INTRODUCTION. 27 



er or heavier, or, in other Terms, to rife or 

 fink, as Occafion requires. 



The Water abounds with Particles of Air, 

 diffus'd through the Whole. What we gene- 

 rally call the G'llh of a Fifh, are a Sort of 

 Lungs, which he opens for the AdrnifTion of 

 Air, and which are formed with fuch a pecu- 

 liar Mechanifm, that they receive it without 

 the lead Intermixture of any Water. The 

 Air indifputably flows into the Bag through 

 them, and by Virtue thereof the Filh afcends : 

 But when he is inclined to fink, he has nothing 

 more to do than to contradl his Bag •, for the 

 Air immediately re-afcends to the Gills, and 

 has a free and uninterrupted PafTage. The 

 Fifli thereupon finks to the Bottom, and the 

 Rapidity of his Defcent is proportioned to the 

 (Quantity of Air that is ejedced. 



Every Kind of Fifli has its Jaws framed 

 for the taking in and macerating its particular 

 Sort of Food. Such as feed upon Weeds and 

 Infe6ls, have feldom any Teeth ; but fuch as 

 prey upon other Fifh, have one, two, or more 

 Rows of Teeth, for the better grinding of 

 their Nourilliment •, in which, as in all other 

 Things, the Wifdom of Providence is mani- 

 feft. The Back-bones of Fifh, for the more 

 eafy bending of their Bodies, confift of very 

 fliort Vertebras, fo joined, that their Tail and 

 Head may be eafily brought together; and 

 their Scales, which in many Species are a- 

 mazingly beautiful, are fo regularly placed la 

 Rows one over another, that they never ob- 

 C 2 flruct 



