10 



ON THE SUUMEnGENCR OF WATEU-IURDS, 



analogous to that of the bird, produced witliout the Kliglitcst deviation fit)ui any 



of the known laws of nature. 

 Tt is true it is executed by 

 a diiferent application of 

 the same pt-inciple Avhich I 

 have supposed to be called 

 into action in the case of 

 the bird, but manifestly 

 only so modified on account 

 of its peculiar formation, I 

 mean from its external cov- 

 ering being rigid. Had it 

 a power of contracting its 

 outward covering, that is, 

 its shell, to one-half its 

 usual bulk, as I have prov- 

 ed the bird to have, it would 

 no doubt exercise it, as 

 the simplest way of produ- 

 cing the desired effect. The 

 water it^takcs in is mani- 

 festly of no value whatever 

 as ballast, for water will 

 not sink in water, and 

 moreover the water at the 

 surface is always warmer than that below, and consequently woidd rather tend 

 to make it float. Its only use therefore must be to compress the air. If it 

 was intended that the bird should use the same means to alter its specific graA-ity, 

 would it not be provided with some special apparatus for the purpose, as we 

 see the Nautilus is? No such provision, however, nor the most distant approach 

 to it exists in the bird; but its external surface is capable of great compression, 

 and is abundantly furnished with powerful muscles, whose combined action would 

 be to compress the body, and they are under the control of the will of the bird. 

 With the knowledge of all these facts before me, I can come to no other 

 conclusion than that the bii*d does so compress its body as to condense the 

 air in its various cavities to such an extent as to render the specific gi'avity 

 of its body, about the same as that of the water in which it swims. 



I have now taken a hasty survey of the whole subject. In the first place 

 I shewed that diving birds possess a wonderful power over their own specific 

 gravity in the water, and I brought, as proofs, records of the habits of certain 

 birds, which had been made without reference to any particular theory, but 

 simply as facts in the history of the birds. I next considered the explanation 

 which is generally given of the phenomenon, and then gave what I hope were 

 satisfactory reasons for considering it insufficient. And lastly, I brought forward 



