A-FRAME DAMS. 



2 5S 



sill, or may be a part of it. A space is left open between the lower boxes for the escape 

 of water, and to assist in keeping the floor clean. The sill closely fits the upper face 

 of the trestles at the point- where the angle is made with the eye-pieces, so as to prevent 

 leakage. The construction may be such as to permit overflow. 



As the top of the trestle will be from 18 to 30 inches wide, it will answer for a 

 walk as required. 



In the head of each trestle is located a pocket-wheel for use in connection with the 

 maneuvering chain. This wheel turns on a shaft attached to the frames. At one 

 edge of the wheel, and forming a part of it, is a ratchet. A pawl, having a tooth which 

 may fit loosely into this ratchet at one end, and having the opposite end formed into 

 a rounded wedge, is pivoted so as to be readily lifted out of the ratchet by depressing 



GENERAL DESIGN OP AN 



A-FRAME DAM OF 



Low LIFT. 



PLAN 



the wedge end. This depression takes place just as the trestles become vertical in 

 raising, by the rounded end being pushed by a stop or projection on the adjacent 

 trestle made for the purpose. As long as the trestles remain touching each other this 

 stop will hold the tooth of the pawl out of the ratchet, and the wheel is free to turn. 

 Let a trestle begin to incline or descend, and the pawl, being released from the stop, 

 immediately falls into the ratchet and arrests the movement of the wheel. The pockets 

 in the wheel are made to fit the chain used for raising and lowering the trestles, and this 

 chain cannot move without also moving the trestle when the pawl is in the ratchet. 



