IN TRODUCTION. 25 



It is eafy to conceive, from what iras l^een 

 fa id, how a Fifh is capable of advancing in a 

 direcfl Line : But the Difficulty that remains, 

 is, how they do to rife and fall in the Water. 

 Now mod Fifh, it muft be obferved, have a 

 Bladder, or fomething equivalent to it, the 

 Ufe of which is quite different from what moft 

 People imagine. This fuppofed Bladder is a 

 Bag of Air, by Virtue of which the Fifli are 

 enabled, according to its Enlargement or 

 Contra(5lion, to rife or fink. 



Nothing is more eafy tO be conceived than 

 this, if we lay down as inconteftable Maxims, 

 equally confident with Experience and good 

 Senfe, i. That a Body will fwim upon the Wa- 

 ter when 'tis lighter than that Quantity of the 

 Fluid whofe Place it fills. 2. That a Body 

 is more ponderous, according as the Parts of 

 it are more compa6l, and contain in them a 

 lefler Quantity of Air *, and lighter, accord- 

 ing as they are porous, and contain in them a 

 larger Quantity of Air. 



Thefe Pofitions allowed, the Body of a 

 Fifh, which is heavier than the Mafs of Wa- 

 ter whofe Place it poflefTes, would unavoid- 

 ably fink to the Bottom, had not the Filh in 

 its Inteftines a VefTel, or Bag, fill'd with Air, 

 which renders him capable of fuflaining him- 

 ic\t in what particular Part of the Water he 

 fees mofl convenient for his Purpofc. This 

 Bag dilates the Fifh, in feme JVIeafure, and 

 makes fbme fmali Improvement to its natu- 

 ral Bulk 5 without any Addition to its Weight. 

 C By 



