CONTRIVANCE A NECESSITY. 149 



it, the question ought rather to be, How is it ever 

 prevented ? The soaring is a much more difficult 

 thing to do than the going onwards. It cannot be 

 done at all in a perfectly still atmosphere. It can 

 only be done when there is a breeze of sufficient 

 strength. Gravity is ceaselessly acting on the 

 bird to pull it downwards : and downwards it must 

 go, unless there is a countervailing Force to keep it 

 up. This force is the force of the breeze striking 

 against the vanes of the wings. But in order to 

 bring these two forces to nearly a perfect balance, 

 and so to "soar," the bird must expand or contract 

 its wings exactly to the right size, and hold them 

 exactly at the right angle. The slightest altera- 

 tion in either of these adjustments produces 

 instantly an upsetting of the balance, and of 

 course a resulting motion. The exact direc- 

 tion of that motion will depend on the degree 

 in which the wing is contracted, and the degree 

 in which its angle to the wind is changed. 

 If the wing is very much contracted, and at the 

 same time held off from the wind, that motion will 

 be steeply downwards. Accordingly this is the 

 action of a Hawk when it swoops upon its prey 



