THE ADAMSON LECTURE
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 energy 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 speculating as to where his money was when it was not in his pocket, and endeavouring to penetrate the mystery which envelopes the transfer of the sovereign backwards and forwards. 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