Page:The Kinematics of Machinery.djvu/532

 510 KINEMATICS OF MACHINERY.

The needle-grinder works at his stone as a part of a machine to which he does not himself give motion. He holds the needles between his fingers and thumb, moving them to and fro, and making them roll on his fingers at the same time in such a way that each needle receives its conoidal point as the envelope to its motion relatively to the grindstone. In modern factories this machinal work of the grinder is to a great extect done away with. The required motion of the needles is obtained by the use of a special mechanism and by giving a special form to the grindstone itself. For some purposes, too, sewing-machines are made entirely self- acting, being driven by power and having their work mechanically guided, and after long years of study the spinner has found her representative in the spinning-machine. None the less, however, must we regard the grindstone, sewing-machine and spinning-wheel as in themselves complete machines. It is possible to do definite work in all three without the direct intervention of man. The grindstone can polish pieces of cylinders, the sewing-machine can stitch straight strips of material, the spinning-wheel can twist and wind up the loose fibres presented to it. The man adds his own action as that of a machine controlled by will to that of the given mechanism ; the living and the lifeless direct-actors together produce, necessarily, a far greater variety of work than was possible for the latter alone.

135.

The Principal Subdivisions of Complete Machines. Descriptive Analysis.

Our examination in the preceding sections of the receptor, communicator and tool has shown us that it is no longer possible to regard these as representing the parts into which complete machines commonly divide themselves. We have found that each of them is absent from some series of cases, so that neither forms a general characteristic of the machine ; and in the end our investigations carried us back once more to the closed kinematic chain, which alone we found to belong to every machine. It cannot be denied that when we stand before the machine itself this abstract idea seems bare and unsatisfactory,