GRAB DREDGES 177 



being controlled by a drum of an ordinary double-cylinder-double- 

 drum reversible engine. 



The manner of operating the grab bucket by means of the double- 

 line system is very simple. When the bucket is lowered to its work 

 both lines run out freely, being kept from overrunning by a brake; 

 thus the full energy of the falling bucket is employed in embedding 

 itself in the material to be lifted. As the bucket can be opened or 

 closed in any position, in submarine works in case the bucket becomes 

 fastened to the bottom, it can be instantly opened to clear itself 

 and another lift taken without further raising the bucket. 



There is no limit to the depth at which the grab bucket dredge 

 can be employed. For ordinary navigation purposes a depth of 

 30 to 35 ft. is the limit of dredging, but at such a depth other dredg- 

 ing machines will give perhaps better results. It is, however, in deep 

 dredging that the grab dredge finds its greatest usefulness; thus, for 

 instance, the dredging for the Buffalo breakwater in 90 ft. of water 

 was very successfully done by the clamshell bucket dredge "Finn 

 MacCooP' illustrated in the following chapter. It is also used for 

 sinking cylinders, shafts and digging pearls, corals and auriferous 

 soils from great depths. 



