[From the Royal Society Transactions, Vol. CLV.] 



XXV. A Dynamical Theory of the Electromagnetic Field. 



(Received October 27, Read December 8, 1864.) 



PAET I. 



INTRODUCTORY. 



(1) THE most obvious mechanical phenomenon in electrical and magnetical 

 experiments is the mutual action by which bodies in certain states set each 

 other in motion while still at a sensible distance from each other. The first 

 step, therefore, in reducing these phenomena into scientific form, is to ascertain 

 the magnitude and direction of the force acting between the bodies, and when 

 it is found that this force depends in a certain way upon the relative position 

 of the bodies and on their electric or magnetic condition, it seems at first sight 

 natural to explain the facts by assuming the existence of something either at 

 rest or in motion in each body, constituting its electric or magnetic state, and 

 capable of acting at a distance according to mathematical laws. 



In this way mathematical theories of statical electricity, of magnetism, of 

 the mechanical action between conductors carrying currents, and of the induction 

 of currents have been formed. In these theories the force acting between the 

 two bodies is treated with reference only to the condition of the bodies and 

 their relative position, and without any express consideration of the surrounding 

 medium. 



These theories assume, more or less explicitly, the existence of substances 

 the particles of which have the property of acting on one another at a distance 

 by attraction or repulsion. The most complete development of a theory of this 



