SLOPE-BACKED TANKS. 153 



air, it will mechanically entangle some of it and 

 thus convey it to where it is required. But the 

 ingenuity of a really careful aquarium-keeper will 

 soon suggest to him various simple means of this 

 kind, by which additional aeration can be obtained 

 if required. The water referred to is of course taken 

 from the tank into which it is made to run. In the 

 volume of ' Science Gossip,' for 1870, the following 

 contrivance is mentioned : " I had been in the habit 

 of drawing off the water to a certain extent daily, but 

 in time it became apparent, as my interest began 

 to flag (as it sometimes will) that this was a somewhat 

 fatiguing process, and therefore one that was likely 

 to be forgotten for a day, thus greatly deteriorating 

 the condition of the water. In order, therefore, to 

 make the tidal arrangement as self-acting as possible, 

 I added the following : At the level of high-water 

 mark I made a hole in the slate back of my tank ; 

 through this I inserted a glass tube (which may be 

 made to fit completely water-tight by placing over it 

 a piece of indiarubber tubing). This tube inside is 

 bent down like a syphon to the level of low water 

 in the tank ; on the outside it communicates with 

 the jar receiving the off-water. Now it will be seen 

 that as soon as the water reaches the level of the 

 tube passing through the back of the tank, it will flow 

 over into the outer tube communicating with the jar, 

 and this tube, by acting as a syphon, will draw off all 

 the water to the lower water level, which in my aqua- 



