vil 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 
