136 



THE FLIGHT OF BIRDS 



but with a difference, to a simple downward glide in 

 still air. In the downward glide, the bird or aviator 

 has to obtain a certain downward velocity by the 

 help of gravity before the air will give him support. 

 In the method of advance I have been describing no 

 downward velocity is required, since the air has an 

 upward trend and resists the pull of gravity. Some- 

 times this method of making headway without any 

 motion of the wings may be seen in mountainous 

 countries, where it is sometimes practised by 

 feathered experts capable of nobler achievements 



Fig. 26. 

 Illustrating advance, with wings held rigid, in the teeth of the 

 wind, the wind having an upward trend. B-D, the body of the 

 bird sloped slightly downward. The arrow represents the wind. 

 Its force acting along F-M is broken up into two forces 

 represented by S-M and R-M. 



even than the Gull. In Algeria I once saw two 

 Eagles sail straight ahead against the wind for about 

 a mile and a half without moving their wings till they 

 reached a high mountain ridge, blowing over which 

 the wind had got an upward trend. Having done 

 their mile and a half, they came back with the wind, 

 beating with their wings, and then repeated their 



