KINETIC THEORIES OF GRAVITATION. 259 



rectly informed that "force is recognized as the agent in every change 

 of motion ; " and therefore we are no nearer the source of this agency 

 after the acceptance the hypothesis than we were before it. 



Still more recently, Professor Tait, in an evening lecture on " Force," 

 delivered September 8, 1876, before the British Association at its Glas- 

 gow meeting, has recurred to his kinetic hypothesis. " Why two 

 masses of matter possess potential energy when separated, in virtue of 

 which they are conveniently said to attract one another, is still one of 

 the most obscure problems in physics. I have not now time to enter 

 on a discussion of the very ingenious idea of the ultramundane corpus- 

 cles, the outcome of the life-work of Le Sage, and the only even appar- 

 ently hopeful attempt which has yet been made to explain the mechanism 

 of gravitation. The most singular thing about it is that if it be true, 

 it will probably lead us to regard all kinds of energy as ultimately 

 kinetic* And a singular quasi-metaphysical argument may be raised 

 on this point, of which I can give only the barest outline. The mutual 

 convertibility of kinetic and potential energy shows that relations of 

 equality (though not necessarily of identity) can exist between the two; 

 and thus that their proper expressions involve the same fundamental 

 units, and in the same way. Thus as we have already seen that kinetic 

 energy involves the unit of mass and the square of the linear unit directly, 

 together with the square of the time unit inversely, the same must be 

 the case with potential energy ; and it seems very singular that poten- 

 tial energy should thus essentially involve the unit of time, if it do not 

 ultimately depend in some way on energy of motiou."t 



This is the unavoidable inference of the kinetic system of force, if 

 consistently maintained. But if there be any induction impregnable, as 

 the generalization of a life-long, a continuous, and an unvarying exper- 

 ience, it is that potential energy does not " involve the unit of time." 

 The carbon that has lain protected in the bowels of the earth for untold 

 ages (certainly for many millions of years) has precisely the same rela- 

 tion to oxygen as the carbon prepared from last year's wood, and 

 holds stored in the same mass the same exactly measurable potential 

 energy. The stone ball that may have lain a thousand years undis- 

 turbed on the brink of a precipice has during that time lost no fraction 

 of its static tension, but will fall with absolutely the same dynamic effect 

 as if thrown up to its seat by a cannon but a moment before. The 

 familiar case of a wound-up clock or watch, with the pendulum or the 

 balance-wheel at rest, is equally irreconcilable with any scheme of kinetic 

 action or of a force involving as a function any " unit of time." As 

 Professor Maxwell points out : " In both cases, until the clock or watch 

 is set agoing, the existence of potential energy, whether in the clock- 

 weight or in the watch-spring, is not accompanied with any visible 



* This was, of course, its very purpose. 



t Nature, September 21, 1876, vol. xiv, p. 463. 



