Dr. J. Croll on the Transformation of Gravity, 2-13 



diminished or is weakened by the work which it performs. 

 If a stone one pound in weight fall from an elevation of 

 1000 feet, one thousand foot-pounds of work will have been 

 performed by gravity on the stone. The stone on reaching 

 the ground is possessed of that amount of energy in the form 

 of motion, all of Avhich has been derived from gravity. But 

 gravity, it is nevertheless maintained, has sustained no loss in 

 communicating this amount of energy to the stone. 



The reasons which appear to have led to this opinion are, I 

 think, mainly the two following : — 1. It has been assumed 

 that the weight of a body is not affected by the work which 

 it performs. 2. The force by which bodies are drawn towards 

 each other does not diminish as they approach, but on the 

 contrary increases. The mutual attraction of the stone and 

 the earth (gravity) does not diminish as the stone descends, 

 but the reverse ; for the stone on reaching the earth's surface 

 is attracted with greater force than it was before it began to 

 descend. From this it is naturally inferred that, whatever 

 may have been lost in the imparting of motion to the stone, it 

 cannot have been gravity. 



In reference to the first of these reasons, it may be remarked 

 that although what we call the permanent weight of a body is 

 not affected by the work performed, still it does not follow that 

 the weight may not be affected at the time the work is being- 

 performed, and still less that there may not be a loss of gravity 

 resulting from the performance of the work. And in regard 

 to the second reason, it does not necessarily follow that, be- 

 cause the force of gravity acting on bodies increases as they 

 approach, there is not a loss of force resulting from their 

 approach. A person, for example, may be growing richer 

 every day, and yet at the same time his daily expenditure may 

 be on the increase. Two electric currents parallel and flow- 

 ing in the same direction mutually attract each other ; and 

 this attractive force increases as they are drawn together ; 

 but we know that there is at the same time a loss of attrac- 

 tive force resulting from their approach. When the two 

 currents are in the act of approaching towards each other, an 

 induced current takes place in each wire in the opposite di- 

 rection to the primary current, the effect of which is to 

 weaken the primary currents, and consequently to diminish 

 the force of their attraction ; so that when the currents are 

 approaching each other the force of their attraction at any 

 given moment is somewhat less than it would otherwise be 

 were the currents stationary. The same is true of magnets. 

 When an electromagnet is drawing towards it a bar of soft 

 iron the attractive force of the magnet for the iron increases 



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