210 Mr. S. T. Preston on some Di/namical Conditions 



8. It is an interesting fact pointed out by Sir William 

 Thomson (Phil. Mag. May 1873) that the distance through 

 which gravity is effective would depend on the distcince through 

 which the gravific particles move before being intercepted by 

 collision with each other (which is equivalent to the mean 

 length of path of the particles). By assuming the distance of 

 the stars to be a multiple of the mean length of path of the 

 particles, it would therefore follow that the stars do not gravi- 

 tate towards each other — this satisfying the condition for the 

 stability of the universe. The assumption of all the bodies of 

 the universe gravitating towards each other is evidently quite 

 inconsistent with stability (as already pointed out by Professor 

 Challis). All that we require to admit is that the effects of 

 gravity hold through as great distances as we have observed 

 them. 



9. The distance through which gravity has been observed to 

 act is well known to be but an infinitesimal fraction of the 

 distance of the stars. It may therefore well be rliat the mean 

 length of path of the particles of the medium producing gra- 

 vity may be but an infinitesimal fraction of this distance. 

 The column of the gravific medium intcrce[)ted between two 

 stars would therefore on the ipJiole be at rest, just as a column 

 of gas is at rest between two bodies a visible distance apart 

 (?'. e. a distance which is a large multiple of the mean length 

 of path of the particles of gas). Le Sage appears to have 

 assumed that the mean length of path of the gravific particles 

 swept through the universe ; or he assumed that streams of 

 matter came from the depths of space and passed entirely 

 through the visible universe into space beyond*. This assump- 



s/5 

 velocity of sound in air by — o~ ; so of other gases. Thus it appears that 



the velocity of a wave in an}-- medium constituted according to the kinetic 

 theory (such as the velocity of a wave of sound in air) is solely dependent 

 on and proportional to the velocity of the particles of the medium ; and 

 this velocity of the wave is independent of the density or pressure of the 

 medium, or of any thing else excepting the velocity of its particles. 



* Le Sage assumed that a continual supply of matter from v^^ithout was 

 necessary for the maintenance of gravity in the visible universe, and that 

 all but a very small fraction of tliis supply passed through inetfectively 

 and was dissipated again in ultramundane space — a means apparently 

 quite disproportionate to the end in view. We observe that imder the 

 principles of the kinetic theory no such supply is necessary, but that all 

 the conditions requisite for gravity may be fulfilled by a medium perva- 

 ding the Ansible universe, and which is at rest as a whole ; or the gravific 

 medium within the bounds of the visible universe may be compared to 

 the air within a receptacle, which is, as a whole, at rest ; and therefore 

 there is no more waste of the matter producing gravity than if mundane 

 matter did not exist. If we imagine, in analogy, a being extremely small 



