HINTS ON SELECTING DREDGES FOR VARIOUS WORK 53 



in, thus feeding the sand to the pump. For harder material the 

 agitator is equipped with knives, which cuts the material as they 

 revolve. For stiff and heavy clay and harder material sufficient 

 space must be left between the blades for large lumps of clay and 

 small boulders to enter the suction pipe. The cutter must not only 

 be of great strength, but must be so attached to the suction pipe 

 as not to be easily detached by the great strain that comes 

 upon it. 



Neither the hydraulic dredge nor ladder type can handle anything 



FIG. 14. Stone Lifter on St. Lawrence River. 



in the way of rock except small boulders. The limit is smaller with 

 the hydraulic dredge than with the endless chain or ladder dredge. 

 A dipper dredge can handle rock and boulders of several yards in 

 size, and the grapple dredge can also handle rock and boulders 

 when fitted with a special stone grapple. But with any type of 

 dredge large stones or boulders must be broken up before they can 

 be removed by dredges. At some places there are now used stone 

 scows or lifters. Fig. 14 is an illustration of such a machine used 

 by the Canadian Government on the St. Lawrence River. This 

 stone lifter, by means of its stone hooks or grabs, will raise boulders 

 weighing as much as 50 tons. 



