Page:The Kinematics of Machinery.djvu/531

 MEN AND ANIMALS AS DIRECT-ACTOBS. 509

machines, as it continually is when that power acts the part of a prime-mover, it is not consistent to leave unnoticed the share taken by the same agency in modifying the work produced by the machine, in taking, that is, the part of a direct-actor. This subject is one to which we have been brought by a method of treatment differing entirely from the old one, under which it found no place. Here I can only enter into it so far as is neces- sary to enable us to come to some definite conclusion as to the completeness of those machines in which human agency is em- ployed in the handling of the work-piece.

In some machines the co-operation of the hand of the worker in the operations to be performed is, from their nature, essential. In the spinning-wheel, for instance, which is one of these, the

FIG. 359.

spinner has herself to regulate and carry out an important part of the form-change which the fibres undergo. Her hand becomes in this way an organ of the machine, in which it forms part of a very complex chaining controlled by the worker's will. A process takes place here, therefore, which corresponds entirely to that described above as occurring, for instance, in the grindstone. The spinning-wheel is driven by the worker's foot also, so that human agency has a twofold action in it. The grinder in Fig. 356 is doubly connected to the machine at which he works in the same way and for the same purposes.

With the sewing-machine the case is precisely similar. In some machines the one hand of the worker drives the mechanism while the other guides the work, in others both hands, often acting in a very complex manner, are required for the latter purpose.