FLIGHT 75 



required to go ahead on the same plane is much lessened. 

 The question of resistance is discussed mathematically in 

 the article " Flight " in Newton's " Dictionary of Birds," from 

 which we take an illustration. A Rook, whose weight and 

 wing-area were found by Sir George Cayley to be, roughly 

 speaking, in the ratio of one pound to the square foot, would 

 be able to glide horizontally whenever it had a velocity of 

 37"3 feet per second, which the bird can easily attain. The 

 need for big pectoral muscles is more obvious when the 

 bird is trying to launch itself from the ground into the air, 

 or when it flies against the wind, or when it carries a heavy 

 weight in its talons. The Great Northern Diver cannot 

 rise from the land at all ; the Cormorant is often seen taking 

 a little run along the rock before it launches itself. 



§ 3. Gliding Flight 



The simplest mode of flight is that of gliding, when the 

 bird having attained to a certain velocity rests on its oars 

 for a while, or having attained a certain height descends to 

 the ground without any stroke of its wings. A gull or a 

 heron or any bird with a large sail-area, having got up a 

 certain speed, glides along in the air without a wing-stroke ; 

 a pigeon launches itself from the dove-cot and glides to 

 the ground ; a hawk swoops from mid-air on its victim, 

 and, missing it, glides up again without effort to a consider- 

 able height. The gliding flight can only be exhibited for 

 a short time ; the bird soon loses velocity or height. The 

 time varies with the velocity previously attained or with the 

 height from which the bird swoops down. Guidance 

 during the gliding can be effected by spreading out the 

 tail feathers, by raising one side of the tail and lowering 

 the other, by moving the head and neck, or by partly flexing 

 the wing on the side to which the bird wishes to turn. If 

 we adopt the hypothesis that birds served an arboreal 

 apprenticeship, it may be that gliding from branch to 

 branch, or from the tree to the ground, was for a long time 

 the main mode of flight. 



