ro THE STRUCTURE AND LIFE OF BIRDS cuHap. 
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! 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. 
