jo THE IMPROVEMENT OF RIVERS. 



In the year mentioned, however, M. Pasqueau introduced the double-stepped liurter, 

 which has practically superseded the former kind, and which provides a second shoul- 

 der up stream of the first one. In this case the entire wicket is pulled up stream, and 

 the end of the prop leaves its support and falls over the second shoulder into a groove. 

 The wicket is then released and the prop slides along the groove which guides it back 

 into position ready for raising. 



The various types of hurters are shown on the corresponding drawing. 



Tripping-bar. This is a bar provided with teeth and moving along just under 

 the feet of the props. It is worked by gearing in the pier or the abutment masonry, 

 the teeth being arranged so as to strike each prop in turn and pull it sideways from 

 the shoulder, thus allowing the wicket to fall. A cushion of water is always neces- 

 sary, as the falling parts will otherwise be damaged or broken by the shock. 



As the total travel of the bar is limited to the distance between any two props, 

 minus the width of the tooth, this arrangement is inapplicable to wide passes, since 

 many props would have to be thrown at once in order to get all of them down within 

 the limit of the travel. About 160 feet is the widest opening to which it can be applied, 

 and it has then to be divided into two parts, one working each way. At first only 

 one tooth engages, but at the last several teeth have to be pulling together. 



The tripping-bar has been in successful use on the Meuse, the Upper Seine, and 

 elsewhere for many years. Some trouble was at first experienced with its use, owing 

 to insufficient power and to the obstruction of gravel, but in the later designs these 

 objections were overcome, the machinery being given a force of 12 to 15 tons, and 

 pulling the teeth through any ordinary obstacle. 



It has not found favor in America, partly owing to the width of the rivers to which 

 Chanoine dams have been applied, and partly to the fact that on the dams where used 

 the bar was weak and was also raised off the floor, as in the French design, so that drift 

 caught under it frequently and unseated it. Its use was therefore abandoned, but 

 the few times in which it was maneuvered showed it to be a valuable device, as the 

 wickets could all be lowered from the shore and with ice against them. To guard against 

 danger from drift, etc., the bar should be made of flat section, say 5 inches wide and 

 | of an inch thick, the teeth being riveted on as bent plates, and it should be recessed 

 so that its top will not project above the masonry. It may slide on rollers or on 

 surfaces provided on the hurters, the latter plan being more reliable, as the rollers 

 may become clogged or broken. The machinery, which should possess a large excess 

 of power, must be arranged to act in either direction, since it has to push the bar back 

 to place after the wickets have been thrown. Guides must be provided every few 

 feet to prevent the bar from lifting up ; these can be bolted to the hurters. 



Where it is desired to use a Pasqueau hurter, which would always be advisable 

 since the bar may at some time become unworkable, the latter may be arranged to slide 

 between the two shoulders, the prop passing over it when the wicket is maneuvered. 



Maneuvers. A wicket dam is usually raised from a service bridge placed on its 



