ON THE POWERS OF CERTAIN WATER BIRDS. 51 t 



an extent as to enable it to maintain the whole of its body just beneath 

 the surface of the water. It is manifest to any one who is at all con- 

 versant with the laws of hydrostatics, that in deep water this can only "^ 

 be accomplished in one of the following ways, by either of which the 

 body of the bird might be supposed to be rendered of, as nearly as 

 possible, the same specific gravity as the water in which it swims : — 



1st. The bird might expel so much air from its body as to remove 

 its power of floating on the surface of the water ; or, 



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. 



With regard to the first of these suppositions, namely, that the 

 bird might expel so much air as to remove its power of floating on 

 the surface, although it is considered by some writers on the subject 

 to be a plausible explanation of the phenomenon, I cannot think that 

 it is the true one ; for, in the first place, we have no evidence that the 

 bird has any power, as to expulsion at least, over the air in the various 

 air cells, which constitute the great bulk of the air vessels contained 

 in its body. Indeed the general impression among anatomists is that 

 it has no power at all over it ; and, even if it had, I cannot think it 

 would be possible for the bird to expel it so quickly as would be 

 necessary to produce such an immediate effect as does actually take 

 place ; nor would the bird have the power of again taking it in so 

 rapidly as it manifestly must do to enable it to float at its ordinary 

 level in so short a time after partial submergence as it in reality does. 



In a paper on this subject the Rev. J. C. Atkinson says : — "I will 

 shoot a Moorhen in the act of diving, and will add to its specific 

 gravity by depositing within its body some twenty or thirty grains of 

 No. 5 shot. Of course then it will sink, and unless my retriever is a 

 rather uncommon one I lose the bird. But no such thing ; the 

 Moorhen comes to the surface immediately, and floats almost as buoy- 

 antly as ever ; and yet whencie and how can the air have been pro- 

 cured, which has been applied to the replenishing of the air vessels 

 and the restoration of the bird's buoyancy." I will endeavour, when 

 considering the second supposition, to give what I believe to be the 

 true explanation of this fact, for it is only as a fact that it is brought 

 forward by Mr. Atkinson, as being opposed to the idea that the bird 

 expelled the air from its body before submergence, and which was the 

 hypothesis of his opponent, Mr. Slaney. I have very Uttle doubt in 



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