338 THE MECHANICAL EQUIVALENT OF HEAT. 



torious naturalist in combating my views, may serve to sup- 

 port this assertion. 



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

 motion can be annihilated, or that, as it is commonly ex- 

 pressed, the quantity of motion once in existence continues 

 unceasingly and without any lessening, and although in this 

 sense the character of indestructibility belongs to every prox- 

 imate cause even, every primary cause, that is, every true 

 physical force, possesses the additional characteristic of being 

 inexhaustible. These characteristics will best admit of being 

 unfolded by the closer consideration of gravity, the most 

 active and widely diffused of the natural forces (primary 

 causes), which, as it were the soul of the world, indestructi- 

 bly 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 contra- 

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

 to imply that, by virtue of its being inexhaustible, the attract- 

 ive power of the earth must be capable of imparting to a fall- 

 ing weight, under certain conceivable circumstances, an infin- 

 ite velocity. But our author himself in several places lets us 

 see that he has a (quite well-founded) mistrust of any so de- 

 cided a conclusion : 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 vir. 

 tually, a certain distance toward the earth, what is the force 

 which the next moment pulls it away again, as it were, in 



