The examples that we have chosen amongst Boureau's appliances for craftsmen will, we believe, be sufficient to explain the principles of their construction. These consist in studying the movements which are normally carried out by the passive hand (usually the left hand, but the right in left-handed people) and to devise an appliance accordingly, the sound hand always becoming the active hand.
Fig. 172.—Leather-cutter's hand.
We could have described many more examples, but we shall only say a few words about the mechanic's hand, which is simply an adjustable spanner which can be automatically closed, terminating in toothed pliers to hold circular objects without the necessity for being screwed up. As a matter of fact, in all the work of a mechanic (sawing, filing, drilling, tightening screws, hammering, forging, and grinding) the left hand is only used for picking up and steadying the article to be manipulated. M. Boureau rightly considers that it is better to entrust this rôle to the artificial hand rather than to contrive to make the latter capable of sawing or of filing by means of the devices which we shall describe further on ([p. 121] and following), ingenious and interesting though these may be.
From these appliances, adapted to certain particular grips, others have been devised for chair caning, soldering, and for enabling factory hands to work starting levers and brakes.
Thus each case must be studied separately and the workman furnished with one or several appliances according to his needs, making the necessary modifications from the existing patterns.
Several of these appliances are attached by a ball-and-socket joint like that described for the leather cutter: this is an intermediate form between the fixed appliance and the jointed appliances which will be described later.
Boureau recommends that the length of the forearm should be such that the artificial appliance reaches only as far as the level of the sound wrist. The work will then gain in precision. We believe that this principle holds good even for the true artificial hand, which should be made 3 to 4 centimetres shorter than the sound hand. But it must be realised that we shall be met with a difficulty, which we have already experienced. Comments are made upon the appearance of the arm and the wearer may sometimes be made to believe that this is due to faulty construction.
For certain special crafts the subject may be studied from another standpoint and an actual tool constructed which carries out the necessary actions like a machine worked by the forearm, so that in these special cases the artificial hand is the active hand.