514 ON THE POWERS OF CERTAIN WATER BIRDS. 



emerge, but, on looking round ami seeing no danger, it suddenly re- 

 laxes the effort which it had till then kept up and its body instantly 

 resuming its usual state, the bird as quickly assumes its natural posi- 

 tion, and floats buoyantly on the water. Let, however, some slight 

 occurrence disturb it, not enough to make it dive, and it instantly 

 sinks itself deeper into the water, and remains submerged until it 

 finds that all danger has disappeared. It runs no risk of being suifo- 

 cated, for the condensed air is just as capable of sustaining life as 

 ordinary air, and will do so just as long as a common inspiration. 

 The bird, too, in this compressed state is able to inspire regularly, 

 though of course in a constrained manner. 



Let us now take one illustration from another class of Nature's works 

 —I mean the Pearly Nautilus. The shell occupied by this curious 

 animal is of considerable size, but has only a small portion of its cavity 

 filled by the body of the animal. The rest of the shell is composed of 

 cells, with the interior of each of which the animal has a direct com- 

 munication by means of what is called the siphuncle or tube. These 

 cells, in their natural state, are filled with air or gas of some kind ; 

 and it is clear that in this condition the animal must float on the sur- 

 face of the water, and cannot while in that state sink. But there is 

 a very curious provision to obviate this inconvenience, for as soon as 

 the animal is frightened, or from any other cause contracts its body 

 within the front part of the shell, water is by this very act forced 

 down the tube, and so into all the cells, and thus compressing the air, 

 the buoyancy of the animal is lessened, and it sinks in the water. 

 When the animal wishes to rise it protrudes the head, and this opens 

 the communication between the cells and the external water, and the 

 air expanding forces out the water, and the animal again floats. Here 

 we find an action analagous to that of the bird, produced without the 

 slightest deviation from any of the known laws of Nature. It is true 

 it is executed by a different application of the same principle which I 

 have supposed to be called into action in the case of the bird, but 

 manifestly only so modified on account of the peculiar formation of 

 the animal ; I mean from its external covering being rigid. Had it a 

 power of contracting its outward covering, that is, its shell, to one- 

 half its usual bulk, as I have proved the bird to have, it would no 

 doubt exercise it, as the simplest way of producing the desired effect. 

 The water it takes in is manifestly of no use as ballast, for water will 

 not sink in water, and moreover the water at the surface is always 



