i8 POPULAR SCIENCE MONTHLY. 



inmates in providing that the water of each pool shall be clean 

 and well aerated. To attain the former end, the water is con- 

 stantly drained from the aquaria and replaced by fresh and fil- 

 tered water ; and to insure proper aeration, the incoming stream 

 is usually passed into the tank in such a way that it draws down- 

 ward with it in its current a cloud of air-bubbles these to sub- 

 divide finely and to be in part absorbed. In the sea- water basins 

 the " reservoir system " has been found most efi:ective in securing 

 the healthf ulness of the water, and is at present in general use. 

 It has certainly an advantage from the standpoint of economy, 

 since by its means a given bulk of water may be used and re-used 

 for months and even years, with better results, indeed, than if a 

 fresh supply of sea water had been employed, for the latter, it is 

 claimed, introduces a constant stream of impurities which can 

 not be removed by filtration. The reservoir system is certainly 

 an easy one to understand. In the basement or cellar of the 

 aquarium building is situated a concreted cistern, whose ca- 

 pacity is ten to fifty times as great as that of the sum of the 

 sea-water tanks throughout the building. In this cool, dark, 

 and uniformly temperatured cistern the water seems to have the 

 power, even in the course of a few days, to purify and "rest," its 

 sediment settling and its air-drinking power becoming restored. 

 It is into this cistern, accordingly, that the water drained from all 

 parts of the building is returned after it has been roughly filtered ; 

 and it is directly from this cistern again that the water is pumped 

 upward as the resupply. By this plan of circulation it is usually 

 arranged that the water of each tank may become changed several 

 times during the day. 



From this review of the general subject we may next pass to 

 the examination of the various aquariums of Europe. 



Naples. First in importance, as has already been noted, 

 stands the aquarium at Naples, highest in rank, also, as a station 

 of marine biological research. Its situation and surroundings 

 are eminently attractive ; it stands in a public garden on the side 

 of the gulf, amid fashionable driveways, surrounded by bright- 

 colored lawns and a wealth of century plants and cactus ; in front 

 are the outlines of distant Capri and the blue waters of the gulf ; 

 in full view is Vesuvius. The building itself is like a huge white 

 palace, conspicuous from nearly every higher j^art of the city. 

 Its main wing, shown in the foreground in the adjoining picture, 

 is the older, dating from 1875, when the station was founded by 

 Prof. Anton Dohrn ; the wing immediately behind it is the newly 

 built physiological laboratory. The aquarium occupies the base- 

 ment of the main structure, and is open to the public daily, al- 

 though to the rest of the building, including the laboratories, 

 library, and rooms of investigators, strangers are not generally ad- 



