STARTING 51 
has a much quicker beat, but even with such a rate 
as his a drop between the strokes is quite possible 
unless he has plenty of way on. And now we are 
getting to the explanation of the big bird’s method 
of rising. Inorder to avoid losing altitude between 
the strokes, he must take care that he has momentum, 
and if he is to have momentum he must be content 
to ascend by a gentle incline. 
The Wing’s Freedom to Rotate. 
Moreover, there is a want of freedom about his 
wing-movements which makes him incapable of 
anything but a very gradual ascent. If he were to 
incline his body steeply upward, after the manner of 
a small woodland bird that, making for a gap in the 
dense spreading boughs overhead, mounts almost 
vertically, he would have to rotate his wings in a 
way that is impossible for him; he would have to 
lower the front margin relatively to the back, or 
else they would beat in such a way as to drive him 
backward instead of lifting him. In fact, he has 
too little freedom at the shoulder. He cannot 
set his wings as a steep ascent requires. The 
small bird’s wings, on the other hand, rotate so 
freely that even when he sets his body with a steep 
upward slant he can still turn them over so that 
they have an up-and-down beat and raise him sky- 
ward. 
But though, speaking generally, the small bird is 
capable of a steeper ascent than the big bird, yet it 
would be a great mistake to imagine that if we were 
to arrange birds according to their weights, from the 
E 2 
