Descended from his flint knife is the abounding variety of steel cutting tools all the way from the razor, concave on both sides, to the axe, doubly convex. As the arts have become more specialized, as artificial power has been introduced, the contrasts of the form of one tool with another have grown more and more striking. The bar which slices metal is stout of build, and rectangular in section, while a lancet is little wider or thicker than a blade of grass. The knives which divide leather, rubber, and rope, differ much from one another; the knife which separates the leaves of a book serves best when dull. Gouges for carving are nicely adapted to the profiles they are to cut; while the exigencies of the power-lathe require its tools to be designed of particular strength and rigidity. Among revolving hand-tools the brace is the most important, enabling the workman to exert great leverage. A minor tool, the gimlet, was formerly more in use than to-day. Now that screws are made with gimlet points they break their own paths. [Illustration: Carving chisels and gouges.] [Illustration: Lathe cutters.] [Illustration: Ratchet bit brace.] From the beginning tool-makers have shown skill in fitting a tool to the hand, as in the Eskimo skin-scraper; this simple adaptation may have arrived in copying the effect of wear. Other good hints have come from observing an implement after its work is done. At the places where mud clung to a plowshare the plow-maker was long ago told at what points to raise his metal; conversely, when a cutter of any kind is unduly worn at any part of its side, there the metal asks to be somewhat narrowed down. [Illustration: Eskimo skin scraper.] [Illustration: Double tool drill cutting boiler plate.] [Illustration: A common drill removes a whole circle of stone.] [Illustration: A ring drill removes much less stone with the same effect.] Annular Drills.