18% THE STRUCTURE AND LIFE OF BIRDS cuHap. 
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 DA, 
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- 
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 
