MOVABLE DAMS. 209 



The first distinct type of movable dam was erected in 1818 on the Lehigh River, 

 in the United States, and was called a bear-trap dam. It consisted of two wooden 

 gates revolving on horizontal axes at the floor level. The down-stream gate pointed 

 up stream, and the up-stream one pointed down stream, the latter resting on the edge 

 of the down-stream gate when raised. The dam was operated by water running under 

 the gates through culverts and forcing them up. The example v/as not copied, and 

 until recent years the type remained practically unknown. 



In the year 1834 M. Poiree, an eminent French engineer, invented the needle 

 dam. This ushered in a new era in navigation, and this type of dam soon multiplied 

 and was improved and modified, and other inventors came forward with new ideas, 

 some good, some bad, until to-day there are numerous systems from which to choose. 



The invention of movable dams was only arrived at after long discussion of ways 

 and means for more successfully operating the apparatus used for closing the chutes 

 in the old stationary dams. The use of needles was then already old. The problem 

 to be solved was how best to widen the passages to accommodate the increased 

 requirements. The experiment of supporting the tops of the needles by a rope was tried, 

 and was in a measure satisfactory for lifts of 2 to 3 feet on passes of considerable width. 

 The rope, which was braced . to the down-stream side of the foundation by strips of 

 wood, was tied to a pier by one end while the other was wound on a windlass. To open 

 the dam it was only necessary to release the line at the pier, when the whole set of needles 

 would float out, being attached to the rope beforehand, as were also the braces. 



This was the status of improvements in fixed dams when the first movable dam 

 was constructed, and it was only natural that iron trestles should supersede ropes in 

 needle dams, that gates sliding on these trestles should later on replace the sluice-gates 

 of the old chutes operated from an overhead bridge, that the swinging wickets formerly 

 used to increase the heights of stationary dams should form the dam itself in after 

 years, and that poutrelles hinged together and supported on trestles should form the 

 curtain dam that was to come into use. 



Classes and Kinds. Movable dams may be divided into two general classes: (i) 

 those requiring extraneous power for their maneuvers, and (2) those operated by the 

 force of the water. Among the first class may be named the various types of "trestle 

 and wicket dam, like the Poiree, Chanoine, Boule, Camere, etc., while the second class 

 comprises the several forms of bear-trap, drum-wickets, etc. The first class is prac- 

 tically the only one so far applied to navigable rivers, and its application until recent 

 years has been confined largely to the wickets of Chanoine and the trestles and needles 

 of Poiree. 



The forms of closing are many, and frequently vary on the same dam; for instance, 

 the pass may be of wickets and the weir of needles, or the reverse may be the case: 

 while more than one type has been applied, even on the same part of a dam ; for instance 

 at the Suresnes dam, in France, the pass is closed by trestles supporting alternate bays 

 of Boul6 gates and CamerS curtains. 



