ONE:It may be remarked as a general rule that the hardness of cutting tools is "inverse as the hardness of the material to be cut," which seems anomalous, and no doubt is so, if nothing but the cutting properties of edges is considered; but all cutting edges are subjected to transverse strain, and the amount of this strain is generally as the hardness of the material acted upon; hence the degree of temper has of necessity to be such as to guard against breaking the edges. Tools for cutting wood, for example, can be much harder than for cutting iron, or to state it better, tools for cutting wood are harder than those usually employed for cutting iron; for if iron tools were always as carefully formed and as carefully used as those employed in cutting wood, they could be equally hard.
