FLIGHT OF BIRDS. 623 



was comparable to a balloon. But this is fallacious. The 

 air must indeed lessen the specific gravity of the bird, but a 

 few mouthfuls of food are sufficient to counteract the 

 lightening. Moreover, in many small birds which are good 

 flyers all the large bones, or all except the humerus, contain 

 marrow, and are therefore not " pneumatic," and the horn- 

 bill, which is but a poor flyer, is one of the most pneumatic 

 of birds. It is likely that the chief importance of the air 

 sacs and air spaces is in connection with respiration and 

 with the regulation of the body temperature. It is certain 

 that in ordinary flight the lightest of birds has to keep 



Fig. 221. — Position of wings in Pigeon at ma.ximum 

 elevation. (From Marey.) 



itself from falling by constant effort. The bird is not com- 

 parable to a balloon, but to a flying machine ; " it has to be 

 not a buoyant cork, but a buoyant bullet." 



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 poetic 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, it breathes through its whole frame and flesh, and glows 

 with air in its flying ; like a blown flame it rests upon the air, subdues 



