i82 THE STRUCTURE AND LIFE OF BIRDS chap. 



acting at A, the comparative length of the lines 

 representing the comparative magnitude of the two 

 forces, then there results a force which is exactly 

 represented both in magnitude and direction by D A, 

 the diagonal of the parallelogram. If, now, we start 

 with a force represented by DA, we can do in imagi- 

 nation, what often happens in reality, break it up 

 into two forces BA and CA, or other forces acting 

 along two other lines, which must, however, con- 



Fig. 49. 



verge on A from opposite sides of DA. This fact 

 is made use of a great deal in sailing. A sail SL is 

 slung obliquely across the boat, and the wind falling 

 upon it tends to move the boat in the direction XY, 

 at right angles to the sail. In the same way when a 

 stone, thrown slantingwise at a window by some one 

 standing some way off, but near the wall of the house, 

 breaks a pane, the fragments of glass are knocked into 

 the room in a direction at right angles to the window. 

 To return to the boat, the force XY can be broken up 



