HYDRAULIC DREDGES 



97 



narrow spiral knives used on the same dredge proved to be more 

 efficient in clay than the first one. The effect of the cutters of work- 

 ing in sand was to wear the blades to a considerable extent, but the 

 bearings kept in good order, the sand being excluded from them by 

 an arrangement by which they are lubricated by water under pressure. 

 From Mr. Apjohn's experience it could be deduced that a cutter 

 with spiral knives is more adaptable for soils hard and compact; 

 while the straight blades seem very efficient in loose soils. The 

 rotary motion of the cutter is imparted by gears and shaftings 

 placed above the suction pipe and operated by special engines. 



FIG. 26. Rotary Cutter at the End of Suction Pipe. 



Fig. 26 shows the rotary cutter at the end of the suction pipe of a 

 hydraulic dredge. 



On the Chicago Drainage Canal was employed a hydraulic 

 dredge designed by Gen. William S. Smith of Chicago. This dredge 

 was provided with a new device for loosening and mixing the material 

 with water. Instead of a rotating cutter at the end of the suction 

 pipe a series of hydraulic jets was used. A pump placed near the 

 bow of the boat supplied water for the jets, which were controlled 

 by hand, the nozzles being fastened to long poles. The jets 

 loosened the muck effectively enough, but at the same time 

 they forced the material away from the suction pipe, so that of the 

 total volume pumped out only a small per cent was solid matter. 

 Various means were tried in an effort to keep the muck near the 



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