Modified Theory of Gravitation. 79 



of the aether will tend to produce an acceleration o£ atomic 

 matter with respect to the aether. From (3) and (4) we find 

 for the acceleration of any free mass m of neutral matter 

 with respect to the gether 



a= |_FB 2 ^cos^(^-W + e), 0, 0| ; . (5) 



while the absolute acceleration A of the nether itself is 

 given by 



In this case then, and (as we easily see) generally for small 

 compressional disturbances of the aether, the ratio 



acceleration of free matter with respect to the 

 aitlier : acceleration of the tether = Fp (7) 



14. There is another aspect of the hypothetical wave- 

 motion which deserves some attention. At any point fixed 

 with respect to the aether, changes of pressure are supposed 

 to occur, accompanied by changes of density, and as we 

 consider the compressibility of the medium in bulk to be 

 very slight, the proportional changes of pressure and of con- 

 stitutive (potential) energy w T ill be great compared with the 

 proportional changes of density. From this it may be 

 surmised that corresponding changes in the velocity of 

 radiation would be involved, and if the amplitude of these 

 changes were not a quite insignificant fraction of the whole 

 velocity, effects might result which would be palpable to 

 observation. On the other hand, all effective wave-lengths 

 are assumed to be so immense that, throughout even an 

 astronomically considerable region, the setherial pressure, 

 and therewith the velocity of radiation, may be treated as 

 uniform at each instant. It is perhaps conceivable that the 

 effect of increased setherial pressure would be to accelerate 

 not only radiation but cdl phenomena in the same proportion ; 

 so that, for example, two universes identically constituted 

 otherwise, but differing from one another as to the pressure 

 prevailing throughout the a3ther and the rapidity of pheno- 

 mena in general, might present identical aspects to their 

 respective inhabitants. But even so, in the question whether 

 the pressure of the aether throughout our universe could 

 be gradually or suddenly changed without giving rise to 



