On Le Sage's Theory of Gravitation. 305 



of gravity may bo equal in all directions. He computes 

 (roughly at about 3,000,000) the number of different direc- 

 tions in which separate streams of particles would require to 

 be moving in order to produce everywhere that sensible uni- 

 formity of pressure which is the characteristic of gravity 

 (page 25). It will be noted that these are all assumptions in 

 themselves entirely arbitrary. He next assumes that the mean 

 velocity of the streams of particles is everywhere the same, 

 and the density everywhere the same. 



4. It will be observed that this theory gives no possible 

 idea as to how such a motion of streams of particles among 

 themselves could be kept up, or naturally maintained. Le 

 Sage attempts to evade the difficulty of the particles encoun- 

 terino; each other by assumin o- them to be so small that " not 

 more than one out of every hundred of the particles meets an- 

 other during several thousands of years." This only removes 

 the difficulty a step further on, without avoiding it. Indeed it 

 may be observed that the theory, in the state in w^hich Le Sage 

 left it, is little more than a series of postulates, some of them 

 almost as unrealizable as graAaty itself. This does not detract 

 from a distinct merit in the origination of the theory ; for it 

 miust be remembered how little dynamical principles were ad- 

 vanced at Le Sage's time, and how few resources he had to 

 draw upon. 



5. I have pointed out (Phil. Mag. Sept. 1877) what (w^hether 

 already observed by others or not) cannot but be regarded as a 

 somewhat startling fact, viz. that no postulates whatever are 

 required for a dynamical theory of gravitation, but that it may 

 be shown that particles of matter in free motion in space must 

 inevitably of themselves arrange their motions so as to produce 

 the effects of gravity — or the special effects of gravity (varia- 

 tion as the square of the distance &c.) must be produced from 

 pure dynamics in the case of a system of particles in free motion 

 in space, w^ithout any necessity for postulates as to the character 

 of the motion at all. This follows from the principles which 

 have been investigated in connexion with the modern kinetic 

 theory of gases, of which Le Sage was ignorant. For it has 

 been demonstrated by Professor Maxwell, in connexion with 

 the kinetic theory of gases, that particles of matter in free 

 collision among each other in space will automatically arrange 

 their motions so as to move uniformly in all directions, i. e. 

 so that an equal number of particles are moving in any two 

 opposite directions (this being the necessary condition for 

 equilibrium of pressure in a gaseous medium *). This cha- 



* I may state tliat T had independently arrived at tliis same result in a 

 paper " On the Mode of the Propagation of Sound on the Basis of the 



