BRIDGE DAMS. 



249 



of the frame. Lengthwise the frames are arranged in groups of two, braced together, 

 and the axes of these groups are 3.80 feet apart. The object of this division was to 

 reduce the width of the curtains to 3.80 feet, in case the width used, 7.61 feet, should 

 be found too great ; but as it has been found convenient, the arrangement of the frames 

 in subsequent dams of this type has been simplified. 



There are two hoisting-chains for each frame, and each chain is divided into two 

 branches, the end of each branch being attached to an upright, thus dividing the 

 strain lifting the frame into four equal portions. On the down-stream side of the 

 uprights a strong wrought-iron hook with angle-irons is attached, for the purpose 

 of raising the frame in case of accident to the chains or to their attachments. This 

 can be done by lowering, along the upright, a chain, the bight of which will be held 



SECTION SHOWING BRIDGES AND OPERATING-CRANE AT POSES DAM. 



securely by the hook. Ring-bolts are attached to the up-stream side of the uprights, 

 so that the frames may be slung below the upper bridge when any repairs are required. 



The suspending chains for the curtains are hooked to rings attached to the two 

 outside uprights of each frame 4.10 feet above the foot-bridge. The two pulleys 

 for rolling the curtains are placed between the intermediate uprights. The lower 

 pulley, holding the down-stream chains, is slightly smaller than the other. This 

 inequality insures a distance between the chains equal to the thickness of the first 

 curtain bar. Besides rolling the curtain, each side of the endless chain can be fixed 

 upon its guide-pulley by a stop carrying a finger, which enters the link of a chain when 



