AXES AND HATCHETS, ANCIENT AND MODERN. 187 



given. Three classes, however, of such tools may be marked out, and 

 into oue 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 ordi- 

 narily 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 retain- 

 ing 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 handi- 

 craft tools, this compound motion is derived from the muscular ac- 

 tions 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- 



