10 THE FLIGHT OF BIRDS 



a direction at right angles 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 b d 

 (fig. 6) represents the plane set with an upward incline 

 and driven horizontally through the air, it can be 

 shown that line d c represents the resistance of the 



Fig. 6. 



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 



