position of the earth and the stone.  The stone is solicited to move but cannot, so long as the muscular strength of the holder prevents the solicitation from taking effect.  The stone, therefore, has potential energy, which becomes kinetic if it is let go, and the amount of that kinetic energy which will be developed before it strikes the earth depends on its position—­on the fact that it is, say, six feet off the earth, neither more nor less.  Moreover, it can be proved that the raiser of the stone had to exert as much energy in order to place it in its position, as it will develop in falling.  Hence the energy which was exerted, and apparently exhausted, in raising the stone, is potentially in the stone, in its raised position, and will manifest itself when the stone is set free.  Thus the energy, withdrawn from the general stock to raise the stone, is returned when it falls, and there is no change in the total amount.  Energy, as a whole, is conserved.

Taking this as a very broad and general statement of the essential facts of the case, the raising of the stone is intelligible enough, as a case of the communication of motion from one body to another.  But the potential energy of the raised stone is not so easily intelligible.  To all appearance, there is nothing either pushing or pulling it towards the earth, or the earth towards it; and yet it is quite certain that the stone tends to move towards the earth and the earth towards the stone, in the way defined by the law of gravitation.

In the currently accepted language of science, the cause of motion, in all such cases as this, when bodies tend to move towards or away from one or another, without any discernible impact of other bodies, is termed a ‘force,’ which is called ‘attractive’ in the one case, and ‘repulsive’ in the other.  And such attractive or repulsive forces are often spoken of as if they were real things, capable of exerting a pull, or a push, upon the particles of matter concerned.  Thus the potential energy of the stone is commonly said to be due to the ‘force’ of gravity which is continually operating upon it.

Another illustration may make the case plainer.  The bob of a pendulum swings first to one side and then to the other of the centre of the arc which it describes.  Suppose it to have just reached the summit of its right-hand half-swing.  It is said that the ‘attractive forces’ of the bob for the earth, and of the earth for the bob, set the former in motion; and as these ‘forces’ are continually in operation, they confer an accelerated velocity on the bob; until, when it reaches the centre of its swing, it is, so to speak, fully charged with kinetic energy.  If, at this moment, the whole material universe, except the bob, were abolished, it would move for ever in the direction of a tangent to the middle of the arc described.  As a matter of fact, it is compelled to travel through its left-hand half-swing, and thus virtually to go up hill.