BIRDS. 491 



in the bones communicate with large air cells, 

 which are distributed in various parts of the body, 

 and which contribute still farther to diminish its 

 specific gravity ; and by means of canals which 

 open into the air passages of the lungs, this air 

 finds a ready outlet when it becomes rarefied by 

 the ascent of the bird into the higher regions of the 

 atmosphere. This air, being contained in the in- 

 terior of the body, which preserves a very elevated 

 temperature, must be constantly in a state of greater 

 rarefaction than the cooler external air; a condition 

 which must contribute, in some slight degree, to 

 render the whole bodv lighter than it would other- 

 wise have been. It appears to me, however, that 

 considerably greater importance has been attached 

 to this circumstance than it really possesses. Many 

 have gone so far as to represent the condition of a 

 bird as approaching to that of a balloon, filled with 

 a lighter gas than atmospheric air; and have been 

 lavish in their expressions of admiration at the 

 beauty of a contrivance, which thus converted a 

 living structure into an aerostatic machine. A 

 little sober consideration will suffice to show that 

 the amount of the supposed advantages resulting to 

 the bird from the diminution of weight, occasioned 

 by the difference of temperature between the air 

 included in its body and the external atmosphere, 

 is perfectly insignificant. Any one who will take 

 the trouble to calculate the real diminution of weight 

 arising from this cause, under the most favourable 

 cirumstances, will find that, even in the case of the 

 largest bird, it can never amount to more than a 

 few grains. 



The conditions in which birds are placed with 

 regard to the density of the surrounding medium, 



