apj^licahle to Lo Sago's Theory of Gravitation. 367 



from interfering with the nniformity of their motions, is tho 

 very condition which corrects and maintains the uniformity of 

 motion in opposition to external disturbing causes. Tliis is 

 the main j)oint which was utterly inexplicable in Le Sage's 

 theory; for it was impossible to see, in the way he put it, how 

 such a motion of streams of particles should be kept up uni- 

 formly in all directions, under continual collisions with them- 

 selves and with mundane matter. By the application of the 

 principles of the modern kinetic theory of gases to the case, 

 this point is completely solved. As before remarked, it is 

 almost startling that the particular form of motion which the 

 particles themselves automatically keep up should be precisely 

 that one which is required to produce gravity (or an effect 

 varying as the square of the distance &c.), — also that it should 

 make the density and mean velocity equal in all parts, which 

 is necessary for the effects of gravity *. 



8. It is an interesting fact to observe that the distance 

 through which gravity vfill act wall depend on the range 

 through which the streams of particles are comparatively un- 

 impeded, i. e. on the mean length of path of the particles. By 

 making, therefore, the mean length of path of the particles 

 less than the average distance of the stars, it would follow that 

 the stars do not gravitate towards each other, which satisfies 

 the condition for the stability of the universe. It is evident 

 (as has been already pointed out by others) that the assump- 

 tion of all the bodies of the universe gravitating towards each 

 other is quite inconsistent with stability; and to the truly 

 philosophical mind any theory which rendered such an assump- 

 tion of instability necessary would be in itself improbable. It 

 is only necessary that the mean length of path of the par- 

 ticles be great enough to produce effects of gravity through- 



* I have been informed by Professor Tait that a book pnblislied in 

 1818 by Dr. I^lair (Regius Professor of Practical Astronomy in the Uni- 

 versity of'lEdinburgh) has come nnder his notice, in which is sketched \ 

 theory of gravitation qnite similar to that of Le Sage. A later edition of 

 the book, under the title 'Scientific Aphorisms,' was published in 1827. 

 Dr. Blair attempts to account for the motion of the particles being kept 

 up uniformly in all directions by assuming them to rebound from tl)e 

 interior of a hollow spherical surface, which is supposed to bound the 

 visible universe. Of course he, like Le Sage, was ignorant of the remark- 

 able fact of the particles themselves automatically arranging their motions 

 so as to move uniformly in all directions, which has been brought to light 

 in the investigations connected with the kinetic theory of gases. The 

 spherical surface is of no use to guide the motions of the particles ; for they 

 do this themselves. It may be observed that Fatio and Redeker deve- 

 loped a similar theory to that of Le Sage, to a certain extent. When we 

 thus find several minds arriving at the same result, this forms an addi- 

 tional aro-ument for its truth. 



>o 



