. The structure and life of birds . aches, otherwise the happy moment is , then, is out of the question for a bird whichis to move at a speed far greater than that of any boat. How, then, is horizontal movement gained ? Tounderstand this a knowledge of the parallelogram offorces is necessary. The proof of the principle involvedwill be found in any book on elementary shall merely try to make clear what is meant. Iftwo forces act upon a body at one point, they combineto make one force. If B A and C A are two forces i82 THE STRUCTURE AND LIFE OF BIRDS chap. acting at A, t
. The structure and life of birds . aches, otherwise the happy moment is , then, is out of the question for a bird whichis to move at a speed far greater than that of any boat. How, then, is horizontal movement gained ? Tounderstand this a knowledge of the parallelogram offorces is necessary. The proof of the principle involvedwill be found in any book on elementary shall merely try to make clear what is meant. Iftwo forces act upon a body at one point, they combineto make one force. If B A and C A are two forces i82 THE STRUCTURE AND LIFE OF BIRDS chap. acting at A, the comparative length of the linesrepresenting the comparative magnitude of the twoforces, then there results a force which is exactlyrepresented both in magnitude and direction by D A,the diagonal of the parallelogram. If, now, we startwith a force represented by DA, we can do in imagi-nation, what often happens in reality, break it upinto two forces BA and CA, or other forces actingalong two other lines, which must, however, con-. Fig. 49. verge on A from opposite sides of DA. This factis made 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 i
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