Page:The Kinematics of Machinery.djvu/513

 MACHINES WITHOUT TOOLS. 491

directly does the work of lifting. But if we suppose the hook re- moved, and the rope itself tied round the body to be lifted, the machine can work precisely as before. The only difference is one of convenience in securing the load, and this obviously does not affect the question before us. The hook cannot therefore really be the tool of the crane or other hoisting machine, for the total re- moval of any essential part of a machine must necessarily render it useless. But it may be suggested in this case the loop of the rope, the improvised sling, is really the hook, differing from the former one only in material and constructive form, not in kind. Let it be so. But now suppose the load removed entirely and the rope allowed to hang by itself, it may hang to such a depth that its own weight becomes as great as that of the former load, and let the crane be set in motion to wind up the empty rope, does it not work precisely as before ? There is still a load to he lifted. The wheels, drums, shafts, pawls, cranks, all move precisely as be- fore. But neither hook nor sling exists, the only weight to be raised is that of the rope itself; the body to be lifted has become a link of the kinematic chain. The tool, in the usual meaning of the word, has completely disappeared.

Let us look at another example the locomotive. The coupling- hooks or other arrangements used for attaching the train to the engine are here usually said to form the tool. But if this be the case is it not extraordinary that such an immense number of differ- ent coupling arrangements should exist, all intended to serve the same purpose, and with any one of which any given locomotive might work ? In this case would a change of coupling-gear alter the machine as a whole ? This must certainly be the case accord- ing to the commonly received theory, for the tool is an essential characteristic part of each machine. But in order to look more thoroughly to the bottom of the matter, suppose the locomotive to be running with its tender only, or still better, suppose the case of a tank locomotive carrying also its own fuel and running entirely by itself. If it come to a steep gradient it may have to exert now exactly as much power as it would do on a level if it were drawing the train ; the coupling-hooks, however, have now absolutely no connection with its work. They certainly do not form in any sense the tool of the machine. It may be said that the latter is now in- complete because it is not carrying goods or passengers, but this