34 DISCOVERIES OF NEWTON. 



strated that " the squares of the periodic times of 

 any two planets are to each other, in the same pro- 

 portion as the cubes of their mean distances from 

 the sun, a law which has been found equally 

 true of satellites and their primaries, if we ex- 

 cept the mutual influence they exert upon each 

 other, termed perturbation. It also led to the 

 important discovery of Newton, that every particle 

 of matter in the universe attracts every other, ac- 

 cording to the law of the inverse square of the dis- 

 tance; a law which, independent of all hypo- 

 theses in regard to the cause of motion, was one 

 of the highest generalizations ever made in the 

 science of Nature. 



In regard to what is called Newton's first law 

 of motion, various and opposite opinions have 

 been entertained. By Leibnitz and other phi- 

 losophers of the last century, it was rejected as 

 hypothetical. According to this law, as announced 

 in the first book of the Principia, " every body 

 perseveres in its state of rest or of motion, 

 unless compelled to change that state by forces 

 impressed thereon." On this law was founded 

 his theory of planetary motion, as developed in 

 the third book, in which he maintains, that 

 when first created, the earth and heavenly bodies 

 were projected into empty space, by the agency 

 of a primitive impulse, communicated at various 

 distances from their centres of gravity ; by which 

 their annual and diurnal revolutions have been 



