FLOATING AND IL\FT1NG 



355 



nects this chamber with the lower pool. When the connection 

 with the upper pool is opened, while that with the lower pool 

 is closed, water from the upper pool fills the chamber under 

 the gate. This causes the downstream leaf to rise, first by 

 flotation and then by the impulse from the flow of the water. 

 The upper leaf is raised by the lower leaf which slides under it, 

 the friction being reduced by rollers. The height to which the 

 gate rises is limited either by stay chains, or by a wood cleat 

 nailed on the under side of the upper leaf. In lowering the gates 

 the operation is reversed, the connection with the upper pool 

 being closed while that with the lower pool is opened. The 

 gate may be made to assume any intermediate position by 



Fig ioi. — The Bear-trap Sluice Gate. 



regulating the extent to which the two valves controlling the 

 inlet and outlet of the chamber under the gate are opened. 



The objections to this form of gate are: (i) the overlap of the 

 upper leaf over the lower one necessitates lifting a considerable 

 amount of water when the gate is raised; (2) the head of water 

 obtainable is only about one-third of the total width of the 

 leaves; (3) the friction between the two leaves, even when re- 

 duced by rollers makes it difficult to operate the gate smoothly; 

 (4) the gate must be made in one section and if the gate is wide 

 one side is apt to go up faster than the other, causing twisting 

 strains; (5) any driftwood or stones which may lodge between 

 the leaves make the lowering of the gate impossible until the 

 obstruction is removed. 



Water can be let out of the reservoir very rapidly with a gate 

 of this character, and the latter can be raised and lowered by 



