16 THE FLIGHT OF BIRDS 
hillock, his jumping-8f place, outside Berlin, he 
was cheered and emboldened by the way the wind 
would catch his glider’s well-designed concavities 
and nearly lift him from the ground. When he 
reached his little hillock he would jump from it 
and make glides of 150 yards and more. 
Lilienthal found that the air that met his glider’s 
concave surface did not act at right angles to the 
chord of the arc (fig. 9), along F, but along a 
line occupying somewhat the position of F’. 
Fie. 9. 
The arrow shows the direction in which the glider is travelling. 
Experiments made by Mr. Wilbur Wright and 
his brother have shown that Lilienthal had called 
attention to a principle on which the aviator can 
base his calculations. In short, with a properly 
rounded surface the lift is greater and the drift is 
less than with a flat one. In order to obtain the 
maximum gain, the best possible curve must be 
discovered, and this can only be done by experiment. 
The writer in Flight already quoted is of opinion 
that each velocity has its appropriate curve. An 
aeroplane that is to be a racer should, he maintains, 
have its surfaces much less curved than a slowly- 
flying one, and undoubtedly the wings of birds 
suggest that he has enunciated a true principle. 
