Dr. J. R. Mayer on the Mechanical Equivalent of Heat. 509 



put fundamental facts of this kind in a clear light ; exactly the 

 opposite, however, is done by the nomenclature at present in use. 

 A few expressions, employed by a very meritorious naturalist in 

 combating my views, may serve to support this assertion. 



"Although/' he says, "it is quite true that in nature no 

 motion can be annihilated, or that, as it is commonly expressed, 

 the quantity of motion once in existence continues unceasingly 

 and without any lessening, and although in this sense the cha- 

 racter of indestructibility belongs to every proximate cause even, 

 every primary cause, that is, every true physical force, possesses the 

 additional characteristic of being inexhaustible. These charac- 

 teristics will best admit of being unfolded by the closer conside- 

 ration of gravity, the most active and widely diffused of the 

 natural forces (primary causes), which, as it were the soul of the 

 world, indestructibly and inexhaustibly upholds the life of those 

 great masses on whose motions depends the order of the universe, 

 while requiring no food from without to call forth its ever renewed 

 activity." 



If these words are intended to contain a material contradic- 

 tion of the views I have put forward, they must be meant to 

 imply that, by virtue of its being inexhaustible, the attractive 

 power of the earth must be capable of imparting to a falling 

 weight, under certain conceivable circumstances, an infinite velo- 

 city. But our author himself in several places lets us see that 

 he has a (quite well-founded) mistrust of any so decided a con- 

 clusion : this is shown in the following, among other passages : — 

 "If we follow up the chain of causes and effects to its first 

 beginnings, we come at length to the true forces of nature, to 

 those primary causes whose activity does not require that they 

 should be preceded by any others, which ask for no nourishment, 

 but which can ever call forth new motions, as it were, out of an 

 inexhaustible soil, and can uphold and quicken those that are 

 already in being." 



Again : " If the moon every moment falls, at least virtually, a 

 certain distance towards the earth, what is the force which the 

 next moment pulls it away again, as it were, in order to give rise 

 to a new falling force? It is precisely its indestructibility and 

 inexhaustibility, its power at all times and under all circumstances 

 to bring about without ceasing, at least virtually, the same effects, 

 that is the essence of every true force or primary cause." 



This "as it were" and "at least virtually," which always 

 slips in at the critical moment, affords room for the suspicion 

 that our author is himself not quite confident of the power of his 

 "true natural causes" to give rise to an inexhaustible amount 

 of motion (of actual exertion of force) ; and the indefiniteness of 

 these expressions is quite characteristic of the Protean part which 



Phil Macj. S. 4. No. 171. Suppl Vol. 25. 2 M 



