244 Dr. J. Croll on the Transformation, of Gravity. 



as the bar approaches ; but there is, notwithstanding, a slight 

 loss of attractive force resulting from the approach of the bar. 

 The attractive force of the magnet for the bar when it is in 

 the act of approaching is less at any given distance than it 

 would otherwise be were the bars stationary at that distance ; 

 for the approach of the magnetized iron weakens the magnet 

 by weakening the current surrounding it. It is also probably 

 true, though it cannot be so easily proved, that a permanent 

 magnet is weakened by the approach of the magnetized iron. 

 May not the same be true of bodies approaching each other 

 under the mutual influence of the attractive force of gravity ? 

 May not a stone when in the act of falling be acted upon by 

 gravity with less force at any given moment than it would be 

 were the stone at rest at that instant ? The point has never 

 yet been determined either by experiment or by observation. 



The fact that the attractive force of bodies increases as they 

 approach towards each other cannot, therefore, be regarded as 

 evidence that no loss of force results from their approach. It 

 may be here also remarked that neither can the fact of this 

 increase of attractive force as bodies approach be regarded as 

 in any way opposed to the law of conservation ; for, as was 

 pointed out by Professor Rankine in his reply to Faraday's 

 objection *. no law of conservation is applicable to the simple 

 force or tendency of two bodies to approach each other, — 

 the physical reason being that the things conserved cannot 

 be two forces ; for the one is never directly convertible 

 into the other. What must be conserved is the force, and 

 the work w T hich it perforins. 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 performed ; and if it be 

 not gravity, it must be something else. 



The generally received explanation is this : when a body is 

 projected upwards, the potential form of energy into which 

 the upward motion of the body is transformed does not con- 

 sist in the simple force of gravity or tendency of the body to 

 descend, but consists in this force or tendency multiplied by 

 the distance through which it is capable of descending. If a 

 stone one pound in weight is at an elevation of 100 feet, then 

 one pound multiplied by 100 feet gives 100 foot-pounds of 

 potential energy possessed by the stone at that elevation ; 

 and this is usually termed the energy of position. Again, 

 when the body has descended to within, say, 20 feet of the 

 ground, it possesses only 20 foot-pounds of potential energy 

 in virtue of position ; for one pound multiplied by 20 feet 

 * Phil. Mag. [IV.] vol. xvii. p. 250 (1859). 



