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words, that all existing forces are but the continuation or prolongation of other forces preceding them, either of the same or other forms, but precisely equivalent in amount; and that no force can terminate its action in any other way than by being transmuted into some other force, either of the same or of some other form; but still, again, precisely equivalent in amount. It was formerly believed that a force might under certain circumstances be _originated_--created, as it were--and hence the attempts to contrive machines for perpetual motion--that is, machines for the _production_ of force. This idea is now wholly renounced by all well-informed men as utterly impossible in the nature of things. All that human mechanism can do is to provide modes for using advantageously a force previously existing, without the possibility of either increasing or diminishing it. No existing force can be destroyed. The only changes possible are changes of direction, changes in the relation of intensity to quantity, and changes of form. The cases in which a force is apparently increased or diminished, as well as those in which it seems to disappear, are all found, on examination, to be illusive. For example, the apparent increase of a man's power by the use of a lever is really no increase at all. It is true that, by pressing upon the outer arm with his own weight, he can cause the much greater weight of the stone to rise; but then it will rise only a very little way in comparison with the distance through which his own weight descends. His own weight must, in fact, descend through a distance as much greater than that by which the stone ascends, as the weight of the stone is greater than his weight. In other words, so far as the balance of the forces is concerned, the whole amount of the _downward motion_ consists of the smaller weight descending through a greater distance, which will be equal to the whole amount of that of the larger one ascending through a smaller distance; and, to produce a preponderance, the whole amount of the downward force must be somewhat greater. Thus the lever only _gathers_ or _concentrates_ force, as it were, but does not at all increase it. It is so with all the other contrivances for managing force for the accomplishment of particular purposes. None of them, increase the force, but only alter its form and character, with a view to its better adaptation to the purpose in view. Nor can any force be ex
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