74 INTRODUCTION. 



volume of contained air is considerably expanded. 

 It was established by experiment, many years ago, 

 by the celebrated naturalist Ray, that, after pricking 

 the air-bladder, fishes were no longer able to rise in 

 the water, but remained constantly at the bottom, 

 like so many of the other tribes which are naturally 

 destitute of this organ ; and fishermen are at pre - 

 sent in the habit of availing themselves of this 

 knowledge, by adroitly pricking the air-bladders of 

 the cod, and other fishes, as soon as they are 

 caught, for the purpose of keeping them at the 

 bottom of their well-boats, and thus of preserving 

 them fresh for the market. On the other hand, it is 

 equally well known, that if fishes have remained long 

 near the surface of the water, exposed to a scorching 

 sun, which produces a great rarefication of the air 

 contained in their air-bladder, they are no longer 

 capable of sinking in the water, but are obliged to 

 remain at the top, till the cool of the evening has 

 again condensed this air, and reduced the bladder 

 to its usual volume, rendering buoyant some other 

 part, at the expense of those by which their vital 

 functions are maintained. 



With respect to those fishes which are destitute 

 of an air-bladder, and which, nevertheless, rise 

 freely in the water, they can effect this only by an 

 effort, although a very slight effort may be con- 

 ceived to be sufficient for the purpose. This is 

 performed, in the ray tribe, by means of their enor- 

 mous pectoral fins, the motions of which act up- 

 wards and downwards, upon precisely the same 



