DREDGING FOR METALS 219 



too much material at one time, and there is a long interval between 

 the loads. 



The hydraulic power dredge is used for gold dredging in New 

 Zealand. It is known as the O'Brien Patent Hydraulic Power Dredge. 

 Water is supplied at sufficient elevation to give gravitation pressure 

 to a Pelton wheel for operating a ladder dredge. Thus this power 

 is used instead of steam or electricity. This motive power saves 

 in the first cost of construction in boilers and engine, fuel, engi- 

 neers and maintenance expenses. When gravitation pressure can 

 be obtained, this system can dredge and treat gravel at a much 

 cheaper cost than steam power. 



Another dredge used in gold mining in New Zealand and Aus- 

 tralia is the Johnson Submerged Jet Dredge. The principle is that 

 of the hydraulic elevator adapted to the requirements of a dredge, 

 and consists of ordinary pontoons divested of boiler, engine and 

 bucket ladder, these being replaced by a hydraulic ally-driven Pelton 

 wheel to work winches, and a hydraulic elevator in place of a bucket 

 ladder to raise materials. The pressure is from gravitation of water. 

 The theoretical supply of the elevator is 1000 tons per hour. 



In Chapter V is mentioned a dredge used for gold mining in 

 California, from which a caisson is lowered to the river bed, and the 

 water pumped from it. Then men enter this caisson and by hand 

 excavate the sand and gravel containing the gold. 



The DuBois hydraulic dredge is also used. In this dredge a 

 caisson is dropped to the river bottom and the suction pipe is let 

 down into the caisson. A diver going down on the outside of the 

 caisson enters it, and stirs up the gravel, causing it to enter the 

 suction pipe. It is stated that in this manner the fine particles 

 of gold cannot float away. Under favorable conditions the diver 

 can remain under water several hours. 



Ladder Dredges. The ladder dredge, taking up the material 

 from the bottom with a minimum of agitation and working slowly 

 and steadily is to-day considered the best for gold dredging. Besides, 

 the buckets retain fully their content, carry a quantity of water to 

 facilitate the work of washing, and deliver the material amidships in 

 a continuous small stream. For these reasons the ladder dredge 

 has always been recognized as the ideal type of dredge for gold 

 mining. 



The ladder dredge employed in mining consists of three different 

 parts, which are: (a) the ladder dredge for the excavation of the 



