. The structure and life of birds . ;Bk :-,s~* Kfl 1 ..• ■■■ . C: •.** 192 THE STRUCTURE AND LIFE OF BIRDS chap. man with his hands tied behind his back may appearto walk with ease, but in the course of a long tramp,it would hamper him much. In stopping-, in risingand till great velocity is attained, a broad expanse ofwing is of use. For the understanding of gliding flight also, it ismost important to bear in mind the law of a bird wishes to descend rapidly, he must partlyflex his wings, so that they may present a lessextended front and, consequently, receive less he
. The structure and life of birds . ;Bk :-,s~* Kfl 1 ..• ■■■ . C: •.** 192 THE STRUCTURE AND LIFE OF BIRDS chap. man with his hands tied behind his back may appearto walk with ease, but in the course of a long tramp,it would hamper him much. In stopping-, in risingand till great velocity is attained, a broad expanse ofwing is of use. For the understanding of gliding flight also, it ismost important to bear in mind the law of a bird wishes to descend rapidly, he must partlyflex his wings, so that they may present a lessextended front and, consequently, receive less he wishes to descend very gradually or maintainhis level or glide upward, he must open his wings totheir full stretch, so as to have the support of as longa front line as possible. The amount of work doneby the front margin and, consequently, the trajectoryof his flight will of course vary with the pace x (fig. 52). The principle just explained can be seen at workin little paper contrivances. Take a piece of papershaped thus. Fig. 53- Fold it along the line AB so that the two sides slopeupwards. Put in a pin along the line with its headnear A. Hold it on a Siant with A at the lower endand let it drop. It will glide some distance and verylikely show an upward tendency ending in a somer-1 See Newtons Dictionary of Birds, p. 265. VII FLIGHT 93 sault. Then put in the pin with its head near B anddrop it as before, but make B the lower end. It willglide by a steep descent to the ground. It is worthwhile improving the cut and balance of these little toystill they behave properly, for they admirably illustrategliding flight: when A leads, a bird with wide-spreadwings is represented ; when B, a bird with wingspartly flexed. Before closing this subject I must refer to the oldfallacy that the bird, owing to its hollow bones, islike a balloon. This has been already dealt with inthe previous chapter (see p. 105). The General Shape of the Bird. If a ship were built on absolutely perfe
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