l6o THE REIGN OF LAW. 



long wings economise the vital force in an- 

 other way. When a strong current of air strikes 

 against the wings of a bird, the same sustaining 

 effect is produced as when the wing strikes against 

 the air. Consequently birds with very long wings 

 have this great advantage, that with pre-acquired 

 momentum, they can often for a long time fly 

 without flapping their wings at all. Under these 

 circumstances, a bird is sustained very much as 

 a boy's kite is sustained in the air. The string 

 which the boy holds, and by which he pulls the 

 kite downwards with a certain force, performs for 

 the kite the same offices which its own weight and 

 balance and momentum perform for the bird. The 

 great long-winged oceanic birds often appear to 

 float rather than to fly. The stronger is the gale, 

 their flight, though less rapid, is all the more easy 

 — so easy indeed as to appear buoyant ; because 

 the blasts which strike against their wings are 

 enough to sustain the bird with comparatively 

 little exertion of its own, except that of holding 



described between the violent exertion required in first rising, and 

 the perfect ease of flight after this first momentum has been ac- 

 quired, is a striking illustration of the true mechanical principles of 

 flight. 



