THE WING OF A BIRD. 79 



point, — its extreme lightness. Here are strong bones, power- 

 ful muscles, stiff, long quills, a wing of large extent made to 

 bear up a heavy bird and to propel him faster than a railroad 

 train can travel ; yet the whole machine weighs but a few ounces 

 even in a bird of the largest size. We have noticed how a 

 membrane stretched out in front and a band of feathers thrown 

 out behind, without any heavy frame to support them, save 

 weight; but the same economy is even more apparent when 

 we observe that the wing-bones themselves are hollow. 



The general shape of the wing is such as to beat down the 

 air with a firm, clean stroke, for which it is concave below to 

 hold the air on the downward, convex above to shed it on the 

 upward beat. We must not conceive of the air as having no 

 weight and no resistance ; en the contrary, every time the wing 

 rises it has to lift all the air above it, and special provisions 

 are made, not only in the shape but also in the structure of the 

 wing, to relieve it of as much weight as possible. The second- 

 aries are movable, and lie between little ridges across the larger 

 fore-arm bone, in which they turn, like oars in rowlocks, edge 

 up on the upward stroke, face down on the descending beat. 



It is generally believed that the hollow bones of a bird help 

 it to rise in the air, and that all a bird's bones are hollow. 

 This is a gross error, as you will see if you examine the leg- 

 bones of a chicken or a duck. Indeed, I do not now remember 

 any bird of this country which has not marrow in its leg-bones. 

 On the other hand, the leg-bones of the ostrich, which cannot 

 fly at all, contain more air-cells than those of our strongest 

 flyers. While nearly all a bird's bones are full of air-cells, 

 none but the wing-bones are hollow, and even these in some 

 of our strongest fliers are solid. The swifts, for example, 

 have long, slender, solid wing-bones. The swallows have 

 only the upper arm-bone hollow. Hollow bones, therefore, 



