218 A TREATISE ON DREDGES AND DREDGING 



are located at the stern. The dredges are of the close bucket type, 

 and there is said to be a great chance for improvement on the diamond- 

 saving end. Such dredging is now being done in several parts of 

 the world. 



Gold Dredging. Dredging is done more extensively for gold 

 than for any other metal or precious stone, and much has been 

 written on the subject. It is impossible to fully cover the subject 

 here. 



Numerous kinds of dredges have been used and are being used 

 for gold dredging, among them being the suction and hydraulic 

 power dredge, the dipper, ladder and the submerged jet dredge. 



It is claimed that the hydraulic suction dredge is not efficient, as 

 the grains of gold, on account of their great weight, drop to the bottom, 

 even when rotary cutters are used on the suction pipe. However, 

 they are used in some sections and in some soils with fairly good 

 results. For instance when the grains of gold are exceedingly 

 small and found entirely in coarse-grained sand the suction dredge 

 has cleaned up the gold. Also when the gold-bearing gravel or 

 sand is underlaid with fissured rock, so that much of the gold cannot 

 be reached with buckets, the suction dredge has cleaned up such 

 bottoms. Mr. Henry G. Granger advocates the use of a hydraulic 

 suction dredge for gold mining, and in a paper before the Am. Inst. 

 of Mining Engineers gives the specifications for such a dredge with 

 a capacity of 1000 cu.yds. per hour, capable of raising large boulders. 

 One consideration against hydraulic suction dredges is the sup- 

 ply of water and the power needed. In bucket dredging, under 

 favorable conditions, about one-eighth part of water to one part of 

 material is lifted, while in centrifugal pump elevating, at least 

 fifteen parts of water to one of material have to be raised, this- 

 being necessary, according to Mr. H. L. Lewis, as the water has to 

 be applied for treatment of the gravel in addition to keeping every- 

 thing in a state of solution for successful elevation. It is evident 

 that this means more power, and shows the necessity of a large 

 amount of water. 



The dipper and grapple dredges have been used for gold, but 

 they are illy adapted to the work, as they disturb the gravel in digging 

 so that much of the gold sinks to the bottom and is lost. It is also 

 impossible to construct a bucket that is watertight, consequently 

 the gold collected at the bottom of the bucket is readily lost through 

 the cracks. Likewise as they are intermittent machines they deposit 



