THE FLIGHT OF BIRDS 303 



simply raised the body in the air, and this would be the case if 

 they merely moved up and down as though there were hinge- 

 joints at the shoulders. But the joints in question allow of a 

 certain amount of rotation, which actually takes place during 

 onward flight. When the down -stroke is made the wing is 

 brought forwards as well as downwards, its under side being 

 inclined so as to face both downwards and backwards. And as 

 a result of this the push of the wing, instead of being entirely 

 expended in raising the bird and preventing it from falling, is 

 only partly employed for this purpose, the rest of it acting back- 

 ward, and therefore propelling the body onwards. And the angle 

 of the wings can be so adjusted that the part of the push acting 

 directly downwards is only just sufficient to maintain the height 

 above the ground already attained, in which case the bird pro- 

 gresses in a horizontal direction, and does not continue to rise. 

 Under these circumstances the hinder part of the body is raised 

 by appropriate muscles when the down-stroke is given, which 

 furthers the end in view. 



During the up-stroke, which is the more rapid of the two, 

 the wing is partly bent, and moved upwards and backwards, 

 the under surface being now directed downwards and forwards. 

 Some of the details of wing-movement may be gathered by 

 reference to fig. 836, taken from serial photographs of flying 

 birds. 



A bird, such as a Lark, rises steeply into the air, not by adjust- 

 ing the wing horizontally, as might be expected, but by inclining 

 its body strongly, which increases the resistance to horizontal 

 movement, and therefore favours upward progress. 



The fact that the wings are not held horizontally but inclined 

 during ordinary flight of course diminishes their efficiency when 

 regarded as organs for preventing the body from falling. If the 

 bird remained in one place this would be a serious matter, but it 

 is in reality of little practical consequence, for it has been shown 

 experimentally that the support given by the air increases very 

 rapidly as speed is got up. Besides which, air-currents may help 

 both in supporting the body and in furthering onward movement. 



Steering and Stopping. This is undoubtedly effected in various 

 ways. The tail is an adjustable rudder, the body can be placed 

 at various angles, and the wings are not of necessity moved 

 together in precisely the same manner. 



