16 



THE FLIGHT OF BIRDS 



hillock, his jumping-off 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'. 



Fia. 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. 



