LIFE IN THE OCEAN. 67 



Ten of these reservoirs are intended for marine animals. The 

 water employed is never changed, but it is kept in continual agitation 

 by circulation, produced by a current of water led from the great pipe 

 which feeds the Bois de Boulogne. This water, being subjected to a 

 strong pressure, compresses a certain portion of air, which, being per- 

 mitted to act on a portion of the sea water contained in a closed 

 cylinder placed below the level of the aquarium, makes it ascend, and 

 enter with great force into a reservoir, into which it is thrown from a 

 small jet. The sea water thus pressed absorbs a portion of the air, 

 which is drawn with it into the reservoir. A tube placed in a corner 

 of the reservoir receives the overflow, and conducts it into a closed 

 carbon filter, whence it passes into a gravelly underground reservoir, 

 returning again to the closed cylinder. The water is once more 

 subjected to the pressure of air, and again ascends to the aquarium. 

 The cylinder being underground, a temperature equal to about 

 sixteen degrees Cent., which is nearly the uniform temperature of 

 the ocean, is easily maintained. During winter, the aquarium is 

 heated artificially. 



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