488 KINEMATICS OF MACHINERY.
similar ternary divisions in other regions of investigation. In many cases too it corresponds distinctly with the natural divisions of par- ticular machines. It is necessary nevertheless, or rather I should say for this very reason, to submit these apparently fundamental conceptions to the test of the strictest possible investigation.
If we examine more in detail the usual treatment of the subject, we find that those parts of the machine most closely connected with the receptor, for the best possible utilisation of the driving force, are very generally considered by themselves in groups, forming what is known as prime-movers, steam-engines, water-wheels, turbines, and so on. By a very similar limitation the group of working parts, or tool, has been taken to be more or less exactly co-extensive with a large class of machines, each one adapted for doing certain particular operations, as spinning, weaving, and printing-machines, machine-tools, &c. Collectively we may call them all tools, by a justifiable use of the word in its most general sense- But if we look more closely at these familiar extensions of the original idea, we shall find that we are treading on very uncertain ground. For if every complete machine must have some motor in the sense used above, must be driven, that is, by steam, air, water, gas, &c., then evidently the lathe and planing machine cannot be complete. Scarcely any of the machines in a factory, indeed, could be considered as complete, none of them would be more than por- tions of complete machines. The common usage therefore, in which they all receive the name machine, must be entirely inaccurate, for it is quite inconsistent with the technical definition of what the machine really is.
Exactly the same difficulty occurs in the case of prime-movers. A very large number of these, considered by themselves, do not possess any part which can be called a tool, for doing work upon other bodies. They also, therefore, cannot be complete machines. Those engineers who devote themselves to the manufacture of steam-engines, turbines, and so on, are makers not of complete machines but only of parts of them. The only really complete steam-engines must be such machines as steam-hammers or crushers, direct-acting rolling-mill engines and so on. All others, no matter how excellent in design or construction, are incomplete machines in themselves and become complete only when combined with other apparatus.
