Januaby 18, 1918] 



SCIENCE 



53 



may think of it in their moments of sophis- 

 ticated philosophizing, there can be no 

 doubt that they, in common with less in- 

 structed people, have a feeling of satisfac- 

 tion and intellectual rest when an adequate 

 mechanical "explanation" is given of some 

 natural phenomenon. 



Newton, with the characteristic boldness 

 of genius, extended the Galilean mechanics 

 of earthly matter to the heavenly bodies, 

 and (as often happens) found in the re- 

 moter phenomena better and more complete 

 confirmation of his theory than in the 

 nearer and more obvious manifestations. 

 With the single additional assumption 

 of the gra\itational force, all the intri- 

 cate wanderings of sun, moon and planets 

 in the celestial sphere fell into a system 

 — simple, orderly and in accord with our 

 commonest experiences of every-daj' life. 

 It is not surprising that to all minds capable 

 of understanding it, Newton's theory car- 

 ried instant conviction. 



Nature and Nature's Laws were hid in night, 

 God said, "Let Newton be," and all was light. 



But the law of gravitation did not enjoy 

 the same independent status in the minds 

 of natural philosophers; from that day to 

 this they have been under temptation to 

 find what we all call an "explanation" of 

 it, while few if any have ever felt the 

 necessity for an explanation of the laws of 

 motion. Newton himself, in the "Op- 

 ticks," speculates as to a possible ethereal 

 explanation of gravitation; and even in 

 the celebrated passage at the end of the 

 "Principia," in which he renounces hy- 

 potheses, the context shows, I think, that he 

 felt strongl}^ the desire for an explanation, 

 but was compelled to forego it because 

 "hitherto I have not been able to discover 

 the causes of those properties of gravity 

 from phenomena." 



The century following Newton was de- 



voted to the development of mechanics and 

 of gravitational astronomy and culminated 

 in the great achievements of Lagrange and 

 Laplace. There was some discussion as to 

 the relative merits of action at a distance 

 and vis a tergo, and some direct attempts 

 to account for grax'itation on the latter 

 basis — notably that of LeSage early in the 

 nineteenth century. But, on the whole, the 

 opinion gained strength that Newton had 

 been right in his view that there was little 

 hope of being able to test such theories by 

 comparison with "phenomena." 



The discovery by Coulomb that mag- 

 netic and eleeti'ic forces conformed to the 

 Newtonian law gave strength to the preva- 

 lent opinion that this law was fundamental 

 in the constitution of the physical universe. 

 The mathematical technique of the subject 

 was highly developed and there was a grow- 

 ing tendency to explain obser\'ed phenom- 

 ena by distance-forces between particles, 

 rather than to seek a more strictly dynam- 

 ical theory to account for such forces. 

 This procedure was certainly defensible 

 upon philosophical grounds, and proved 

 its utility in many problems of mathemat- 

 ical physics. It was the prevailing fashion 

 in the early part of the nineteenth century. 



Thus it was entirelj- natural that Am- 

 pere, when he heard in 1820 of Oersted's 

 discovery, should have based his investiga- 

 tion of electrodynamics upon the Newton- 

 ian model, by using current-elements act- 

 ing upon each other by forces in the line 

 joining them. Again the law proved to be 

 that of the inverse square ; but the fact that 

 tlie attracting elements were directed quan- 

 tities added many difficulties which, in the 

 state of mathematical science at that time, 

 gave ample scope to the "Newton of elec- 

 tricity" for the display of his genius. 

 These vector relations involve an indeter- 

 minateness which later gave rise to many 

 rivals to Ampere's theory; other expres- 



