106 THE STRUCTURE AND LIFE OF BIRDS chap. 



beneath the cruel light of fact. A bird can carry only 

 a very small amount of air in its sacks and bones, 

 and the difference in weight between a few cubic 

 inches of heated or cold air is too infinitesimal 

 to be worth considering. The fact that an eagle 

 may sometimes be seen carrying off a lamb 

 ought to convince any one that the saving of the 

 tiniest fraction of an ounce of weight would make 

 practically no difference. True, air within the 

 bird, whether heated or not, will expand its volume, 

 and lessen its specific gravity, 1 but it could not 

 help it to rise, and this is the real difficulty. Moreover, 

 many birds which fly to perfection, for instance the 

 swallow, have all their large bones solid. If by any 

 means a bird attained the lightness of aballoon,he could 

 not fly. A balloon drifts with every gust ; steering is 

 impossible ; the wind chooses its course. A machine 

 which is light as air can have no strength to gain a 

 velocity other than that of the air-current in which 

 it moves. The bird-balloon, as light as the wind and 

 as strong as iron, is a figment of the imagination. 



What, then, is the true explanation of the aeration 

 or pneumaticity of birds' bones ? It is impossible that 

 it can be of use in the regulation of temperature, since 

 the air cannot be expelled from them at will. But the 



1 It may be well to explain what is meant by specific gravity. 

 The weight of water is taken as the unit. When it is said that 

 the specific gravity of gold is 19, it is meant that a cubic foot of 

 gold weighs 19 times as much as a cubic foot of water. Thus, 

 when a bird inflates his sacks with air, his weight increases by 

 the weight of the air breathed in, but his specific gravity is 

 lessened. An average cubic inch of him does not now weigh so 

 much ; his weight in proportion to his bulk has gone down. 



