Page:The Kinematics of Machinery.djvu/533

 DRIVER AND WORK-PIECE. 511

and does not promise to be of much practical assistance in the work of the machine designer. But in every possible case, however complex, it is of the greatest importance to be able to lay hold clearly of the general underlying principle, and thus to recognise the cause of the non-success of certain combinations which have rested on a defiance of this principle, a destruction of the closure of the chain. The natural wish remains, however, to go beyond this general principle, and to determine at least the more important lines along which the development and application of the principle take place. The three old subdivisions have certainly been of some assistance in this respect, and it was in no way our intention in criticising them to oppose such a desire. But before we could meet it, it was necessary thoroughly to clear the ground, and obtain a rigid, logical basis upon which we could rest when we turned to more detailed matters. Now, however, that we have attained such a position, we may proceed to examine the distin- guishing characteristics of certain parts, or groups of parts, which seem to serve very definite functions, within the machine itself.

Our investigation has shown, in the first place, that two parts appear distinctly as forming portions of the great majority of machines, which hitherto have been generally considered to be external to them the driver and the work-piece. In the steam- engine we recognise the former at once in the driving column of steam ; the latter is less distinct, it may sometimes be the fly-wheel shaft, sometimes a toothed- wheel upon it, sometimes a belt. With the lathe the case is reversed, the work-piece we see directly, the driver is not so obvious. In general the driver is most easily recognised in the prime-mover, the work-piece in the direct-actor. This shows itself very distinctly in the names of the different machines, as steam-engine, gas-engine, water-wheel, &c., among the prime-movers, paper-machine, rivet-machine, &c. among the direct-actors. Thus in name, at least, if not in theory, driver and work-piece have been considered as parts of the machine.

We must also consider that mechanism in the machine which connects the required change of place or form, or both, in the driver with the similar changes in the work-piece, as forming one of its essential parts or groups of parts. We distinguish for instance the piston-engine from the steam re-action-wheel, the bucket-wheel from the turbine, the shingling-hammer