VII 



FLIGHT 255 



pair that lower the wings sometimes weighing one 

 fifth of the whole weight of the body), and the entrails. 

 High up, just under the backbone, come the lungs 

 with their spacious air-sacks. This arrangement is no 

 doubt advantageous. Imagine a flying-machine with 

 wings springing from a point many yards above the 

 engine which supplied the motive power. It would 

 have a constant tendency to right itself if it capsized. 

 In the same way the bird is helped in balancing by 

 the fact that his centre of gravity is low down, but to 

 a much less extent, since the point lies only a little 

 below the wings when expanded horizontally. The 

 lower the weight lies, the greater the space through 

 which it must be raised before a capsize can take 

 place. And owing to the way the wings work, it must 

 lie, as I hope to show soon, mainly behind the shoulders. 

 But the power of recovering balance at any moment 

 by making the appropriate movements is quite as 

 important as the exact position of the centre of 

 gravity. A bicyclist never ceases to make the neces- 

 sary adjustments, though he may be unconscious of 

 the fact. And if a lark be carefully watched from 

 below as he rises he can be seen to be perpetually 

 moving his tail to left or right, thus maintaining his 

 balance and at the same time keeping his head to the 

 wind. If there is a dead calm, he trusts more to 

 movements of his head. Moreover, a bird uses, I 

 believe, his power of bending to right or left at the 

 waist and so shifting his centre of gravity. To recover 

 equilibrium, he might give a harder .stroke with one 

 wing or the other, but it is not certain that the wings 

 ever beat unequally. When at the end of the stroke 



