. The structure and life of birds . hirds as great again, a thickness ofwall only one third as great We can now seewhy small birds have so little aeration. In their case,there would be no great reduction of weight since theexterior shell of the bones forms a great part of theirbulk. In the case of a larger bird, with bones manytimes multiplied in size, but the thickness of the wallsincreased comparatively little, the removal of the 1 The girths are in the ratio of 25 : 42 ; while 3 14 representsthe ratio of the thickness of the walls, the measurements beingi§q and j4s of an inch. VI FO
. The structure and life of birds . hirds as great again, a thickness ofwall only one third as great We can now seewhy small birds have so little aeration. In their case,there would be no great reduction of weight since theexterior shell of the bones forms a great part of theirbulk. In the case of a larger bird, with bones manytimes multiplied in size, but the thickness of the wallsincreased comparatively little, the removal of the 1 The girths are in the ratio of 25 : 42 ; while 3 14 representsthe ratio of the thickness of the walls, the measurements beingi§q and j4s of an inch. VI FORM AND FUNCTION 11 marrow will be a great advantage. This will W clearif we take two cubes, a side of one of which is twicethe length of a side of the other. Then the face is four times as large and the cubiccontents eight times as large. This will be true of otherfigures besides cubes, so that if the average girth of onebone be double that of another, and if the length also bedouble, its cubic contents will be approximately eight. Fig. 2q.—Cubes. times as great; and as the walls do not thicken in pro-portion to the increased girth, nearly all the enlargedinterior can be filled with air. Clearly, then, a largebird has much more to gain by dispensing with marrowthan a small one. Thus the Eagle has gained in point of lightness. Itmust also have gained in point of strength, for in-creased length of wing means an altogether dispro-portionate increase of work. The longer the wing, I ii4 THE STRUCTURE AND LIFE OF BIRDS chap. the greater the pace at which its extremity will move ;if the velocity is doubled, it is well known that the re-sistance of the air is far more than doubled, so that anincrease of strength is required that is altogether outof proportion to the increase of length. This will bemade clearer, when we come to the subject of flight(see p. 175). The Hornbills are a puzzle. The extreme short-ness of the hand bones, a ridiculous anticlimax follow-ing upon so grand an u
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