KINETIC OR MECHANICAL VIEW OF NATURE. 83 



phenomena of tension and pressure (magnetic action) 

 as also of motion of its parts (electro-magnetic action). 

 Now in a medium which is so constituted i.e., which 

 possesses elastic mobility of its parts we know that 

 by a whirling or vortex motion phenomena of pressure 

 and tension can be produced in certain parts, and the 

 questions accordingly presented themselves to Maxwell, 

 How by such tension and pressure in certain parts of the 

 medium can magnetic phenomena be represented ? and 

 How can the vortices communicate motion to, or receive 

 motion from, the interlying movable particles of the 

 medium ? He succeeded in working out a very complete 

 model of such a medium, representing by its mechanical 

 motions both magnetic and electro-magnetic phenomena. 

 Especially was he successful in visualising Faraday's 

 lines or tubes of force, and endowing them with me- 

 chanically measurable forces. Maxwell admits that " his 

 conception . . . may appear somewhat awkward. I 

 do not," he says, " bring it forward as a mode of con- 

 nection existing in nature. ... It is, however, a mode 

 of connection which is mechanically conceivable and 

 easily investigated ; ... so that I venture to say that 

 any one who understands the provisional and temporary 

 character of this hypothesis will find himself rather 

 helped than hindered by it in his search after the true 

 interpretation of the phenomena." l 



1 ' Collected Papers,' vol. i. p. 486. 

 At the end of his paper on physical 

 lines of force, Maxwell touches on 

 the philosophical question, "how 

 much evidence the explanation of 

 phenomena lends to the credibility 

 of a theory, or how far we ought to 



regard a coincidence in the mathe- 

 matical expression of two sets of 

 phenomena as an indication that 

 these phenomena are of the same 

 kind. We know that partial co- 

 incidences of this kind have been 

 discovered ; and the fact that they 



