658 A Fluid Analogue for the ^ Ether. 



surrounding region. Moreover the energy or! wave-motion 

 constituting this pulse would be associated with the critical 

 wave-length as a predominant wave-length ; that is to say, 

 the greater part of its energy is associated with waves whose 

 wave-lengths differ little from the critical wave-length *. It 

 thus appears that the apparent inertia of a solid mass moving 

 steadily in, or on the surface of, a fluid would be greater and 

 greater for increasing values of the speed of the mass for all 

 speeds up to the minimum wave-speed. When a uniform 

 speed equal to the minimum wave-speed in the fluid is 

 attained by the solid, its apparent inertia then becomes 

 infinite. The apparent increase in the inertia of a solid 

 with speed, is in reality due to the reaction of the fluid 

 set in motion by the solid which would act in a direction 

 tending to stop the motion of the solid. Whether it would 

 also tend to produce a contraction of the solid in the 

 direction of its motion has not yet been definitely established 

 by analysis, though this seems to be very probable. 



The above considerations clearly indicate that there is 

 a very close general similarity between the apparent 

 increase in the inertia of bodies moving in a fluid, and 

 the corresponding apparent increase of inertia of electrons 

 moving through aether at speeds increasing up to, and also 

 equal to, the velocity of light. In addition to this general 

 resemblance between the aether and an ordinary fluid with 

 respect to the motion of solid bodies within each, we have, 

 in the X-rays, evidence of the existence of wave-disturbances 

 ■of very large amplitude in the aether arising from the motion 

 through it of particles moving at the speed of light, corre- 

 sponding closely to the waves of large amplitude in water 

 arising from the motion of solid bodies in water at a speed 

 equal to the minimum wave-speed. 



The analogy which we have been examining suggests 

 other points of comparison between the interactions of 

 matter and aether on the one hand and the interactions 

 of ordinary solids and fluids on the other. With respect 

 to the physical quantities— momentum, force, energy, 

 associated with the bodies moving in an invisible fluid such 

 as the aether, the interpretation of the analogy is clear, 

 or is virtually indicated in what precedes with reference to 

 apparent increase of inertia. One additional point might, 

 however, be mentioned in connexion with the motion of 

 bodies in fluids. If the motion of a body on the surface 

 of a fluid be an accelerated motion, this would evidently 



* Proc. Roy. Soc. A, vol. lxxxix. pp. 583-4 (1914). 



