun with equal force, and 

 that the Sun, Earth, Moon, and Planets all act on 

 one another with mutual attractions, seemed to 

 violate the supposed philosophic principle that 

 matter cannot act where it is not. Descartes's 

 doctrine died hard among the mathematicians and 



