Philosophical State of the Physical Sciences. 501 



He supposed that bodies were pressed toward one another by the 

 everlasting pelting of ultramundane atoms, inward bound from 

 the immensity of space beyond, the faces of the bodies which 

 looked towards each other being mutually screened from this 

 bombardment. It was objected to this hypothesis, which intro- 

 duced Lucretius into the society of Newton and his followers, 

 that the collision of atoms with atoms and with planets, would 

 cause a secular diminution in the force of gravity. Le Sage 

 admitted the fact. But as no one knew that the solar system 

 was eternal, the objection was not fatal. As the necessity for 

 giving a mechanical account of gravitation was not generally 

 felt at the time, the theory of Le Sage fell into oblivion. In 1873 

 Sir William Thomson resuscitated and republished it. He has 

 fitted it out in a fashionable dress, made out of elastic molecules 

 instead of hard atoms, and has satisfied himself that it is con- 

 sistent with modern thermodynamics and a perennial gravi- 

 tation. 



Let us now look in a wholly different quarter for the mecha- 

 nical origin of gravitation. In 1870 Prof. Guthrie gave an 

 account of a novel experiment, viz. the attraction of a light body 

 by a tuning-fork when it was set in vibration. Thomson re- 

 peated the experiment upon a suspended eggshell and attracted 

 it by a simple wave of the hand. Thomson remarks "that what 

 gave the great charm to these investigations, for Mr. Guthrie 

 himself, and no doubt also for many of those who heard his ex- 

 positions and saw his experiments, was, that the results belong 

 to a class of phenomena to which we may hopefully look for 

 discovering the mechanism of magnetic force, and possibly also 

 the mechanism by which the forces of electricity and gravity are 

 transmitted." By a delicate mathematical analysis, Thomson 

 arrives at the theorem that the " average pressure at any point 

 of an incompressible, frictionless fluid originally at rest, but set 

 in motion and kept in motion by solids, moving to and fro, or 

 whirling round in any manner, through a finite space of it," 

 would explain the attractions just described. Moreover he is 

 persuaded by other effects besides those of light, that, in the 

 interplanetary spaces and in the best artificial vacuum, the me- 

 dium which remains has "perfectly decided mechanical qualities, 

 and, among others, that of being able to transmit mechanical 

 energy in enormous quantities;" and he cherishes the hope that 

 his mathematical theorems on abstract hydrokinetics are of some 

 interest in physics as illustrating the great question of the 

 eighteenth and nineteenth centuries — Is action at a distance a 

 reality, or is gravitation to be explained, as we now believe 

 magnetic and electric forces must be, by action of intervening 

 matter ? 



