Oil) 
on of Air in the Bodies of all fiflies, to ferve their ufe, accord- 
ing to the depth of water they are bred and live in ; Perhaps by 
feme Mufcle the fifh can contraft this bladder beyond the pref- 
fureof the weight of water: Perhaps the fifli can by its fides cr 
fome other defence keep off the preffure of the water, and give 
the Air leave to dilate irs felf. In thefe cafes the fifli will be help* 
ed in all intermediate diftances, and may rife or fink from any Re- 
gion of the water without moving one Finn. 
It were worth obferving , what fifhes want bladders, and if 
the bladders of feveral fifhes are not of different fbapes or big- 
nefs, and how they are in Sea-fi(hes that live in great depths, 
and whether any amphibious creatures have them , or any thing 
analogous ; as the Lungs may be , gr other Cavities. By an in- 
quiry into thefe and other particulars this conjefture may be either 
fortified or refuted. 
So far this ConjeSture : In reference to which, when it was pro- 
pounded to the Honourable Robert Boyle, he, reflefting upon the 
manner how a Fifih comes to rife or fink in water, foon bethought 
himfelf of an Experiment probably to determine, Whether a Fiffi 
makes thofe motions by conftrifting or expanding himfelf f The 
Experiment by him fuggefied was. To take a Bolthead with a wide 
neck, and having fil I'd it almoft full with water, ro put into it 
fome live fifli of a convenient fize, thac is, the biggeft that can be 
got in, as a Roch^ Perch, or the like; and then to draw out the 
neck of theBolthead as (lender as you can^and to fill that alfo almoft 
with water; Whereupon the fifh lying at a certain depth in the 
water of the Glafs, if upon his finking you perceive the water at 
theflender top does fubfide, you may inferr, he contrads himfelf 5 
and if,upon his rifing,the water be alfo raifed,you may conclude,he 
dilates himfelf 
Am 
