8 THE FLIGHT OF BIRDS 
at right angles to theurface he presents to it. 
And this force is broken up into two, the one 
supporting him, the other tending to drive him 
backward, or, as it is now briefly and clearly put, 
the force is resolved into lift and drift. 
Fia,. 4. 
Boat tacking.—w, the wind which acts at right angles to sr 
(the sail), towards x. The foree is broken up into two forces, 
acting towards D and L. 
A paper kite supplies us with another and perhaps 
still more apt illustration. If the air is still, the 
kite-flyer may supply a force by running with the 
string; if there is a wind, he has merely to hold 
the string. Let us imagine that the wind is blowing 
horizontally. It will, nevertheless, lift the kite, as 
long as the string is held firmly. It strikes against 
the oblique surface which the kite presents, and 
