Faraday-Maxwell Mechanical Stress. 643 



the conductor is in a vacuous space, so that the dielectric is 

 simply aether, the conclusion to which our assumption leads 

 may be expressed by saying that where the conductor is, the 

 aether is not, the surface of the conductor being^ that boundary 

 where the aether ceases and the substance of the conductor 

 begins. We are thus treating the conducting body as foreign 

 to and wholly distinct from the aether. 



4. On the other hand, suppose that we are dealing with a 

 medium in which it is possible for a " strain-figure " to exist: 

 this term being understood to mean a distribution' of strain 

 which requires no independent source of stress to maintain 

 it, but is entirely self-sustaining. Such a strain-figure is at 

 least a possible aspect of the modern electron, whether 

 positive or negative. It is necessary to conceive of it as 

 freely mobile through the aether, the displacement of the 

 strain-figure from some given position to a neighbouring- 

 position being equivalent to imposing a differential strain 

 upon the aether. When the aether in the neighbourhood of 

 the strain-figure is stressed in a definite manner by some 

 independent agency, it is at once evident, on applying the 

 principle of virtual work, that the tendency of the strain- 

 figure to move through the medium in one direction or 

 another depends essentially on the type of the strains of which 

 the strain-figure is made up. For example, when a material 

 body is regarded as an assemblage of self-equilibrating 

 aetherial strain-distributions (electrons), it must not be 

 assumed that an excess of aetherial pressure* on one side 

 will necessarily tend to displace the body through the aether 

 towards regions where the pressure is less ; this will only be 

 the case if the constitution of the body is such that the region 

 which it occupies contains less aetherial substance than a like 

 volume of free aether f. If the region occupied by the body 

 contains an excess of aetherial substance, the body will tend 

 to move from places of lower towards places of higher aetherial 

 pressure. In the intermediate neutral case, the existence of 



* Without assuming any special constitution for the aether, we may 

 take the hydrostatic pressure to he one of the constituents into which 

 the stress at each point is resolved. 



t Let the defect of aether in the region occupied by the body be 

 measured by the mass vp, where p is the density of free aether, and v is 

 of the nature of a volume. Then a in'essure-gradient in the aether causes 

 a force whose components are 



_ /d/"> dp dp\ 



v Xdx' by' &r 



to act on the body, tending to displace it with respect to the aether. 

 This result is easilv proved, and is equivalent to equation (3) p. 76 of 

 Phil. Mag. January 1909. 



2X2 



