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. In order 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 



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