242 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1908. 



potential energy works admirably, and is very nnlikely to be super- 

 seded ; but, regarded from a philosophical point of view, the concep- 

 tion of potential energy is much less satisfactory and stands on quite 

 a different footing from that of kinetic energy. When we recognize 

 energy as kinetic we feel that we know a great deal about it ; when we 

 describe energy as potential we feel that we know very little about it, 

 and though it may be objected that from a practical point of view 

 that little is all that is worth knowing, the answer does not satisfy an 

 inquisitive thing like the human mind. 



Let us consider a commercial analogy and compare kinetic energy 

 to money in actual cash and potential energj^ to money at our credit 

 in a bank, and suppose such a state of things to exist that when a man 

 lost a sovereign from his pocket it was invariably collected, he did 

 not know how, and placed to his credit in a bank situated he knew 

 not where, from which it could always be recovered without loss or 

 gain. Though the knowledge that this was so might be sufficient for 

 all commercial purposes, yet one could hardly suppose that even the 

 most utilitarian and matter-of-fact of men could refrain from specu- 

 lating as to where his money was when it was not in his pocket, and 

 endeavoring to penetrate the mystery which envelops the transfer 

 of the sovereign backward and forward. Well, so it is with the 

 physicist and the conception of different forms of potential energy; 

 he feels that these conceptions are not simple, and he asks himself 

 the question whether it is necessary to suppose that these forms of 

 energy are all different ; may not all energy be of one kind — kinetic ? 

 and may not the transformation of kinetic energy into the different 

 kinds of potential energy merely be the transfer of kinetic energy 

 from a part of the system which affects our senses to another which 

 does not, so that what we call potential energy is really the kinetic 

 energy of parts of the ether which are in kinematical connection with 

 the material system? Let me illustrate this by a simple example. 

 Suppose I take a body. A, and project it in a region where it is not 

 acted on by any force. A will move uniformly in a straight line. 

 Suppose, now, I fasten another body, B, to it by a rigid connection, 

 and again project it. A will not now move in a straight line, nor 

 will its velocity be uniform ; it may, on the contrary, describe a great 

 variety of curves, circles, trochoids, and so on, the curves depending 

 on the mass and velocity of B when A was projected. Now, if B 

 and its connection with A were invisible so that all we could observe 

 was the motion of A, we should ascribe the deviation of A's path from 

 a straight line to the action of a force, and the changes in its kinetic 

 energy to changes in the potential energy of A as it moved from 

 place to place. This method is, however, the result of our regarding 

 A as the sole member of the system under observation, whereas A is 

 in reality only a part of a larger system ; when we consider the sys- 



