SHUTTER DAMS. 251 



the lever is lowered. Finally, the uprights have on their up-stream faces iron claws, 

 which serve as stops to the rolled curtains. 



Each curtain corresponds to an opening 7.61 feet wide and 17.55 f ee t high m the 

 deep passes. The bars are of yellow pine, each 0.25 foot high, with one-sixteenth of 

 an inch play between them to allow for swelling. Their length is 7.47 feet; giving a 

 play of o. 1 3 foot between two neighboring curtains. This, interval can be closed by 

 a joint cover if the dam requires to be made tight. The thickness of the upper bar is 

 0.13 foot, and it increases progressively downward. The upper bar, exposed to shock 

 from floating bodies, is strengthened by an angle-iron. The hollow cast-iron rolling 

 shoes are heavy enough to cause the curtain to sink easily into the water when unrolled. 

 The rows of hinges are of bronze, so as not to rust. They have strong flanges, and 

 their pins are of drawn phosphor-bronze. All the handling machinery can be carried 

 on cars rolling on the service-bridge tracks. 



With the suspension above described and in use at Port Mort the frames can be 

 removed for repairs as follows: Lifting-jacks are placed under the cross-pieces uniting 

 the two suspending rods of a frame above the down-stream roadway. Each jack rests 

 upon a platform arranged for this purpose in the horizontal bracing of the roadway. 

 After placing the jack and removing the wedges which prevent the lifting, the jack 

 is screwed up, care being taken to wedge the ends of the cross-piece as it moves up. 

 This wedging serves to sustain the lifted frames. The chains from the windlass on the 

 upper bridge are then hooked on, and the frames are rotated to a horizontal position 

 and made fast to the under side of the upper bridge. 



The dam is operated by four electric cranes, traveling on the bridges, the power 

 being supplied by turbines. 



The cost per foot run was as follows : 



' Masonry foundations $813 . 72 



Ironwork: 



Upper bridges 114.09 



Frames 53 . 54 



Curtains, etc 25 . 68 



Total $1007 . 03 



The employment of the existing type of bridge dam on navigable streams in 

 America is not a probability, because navigation requires considerable ' ' headroom," and 

 most of the rivers vary too greatly between low and high water. On the Ohio, for 

 example, few bridges are less than 100 feet above low water. 



SHUTTER DAMS. 



General. The development of the shutter dam is due to M. Thnard, who con- 

 ceived the idea of the type from certain weirs on the river Orb, in France, which had 

 been provided with small movable shutters during the eighteenth century. 



