OBJECTIONS CONSIDERED. 339 



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 in- 

 definiteness of these expressions is quite characteristic of the 

 Protean part which the force of gravity plays in writings on 

 natural science. The most arbitrary explanations are given 

 of this word, and then, when facts no longer admit of any 

 thing else, a retreat is sought in the Newtonian conception. 



Gravity being called a force, and at the same time the 

 term force being connected, in accordance with the common 

 use of language, with the conception of an object capable of 

 producing motion, leads to the false assumption that a me- 

 chanical effect (the production of motion) can be produced 

 without a corresponding expenditure of a measurable object ; 

 and here is likewise plainly the reason why our author could 

 neither keep clear in his facts nor consistent in his reasoning. 

 If once the production of motion out of nothing is granted, 

 the annihilation of motion must also be admitted as a conse- 

 quence ; and the magnitude of motion must, in accordance 

 with this assumption, be simply proportional to the velocity, 

 or =Mc, and the "quantity of motion once in existence" 

 must be =+Mc McQ. But notwithstanding his "inex- 

 haustible forces," the writer referred to expressly declares 

 that motion is indestructible ; but, instead of stating his opin- 

 ion as to what becomes of motion which disappears by fric- 

 tion, he says in another place again that it remains " unde- 

 cided " whether the effect of a force (the amount of motion 

 produced by it) is measured by the first or by the second 



