The title Machine has not hitherto been used logically. Commonly it is applied only in those cases where force or motion appears continuously or to some large extent. Many would not call the balance which we have just mentioned a machine, in consquence of the narrowness of the limits within which, its motion is confined; but force and motion are employed in it in exactly the same way as in other machines; it certainly ought there- fore to receive the same name. We may much rather say that the Engineer's measuring instruments, the theodolite, level and so on, are not machines. Here indeed mechanisms, in exactly the above de- scribed meaning, are used, and forces must be applied to these in just the way we have supposed in order that they may be used. The forces however are very small, and the mechanisms are only used at intervals, so that the name Instrument may properly be preferred for them. But the title Machine is even here not incorrect, as one may convince oneself by looking at the English giant telescope with its massive foundation and all the appliances for working it. With all these it differs in degree only, and not in kind, from a little pocket telescope. To such machines as occur naturally, also, the name is denied by many. Two blocks of stone which, like " dog- knee " levers, grip a third between them, may form kinematically the same combination as the train shown in Fig. 11; the so- called Eocking-stones, which have been weathered into existence in many places, are formed like balance beams; the Geysers of Iceland act in a way to a certain extent resembling the steam-engine, forcing the water through distinct vertical tubes formed by stalactitic deposits; from all these we cannot withhold the name of Machine. I mention these things, however, merely to show the avail- ability of the word for our purpose, for the strictly scientific mean- ing of the name employed cannot be a matter of indifference to us. It is far from my intention to urge the employment of the name in cases in which its use is of no importance. But the examples just given come as well within our definition of a machine as within the above demonstration of its general nature. They show also that, in spite of the non-employment of the name, it is yet perfectly correct in the circumstances we have supposed; it serves, that is to say, to indicate that they jointly possess the characteristics summed up in our definition.
We have already seen how a mechanism becomes a machine.
