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SCIENCE PROGRESS. 



there was fixed at the top a glass tube, with a horizontal 

 arm. Any change in the volume of the fish was of course 

 indicated by a movement in the liquid in this tube. He 

 found that as the fish swam upwards its volume expanded, 

 as shown by the movement of the water in the horizontal 

 tube, and conversely as it swam downwards. The move- 

 ments of the water in the tube always corresponded 

 exactly to the height of the fish, just as if the latter had 

 been a " Cartesian Diver," pulled up and down by a 

 thread. Similar results were obtained when, by means of 

 slightly different experimental arrangements, the pressure 



Fig. 2. 



inside the vessel was artificially raised or lowered. Thus 

 the fish made no use of its muscles to compress its air- 

 bladder. Compression of the air-bladder, when it did 

 occur, seemed to be merely accidental and momentary, as 

 when the animal made a sudden and violent movement. 

 Even with a species possessing a specially muscular swim- 

 ming-bladder the result was precisely the same. If the 

 fish was confined in a closely fitting wire cage, so that it 

 was unable to move its tail or fins (Fig. 2), or if it was 

 anaesthetised with ether, it behaved, when subjected to 

 differences of pressure, exactly like a " Cartesian Diver". 



