372 



SCIENTIFIC THOUGHT. 



All changes of these states, and all phenomena depen- 

 dent thereon, are therefore consequences of these inter- 

 actions. But bodies exert such mutual actions when in 

 contact as well as from a distance, and it was evident 

 that a beginning had to be made with the latter in 

 order to gain a clue for the investigation of the former ; 

 this being especially needful whenever the spatial rela- 

 tions of bodies escape observation, as is the case with 

 bodies which are in contact. And so it has really 

 happened, inasmuch as a beginning was made by ex- 

 amining the mutual action of cosmic bodies i.e., with 

 the phenomena of gravitation. To this first field of 

 research viz., the phenomena of gravitation there was 

 then added the investigation of electric and magnetic 

 interactions, as next to gravitation these are the only 

 actions which take place from one body to another at 

 measurable distances, these actions being themselves 

 measurable. Now for a long time Newton's doctrine 

 of gravitation furnished the leading idea for nearly all 

 theories of electricity and magnetism, till a new clue 

 was gained through Oersted's and Ampere's discoveries 



the form of a velocity. It had 

 for Weber a theoretical as well 

 as a practical meaning, for it en- 

 abled him to effect a connection 

 between the electro-magnetic and 

 the electro-static or absolute sj'stem 

 of measurements. When he suc- 

 ceeded in measuring this quantity, 

 it was found that the figure for the 

 constant, which meant a velocity, 

 was practically the same as that for 

 the velocity of the propagation of 

 light. Weber himself does not seem 

 to have attached any physical mean- 

 ing to this coincidence : later he and 

 Kirchhoff remarked that under cer- 



tain conditions an electrical wave- 

 motion might take place in an 

 electrical conductor, and that the 

 velocity of the propagation of this 

 would coincide with that of light 

 (see Kirchhoff in ' Annalen der Phy- 

 sik und Chemie,' 1857 ; and Weber, 

 ' Electrodyn. Maasbest. ,' 1 864). It 

 was reserved for Clerk Maxwell to 

 point to the real physical interpre- 

 tation of Weber's constant. Of 

 this I shall speak in a later chapter 

 (see Maxwell's memoir ' On Physical 

 Lines of Force,' 1862, reprinted in 

 ' Scientific Papers,' voL i.) 



