, 5 6 THE J.MPRO\'EMl-:\ 1 < >/' RIVKRS. 



In place of the chain-wheel and pawl a latch can be used, placed and removed by 

 hand at the time of the maneuvers. 



In addition to the maneuvering chain, which may be connected with or discon- 

 nected from the trestles at will, there may l>c j -laced between each adjacent trestle a 

 few feet of chain, called the fixed chains. These are fastened to the trestles by eye- 

 iHilts. and their length is determined by the number of trestles it is desired to raise 

 simultaneously that is, by the power of the crab' but they must be sufficiently 

 long to j>ermit the trestles to lie flat when down. 



Maneuvers. On the lock wall or pier is located a chain crab for maneuvering. 

 The chains which pass over the pocket-wheels in the trestles are operated by this crab. 

 The last trestles arc made fast to the ends of the chains. 



The methods of lowering and raising are the same for a pass as for a weir. To 

 lower the dam the trestle next the abutment is unhooked from the masonry and 

 pulled toward the abutment, the chain being unwound at the same time on the crab, 

 until it tightens the fixed chain (in case one is used) between it and the next trestle 

 and starts that trestle downward. As this occurs the pawl will engage with the ratchet, 

 and lock the next to the last trestle on the chain. The unwinding goes on con- 

 tinuously, and when the next fixed chain is stretched it will start a third trestle, and 

 so on until all are down. In case no fixed chain is used, a man standing on top makes 

 the connection between the chain and trestle. 



To raise the trestles the chain on the crab is wound in, bringing up the first one 

 (being the last lowered) and starting several others. When the first becomes vertical 

 and strikes the masonry, its pawl is lifted out of the ratchet by a stop made for the 

 purpose, and thus the trestle is released from the chain without stopping the crab. 

 The continuation of the winding brings up the second trestle, which is released from 

 the chain when its pawl strikes the stop on the first. All trestles are thus raised, 

 after the first, by winding in a length of chain equal to that of the short chains con- 

 necting the trestles, or equal to the spacing given when the dam was lowered. Where 

 a continuous chain is not used the trestles may be raised and lowered precisely as are 

 those of wicket and needle dams, each trestle being connected with its neighbor as 

 brought up. 



To regulate the pool, in either case, it is only necessary to lower a sufficient 

 number of trestles on the weir next the abutment. In these, which are most liable to 

 be used for pool regulation, the fixed chains may be lengthened so as to give less load 

 on the crab in order that the operation of raising may be performed by the watch- 

 man alone when necessary. 



Advantages Claimed. The advantages claimed for this style of dam over those 

 formed by needles, gates, or wickets are: 



The dam, being raised and lowered across the current, can be ojjerated either 

 wholly or partially under great heads of water with two or three men. 



In raising, the dam is complete when the trestles arc up, while in other forms it 



