FLIGHT. 



651 



not comparable to a balloon, but to a flying machine ; 

 " it has to be not a buoyant cork, but a buoyant bullet." 

 In short, the air-sacs increase the bird's respiratory content, 

 secure more perfect aeration of the lungs, and probably 

 aid in regulating the body temperature. 



Ruskin has compared the flight of a bird to the sailing of a boat. 

 "In a boat the air strikes the sail ; in a bird the sail strikes the air ; in 

 a boat the force is lateral, and in a bird downwards ; and it has its sail 

 on both sides." But, as he says, the sail of a boat serves only to carry it 

 onwards, while wings have not only to waft the bird onwards, but to 

 keep it up. To carry 

 the weight of the bird the 

 wings strike vertically, 

 to carry the bird onwards 

 they strike obliquely ; 

 sometimes (.he direction 

 of the stroke is more 

 vertical, and then the 

 bird mounts upward ; 

 sometimes it is more 

 oblique, and then the 

 bird speeds onwards ; 

 usually both directions 

 are combined. The 

 raising of the wing after 

 each stroke requires rela- 

 tively little effort, the 

 resistance to be overcome 

 being very slight. In 

 steering, the feathers of 

 the tail often bear to the 

 wings a relation compar- 

 able to that between 

 rudder and sail. 



Modes of flight. There 

 are three chief modes of 

 flight : 



1. By gliding or skimming, during which the bird has its wings 

 spread, but does not flap them, depending for its movement on the 

 velocity acquired by previous strokes, by descending from a higher 

 to a lower level, or by the wind. This may be readily observed in 

 gull and heron, in a pigeon gliding from its loft to the ground, or in 

 a falcon swooping upon its quarry. 



2. By active strokes of the wings, in which the wings move down- 

 ward and forward, backward and upward, in a complex curve. This 

 is of course the commonest mode of flight. 



3. By sailing or soaring with motionless spread wings, in which the 

 bird does not necessarily lose in velocity, or in vertical position, as is 

 the case in gliding. It is illustrated by such birds as crow, falcon, 



FIG. 325. Position of wings in pigeon at 

 maximum elevation. From Marey. 



