272 Prof. W. McF. Oit on 



when the aether has once assumed the equilibrium state 

 appropriate to given positions of the electrons or conductors, 

 the nuclei as well as the surrounding sether are in equilibrium, 

 and there is no tendency for anything to move unless the 

 continuity of the medium can be broken down in some way. 



5. It is held by Larmor that a model of the electric field 

 may be complete without exhibiting a direct mechanism by 

 which electrostatic attractions and repulsions are transmitted 

 across the sether from the surface o£ one conducting region 

 or electron to that of another*. As these attractions and 

 repulsions make their appearance as coefficients of the virtual 

 displacements of electrons, such displacements being made on 

 the supposition of " free mobility," it is, I think, evident that 

 the reason his theory does not show the transmission of these 

 forces through the aether is that it does not show any ma- 

 chinery by which the mysterious " mobility " of the electrons 

 is provided and the constructive processes involved in their 

 motion are performed. 



6. Thus the illustration of the origin of the static mechan- 

 ical attraction between positive and negative electrons given 

 in the latest exposition of the theory f appears inadequate for 

 the purpose. Here an ideal canal in the sether is imagined 

 as filled up by a flexible wire of infinite torsional rigidity, 

 and in continuous connexion with the surrounding sether ; 

 the rotational displacement of any cross-section C round its 

 axis by an impressed torque is transmitted all along the wire, 

 and thence to the surrounding sether, and two complementary 

 electrons are thus developed at its ends A and B, which 

 persist as long as the rotational displacement of C is maintained. 

 The author then states that the tangential tractions which the 

 surrounding sether exerts on the surface of the wire form 

 a system of forces statically equivalent by the principle of 

 virtual work to an attraction between its ends, and that the 

 mechanical attraction between the electrons may in this 

 manner be considered as transmitted by the wire. This 

 cannot be the case, however ; the tangential tractions are 

 proportional to the strains, and therefore so also is their 

 resultant, if any, whereas the electrostatic force to be ex- 

 plained is proportional to the square of the strain ; any 

 mechanical force on the electrons would in fact be reversed 

 by reversing the torque imposed on the wire, that is by 

 interchanging the positions of the positive and -negative 

 electrons. This consideration, taken in conjunction with 

 those of symmetry, shows in fact that if the wire be originally 



* For the reasons assigned, cf. Phil. Trans. 1897, A. p. 212. 

 f ' ./Ether and Matter,' p. 329 



