RECENT CONSTRUCTION OF MOVABLE DAMS. 269 



and Mirowitz, are in operation. A brief description of them is given in the following 

 paragraphs. 



Dam No. i at Troja. This dam lies just below Prague, and will provide the last 

 pool of the system. It was commenced in 1899 and consists of three openings, two of 

 122 feet and one of 153 feet in width, closed with needles from 12.2 feet to 15.3 feet 

 long, supported by fixed bars. The general construction and details of the dam and 

 of the lock are similar to those at Klecan. The lift of the lock is given as 17.7 feet. 

 A chute is provided for rafts 39 feet wide and about 1400 feet long, with slopes varying 

 from i in 200 to i in 100. 



Dam No. 2 at Klecan. Work was commenced here early in 1897. The level of 

 the pool above the original normal stage of water was to have been n.8 feet, but as 

 this was considered too great for needles, it was changed to 10.2 feet. The lift of the 

 lock is given as 10 feet. The openings are three in number, two weirs of 127 feet 

 each, and a pass of 131 feet, built chiefly with granite foundations and with wide 

 aprons to guard against undermining. They are all closed with needles supported by 

 swinging escape-bars on the Kummer system. 



The trestles are spaced 4 feet i inch apart and connected to each other by fixed 

 chains, and are 12.1 feet, 13.5 feet, and 15.4 feet high over all. They are protected 

 when down by a sill 2 feet high. The aprons or floors are of iron and hinged to the 

 trestles. When lowering, a small crab is taken out on the dam and attached to each 

 trestle-head in turn, and the preceding trestle is lowered by connecting its chain with 

 the crab. Another method is to place the crab on the pier and take a long chain out on 

 the foot-bridge. The trestle-chains are then attached in turn to its end, and can thus 

 all be lowered without moving the crab. 



The needles are of larchwood, and provided with iron handles, hook-shaped. They 

 are 10.8 feet, 12.0 feet, and 13.0 feet long, and measure 3.7 inches, 3.9 inches, and 

 4.7 inches square, and when wet weigh 46 Ibs., 55 Ibs., and 72 Ibs., respectively. Thir- 

 teen are used to each bay, all connected to a rope. 



The lock is of cut stone and rubble masonry combined, and is 722 feet long over all, 

 divided by a pair of gates into two chambers, one about 220 feet long and 36 feet wide, 

 the other about 460 feet long and 65^ feet wide. The width between the walls at each 

 end is 36 feet, and one chamber wall is built on the same straight line from end to end. 

 The filling or emptying of the two chambers as a whole takes 18 minutes This is 

 accomplished by means of culverts in the walls, closed by vertical wickets. The 

 middle gates are provided with butterfly valves. 



All the gates are composed of steel frames sheathed with wooden plank, and are 

 operated by spars and racks placed under the coping. The upper gates arc 9^ feet 

 high, and the two lower pairs 19$ feet high. Their tops are all 16 inches above pool. 



On the right bank a chute for rafts was built 39 feet in width and composed of a 

 long and a short slope. As first constructed, the latter was made i in 10, and 134 feet 

 long; this, however, was found to be too steep, and it was changed to a mean slope 



