3B3 



back, would be incapable of performing any motious of progression : this 

 happens when this bladder is burst or punctured. Constrictor muscles 

 expel the gas which it contains, and force it into the stomach, or oesopha- 

 gus, when the animal wishes to sink. This expulsion becomes imprac- 

 ticable, if the gas undergoes considerable expansion, from the application 

 of heat, and resists the compression that is applied to it. Hence, during 

 the fry-time, fishes after remaining long on the surface of the water, ex- 

 posed to the heat of the sun, become unable to sink, and are easily 

 caught*. 



As the fish is entirely surrounded by a medium which presents, on 

 every side, an equal resistance, the velocity which he might have acquired, 

 by striking the fluid behind, with his tail, would be lost, from the resist- 

 ance of the water which he would have to displace forward, if, immedi- 

 ately after striking with his tail, he did not bring it back into a straight 

 line, so as to present to the fluid, only the inconsiderable breadth of his 

 body; the velocity with which he moves is, besides, very inferior to that 

 with which he uses his tail. This part being brought into a straight line, 

 the fish contracts it to the smallest dimensions, at the same time that he 

 brings it to the other side ; he then expands it and strikes the fluid in a 

 contrary direction, in a line between the two oblique impulses which both 

 strokes have given to it. The fish turns horizontally, and directs himself 

 towards the side he chooses, by striking more powerfully, or with great- 

 er quickness, on one side than on the other, or by striking only on one 

 side. 



Fishes without an air bladder, are reduced to live at the bottom of the 

 water, unless they have a flat body and are furnished with horizontal fins, 

 so as to enable them to strike a considerable surface of water, in a power- 

 ful manner, as in the case with rays y whose wide fins are not inaptly term- 

 ed wings, the motion of these fishes, in water, precisely resembling that 

 of birds in the air, with no other difference but that of the different densi- 

 ty of the medium in which they move, as will be shown in treating of the 

 motions of progression peculiar to this class of animals. 



CXCI. Of flying. ' A bird, in rising, or in moving in the air, has to 

 use much more force and with much greater velocity, than a fish in 

 swimming. He has not the power, like the latter, of placing himself in 

 equilibrio with the fluid in which he moves, by means of an internal or- 

 gan that renders his specific gravity equal to that of the medium he is in. 

 This medium, besides, presents less resistance to the powers which strike 

 it to obtain a point of support. 



Though birds are unable of becoming as light as the air, it is, how- 

 ever, in their power to obtain a specific gravity, not much exceeding that 

 of the atmosphere, Nature has rendered them very light, by providing 

 them with very capacious lungs, capable of great dilatation, from the re- 

 markable-mobility of the parietes of the chest, and by extending the lungs 



* The nature of the air contained in the air bag of fishes has been investigated by 

 PRIESTLEY and Fotrncnoz, and lately by M. 13toT. According to the accurate and ex- 

 tended experiments of the last named inquirer, it would appear to consist entirely of 

 oxygen and azote. The proportion of these gases varies according to the species and 

 depth at which they are caught ; that of oxygen increasing- with the depth of water 

 from an almost insensible quantity, until it amounts to 87 parts in a hundred of the 

 whole air. It would appear from the experiments of M. Biot, that this ail- is a secretions 

 from the sac which contains it-~Auth9r's Note. 



