F. W. HE ABLE Y : WIND AND FLIGHT. 117 



The principle is the same when the wing- beats the air, 

 and when the bird gliding- with outspread wings inclines 

 his body slig-htly upward, so that the air may act on his 

 whole outspread surface. The accompanying photograph 

 (Fig. 1) shows the position of the wing at important phases 

 of the stroke. 



Gliding. 



Gliding introduces us to another wonderful fact in the 

 action of air in motion. We can understand this best by 

 considering what hapj)ens when a boat is tacking, sailing- 

 close to the wind, so that the air impinges on the sail at 

 an acute angle. It strikes against the foremost part of 

 the sail, and, being- prevented from continuing in its 

 natural coiu-se, rushes along the sail — as shown by the 

 arrows in the diagram — and thus, if the sail is a broad 



w 



w 



Fig. 2. — Diagram illustrating Gliding. s l, sail ; w, wind. 



one, its front margin does all the work. To apply this to 

 gliding flight — it is the front margins of the wings that 

 support the bird when he holds them outstretched, inclined 

 only slightly to the horizontal. Consequently, the greater 

 the extent of the front margin, the better able will he be 

 to maintain his level or even to ascend. If he wishes to 

 descend as he glides, he will partly flex his wings. 



