NATURAL THEOLOGY. 145 



B. How perfectly is the object accomplished by 

 this admirable contrivance ! A millwright would be 

 puzzled to say how two surfaces could be made to 

 grind continually upon one another without requiring 

 to be picked ; always wearing away — and yet for 

 years preserving nearly the same projecting elevation 

 upon the surface. We here see the solution of this 

 mechanical problem. 



A. Another remarkable structure appears in the 

 other teeth, viz. the cutting teeth, for the bene- 

 fit of certain animals. It is seen in the gnawing 

 animals, such as the squirrel, mouse, rat, &c. As 

 these animals have occasion for sharp front teeth, 

 there is, first, a curious provision for this purpose. 

 The enamel is only upon the forward part of their 

 gnawing teeth. The back part is of a softer kind of 

 bone. The effect is, that the back naturally wears 

 down and leaves the enamel in the form of a sharp 

 edge. The front teeth of these animals resemble a 

 bevelled tool, or a chisel, and it is owing to this happy 

 construction. But they must necessarily wear away 

 very fast, and wonderful as it may seem, it is said 

 they run back nearly the whole length of the jaw, 

 under the roots of the other teeth, in sockets pro- 

 vided for that purpose, from which they shoot up, as 

 the edges wear down. It is exactly the same con- 

 trivance as the pencil cases in which the pencil is 

 made to screw out, as the pencil is cut away. 



B. What inexhaustible varieties of creative kind- 

 ness and skill ! I suppose, if it were not the nature 



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