STABILITY 35 



And so, if he wishes to maintain his level or rise, 

 he must extend his wings to the utmost; if, on 

 the other hand, he wishes to descend rapidly, a 

 partial flexing is the measure to adopt. To the 

 aviator such a method is only to a very limited 



Fig. 13. 

 Swift, gliding with wings fully expanded. 



Fig. 14. 

 House-Martin, gliding with wings partly flexed. 



extent possible ; he cannot half-flex his planes. 

 Nevertheless, he is able to slide down a steep incline, 

 much as a bird does. By means of his fore-and- 

 aft balancing apparatus, he sets his aeroplane 

 at the proper angle and shuts off steam. If he is 



D 2 



