. Die Struktur und das Leben der Vögel. Der Luft, travel von seinem Kurs durch das Vorderteil gedreht worden ist. Je schneller das Boot segelt, und, auch, das näher zu thewind sie segelt, das zutreffende dieses ist; der Enge wird der Rand des Segeles sein, das wirklich arbeitet. Das nennt man das Gesetz von Avanzini. Er hält mit Rücksicht auf den Vogelflügel, der sich beim Abschlag sowohl schnell vorwärts als auch abwärts bewegt und natürlich die Weiterbewegung des ganzen Vogels teilt. Es istrichter der schnell bewegenden Extremität als der unteren inneren Teil, und dies macht die beachtlich in der langen Flügel, insbesondere t

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. The structure and life of birds . of air, which has been turned from its course by the forepart.The faster the boat sails, and, also, the nearer to thewind she sails, the truer this will be ; the narrowerwill be the margin of sail that really works. Thisis called the law of Avanzini. It holds with regardto the birds wing, which during the down stroke movesrapidly forward as well as downward, and, of course, shares the onward movement of the whole bird. It istruer of the swiftly moving extremity than of theslower inner part, and this accounts for the remarkableway in which long wings, notably those of the Swiftand Gannet, narrow towards their ends. ProfessorPettigrew made some experiments which illustratethis. He cut away the hinder part of a Bluebottleswings and apparently it could fly equally well. Inthe same way with Sparrows, the removal of the samepart of the wing seemed to do little damage. Still todescribe the flight after these mutilations as perfectis to go too far. No failing ma) strike the eye. A vn FLIGHT 191. 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 Avanzini.If a bird wishes to descend rapidly, he must partlyflex his wings, so that they may present a lessextended front and, consequently, receive less support.If 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 ^ (fig. 52). The principle just explained can be seen at workin little paper contrivances. Take a piece of papershaped thus

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• bookcentury1800
• bookdecade1890
• bookyear1895
• bücherubjektvögel