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 Xewton'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 tain conditions an electrical wave- 



for Weber a theoretical as well motion might take place in an 



as a practical meaning, for it en- electrical conductor, and that the 



abled him to effect a connection velocity of the propagation of this 



between the electro-magnetic and would coincide with that of light 



the electro-static or absolute system ^see Kirchhoff in ' Annalen der Phy- 



of measurements. When he sue- sik und Chemie,' 1857; and Weber, 



ceeded in measuring this quantity, ' Electrodyn. Maasbest.,' 1864). It 



it was found that the figure for the was reserved for Clerk Maxwell to 



constant, which meant a velocity, point to the real physical interpre- 

 was practically the same as that for ' tation of Weber's constant. Of 



the velocity of the propagation of this I shall speak in a later chapter 

 light. Weber himself does not seem ! (see Maxwell's memoir ' On Physical 



to have attached any physical mean- Lines of Force,' 1862, reprinted in 



ing to this coincidence : later he and ' Scientific Papers,' vol. i. ) 

 Kirchhoff remarked that under cer- 



