AERA TION OF AQUARIA. 163 



presenting new surfaces to the oxygenating influ- 

 ences of the air. In fresh-water aquaria we need 

 this mechanical aeration and circulation in the same 

 degree, perhaps, although we there imitate the quiet 

 condition of our still ponds and tarns. In our small 

 parlour aquaria, constructed to maintain marine crea- 

 tures, we also require its assistance, such as may be 

 produced by the means already described ; but it 

 should be remembered that here we are only imitating 

 the natural conditions of rock-pools. On the other 

 hand, in the huge tanks seen in every large public 

 aquarium, it is sought to imitate the conditions of 

 the open sea. 



A most important mechanical contrivance for 

 aerating aquaria was invented by the late Mr. G. 

 Hurwood, of Ipswich, in 1859. It consisted of an 

 arrangement by which "the pressure of a stream of 

 fresh water, such as exists in the pipes of waterworks 

 in towns, or such as can be got from a high cistern 

 already existing in a dwelling house, may be em- 

 ployed to compress air, which compressed air in its 

 turn forces a current of sea water into an aquarium." 

 This contrivance was first successfully adopted on 

 a large scale at that erected in the garden of the 

 Acclimatisation Society of Paris, in 1859, shortly 

 after Mr. Hurwood invented it. It was afterwards 

 partly applied by Mr. Lloyd to the Hamburg Aqua- 

 rium, of which he then had the charge. The sea 

 water in the Hamburg institution was "circulated 



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