KINETIC OR MECHANICAL VIEW OF NATURE. 87 



writings he adopted a different and more general process 

 of reasoning. If electrical and magnetic as well as 

 optical phenomena are produced by the motions of the 

 parts of a medium possessed of certain mechanical pro- 

 perties, this medium represents a mechanical system, and 

 must therefore he subject to the general laws which 

 regulate all mechanical systems. These general laws are 

 laid down in dynamics, where it is shown that a complete 

 knowledge of the behaviour of such a system can be 

 reduced to the knowledge of the distribution in it of a 

 quantity called Energy. 



I intend in the next chapter to trace historically the 



53. 

 Conse- 



on the lines 



drops this somewhat crude device, 

 as well as the older theory of par- 

 ticles acting at a distance, with 

 forces which, according to Weber, 

 depend on their velocities, and starts 

 from " the conception of a compli- 

 cated mechanism capable of a vast 

 variety of motion, but at the same 

 time so connected that the motion 

 of one part depends ... on 

 the motion of other parts, these 

 motions being communicated by 

 forces arising from the relative dis- 

 placement of the connected parts, in 

 virtue of their elasticity " (Papers, 

 vol. i. p. 533). He further says : 

 " I have on a former occasion at- 

 tempted to describe a particular 

 kind of motion and a particular 

 kind of strain, so arranged as to 

 account for the phenomena. In 

 the present paper I avoid any hy- 

 pothesis of this kind ; and in using 

 such words as electric momentum 

 and electric elasticity in reference 

 to the known phenomena of the in- 

 duction of currents and the polar- 

 isation of dielectrics, I wish merely 

 to direct the mind of the reader to 

 mechanical phenomena which will 

 assist him in understanding the 



electrical ones. All such phrases 

 in the present paper are to be con- 

 sidered as illustrative, not as ex- 

 planatory. In speaking of the 

 energy of the field, however, I 

 wish to be understood literally. All 

 energy is the same as mechanical 

 energy, whether it exists in the 

 form of motion or in that of elas- 

 ticity, or in any other form. The 

 energy in electro-magnetic phe- 

 nomena is mechanical energy. The 

 only question is, Where does it 

 reside ? On the old theories it 

 resides in the electrified bodies, 

 conducting circuits, and magnets, 

 in the form of an unknown quality 

 called potential energy, or the power 

 of producing certain effects at a 

 distance. On our theory it resides 

 in the electro-magnetic field, in the 

 space surrounding the electrified 

 and magnetic bodies, as well as in 

 those bodies themselves, and is in 

 two different forms, which may be 

 described without hypothesis as 

 magnetic polarisation and electric 

 polarisation, or, according to a very 

 probable hypothesis, as the motion 

 and the strain of one and the same 

 medium " (p. 563). 



