Page:The Kinematics of Machinery.djvu/249

 FORCE-CLOSURE IN PRIMITIVE MACHINES. 227

himself and his fellows. The idea of making the forces of nature do his work was far beyond him. He was contented, happy, if by any exertions he could even in scanty measure carry out his intentions. Force-closure was his most ready' auxiliary in effecting the desired action among the bodies which he had chosen for his purpose.

The twiiiing-stick of the fire-drill, the most early form of what we know as a " turning-pair," was not only force-closed by pressure in the direction of its length but also in all transverse directions, the hands set it in motion by force-closure, the piece of wood beneath it was kept stationary by force-closure. The introduction of the cord and the upper bearing-piece for the twirling-stick (Fig. 161) marked a great machinal advance, for by the addition of two new kinematic elements it enabled the whole muscular force of the worker to be applied to the pulling backwards and forwards of the cord, while formerly his hands had to be pressed together as well as moved to and fro. The cord itself, however, is again a force-closed element, and is kinematically paired with the stick by force-closure.

If we trace this twirling mechanism onwards from the boring- tool of the lake-villagers, and of Homer's ship-carpenters, who used it almost unchanged, to the ancient lathe, we see that it has made notable progress. The double force-closure of the bearings of the revolving-piece has been superseded by pair-closure, by the addition of the second head-stock. The motion of the driving- cord has been greatly improved. Not only is the upper end of the cord made to move in a definite manner by the spring-beam, and the lower end by the treadle, (for which purpose two additional links have been added to the kinematic chain) but the backward motion of both is effected by energy stored in the spring. The workman has thus to produce directly only the one motion, for- wards. The machine being arranged so that this can be done by the foot, the hands are at liberty to hold and direct the turning-tool. But again the new element, the spring, is force-closed, and the action of the foot upon the treadle, whether driving it downwards or driven by it upwards, is force-closed also. The Kalmuck lathe already mentioned may be regarded as one of the too little known intermediate steps between the Homeric drill and the Italian lathe ; although it may here be difficult to estimate separately the

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