26 NATURE'S TEACHINGS. 



than those of the mast, but they are very much thinner, and 

 taper away from the base, where the greatest strain exists, to the 

 end, where they come to the finest imaginable edge. This 

 modification of structure enables the outer shell of the quill to 

 be exceedingly thin and light, and, moreover, gives to the 

 whole quill an elasticity which is quite wonderful, considering 

 its weight and strength. 



Then, in the iron mast the exterior is quite smooth, whereas 

 in the Porcupine quill it is regularly indented, exactly on the 

 principle of the corrugated iron, which combines great strength 

 with great lightness. And I cannot but think that our iron 

 masts might be made both lighter and stronger if the shell 

 were thinner, the internal ribs made like those of the Porcu- 

 pine quill, and the shell corrugated instead of being quite 

 smooth. The internal cells of the quill are, of course, not 

 needed in the mast, as they are intended for nutrition, and not 

 for strength. 



BEING on this subject, we may take the shape of the Porcu- 

 pine quill, and compare it with that of the ship's yard. It 

 will be seen that the two are so exactly similar in form that 

 the outline of one would answer perfectly well for the other. 

 The only perceptible difference is, that in the ship's yard both 

 ends are alike, whereas in the Porcupine quill the end which 

 is inserted in the skin is rounded and slightly bent, while the 

 other end is sharply pointed. 



The principal point to be noticed in the form of both quill 

 and yard is, that they become thicker in the centre, that being 

 the spot on which the greatest strain comes, and which, in con- 

 sequence, needs to be stronger than any other part. While 

 holding and balancing the pole which Blondin uses to preserve 

 his balance when walking on the high rope, I was struck with 

 the fact that the pole, which is heavily weighted at each end, 

 had to be strengthened in the middle, exactly on the principle 

 of the Porcupine quill and the ship's yard. It could not, of 

 course, be thickened, as the hands could not grasp it, but it 

 had to be furnished with additional strengthening. And the 

 necessity of such strengthening is evident from the fact that 

 on one occasion the pole did break in the middle, so that any 

 one of less nerve and presence of mind must have been killed. 



