FISHES IN GENERAL. 235 



chores of Spain. The cachalot is said, in the same manner, 

 to pursue a shoal of herrings, and to swallow thousands at a gulp. 



This may be one cause of the annual migrations of fishes from one 

 part of the ocean to the other ; but there are other motives which 

 come in aid of this also. Fishes may be induced to change the place 

 of their residence, for one more suited to their constitutions, or more 

 adapted to depositing their spawn. It is remarkable that no fish are 

 fond of very cold waters, and generally frequent those places where it 

 is warmest. Thus, in summer, they are seen in greatest numbers in 

 the shallows near the shore, where the sun has power to warm the 

 water to the bottom ; on the contrary, in winter, they are found to- 

 wards the bottom, in the deep sea; for the cold of the atmosphere is 

 not sufficiently penetrating to reach them at those great depths. Cold 

 produces the same effect upon fresh-water fishes ; and when they are 

 often seen dead after severe frosts, it is most probable that they have 

 been killed by the severity of the cold, as well as by their being ex- 

 cluded by the ice from the air. 



All fish live in the water ; yet they all stand in need of air for their 

 support. Those of the whale kind, indeed, breathe the air in the 

 same manner as we do, and come to the surface every two or three 

 minutes to take a fresh inspiration ; but those which continue entirely 

 under water, are yet under a necessity of being supplied with air, or 

 they will expire in a very few minutes. We sometimes see all the 

 fish of a pond killed, when the ice every where covers the surface of 

 the water, and thus keeps off the air from the subjacent fluid. If a 

 hole be made in the ice, the fish will be seen to come all to that part, 

 in order to take the benefit of a fresh supply. Should a carp, in a 

 large vase of water, be placed under an air-pump, and then be de- 

 prived of its air during the operation, a number of bubbles will be 

 seen standing on the surface of the fish's body ; soon after, the ani- 

 mal will appear to breathe swifter and with greater difficulty; it will 

 be seen to rise towards the surface, to get more air ; the bubbles on 

 its surface begin to disappear ; the belly, that was before swollen, will 

 then fall of a sudden ; and the animal sinks expiring and convulsed 

 at the bottom. 



So very necessary is air to all animals, but particularly to fish 

 that, as was said, they can live but a few minutes without it ; yet no 

 thing is more difficult to be accounted for than the manner in which 

 they obtain this necessary -supply. Those who have seen a fish in 

 water, must remember the motion of its lips and its gills, or at least of 

 the bones on each side that cover them. This motion in the animal 

 is, without doubt, analogous to our breathing ; but it is not air, but 

 water, that the fish actually sucks in and spouts out through the gills 

 at every motion. The manner of its breathing is thus : the fish first 

 takes a quantity of water by the mouth, which is driven to the gills; 

 these close and keep the water so swallowed from returning by the 

 mouth ; while the bony covering of the gills prevents it from going 

 through them, until the animal has drawn the proper quantity of an 

 from the body of water thus imprisoned : then the bony covers open, 

 and give it a free passage ; by which means also the gills again are 

 opened, and admit a fresh quantity of water Should the fish be pre- 

 vented from the free play of its gills, or should the bony covers be kept 



