152 Dr. S. Tolver Preston on some 



then an enormous number of them can exist in the unit of 

 volume combined with an enormously great length of path. 



In general, in putting forward a theory of this kind, we 

 should see no improbability in the assumption of either a very 

 great or a very small number. Our objection to uncommonly 

 great or uncommonly small numbers rests in fact upon custom, 

 and regularly disappears as soon as the theory in question has 

 further introduced itself. 



There exists in space field enough, when necessary, for 

 finer material, as our conceptions are not limited in the 

 direction of smallness, and the smaller the particles, the 

 quicker is their natural speed of motion, and the more intense 

 the enclosed store of concealed energy : also the whole 

 arrangement becomes all the less appreciable by our senses. 

 The effects — called gravitational effects — on the other hand, 

 do not escape detection by our senses ; and reasoning from 

 these effects, we trace and infer the invisible causes which lie 

 at the basis of these effects. 



Evidently there exists just as little an obstacle in space to 

 smallness of size as to any given velocity of motion, and there 

 are reasons for supposing that gravity must propagate itself 

 with great velocity. Precisely because the normal velocity 

 of the atoms is great, the material concerned in producing 

 gravity can be very limited in quantity, and notwithstand- 

 ing that exert a very considerable pressure. The atoms are 

 therefore to be assumed very small, almost points, the con- 

 dition adapted for a great length of path. The analogy of 

 this gravitation mechanism (at least in principle) with the 

 generally assumed structure of our atmosphere, may be 

 regarded as a recommendation to the theory. 



A further objection of Maxwell's, that according to this 

 theory the action of gravity could only be kept up by an 

 enormous expenditure of external work little short of ruinous, 

 applies in fact to the theory of Le Sage in the form presented 

 by Lord Kelvin ; also to the theories of Isenkrahe and Bock 

 considered further on ; not, however, to the theory set forth by 

 the present writer, because, according to this latter theory, the 

 maintenance of the motion of the aether atoms demands just 

 as little an expenditure of energy as the maintenance of the 

 motion of the molecules of a gas in the ordinary gas theory. 

 Moreover, the " shelter " of one mass by another is explained 

 without any absorption of energy. 



The large store of energy contained in the aether atoms is 

 moreover of use for the explanation of the most varied natural 

 phenomena ; and it may be observed that the intervention of 



