184 THE STRUCTURE AND LIFE OF BIRDS cuap. 
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 during each down stroke by muscular effort.! 
The Bird in Motion—Support Given by the Air. 
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 toa 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 jn to fill the 
1 See p. 257. 
