. The structure and life of birds . ock. This isas much as to say that it is the aim of the oarsmanto displace as much water as possible. It is only,however, by making the displacement of water apreliminary object, that he gains a fulcrum by whichto move his boat. We must now consider the working of both wingsat once. In order to understand this we may imaginea boat rowed by oars employed as levers of the sameorder as a birds wings. The rowlocks would be inthe middle of the boat, and the oarsman would sit oneither side holding the oars between rowlock andblade. They would have to face the bows
. The structure and life of birds . ock. This isas much as to say that it is the aim of the oarsmanto displace as much water as possible. It is only,however, by making the displacement of water apreliminary object, that he gains a fulcrum by whichto move his boat. We must now consider the working of both wingsat once. In order to understand this we may imaginea boat rowed by oars employed as levers of the sameorder as a birds wings. The rowlocks would be inthe middle of the boat, and the oarsman would sit oneither side holding the oars between rowlock andblade. They would have to face the bows, and this, N 2 180 THE STRUCTURE AND LIFE OF BIRDS chap. perhaps, would be the only advantage. But it is quitepossible to propel a boat in this way, and such asystem of rowing would illustrate what takes place inflight. True, the blades of a birds oars facedifferently, so that, while they propel him, they atthe same time raise or maintain him in the air. Butthe system of leverage is the same. This diagram isa further Fig. 47. A* and Z are rigid rods representing the birds wings hinged at Ito V Vthe birdsbody ; a b and c b are the muscles which lower the wings. The shortening of a band c b will cause Y 3 to rise, since the air resists the descent of X and Z. AfterAlix, Appareil Locoiiioteur des Oiscaicx. Horizontal Flight. Why does a bird advance horizontally when itworks its wings up and down ? The common metaphorwhich makes them oars is picturesque, but may be, asI have shown, misleading. Vergil, who, in describingDaedalus wings, uses the expression Remigiumalarum, doubtless never intended to commit himselfto any theory of flight. If in a scientific work, wings VII FLIGHT arc spoken of as oars, it must be borne in mind thatthey arc oars of a peculiar kind. The French arcfond of the terms vol rame and vol a voiles,which have the merit of neatly distinguishing ordinaryfrom sailing flight. Wings work by movement up and down. If theyfaced as the blades of an oar fa
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