392 On the Mechanical Theory of Gravitation. 



May not analogous considerations apply to gravity (still 

 regarded by sonic as the result of a mystic "action at a dis- 

 tance"), and which, as an actual fact, now remains as the 

 last remnant of spiritualism to be expelled from physics. In 

 relation to this subject, Sir William Thomson, in an important 

 development of Le Sage's theory of gravitation, published in 

 the Philosophical Magazine for May 1873, makes the follow- 

 ing suggestive remark, viz. that, assuming certain possible 

 conditions to exist, " crystals would generally have different 

 permeabilities* in different directions, and would therefore 

 have different weights accordino; to the direction of their axes 

 relatively to the direction of gravity. No such difference has 

 been discovered ; and it is certain that, if there is any, it is 

 extremely small. Hence the constituent atoms, if seolotropic 

 as to permeability, must be so but to an exceedingly small 

 degree " (page 331) f. 



Might not this point be worthy of experiment ? Although 

 the discovery of some such minute specific differences would 

 not, in point of principle, be necessary to the establishment of 

 the truth of the existence of an explanation for gravitation, yet 

 a discovery of this kind would be highly interesting as a con- 

 firmation of the truth of the particular explanation afforded 

 by the kinetic theory of gases (which depends on the permea- 

 bility of gross matter by a gaseous aether, whose minute com- 

 ponent particles have an extremely long mean path). The 

 groundwork of this explanation (i. e. the one first started in 

 very crude form by Le Sage, of Geneva) is now, it may be 

 observed, generally regarded by competent judges as the 

 only conceivable rough basis on which an intelligible mecha- 

 nical theory of gravitation admits of being built. 



The main object of this paper is to call more particular 

 attention to the desirability of an experimental search in rela- 

 tion to the mechanical theory of gravitation, or to the expe- 

 diency of a more precise and exact observation of gravific 

 effects under diverse conditions of form, structure, &c. of 



* It should be kept in view, that one of the essential conditions of Le 

 Sage's theory is that gross matter should be permeable by the minute par- 

 ticles of a finer material, or aether. 



t Sir William Thomson remarks further as follows in relation to this 

 point, viz.: — "A body having different permeabilities in different direc- 

 tions would, if of manageable dimensions, give us a means for drawing 

 energy from the inexhaustible stores laid up in the ultramundane cor- 

 puscles — thus: first, turn the body into a position of maximum weight; 

 secondly, lift it through any height ; thirdly, turn it into a position of 

 maximum weight; fourthly, let it down to its primitive level. It is 

 easily seen that the first and third of those operations are performed with- 

 out the expenditure of work ; and, on the whole, work is done by gravity 

 in operations 2 and 4 " (page 331). 



