[ 364 ] 



XLIX. 0)1 some Dijnamical Conditions applicable to Le Sage's 

 Tkeory of Gravitation. By S. ToLVEii Preston*. 



No Il.t 



1. rriPIE explanation of gravitation has now risen to the rank 

 -J*- of one of the foremost problems of modern science ; 

 indeed the day for the ascription of occult qualities to matter 

 is now gone^ and phenomena, demand an explanation by the 

 reason. The ascription of an occult quality, so far from throw- 

 ing light upon a phenomenon, only serves to darken it. The 

 effects of gravity (like all other physical effects) being effects 

 of motion, have, like other physical effects, to be explained. 

 A rational explanation has to be given for the motion of ap- 

 proach of two masses. If we were to make an exception of 

 this case, it might be argued that we might make an exception 

 of other cases ; and since ad physical effects are effects of mo- 

 tion, we should thus in principle have nothing to explain at all. 



2. The absolute necessity for giving an explanation of gra- 

 vity being admitted, we may inquire what has hitherto been 

 done in this respect. The only theory worthy of serious con- 

 sideration, or which has stood any test at all, is the theory put 

 forward by Le Sage of Geneva. An immense advance in dy- 

 namics has been made since his day. It therefore behoves any 

 one to ask how far the principles put forward by him admit of 

 being improved and modified according to modern advances 

 in dynamics. 



3. To any one who has carefully read Le Sage's theoryj, it 

 will be evident that the theory consists mainly in a series of 

 postulates or conditions arbitrarily assumed so as to be adapted 

 to produce the results required. Le Sage assumed (1) the 

 movement of streams of particles coming from an indefinite 

 distance in space and converging towards the visible universe. 

 He even calculates (by a given velocity of motion) the distance 

 those particles would require to have come which produce gravi- 

 tation, at a remote epoch of 10,000 years (page 22). He there- 

 fore calls the particles, from the enormous distance he supposes 

 them to come, ^^ultramundane particles." Next he assumes, 

 quite arbitrarily, that the particles move uniformly or equally 

 in all directions J this being necessary in order that the action 



* Communicated by tlie Author. 



t The other part may be found in the ' Philosophical ^lagazine ' for 

 September 1877. Certain points are recapitulated in the present paper for 

 completeness. A paper " On the Mode of the Propagation of Sound " 

 (Phil. Mag-. June 1877) also bears on the subject. 



X The theory is given in a work entitled JDcux Traitcs de Physique Me- 

 canique, by Pierre Prevost, 



