given. Three classes, however, of such tools may be marked out, and into one or other of these it is probable all those tools which can properly be defined as tools with cutting-edges may be arranged.
A first class will comprehend tools which meeting the work at a particular angle continue the path of each portion of the edge in the same straight line. Axes, adzes, gouges, chisels, and planes (as ordinarily used by carpenters), belong to this class. Such tools are brought into action either by impact or by direct thrust. The adaptation of machinery to tools in this class is easy, because the cutting-edge has to describe only a straight line, and this done once, if the place of application be removed, a repetition of impact or thrust in the same direction will suffice.
Fig. 1.—Adze of Flint.
A second class will comprehend tools which, while as a rule retaining the angle at which they are applied to the work, the path of any portion of the tool is not a straight but a curved line. Tools of Class 2 are seldom acted upon by direct impact, or simple thrust. To adapt them to machine-work requires either a compound motion in the tool, or a motion compounded of the tool and work. When used as handicraft tools, this compound motion is derived from the muscular actions of the body of the workman, or the mechanical contrivances of construction in the tool. Knives, shears, razors, and saws, belong to this class. And to this class belong those tools in which what are mechanical contrivances for causing a "draw cut" are introduced, e. g., certain garden and pruning shears, also, hay and bread cutting knives. There is a motion in the human jaws which gives to the cutting teeth this "draw cut," and so they separate what is between them as draw-cut scissors might do. Indeed, all tools in this class operate most efficiently when acting upon the "draw-cut" system.
Hence, while certain of the human teeth belong to Class 1, others belong to Class 2. The contrivance in the jointing of the lower jaw to the upper in man is a compound one, adapting itself to three motions, one or other of which is found in many tools. There is up--
