10 THE FLIGHT OF BIRDS 
a direction at right aff€les to the plane, and the 
force so acting is, of course, resolved into two, one 
tending to raise the plane, the other resisting its 
horizontal progress. The question, then, which we 
wish to decide is: In what proportion is the force 
of the wind divided between the two components, 
between lift and drift? Now, supposing that BD 
(fig. 6) represents the plane set with an upward incline 
and driven horizontally through the air, it can be 
shown that line p c represents the resistance of the 
ot mamta 
Fia. 6. 
Cc 
air to its onward progress and B C, a much longer 
line, the support given by the air. In fact, when 
the plane is inclined but slightly upward the support 
it gets from the air is far greater than the resistance, 
a fact that can be proved by experiment. The 
mathematical proof that D c represents the drift 
(or resistance), and B C the lift, I give in fig. 7. 
It is now apparent that, as the angle of inclination 
to the horizon is more and more reduced, the pro- 
portion of lift to drift becomes greater and greater. 
Why not, then, reduce the angle till the resistance 
of the air to horizontal progress becomes a_negli- 
gible quantity ? But obviously there is a limit 
to the process. If the plane has so slight an incline 
that it is almost horizontal, the air will offer but 
little resistance, and however big a proportion of 
this we may allot to lift and however small a one 
