210 NATURE'S TEACHINGS. 



and, although the fury of the tempest has been such that the 

 waves have washed completely over its summit, it stands as 

 firmly as it did when it was finished in 1760. 



WHETHER the original inventor of the " dovetail " took his 

 idea from Nature I cannot say, but he certainly might have 

 done so. On the left of the illustration is part of a human 

 skull. 



The skull is not, as many persons seem to think, made of a 

 single bone, but it is composed of many bones, united by 

 " sutures," which are, in fact, natural dovetails. Although in 

 early life these sutures are comparatively loose, they hold the 

 various parts together so firmly, that if the head be violently 

 struck, the bones may break, but the sutures do not give 

 way. 



Perhaps some of my readers may ask how it is possible to 

 take a skull to pieces without cutting it or fracturing the 

 su ures. It is done in a way equally simple and ingenious. 

 The skull is filled through the opening with dried peas, and 

 then sunk under water. The peas expand with the moisture, 

 and, as they exert an equable force in all directions, they 

 slowly and quietly pull the sutures asunder, without injuring 

 the bones. 



THE DAM. 



IN many human operations, where a certain depth of water 

 is required in a running stream, the reasoning powers of man 

 have enabled him to attain his object by building a dam, or 

 obstacle across the stream, which forces the water to rise to its 

 level before it can find a passage. Such, for example, are the 

 Locks which render rivers navigable, and allow even the 

 heavily laden barges to traverse miles of water which would 

 otherwise have been closed to them. 



Those mills, again, which are worked by water need that a 

 sufficient amount of water should be ready in order that it may 

 by its weight force the wheel round. Such a Dam is shown on 

 the right hand of the illustration, the height to which it raises 

 the water being shown by the level of the stream below the 

 Dam, and that of the water as it tumbles over in a miniature 

 cascade. 



