Modified Theory of Gravitation. 77 



It will be necessary later to examine the forces which 

 material bodies might be expected to exert on one another 

 owing to their motion through the aether. 



10. Properly speaking, there are two distinct ways in 

 which matter may be conceived o£ as moving with respect to 

 the aether. . For simplicity, consider only a single electron, 

 and let us agree to give the name ; ' nucleus" to a certain 

 definite central region of the electron. At any instant the 

 nucleus comprises a certain identical portion of aetherial 

 substance, and if this portion were bodily displaced, with 

 respect to the surrounding aether (which thereby became 

 strained), it might be said that the electron had suffered a 

 displacement of like amount. But such displacement, even 

 through distances far less than the diameter of the electronic 

 nucleus, would presumably be met by an opposing stress of 

 enormous magnitude, arising from the deformation of the 

 circumnuclear aether ; while the alternative type of electronic 

 motion, involving only transference of strain from one part 

 of the aether to another, can take place freely without 

 evoking any opposing stress. (The forces mutually exerted 

 by electrons and assemblages of electrons not coming here 

 into consideration.) We shall suppose that when, from any 

 cause, matter experiences a tendency to move, or to change 

 its motion with respect to the aether at large, the resulting 

 motion may, with sufficient accuracy, be treated as exclu- 

 sively of the strain-transference type assumed in the last 

 paragraph. It should be remarked that, whereas vortex 

 motion would be involved in a bodily displacement of that 

 identical portion of aetherial substance which is instan- 

 taneously co-extensive with the nucleus of an electron, the 

 type of motion dealt with in § 9 above is irrotational. The 

 latter motion may be supposed to be entirely distinct from 

 that which, arising from the charge of the moving electron, 

 constitutes a surrounding magnetic field. Such a supposition 

 is justifiable if, as I imagine, a magnetic field involves no 

 bodily motion of the aether. 



11. Let us now conceive the aether to be traversed by 

 trains of compressional-raref actional waves, and consider 

 first how the motion of matter will be directly affected 

 thereby. As already stated, the aether is regarded as so 

 slightly compressible that, where compressional waves are 

 concerned, any stresses which may be involved, over and 

 above the variations of normal pressure, are relatively in- 

 significant : the aether, in regard to such waves, behaving 

 like a compressible fluid, whose motion is sensibly irrotational. 

 To fix ideas, let the wave-motion be limited to a single 



