72 Dr. C. V. Burton on a 



by the presence of atomic matter, is assumed to be so 

 nearly incompressible that the bulk-modulus of elasticity 

 enormously transcends every other elastic modulus ; so that, 

 in dealing with any compressional-rarefactional disturbances, 

 we may treat the medium as a fluid. 



In two [ particulars, however, there is an essential difference 

 between the views now tentatively put forward and the 

 more familiar form of pulsatory hypothesis. These differences 

 are indicated in the following five paragraphs. 



2. It is a well-known hydrodynamical result * that if two 

 spheres, placed at some distance apart in a frictionless in- 

 compressible fluid, are by any means caused to execute 

 periodic pulsations (increase and decrease of volume), the 

 pulsation-period being the same for the two spheres, the 

 average force which either sphere causes to be exerted upon 

 the other will be an attraction, a repulsion, or zero, according 

 as the phase-difference of the pulsations is less than, greater 

 than, or ; equal to a quarter-period. Hence, if we are to 

 construct on these lines a model illustrative of gravitation, 

 we must assume agreement of phase amongst all the pulsating 

 centres, or at least we must suppose that the phases of the 

 various centres are more nearly in agreement than would be 

 the case if the phase-distribution were purely random. This 

 seems to suggest that the source of pulsatory motion is not 

 to be sought for in the free vibrations of individual atoms or 

 electrons, but rather in something external to these, and 

 acting on them all in common. 



Let us suppose, for example, that free rather is very nearly 

 incompressible, and that every region where atomic matter 

 exists is a region of somewhat enhanced compressibility. 

 On this supposition, an increase of pressure applied by any 

 ideal means to all the rather within a given volume would 

 cause only a very minute contraction except where atomic 

 matter was present ; the expansion resulting from a diminu- 

 tion of the pressure applied to the rather being similarly 

 localised. 



3. It appears that we might thus account for attractions 

 of gravitational type by supposing the setherial pressure to 

 be undergoing a secular change. A gradually increasing (or 

 decreasing) pressure would cause every region containing 

 atomic matter to behave as a sink (or as a source) in the 

 medium. For our purpose, however, accelerated inflow (or 

 outflow) of rather at each sink (or source) would be 

 necessary ; that is to say, the strength of each sink (or 



* Hicks, loo. cit. Cf. also Basset's " Hydrodynamics," vol. i. chap, xi., 

 where farther references are o-iven. 



