1 84 THE STRUCTURE AND LIFE OF BIRDS chap. 



given, is difficult to say. He speaks of it as working 

 like the screw of a steamer. If this is taken to mean 

 that the surface of the wing during the down stroke 

 has much the same incline as the blade of a screw, the 

 description is no doubt correct : the opposite wing will 

 correspond to a blade of a screw revolving in the 

 opposite direction : both tend to drive the bird 

 forward and, at the same time, lift it, as the descending 

 blades of the screw of a steamer propel and lift the 

 ship. But it is impossible to go beyond this and 

 compare such fractional rotations as the wing is 

 capable of to the complete revolutions of a screw : 

 during the up stroke the wing is certainly not a screw- 

 blade propelling the bird in a forward direction. 



In horizontal flight the hind part of the body is 

 raised duri-ng each down stroke by muscular effort. 1 



The Bird in Motion — Support Given by the A ir. 



If a bird flies at a great pace he derives far more 

 support from the air than if he flies slowly. He is 

 perpetually coming to fresh columns of air, each 

 series of which is able to sustain his weight for a 

 moment. If a gull be tied to a string, it is said that 

 he cannot support himself when he comes to the end 

 of his tether, and though some other birds are not 

 under these circumstances reduced to complete help- 

 lessness, the downfall of the gull is a significant fact : 

 since he has no onward movement, the air gives his 

 body little or no support, and, besides this, the air 

 which his wings are beating has rolled in to fill the 

 1 See p. 257. 



