Modified Theory of Gravitation. 73 



source) must be steadily increasing or steadily decreasing 

 with the time — no matter which. The suggested possibility 

 of accounting for gravitation by means of a slow secular 

 change taking place throughout the universe appeared at 

 first sight very alluring ; but calculation shows at once that, 

 feeble as gravitational forces are, they are incomparably 

 greater than could be accounted for in this way. 



4. What, then, would be the result of periodic or quasi- 

 periodic fluctuations of the aetherial pressure ? Every 

 portion of atomic matter would constitute a centre of 

 pulsatory motion in the aether ; and throughout any region 

 not too immense the pulsations would agree sensibly in phase. 

 It is by assuming the existence of a fluctuating aetherial 

 pressure that I have here sought to obtain a consistent 

 illustration of gravitational . attraction ; and, as furnishing 

 most readily an assignable cause for such fluctuations of 

 pressure, the propagation of compressional-rarefactional 

 waves through the nearly incompressible aether is suggested. 

 These waves are supposed to travel with so great a velocity 

 that, even though all effective periods involved be very small, 

 the effective wave-lengths, measured even by astronomical 

 standards, are very great. Though the origin of the assumed 

 wave-trains were unexplained, these might still be regarded 

 as contributing to the explanation of gravity ; for when once 

 the constitution of a dynamical system has been defined, the 

 mere supposition that the system is in motion is hardly to be 

 viewed as a piling-up of hypotheses. And if we regard the 

 question broadly, extending our consideration to that wider 

 range of space and motion wherein the whole of our explorable 

 universe must needs be treated as an infinitesimal volume- 

 element, sensibly homogeneous as regards its setherial 

 content, we may admit our complete ignorance of this 

 greater universe, and of the forms of disturbance which 

 might emanate from it. One might perhaps conceive of 

 compressional waves as proceeding from a more primitive 

 and chaotic condition of the " primal aether," from which an 

 aether such as we know, with electromagnetic qualities, may 

 be in process of formation. As regards the sensible uni- 

 formity in time which must characterize the primary dis- 

 turbance, if so unchanging a phenomenon as gravitation is 

 thus to be accounted for, we may think of it as a quality 

 naturally to be looked for in any activities wdiose scale is 

 sufficiently vast ; just as the energy of the radiation passing 

 into space from the sun varies but little in the course of an 

 hour, or as the turbulence of an ocean presents much the 

 same aspect from minute to minute. 



