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 consequence, 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. Bearing in mind, then, that in a rod or pole the centre is the part which most requires to be strengthened, we can see, in cases too numerous to mention, how art has followed, though perhaps unconsciously, in the footsteps of nature. Take, for example, the beam of a steam-engine, such as is given in the sketch, and for which the great engine at Chatham acted as model. The reader will observe that in this case the beam is gradually thickened towards the centre, the ends, where the strain is slightest, being comparatively small. Another point also must be noticed. Equal strength could have been obtained had the beam been solid, but at the expense of weight, and consequent waste of power. Lightness is therefore combined with strength by making the beam consist of a comparatively slight centre, but having four bold ridges, as shown in the section given in the accompanying illustration. This plan, as the reader will see, is exactly the same as that which is adopted in the iron mast and porcupine quill, except that the ridges are external instead of internal. The same mode of construction is employed in ordinary cranes, the principal beam of which is almost identical in form with that of the engine, both being thickest in the centre, and both strengthened with external ridges. There are also other analogies between the hollow mast and natural objects. Keeping still to the animal world, we find the quill feathers of the flying birds to supply examples of the combination of great strength with great lightness and very little expenditure of material. Their wing bones, too, are hollow, communicating with the lungs, and are consequently light as well as strong.