452 Mr. J. Croll on certain Hypothetical Elements 



of what becomes of the motion externally imparted to the stone 

 when throw nupwards, or where the increase both of attraction 

 and motion is derived as it descends. If the attraction theory 

 be correct, then there is a destruction of force in the one case, 

 and a creation of force in the other ; and if so, then the principle 

 of conservation of force is violated. 



Professor Briicke tries to answer Faraday's objections in the 

 following manner : — (l Let the mass A," he says, " be separated 

 from the mass B by an external force ; while this separation 

 takes places the attraction diminishes, the attractive forces being 

 in the inverse ratio of the squares of the distances. Where 

 abides the force which is here destroyed ? The reply is : If the 

 mass A be left to itself, it moves back towards B, and when it 

 has arrived at its original position it will be attracted by B with 

 the same force as before ; besides this, it has attained a velocity, 

 half the square of which, multiplied by the mass of A, is exactly 

 equal to the work which was formerly expended in removing it 

 from B. There is therefore no force destroyed by the change 

 which the external cause has wrought ; but just as much force 

 appears at the end as was expended in producing the change"*. 



It will be easily perceived that this never touches the objec- 

 tion. Faraday will admit that when A has returned to its original 

 position, it will have received back all the force that was lost. 

 When A is removed from B by an external force, the motion 

 disappears without producing any apparent effect ; it does not 

 appear under any other form of force ; and when A approaches 

 B, motion is produced without the expenditure of any force. 

 There is therefore an unaccountable loss of force in the one 

 case, and as unaccountable a gain of force in the other case. 

 Consequently when A reaches B it will be in the same state as 

 when it left ; for the loss in the one case is compensated by the 

 gain in the other. But this never explains what became of the 

 force which was lost in the first case, or whence was derived 

 the force which was gained in the latter case. If a man who 

 had lost in some unaccountable manner £\0 the one day and 

 gained in an equally unaccountable way £\0 the next day, were 

 to inquire what became of the money lost on the first day, and 

 whence came the money gained on the second, it would be no 

 answer whatever to tell such an individual that he was just as rich 

 at the end of the second day as he was at the beginning ef the first. 

 It would of course be a somewhat satisfactory answer to be told 

 that the money gained the second day was what had been lost 

 the first ; and this no doubt is the idea Professor Briicke wishes 

 to convey. When the stone is thrown upwards, he supposes 

 that the motion or energy imparted becomes stored up in the 



* Phil. Mag. February 1858. 



