70 
THE OCEAN WORLD. 
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 permitted to act on a portion of the sea water contained in 11 
closed cylinder placed below the level of the aquarium, makes d- 
ascend, and enter with groat force into a reservoir, into which it 
is thrown from a small jet. The sea water thus pressed absorbs o 
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 conduct 
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 th® 
aquarium. The cylinder being underground, a temperature equal to 
about sixteen degrees Cent., which is nearly the uniform temperature 
ot the ocean, is easily maintained. Curing winter, the aquarium d 
heated artificially. 
