318 



SCIENTIFIC THOUGHT. 



11. 



Newton's 

 ' Principia. 



work, however, was conceived in the highest philosophic 

 spirit, inasmuch as the enunciation of the so-called law of 

 gravitation required the clear expression of the general 

 laws of motion. In the first and second parts of the 

 work the discoveries of Galileo and Huygens were ab- 

 sorbed, generalised, and restated in such terms as have up 

 to our age been considered sufficient to form the basis for 

 all purely mechanical reasoning.^ In the latter part the 

 new rule, corresponding to Kepler's empirical laws, is 

 represented as the key to a system of the universe. The 

 great outlines of this system are boldly drawn, and the 

 working out of it is left as the great bequest of Newton 

 to his successors. At the end of the eighteenth century, 



^ The most recent historian of 

 the subject is Prof. Ernst Mach of 

 Prague, whose ' Meehauiii in ihrer 

 Entwickelung, historisch - kritisch 

 dargestellt,' 2nd ed., 1889, I cannot 

 praise too highly. It has been 

 translated into English by M'Cor- 

 mack (Chicago and London, 1893). 

 Referring to Newton, he saj's : 

 " Newton has with regard to our 

 subject two great merits. Firstly, 

 he has greatly enlarged the hori- 

 zon of mechanical physics through 

 the discovery of universal gravi- 

 tation. Further, he has also com- 

 pleted the enunciation of the prin- 

 ciples of mechanics as we now ac- 

 cept them. After him an essen- 

 tially new principle has not been 

 established. What after him has 

 been done in mechanics refers to 

 the deductive, formal, and mathe- 

 matical development of mechanics 

 on the ground of Newton's prin- 

 ciples" (p. 174). " Newton's prin- 

 ciples are sufficient without the 

 introduction of any new principle 

 to clear up every mechanical prob- 

 lem which may present itself, be 



it one of statics or of dynamics. 

 If difficulties present themselves, 

 they are always only mathematical, 

 formal, not fundamental " (p. 2-39). 

 " All important mathematical ex- 

 pressions of modern mechanics were 

 already found and used in the age 

 of Galileo and Newton. The spe- 

 cial names . . . have sometimes 

 been fixed much later. Still later 

 came the adoption of uniform 

 measures, and this process is even 

 yet incomplete " (p. 252). In this 

 country it is one of the great mer- 

 its of Thomson and Tait's ' Nat- 

 ural , Philosophy ' that they ' ' re- 

 stored " the teaching of mechanics 

 and placed it on the original foun- 

 dations afforded by Newton's laws 

 of motion, in his own words, as 

 " every attempt that has been 

 made to supersede them has ended 

 in utter failure" (Preface), and, 

 though they " are only temj^or- 

 arily the best," there does not 

 exist, " as yet, anything nearly as 

 good " (Tait in article " Mechanics," 

 ' Ency. Brit.,' 9th ed., p. 749). 



