, s , THE IMPROVEMENT OF RIVERS. 



The Thenard shutter consists of a panel hinged to a floor at the 1>ottom and sup- 

 ported by a prop near its middle on the lower side, the shutter lying down behind a 

 sill when lowered. To facilitate the lowering M. Tlu'nard introduced a bar provided 

 with projections, and capable of being pulled or pushed by means of gearing in the 

 masonry. It was arranged so that the projections would strike the props one at a 

 time and pull them from their supports, thus allowing the shutters to fall. This was 

 known as the tripping-bar. It was found difficult to raise them, however, against 



'DETAILS THENARD DAMS. 



SECTION-POIREETHENARD DAM 



any head, and f<>r 1hat reason counter-shutters were introduced up stream. These 

 were arranged to rise down stream, or in the opposite direction to the main shutters, 

 being held in an upright position by chains. 



Maneuvers. - To raise the dam, the counter-shutters, which were kept in place 

 on the floor by latches worked by ,-i tripping-rod, were released, and the force of the 





