DREDGING FOR METALS 227 



the belt, along which they travel and are tipped over when the belt 

 revolves around the guiding drum at the upper end of the beam. 

 Provisions are made to prevent the tipping of the materials on the 

 sides at the beam. The beam is made 50 or 60 ft. long, thus allowing 

 the dumping of the material at some distance from the back of the 

 dredge. 



These ladder dredges may be operated either by steam engines 

 or by electric motors. Where two or more dredges are in close 

 proximity to each other it is more economical to locate a power 

 station on shore and transmit electricity to the dredges than to have 

 engines or boilers on board. 



These dredges as a rule do not work in rivers, but in trenches 

 contiguous thereto. In most cases a pit is dug deep enough for the 

 dredge to float in and of sufficient dimensions to allow it to turn 

 round. Very little water is required to float the dredge and to 

 operate it, as the water which is pumped for washing purposes 

 comes back to the pit. The amount of water is determined by 

 seepage and the cleanness of the gravel dug. The face of the pit 

 must be kept square and its corners worked up, otherwise it will 

 gradually narrow and ground will be left behind and lost. The 

 dredge works ahead, undermining the banks, and the material will 

 naturally fall to the buckets, when good work will result, and the 

 dredge can then run for an hour or more with full buckets without 

 being moved. 



It is very important to keep the dredge running as near 24 hours 

 a day as possible, for when a dredge stops, the producing part of 

 the plant stops, while the expenses continue. Well-managed dredges 

 are making a monthly average of from 80 to 90 per cent of the 

 possible running time, including that lost for cleaning up of the gold 

 tables, an operation which takes from 5 to 6 hours every fifteen 

 days. This time, however, is not all dead loss, as it can be used to 

 . advantage in general repairs. 



Of Australian dredges Messrs. Marks say: "The sluice-box or 

 tables are cleaned up weekly, and the clean-up is conducted as 

 follows: In either case the ripples or pieces of expanded metal 

 are lifted and the lighter material caught in the sluice-box or tables, 

 is run off the mats by a gsntle stream of water, the gold, heavy 

 sands, and pieces of metal of any description alone remaining. 

 The mats being run clear, the water is turned off, and the calicoes 

 {which are forming the upper stratum of the mattress of fibrous 



