320 THE INFLUENCE OF INANIMATE SUKKOUNDINGS. 



column of water, theoretically estimated as that of many atmo- 

 spheres, than we human beings, in the normal condition of our 

 bodies, are aware of the weight above us of one atmosphere ; 

 and simply for this reason, because the pressure is equal on all 

 sides, and because they are themselves permeated by fluids 

 which, as is well known, are almost incapable of compression. 

 The weight of a high column of water can never affect an 

 animal at the bottom, excepting when the animal has cavities 

 in its body which are filled either with a fluid of less density 

 than the water, or with gases. In the latter case particularly, 

 the pfiects are, as is well known, easy to observe, since gases are 

 in a high degree compressible. Divers who plunge into great 

 depths — as, for instance, the pearl-divers in the Indian seas — 

 or, on the other hand, people who climb mountains to a great 

 height, often sufier severely from the difference of pressure in 

 the external atmosphere and the tension of the air in their 

 lungs, or the pressure in the internal vessels : the divers 

 because the increased pressure causes compression of the air in 

 the lungs ; the cKmbers because, on the contrary, the heavier 

 air in the lungs tends to expand under the reduced atmospheric 

 pressure at a great height. When a man hag accustomed 

 himself to the lighter atmosphere of high mountains — i.e. to the 

 smaller pressure — -he frequently finds it more healthy and agree- 

 able than that of the plain, or at least equally so. Birds, which 

 often come down to the plain from the giddiest- heights with 

 extreme rapidity, must evidently be capable of accommodating 

 themselves much more promptly than man to the alteration of 

 pressure, since not their lungs merely but their pneumatic bones . 

 and all the other air-cavities of the body are fiUed with air. 



On the other hand, there are animals which have cavities 

 filled with gases in their body, but which are not capable of 

 effecting a change from the compression which must be the 

 condition of such gases at considerable depths, so rapidly as 

 birds nor even as man ; not so rapidly indeed as quick alterna- 

 tions in the external pressure would require. This is the case 

 with fishes provided with a swimming-bladder. An interesting 

 instance is afforded by the little fish of the Lake of Constance 

 known as the Kilch. These fish, allied to the Trout family, 



