220 A TREATISE ON DREDGES AND DREDGING 



materials from the bottom; (b) the apparatus for washing the dc'hris 

 and collecting the gold; (c) the conveyor for the disposal of the 

 tailings. Each one of these will be described separately, but all 

 are mounted on a single pontoon. 



The pontoon of a placer dredge is designed to work in shallow 

 water and consequently is built with a flat bottom in order to have 

 the least draft. It is provided with a central well for the ladder and 

 the walls of the pit are firmly held together by the truss of a gantry 

 used for the raising and lowering of the ladder. The pontoon is gener- 

 ally built of wood, but when the dredge must be shipped to distant 

 points or where skilled labor is scarce, the pontoon is made of 

 steel plates and girders built up in sections of such weight as to 

 be easily transported and joined together. About the center of 

 the pontoon is located the tower supporting the upper end of the 

 ladder. 



The ladder is built up of steel beams and plates riveted together 

 and forming a solid support for the loaded buckets that slide on 

 its upper surface. To facilitate the sliding several rollers are intro- 

 duced. The lower end of the ladder carries the tumbler of polygonal 

 shape, around which revolve the buckets, while as usual the driving 

 tumbler is at the upper end of the ladder. Ladders are made of 

 different lengths, varying with the depth at which the dredge is 

 designed to work. In any case it is preferable to have the ladder 

 of such length as to work under an angle of 45. Buckets are 

 made from 3 to 13 cu.ft. capacity. The Bucyrus Co. build placer 

 dredges of three sizes, having buckets of 3, 5, and 7 cu.ft. capacity 

 respectively. Buckets are constructed of steel plates reinforced at 

 the mouth by another steel plate of greater thickness; they are 

 built with wide mouth in order that their contents may fall more 

 rapidly when the point of discharge is reached. In some dredges 

 the buckets are attached one to another so as to form a continuous 

 series succeeding each other on the consecutive faces of the revolv- 

 ing tumbler. In other dredges the buckets are alternated with 

 the links of the chain so that a bucket and a link succeed each 

 other upon the consecutive faces of the tumbler. The links are 

 made of solid steel bars connected by steel pins. The bucket chain 

 moves along the ladder owing to the revolving of the driving 

 tumbler being driven by special engines and the power transmitted 

 in any one of the ways indicated in the general discussion of the 

 ladder dredge. The engine for lifting the ladder and other 



