136 THE FLIGHT OF BIRDS 
but with a difference, to a simple downward glide in 
still air. In the downwWrd 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 
R M 
Fia. 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 
