Marine A quaria 2 5 



panying illustration. Strong circular currents are formed in the imme- 

 diate vicinity of the cylinder, while at the far end the water moves 

 very gently. There is also a noticeable upward motion from the bottom 

 of the tank. 



The speed of rotation and the corresponding strength of currents may 

 be regulated by the speed of the motor, controlled through a rheostat, 

 and by means of a set of pulleys of different diameters. The dimensions 

 of the cylinder also affect the strength of the current produced and they 

 therefore should vary according to the size of the aquarium used. The 

 cylinder shown here is 4 inches in diameter and 6 inches long. About 

 one inch is left above water. The tank is 25 x 15 x 14 inches. The 

 bottom of the cylinder (B) 

 is a celluloid disk. Non- 

 corrosive material should 

 also be used for the suspen- 

 sion rod (C) and brace wires 

 (W). The diameter of the 

 pulley (P) is 12 inches. 



The water may be with- 

 drawn from the tank through 

 a siphon (S) the upper end 

 of which is placed inside the 

 revolving cylinder. When 

 the cylinder is in rotation 

 small organisms are never 

 actually drawn against its 



wall, because the centrifugal force throws them away from it. They 

 are then caught up in the circular currents and soon find themselves in 

 quieter waters at the far end of the aquarium. In this manner the 

 water in the tank may be changed without losing its inhabitants. 



When desirable, a constant flow of water may be supplied by placing 

 the lower end of the overflow siphon (S) in a vessel (V) , adjusted so that 

 the top of it is level with the water in the aquarium. The water is 

 introduced from a reservoir, in which it is kept at a constant level. If 

 necessary the water from a laboratory faucet may be filtered through 

 glass wool to remove sediment and other foreign matter. 



Imitation of tidal movement. The arrangement imitating tidal move- 

 ment of water and permitting attached forms to be subject to rhythmical 

 changes in hydrostatic pressure consists of a series of tall jars in which 

 the equilibrium is maintained through a system of tubes (T, Ti) (Fig. 

 30). An automatic siphon (S) in jar (A) controls the level in all the 

 jars. After the level in the controlling jar has reached the highest posi- 

 tion the water begins to empty through the siphon drawing it from all 



Fig. 30. — The arrangement imitating tidal 

 changes. A, controlling jar; S, automatic siphon; 

 T, T lf glass tubes connecting the jars. 



