494 BIRDS. 



falling by frequent effort, while some of the heaviest birds fly 

 most swiftly. The bird is not comparable to a balloon, but to 

 a flying machine ; " it has to be, not a buoyant cork, but a 

 buoyant bullet." On the other hand, there is no doubt that 

 the hollowness of the bones makes it possible to combine 

 much strength with little weight, while the heated air which 

 fills the air-sacs around the lungs must make it easier for the 

 bird to raise itself from the ground. 



The form of the bird is also important, for the motion of 

 a body through the air, and the measure of support which the 

 air gives to it, vary greatly according to the form of the 

 body. Just as the speed of a boat varies with its shape, 

 so is it with the flight of a bird. Buffon noted that eagles 

 disappeared from sight in about three minutes, and it seems 

 that a common rate of flight is about fifty feet per second. 



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 the direction of the stroke is more 

 vertical, and then the bird soars or hovers ; sometimes it is 

 more oblique, and then the bird speeds onwards ; usually 

 both directions are combined. The raising of the wing after 

 each stroke is relatively effortless, the resistance to be over- 

 come being very slight. In steering, the feathers of the 

 tail often bear to the wings a relation comparable to that 

 between rudder and sail. 



There are many different kinds of flying. The humming- 

 birds flutter beside the flowers with rapidly vibrating wings ; 

 the kestrel hovers " in the face of a stiff gale or in a perfect 

 calm ;" the albatross sails in the wind as a kite does, " its 

 own weight answering the purpose of the string." There is 

 no motion more marvellous or more beautiful than the 

 flight of a bird. It is harmonious with the bird's true 

 nature. For there is more than poetical insight in Ruskin's 

 description :— " The bird is little more than a drift of the 

 air brought into form by plumes ; the air is in all its quills, 



