t 4^3 ) 
Bladder to the Stomach , which fliews that Opini- 
on not right. There is more reafon to believe 
its u(e is by dilating it felf, to render the Fifh 
lighter, as occafion ftrves, for Swimming : So that 
when this Bladder is dilated, or the ^Asr contained 
therein weighs lefs than an equal quantity of Water • 
if it be dilated a little, the Filh weighs as much 
as fo much Water and if it be comprefTed, the 
,Fifti weighs more than an equal quantity of Wa- 
ter for the Air Comprefled , weighs more than 
when 'tis Dilated. 'Tis a Rule in Hydroftaticks , 
that a Body that weighs more than its equal Bulk of 
Water muft fmk to the bottom,* if it weighs lefs 
it will rife to the top ; if it weighs exadly equal 
to fo much Water, it will ftay where-ever 'tis pla- 
ced. Now the Air of the Bladder, by being Com- 
prefled or Dilated, by means of the Mulcles of 
the Fiili , caufts the Filh to be more or left hea- 
vy. That: it is this Bladder that makes the Fiffi 
Swim is plain for if it be by any means burfl:, 
fo ^at it cannot .be Extended , the Filh can no 
more raife it felf in the Water , but keeps conti- 
nually at the bottom. The Fins and Tail affift 
thecn in their pafFage through the Water, whether 
they will but the Dilatation of the Air in the 
Bladder makes them capable of Swimming after the 
fame manner , as the Dilating of the Lungs and 
Thorax of a Man bears him up in the Water. 
.Flat Fifh , (uch as Soles , have none of this 
Bladder ^ for they are able , by reafon of their 
Breadth , to keep themfelves up in the Water. 
Cray-fi(h » and other Shdl-filh '^'^Vft k likewlfe , 
Pv. r r a for 
