Sr aes me CEI OT BR PPE AAA AE ig a hele eee ah ENES Sear) aie ES EE anh ET FINA N lea oa 
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ADDRESS OF PROFESSOR LOVERING. 653 
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. 
n 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 consistent with modern thermo-dynamics and a perennial gravi- 
tation. 
Let us now look in a wholly different quarter for the mechanical 
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 repeated 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 expositions and saw 
his experiments, was, that the results belong to a class of phe- 
homena to which we may hopefully look for discovering the mech- 
= 4nism 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 spaees and in the best 
artificial vacuum, the medium which remains has “perfectly de- 
cided 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 
steat 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? 
` In 1869 and 1873, Prof. Challis of Cambridge, England, pub- 
lished two works on the Principles of Mathematical Physics. 
They embody the mature reflexions of a mathematical physicist 
