PORTION OF PORCUPINE QUILL. SECTION OF ENGINE BEAM. COMPLETE QUILL. IRON YARD AND YARD-ARM. BAMBOO. ENGINE BEAM. SECTION OF PORCUPINE QUILL MAGNIFIED. SECTION OF IRON MAST.] But were the quill merely a hollow tube filled with pith, it would be too weak to resist the strain to which it is often liable. Consequently it is strengthened by a number of internal ribs, composed of the same horny material as the outer coat, and arranged in exactly the same way as those of the mast. There are yet other points in the structure of the Porcupine quill which might be imitated with advantage in the mast. In the first place, the internal ribs are much more numerous 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 Porcupine 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 Porcupine 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.