Page:Popular Science Monthly Volume 4.djvu/708

688 barblets from the other side of the next barb they cross at right angles, and each hook falls directly over one of the straight barblets and fastens to it.

This is a very beautiful adaptation, but what is the use of it? Why could not the whole vane be made in one piece? It is commonly said that its purpose is the same as that accomplished by the overlapping of the feathers, to form valves which shall allow the air to pass in one direction but to resist its passage in the other. Unfortunately, the hooks are arranged in just the wrong way for this, for pressure from above tightens them, while pressure from below tends to loosen them, although they are too firmly fastened to be easily unfastened. The true use of the separate pieces seems to be to secure that all-important property, the greatest strength, by the least use of material; and it is done in precisely the way that men have employed for securing the same end.

A scantling of wood placed on its edge will support a much greater weight than one placed on its side; so, in laying a floor, instead of laying all the boards on their sides, which would not be strong enough, or placing them all on their edges, which would use too much wood, the plan is to place on edge a sufficient number, and on these to lay the floor of boards on their sides.

The method employed in the feather is still better than this: the shaft of each barb is flattened vertically, but, instead of a separate floor laid on these, the top of each rafter, or barb, is split, and these split portions are bent down and bound to each other by the hooks already spoken of, arching over the spaces between the barbs, in exactly the way that the arches of masonry span the spaces between the iron girders of a fire-proof floor.

Before we shall be prepared to understand the way the feathers act in flight, we must examine the way in which they are placed in the wing. The anterior edge of the wing is a firm rim of bone, and the quills are fastened to this rim, with the flexible end of each feather projecting backward with nothing but adjacent feathers to support it, so that the posterior border of the wing bends very easily. The feathers are of such a shape that the wing is convex on its upper surface, and concave below.