. The structure and life of birds . e use of a great deal in sailing. A sail SL isslung obliquely across the boat, and the wind fallingupon it tends to move the boat in the direction XY,at right angles to the sail. In the same way when astone, thrown slantingwise at a window by some onestanding some way off, but near the wall of the house,breaks a pane, the fragments of glass are knocked intothe room in a direction at right angles to the return to the boat, the force XY can be broken up FLIGHT 183 into two forces, one represented by XM, the other byXR. But the force XR will have litt
. The structure and life of birds . e use of a great deal in sailing. A sail SL isslung obliquely across the boat, and the wind fallingupon it tends to move the boat in the direction XY,at right angles to the sail. In the same way when astone, thrown slantingwise at a window by some onestanding some way off, but near the wall of the house,breaks a pane, the fragments of glass are knocked intothe room in a direction at right angles to the return to the boat, the force XY can be broken up FLIGHT 183 into two forces, one represented by XM, the other byXR. But the force XR will have little effect since thewater offers great resistance to the movement of aboat sideways through it ; at any rate if it has a keel ora centreboard. The force XM will cause the boat tomove in the direction she is meant to go. This is anexcellent illustration of what happens in the flight ofa bird. When the wing is descending its frontmargin is lower than its hind margin ; it is turned sothat the long feathers slope upwards. For simplicity. x Y Fig of wing ; X, front margin ; Y, hind margin ; W = wind caused by the down stroke ; BA, resultant force, which is resolved into BC, BD. we must imagine that the air is quite still. The wingdescends with great velocity through it, making itsown wind like a bicyclist. The resistance of theair will be equal to a wind blowing vertically upward(W in Fig. 50). When it strikes the inclined surfaceof the wing its force will act in a direction at rightangles to it (BA). The force BA may be dividedinto two forces, BD, which supports the bird, BC,which drives him onward. Whether Professor Fettigrews description of thewings movement agrees with the account I have 184 THE STRUCTURE AND LIFE OF BIRDS chap. given, is difficult to say. He speaks of it as workinglike the screw of a steamer. If this is taken to meanthat the surface of the wing during the down strokehas much the same incline as the blade of a screw, thedescription is no doubt correct
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