﻿26 Mr. G. A. Schott on the 



spheroid (§ 5), that the stress is a pressure, and that the 

 surface of the electron is a surface of constant pressure. 



This assumption has the advantage that the resultant action 

 of the aether on the electron vanishes, so that the electron 

 in moving through the aether experiences no resistance on 

 account of its expansion. Nevertheless the aether resists the 

 expansion of the electron with a pressure, which, as we have 

 seen, depends on the size of the electron and on its velocity of 

 translation. 



The electron in expanding exerts an equal pressure on the 

 surrounding aether and must be supposed to push it outwards. 

 We can only form a mechanical conception of this action in 

 two ways : — 



(1) The aether fills the space occupied by the electron ; in 

 this case it must be expansible. 



(2) The electron is an empty space surrounded by aether ; 

 in this case the aether need not be expansible. 



§ 7. In the first case an expanding electron produces a 

 pressure at a distant point varying approximately inversely 

 as the distance, and in consequence acts on a second distant 

 electron with a force inversely as the square of the distance, 

 but the force is a repulsion. This is in accordance with the 

 view expressed by Maxwell that gravitational attraction 

 cannot be transmitted by means of stress in a medium. If 

 the aether be expansible, its bulk-modulus must be so great 

 that inequalities of pressure due to expansion are carried 

 away by waves almost instantaneously. The first case becomes 

 practically identical with the second for our purpose. 



§ 8. Let us assume for the sake of definiteness that the 

 aether is a continuous isotropic medium, which possesses 

 inertia and rotational elasticity, and may, or may not, possess 

 volume elasticity. This is the type assumed by Larmor. 



In such a medium permanent rotational motions are 

 impossible ; any permanent motion is irrotational. 



The expanding electron in this aether virtually constitutes 

 a hydrodynamic source ; two sources attract each other with 

 a force proportional directly to their strengths and to the 

 density of the medium, and inversely to the square of the 

 distance between them. Therefore two expanding electrons 

 attract each other according to the same law. 



If a, a' be their radii, and fi the density of the medium, 



the hydrodynamic attraction is — — —^ . 



When we consider two systems of n,n' electrons respectively, 

 at a distance large compared with the dimensions of either 



