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 
Fie. 13. 
Fia. 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 
DsZ 
