KINETIC THEORIES OF GRAVITATION. 267 



either by experiment or by observation/ 1 * There is no conceivable 

 reason why the falling stone should not " be acted upon by gravity with 

 less force" than if it were at rest, (or why our fifth and sixth propo- 

 sitions should be true,) excepting the cogency of ascertained fact. 

 Natural philosophy is an experimental science ; and this point has been 

 determined both " by experiment and by observation." The seusit-ive gal- 

 vanometer needle of Faraday would have betrayed a sigh of commuting 

 energy in the falling weight employed, while the actual increments of 

 vis viva were really so large that they would have been a notable duty 

 for the coarsest scales. And the ceaseless fall of planets from the tan- 

 gents of their orbits, without any reduction of their own centripetal 

 tensions, or of their satellitic control, is a constant and conclusive 

 observation to show that this law or condition of gravitation — embraced 

 in our sixth proposition — has not (as above suggested) been " assumed." 



Mr. Croll continues: " But if the force of gravity does not sustain 

 any loss as work is performed by it, what then is it that is supposed to 

 sustain the loss"? Some form of energy must diminish as work is per- 

 formed ; and if it be not gravity it must be something else. The 

 generally received explanation is this : when a body is projected upward, 

 the potential form of energy into which the upward motion of the body 

 is transformed does not consist in the simple force of gravity or tend- 

 ency of the body to descend, but consists in this force or tendency 

 multiplied by the distance through which it is capable of descending 



. . . This mode of viewing the matter, it is perfectly true, com 

 pletely meets the mathematical and mechanical conditions of the prob- 

 lem ; but for this very reason it seems to me to hide somewhat the 

 real physical nature of the process. [!]... Space and time are 

 conditions, but conditions absolutely necessary to the transformation of 

 potential energy into kinetic, and of kinetic energy into potential ; but 

 they themselves cannot be forms of energy. But if it be true that the 

 mere force of gravity or tendency of the stone to fall to the ground is 

 not the potential energy, but that this potential energy is the force 

 multiplied by the space through which it can act, then space must 

 become a form of potential energy. This is evident; for the potential 

 energy in this case consists' of two factors, one of which is the space 

 through which the force acts. It thus becomes just as much a form of 

 energy as the other factor, viz, the force." t 



The conclusion that space is " a form of" action, because all action is 

 necessarily conditioned by space, does not appear so " evident." Mr. 

 Croll correctly states : " But it is not in reference to gravity alone that 

 this space-condition is essential to the transformation of potential into 

 kinetic energy. It is as we shall shortly see, a condition absolutely 

 necessary to the transformation of energy under every possible form. 

 In the unbending of a spring the amount of work which can be per- 



* Loco citat., pp. 242-244. t Loco citat., pp. 244, 245. 



