1733. Operative instruments are used in labours of a comparatively light kind. They may be used in general with one hand, and commonly bring into action but a part of the muscular system; the scythe, however, is an exception. They are similarly constructed to tools, and act on the same principles, differing from those only in being generally reducible to levers of a third kind, or those in which the power or hand is between the weight, or matter to be cut or separated, and the fulcrum or arm, as in cutting off a shoot with a knife. But in clipping, the fulcrum is between the hand and the weight or object to be clipped off, and therefore shears act as wedges moved by levers of the second kind. The materials of installments are in general the same as tools, but the handles of knives are usually made of horn, bone, ivory, or a species of sea-weed, instead of wood, and the greatest attention is requisite as to the iron and steel of the blades.
