250 THE STRUCTURE AND LIFE OF BIRDS cHaP. 
change of balance is effected the difficulty begins. 
We might naturally suppose that one wing beats 
harder than the other; in the turn to the left the 
right wing would work with greater energy, so as to 
throw the body on to its left side. That being so, it 
is puzzling at first, when we see the turn made, to 
find that the right wing which should give the harder 
stroke is held aloft while the left wing is lowered. 
This, however, is not an insuperable difficulty. Each 
wing makes the same angle with the body, so that a 
line connecting the tips would either pass through the 
shoulder-joints or be parallel to a line passing through 
them. It is only the rolling over of the body on to 
its side that causes them to point upward and down- 
ward respectively. This being the position in which 
we see the wings as the bird wheels, can the right 
have struck harder than the left in order to effect the 
turn? Possibly it may have. In ordinary flight, as 
we have seen, the body at once rises in response to 
the beat of the two wings together, and thus they are 
brought home to their lower limit, without the stroke 
having to be pulled through. The right wing, then, 
if it gave the stronger beat, might instantaneously 
raise the body on the right side and produce the 
desired result, the change of balance; when, very 
possibly, all we should see might be the bird sailing 
on with the right wing pointing upward and the left 
downward. Still there would be an instant at which 
the wings would form unequal angles with the body, 
and what evidence of it can we obtain? If a Gull be 
watched as he slowly wheels, there seems to be ab- 
solutely no inequality of angle; when a rapid turn 
