228 



A TREATISE ON DREDGES AND DREDGING 



matter) are lifted and washed one by one into a gold-box or tub 

 until quite clean. When all the mats have been washed, they 

 are relaid as before, preparatory to starting work. 



The action of the buckets is slow but gradual. In the close- 

 connected type the number per minute ranges from 18 to 30, 

 while with link type 12 to 14 are delivered. The speed of the 

 ladder is generally about 50 ft. per minute, varying with the hard- 

 ness of the ground. Sometimes it is reduced to less than 40 ft., 

 and again is increased to 75 ft. These dredges are now built to 



FIG. 76. Placer Dredge. 



excavate to a depth of 65 ft. Fig. 76 illustrates this type of dredge. 



The following description of the "Hunter Dredge" and of its 

 work, and the cost of excavating gravel with it, is condensed from 

 an article in Engineering Contracting: 



"This dredge was built by the Western Engineering and Construc- 

 tion Co., of San Francisco, Cal., for the Oro Water, Light and Power 

 Co., of Oroville, Cal. The dredging machinery proper was manu- 

 factured by the Bucyrus Co. of South Milwaukee, Wis. Operation 

 of the dredge was commenced in August, 1907, working placer 

 gravel 38 ft. deep from the water line to bed rock. 



"The entire plant embodies the latest type of construction, 



