foil when undue pressure is brought to bear upon it. Moreover, the
operation of sharpening would be extremely difficult.

So the blade of the chisel is merely faced with a thin plate of hardened
steel, the remainder being of softer material.

Now, it is not at all likely that the unknown inventor of the modern
chisel was aware of the analogy between art and nature, and would
probably have been very much surprised if anyone had stated that he had
borrowed his idea from the incisor teeth of the water-rat.

Yet he might have done so, for these teeth are almost wholly formed of
ordinary tooth matter, and are faced with a thin plate of hard enamel,
which exactly corresponds with the hardened steel facing of a chisel.

Any of my readers who possess skates will find, on examination, that the
greater part of the blade is, in reality, soft iron, the steel, which
comes upon the ice, being scarcely a fifth of an inch in length. The
hardened steel allows the blade to take the necessary edge, while the
soft iron preserves the steel from snapping.

Should the skate have been neglected and allowed to become a little
rusty, the line of demarcation between the steel and the iron can be
distinctly seen. Similarly, in the beaver and the water-rat, the
orange-yellow colour of the enamel facing causes it to be easily
distinguished from the rest of the tooth. In most of the rodents the
enamel is white, and the line of demarcation is scarcely visible.

Now we have to treat of a question of mechanics.