212 PHILOSOPHICAL TRANSACTIONS. [aNNO 1675. 



of the weight or pressure of the incumbent water, the nearer it is to the top. 

 Hence also it occurred to me, that fishes, by reason of the bladder of air that 

 is within them, can sustain or keep themselves in any depth of water. For the 

 air in that bladder is like the bubble, more or less compressed, according to the 

 depth the fish swims at, and takes up more or less space; and consequently the 

 body of the fish, part of whose bulk this bladder is, is greater or less according 

 to the several depths, and yet retains the same weight. The rule de insidentibus 

 humido, is, that a body that is heavier than so much water as, is equal in quantity 

 to the bulk of it, will sink; a body that is lighter will swim; a body of equal 

 weight will rest in any part of the water. Now by this rule, if the fish in the 

 middle region of the water, be of equal weight to the water that is commensu- 

 rate to the bulk of it, the fish will rest there without any tendency upwards or 

 downwards : but if the fish be deeper in the water, the bulk of the fish becoming 

 less by the compression of the bladder, and yet retaining the same weight, it 

 will sink and rest at the bottom: and, on the other side, if the fish be higher 

 than that middle region, the air dilating itself, and the bulk of the fish conse- 

 quently increasing, but not the weight, the fish will rise upwards, and rest at the 

 top of the water. Perhaps the fish, by some action, can emit air out of this 

 bladder, and afterwards out of its body, and also, when there is not enough, 

 take in air and convey it to this bladder ; and then it will not be wondered, that 

 there should be always a due proportion of air in the bodies of all fishes, to serve 

 their use, according to the depth of water they are bred and live in : perhaps by 

 some muscle the fish can contract this bladder beyond the pressure of the weight 

 of water; perhaps the fish can by its sides or some other defence keep off the 

 pressure of the water, and give the air leave to dilate itself. In these cases the 

 fish will be helped in all intermediate distances, and may rise or sink from any 

 region of the water, without moving one fin. 



So far this conjecture: in reference to which, when it was propounded to the 

 Hon. Robert Boyle, he, reflecting on the manner how a fish comes to rise or 

 sink in water, soon bethought himself of an experiment probably to determine, 

 whether a fish makes those motions by constricting or expanding himself? The 

 experiment by him suggested was; to take a bolthead with a wide neck, and 

 having^ filled it almost full with water, to put into it some live fish of a convenient 

 size, that is, the largest that can be got in, as a roach, perch, or the like ; then 

 to draw out the neck of the bolthead as slender as you can, and to fill that 

 also almost with water: whereupon the fish lying at a certain depth in the water 

 of the glass, if upon his sinking you perceive the water at the slender top 

 subsides, you may infer that he contracts himself, and if, upon his rising, the 

 water be also raised_, you may conclude that he dilates himself. 



