A A Con^eStme concermng the Bladders of Air that Are found in 
. ... Fijhes , eommumuted by hA\ and illufirated by an Exferh 
i-r ment fu^gejled by the Honorable Robert Boy le. 
EfleftingonthacQueftion, Whether Liquids gravitate upon 
Bodies lumierfed or not ? I came to a RefolutioQ in my own 
thoughts, that they do gravitate j and one of thegreateft inftan- 
ces thacdidoccurr to me was, that a bubble of Air, rifing from 
the boitoaij does dilate ic feif all the way to the top i which is 
caufed by the leflfening of the weight or prelTureof the incumbent 
water, the nearer it is to the top.- Upon confideration of that in- 
ftance, the following conjeflure preftnted its felf to my thoughts 5 
That fifhes by reafon of the bladder of Air that is within them can 
fuftein or keep tliemfelves in any depth of water. For the Air in 
that bladder is like the bubble,moreor lefs comprefled, according 
to the depth the fifli fwims at, and takes up more or lefs fpace5 
and confequenrly the body of the fifli, part of whofe bulk this 
bladder is, is greater or lefs according to the feveral depths, and 
yet retains the fame weight. The Rule de injidentibus humido^ is, 
that a Body that is heavier than fo much warer as is equal in quan- 
tity to the bulk of it, will fink ; a Body that is lighter, will fwim^ 
a Body of equal weight, will reft in any pare of the water. 
Now by this Rule, if the filh in the middle Region of the water 
be of equal weight to the water that is commenfijrate to the bulk 
of it, the fifli will reft there without any tendency upwards or 
downwards : And if the fifli be deeper in the water, the bulk of 
the fifh becoming lefs by the compreflion of the bladder, and yet 
retaining the fame weight, it will fink and reft at the bottom: 
And on the other fide, if the filli be higher than that middle Regi- 
on, the Air dilating its felf, and the bulk of the fifli confequently 
increafing, but not the we^ght^ the fifii will rife i5pwards,andreft at 
the topof the water. 
Perhaps the fifhby fomeaftion can emit Air out of this blad- 
der, and afterwards out of its body, and alfo, when there is not 
enough, take-in Air and convey it to this bladder; and then it 
will not be wondred, that there fiiould be alwayes a fit proporti- 
on 
