The American chuck consists of a pair of metal jaws fixed at their base into a cylinder and appearing somewhat like the petals of a long corolla. Another cylinder is screwed over the first to control the opening and shutting of the jaws. When this cylinder is unscrewed the jaws open and the handle of the tool can be inserted, when it is screwed up it closes the jaws and makes them grip the handle. [Illustration: FIGS. 180 to 185.--_The American Chuck._ The pincers are composed of two jaws with vertical cylindrical grooves (to fit upon a handle), joined above by a ring (Fig. 185) and coupled by a spring C which keeps them apart. The pincers fit into a piece B (Fig. 182) cut on the outer side with a screw thread (Figs. 182 and 183) upon which the piece A is screwed up or down (Figs. 180 and 181). When screwed towards the point of the pincers it presses on the two jaws and closes them. When screwed in the opposite direction the pincers open automatically.] If the jaws open widely, the wooden handle of a tool can be held, but if the opening is small the unmounted tool must be fitted into them (Figs. 186 and 187). This method is specially useful for files, as it frequently happens that several files are required for the same piece of work and they can be changed rapidly. [Illustration: FIGS. 186 and 187.--File fitted into the American chuck.] It must, however, be insisted upon that the principle of giving a passive rôle to the artificial hand is to be preferred. (_b_) _Appliances with Mobile Joints._--In the course of work the direction of the wrist is changing at every instant, flexion, extension, pronation, and supination occurring, sometimes in order to move around the object, sometimes in order to maintain a suitable position when the movements of the shoulder and elbow vary the direction of the forearm. The consequence of this is that the worker learns to turn the piece of work around with his sound hand. To get over this difficulty passive joints are inserted at the wrist, which allow the appliance to move when it is pressed against the piece of work and to take up the direction which suits the inclination of the forearm. [Illustration: FIGS. 188 to 190.--Agricultural hook and ring. (Gripouilleau.)] [Illustration: FIG. 191.--Combined hook and ring. (Boureau.)] [Illustration: FIGS. 192 to 194. 1. Tram driver's and chauffeur's bell. 2. Method of use by a tram driver. 3. Management of a motor car lever. Pressure at the extremity and traction.]