THE MASTERY OF THE AIR 247 



there is 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 considerable 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 



