Moaeh of the Aether. 319 



we obtain the equation 



ee + c 2 curl curl e = 0, 



which is no other than the equation of motion of MacCullagh's 

 aether,* the specific inductive capacity corresponding to the 

 reciprocal of MacCullagh's constant of elasticity. In the 

 analogy thus constituted, electric displacement corresponds to 

 the twist of the elements of volume of the aether ; and electric 

 charge must evidently be represented as an intrinsic rotational 

 strain. Mechanical models of the electromagnetic field, based on 

 Fitz Gerald's analogy, were afterwards studied by A. Sommerfeld,f 

 by K. Keiff,J and by Sir J. Larmor. The last-named authorll 

 supposed the electric charge to exist in the form of discrete 

 electrons, for the creation of which he suggested the following 

 ideal processIF : A filament of aether, terminating at two 

 nuclei, is supposed to be removed, and circulatory motion is 

 imparted to the walls of the channel so formed, at each point 

 of its length, so as to produce throughout the medium a 

 rotational strain. When this has been accomplished, the 

 channel is to be filled up again with aether, which is to be 

 made continuous with its walls. When the constraint is 

 removed from the walls of the channel, the circulation imposed 

 on them proceeds to undo itself, until this tendency is balanced 

 by the elastic resistance of the aether with which the channel 

 has been filled up ; thus finally the system assumes a state of 

 equilibrium in which the nuclei, which correspond to a positive 

 and a negative electron, are surrounded by intrinsic rotational 

 strain. 



Models in which magnetic force is represented by the 

 velocity of an aether are not, however, secure from objection. 

 It is necessary to suppose that the aether is capable of flowing 

 like a perfect fluid in irrotational motion (which would corre- 



* Cf. p. 155. t Ann. d. Phys. xlvi (1892), p. 139. 



I Reiff, Elasticitat und Elektricitdt, Freiburg, 1893. 

 Phil. Trans, clxxxv (1893), p. 719. 



|| In a supplement, of date August, 1894, to his above-cited memoir of 1893. 

 H Phil. Trans, clxxxv (1894), p. 810; cxc (1897), p. 210; Larmor, Aether 

 .and Matter (1900), p. 326. 



