100 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



moved from the outlet as possible. At D some thin air-tubes are con- 

 nected with the spigot (or rather soldered to it), through which a suffi- 

 cient quantity of air is brought in from the water flowing through the 

 tube, and rises and bubbles at F. The more powerful the current of 

 the water, and the higher the pressure in vessel No. 3, all the more 

 abundant will be the quantity of air which enters the water. The water 

 flows out above a perforated inner lid, where at E a connection is made 

 with the vessel destined for the outflowing water by means of a rubber 

 tube. The water first of all flows into a perforated vessel containing 



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Fig. 3. 



fine gravel — the filter — and thus enters the vessel No. 2 in a purified 

 condition. By means of a simple suction and forcing pump, A, with the 

 sucker B, the water is, through the tube 0, pumped into the raised 

 vessel. In the center of the firm outer lid there is a perforated stopper, 

 «, and below it a large opening in the inner lid, which can also be 

 closed by a perforated lid ; this is intended for filling, removing, and 

 observing the transporting vessel No. 1. A floating lid, e, in vessel No. 3 

 indicates the height of the water by a plummet, ft, connected therewith. 

 Through the lid a the quantity and temperature of the water in vessel 

 No. 2 can be observed, and, if necessary, it can be cooled off by means 

 of ice. 



In this manner all the necessary conditions are fulfilled. The shak- 

 ing of the water has been rendered impossible, as in consequence of the 

 outlet-pipe, fastened between the upper and inner lids, the vessel is 

 always entirely filled with water, and is still sufficiently provided with 

 air. The introduction of water with the air at the bottom of the vessel 

 serves to expel the carbonic acid and thus constantly renews the water. 

 Whilst the raised vessel can be pumped full in 15 minutes, it takes the 



