The division of forces into dynamical, statical, and kinetic has been long recognized by all competent judges as very good and satisfactory. But our men of progress, in the innocent belief that, before they appeared on the scene, everything in this world was darkness, have changed all that. All forces are now stated to consist in nothing but "mass animated by velocity." Dynamical forces are rejected, it would seem, because they imply what modern science cannot, or will not, understand—i.e. real production (they call it creation) of movement. On the other hand, statical forces are not masses animated by velocity, and thus are set aside because they originate no real movement. Such is the consistency of our progressional friends.
Yet so long as all effect will need a cause, there can be no doubt that statical forces must be real forces. Two weights balancing one another at the ends of a lever certainly act on one another, as every one must admit who observes the change produced by taking away one of the two. A weight which actually prevents another weight from falling surely exerts a positive influence on it, and therefore displays power and brings forth an amount of action. So also, when a weight is at rest on a table, gravity does not remain dormant with regard to it, but urges it toward the table with unyielding tenacity. Hence the table must continually react in order to keep the body at rest. It is evident, therefore, that the weight, while at rest on the table, exerts its powers and is engaged in real action; for nothing but real action can awaken real reaction. Again, when we try to raise a weight, we feel that we must overcome a real resistance; and when we support a weight, we feel its action upon our limbs. Hence pressure is a real force, though it be not mass animated by velocity; and the same is evidently to be said of traction, torsion, flexion, etc. It is, therefore, impossible to ignore statical forces.
That dynamical forces are likewise indispensable in science I think it would be quite superfluous to prove. Rational mechanics is wholly based upon them, and no phenomenon in nature can be explained without them. If modern science finds it difficult to understand the production of local movement, let her consider that, after all, it would be less damaging to her reputation to confess her philosophical ignorance than to deny what all mankind hold and know to be a fact.
From this short discussion we may safely conclude, with the old physicists, that there are in nature dynamical and statical as well as kinetic forces, and that the word force should be uniformly used in philosophy as expressing a quantity of action measured by the quantity of its effect, or by something equivalent. But we have not yet done with our advanced theorists.
It is curious that, after having reduced all forces to "mass animated by velocity," they have not hesitated to introduce into science a force which is neither mass animated by velocity nor a common statical force, but something quite different, to which they gave the name of "potential energy." The first to imagine this spurious force was, if I am not mistaken, the German Dr. Mayer, one of the great leaders of modern thought, who, considering that a body raised from the floor would, if abandoned to itself, fall down and acquire a momentum calculated to do an amount of work, conceived the raising of the body as equivalent to a communication of latent energy destined to become visible at any time in the shape of movement as soon as the body is left to itself. Such an energy, as still unevolved, was called "potential energy."
"If we define 'energy' to mean the power of doing work," says a well-known English professor, "a stone shot upwards with great velocity may be said to have in it a great deal of actual energy, because it has the power of overcoming up to a great height the obstacle interposed by gravity to its ascent, just as a man of great energy has the power of overcoming obstacles. But this stone, as it continues to mount upwards, will do so with a gradually decreasing velocity, until at the summit of its flight all the actual energy with which it started will have been spent in raising it against the force of gravity to this elevated position. It is now moving with no velocity—just, in fact, beginning to turn—and we may suppose it to be caught and lodged upon the top of a house. Here, then, it remains at rest, without the slightest tendency to motion of any kind, and we are led to ask, What has become of the energy with which it began its flight? Has this energy disappeared from the universe without leaving behind it any equivalent? Is it lost for ever, and utterly wasted?... Doubtless the stone is at rest on the top of the house, and hence possesses no energy of motion; but it nevertheless possesses energy of another kind in virtue of its position; for we can at any time cause it to drop down upon a pile, and thus drive it into the ground, or make use of its downward momentum to grind corn, or to turn a wheel, or in a variety of useful ways. It thus appears that when a stone which has been projected upwards has been caught at the summit of its flight and lodged on the top of a house, the energy of actual motion with which it started has been changed into another form of energy, which we denominate energy of position, or potential energy, and that, by allowing the stone again to fall, we may change this energy of position once more into actual energy, so that the stone will reach the ground with a velocity, and hence with an energy, equal precisely to that with which it was originally projected upwards."[271]
Such is the theory. It is scarcely necessary to say that the whole of it is a delusion. First, the velocity imparted to the stone is not a working power, but only a condition for doing work, as I shall presently show; and, therefore, it cannot be styled "energy."
Secondly, when the stone is caught at the summit of its ascent, and (according to the strange phrase of the author) is moving with no velocity, it possesses nothing more than it possessed when lying on the ground. Its elevated position is only a new local relation, which confers no power, either actual or potential. It is indeed possible to let the stone drop down; but then its fall will be due to the action of the earth, and consequently to extrinsic causation, not to anything possessed by the stone on account of its elevated position.
Thirdly, the words "potential energy" cannot be coupled with one another without absurdity; for "energy," according to all, means power to act, whilst "potential" means liability to be acted on. Hence "potential energy" would mean either a power to act which is ready to be acted on, or a power which is to be acquired by the body through its being acted on. The first alternative confounds act with passive potency, and action with passion; the second assumes that the velocity to be acquired by the body is a real working power, which it is not.
Fourthly, it is against reason to admit that "the energy of actual motion is changed into another form of energy." For where is the causality of the change? The only causality concerned with the modification of the upward movement of the stone is the action of gravity; and this, being directly antagonistic to the ascensional velocity, tends to destroy, not to transform, it.