254 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



of the water flowing through the flume, in the direction of the arrow, 

 serves to swing open the outer valve, and to keep it open until the pond 

 is emptied or the tide turns. In the latter event the outer valve closes 



Fig. i. 



automatically until the level of the water without again falls below that 

 of the water remaining in the pond when its operation is resumed. 



Fia. 



