262 THE WORLD MACHINE 



days mathematicians might work out the origin of the earth 

 and of the solar system as well. 



He explained the tides. Poseidonius had done that in a 

 way ; Newton gave the mathematical theory of the tidal flow, 

 calculated how much was due to the moon, how much to the 

 sun, explained why it was that the spring and autumnal tides 

 were higher than at other times, by the conjunction of the 

 attractive forces of the two bodies. Further consequences of 

 his discoveries : he perceives that from their observed shapes 

 it was possible to estimate the time of rotation of the planets 

 that is, the length of their " day." Thus Jupiter, despite 

 its huge bulk, a thousand times that of the earth, has a much 

 more violent rate of revolution ; its day is less than half our 

 earthly day. 



And these are but the important matters of this kindling 

 book. It was a wonderful advance ; you perceive that there 

 was comparatively little left for those who were to come after. 

 This is not saying that he made no mistakes ; they were as- 

 tonishingly few. It is not saying that he had wholly exhausted 

 the subject ; Laplace, Lagrange, and many another were yet 

 to come ; but the fact remains that in the essential theory 

 neither discovery nor subsequent investigation has effected any 

 radical change. The system of the world now taught to school 

 children is the system of Newton. Perhaps the larger number of 

 present-day works upon physical astronomy are a digest of the 

 Principia. 



The work appeared within two years after it had been begun. 

 Originally the Royal Society intended, to defray the cost of its 

 publication ; its funds ran low ; Halley put the book through 

 the press out of funds from his own purse. He became thus 

 not merely the godfather of the Principia ; it was his ward as 

 well. The outlay involved a sacrifice ; it is pleasant to know 

 that eventually he made well from his venture. It was written, 

 of course, in Latin. Like Bacon, unlike Galileo or Bruno, 

 Newton did not trust to his mother-tongue. England was then 

 regarded by the rest of Europe as a half -barbarous land. Even 

 Shakespeare was unknown. An enlightened continental as little 

 thought of learning English then as he might think of acquiring 

 Hungarian now. 



The Principia brought its author neither to the torture 

 chamber nor the stake ; they gave him a Government post 



