THE MASTERY OF THE AIR 247 



no wind against the bird and if the bird is not 

 rising, the work of rowing with its wings in 

 the elastic air is not hard. A ship has the 

 advantage that it floats in the water, whereas 

 the bird cannot float in the air; but the ship 

 has the disadvantage that the water offers con- 

 siderable resistance to a body passing through 

 it, whereas the air offers little resistance to a 

 smooth body passing quickly through it. 



The second kind of flight is gliding, seen 

 when a bird, having got up a certain speed, 

 rests on its oars, and holding its wings taut 

 glides along, or when a bird launches itself 

 from a tree and with outstretched, but un- 

 moving wings, glides to the ground. When a 

 bird glides along after getting up speed it is 

 bound to sink, but this may be counteracted 

 for a time if an ascending current of air beats 

 up against the bird's outstretched wings from 

 below. We often see this when a gull flying 

 from the fields seawards meets just above the 

 edge of the cliffs an ascending landward 

 breeze. In this case there is a transition to 

 the third kind of flight, called "sailing." 



SAILING FLIGHT. When an albatross goes 

 up one side of the ship, keeping pace with 

 the vessel, without a stroke of its wings, we 



