CONTRIVANCE A NECESSITY. 151 



forces concerned in flight, that the common ex- 

 planation of Birds being assisted by air-cells for 

 the inhalation and storage of heated air, must 

 not only be erroneous, but founded on wholly false 

 conceptions of the fundamental mechanical prin- 

 ciples on which flight depends. If a Bird could 

 inhale enough warm air to make it buoyant, its 

 power of flight would be effectually destroyed. 

 It would become as light as a Balloon, and con- 

 sequently as helpless. If, on the other hand, 

 it were merely to inflate itself with a small 

 quantity of hot air insufficient to produce buoy- 

 ancy, but sufficient to increase its bulk, the 

 only effect would be to expose it to increased 

 resistance in cleaving the air. It is true, indeed, 

 that the bones of Birds arc made more hollow 

 and lighter than the bones of Mammals, because 

 Birds, though requiring weight, must not have too 

 much of it. It is true, also, that the air must have 

 access to these hollows, else they would be unable 

 to resist atmospheric pressure. But it is no part 

 whatever of the plan or intention of the structure of 

 Birds, or of any part of that structure, to afford bal- 

 loon-space for heated air with a view to buoyancy. 



