512 ON THE POWERS OF CERTAIN WATER BIRDS, 



my own mind that the muscular system of the bird would enable it 

 so to compress its body as to expel sufficient air to make it of the 

 same specific gravity as water, were the various air cavities so arranged 

 as to allow of free egress and ingress. But, unfortunately for this 

 theory, such is not the case, and I much doubt whethei the removal of 

 all the air in the numerous air cells, the bones and feathers, would be 

 possible even under the air pump. Another argument, also, which bears 

 strongly against this idea is, that were the bird to get rid of the air 

 from the air cavities, it could only do so through the lungs, which 

 thus must be in the same condition, and the bird would necessarily 

 become suffocated for want of the quantity of air absolutely essential 

 for respiration, and which the bird can do without worse than any 

 other animal ; for its circulation, and consequently its respiration, is 

 very rapid, and it is this which enables it to keep up its natural heat 

 under circumstances that would be fatal to animals otherwise consti- 

 tuted. No ; every bird on diving has the power, if it sees reason to 

 exercise it, of arresting its own progress upward, so that it shall at 

 first only shew its head and neck, or only its bill, above water, and it 

 can in this state take in a fresh supply of air, and this, too, in a single 

 second, sufficient to enable it to take a long dive before again coming 

 up, as every one must often have observed. How would it be possible 

 for the bird to perform all this if it had gone down with such a re- 

 duced quantity of air as must have been the case had this been the 

 mode of accomplishing its purpose. 



I will now dismiss this hypothesis and proceed to consider the 

 second mode. 



2nd. It might so compress its body as to condense the air in the 

 various cavities to such an extent as to place it in the required con- 

 dition. 



It is a well known fact that if you condense a cubic foot of air into 

 a vessel already containing another cubic foot under the ordinary at- 

 mospheric pressure, you do not increase the buoyancy of the vessel in 

 water by the additional quantity of air, but the contrary — you lessen 

 it, and make it sustain less weight in the water by somewhere about 

 535 grains. Now, I cannot help thinking that here we have the true 

 solution of the difficulty in question. No one, I imagine, will deny 

 that if the bird has the power of compressing itself to a sufficient ex- 

 tent, it must sink instead of swim. Let us now see whether it would 

 be possible for a bird to compress itself to such an extent as to be in 



