EXCAVATION OF SUBAQUEOUS ROCKS 



25 



the rule adopted by the makers being to make the ram 1 ton in" 

 weight for each meter of depth. The whole force of the impact is; 

 concentrated on a few square inches of rock surface, so that the 

 hardest rock is readily split and pulverized. 



The ram falls through a well in which is a fixed hardwood guide. 

 This guide is short and loose fitting, and it therefore permits the 

 cutter a certain degree of freedom in an angular direction. The 

 guide is mounted so that it can be adjusted vertically to suit different 

 depths of water and is provided with renewable wearing plates and 

 spring cushions to deaden any shock. 



FIG. 4. Lobnitz Rockcutter. 



The cutter bars are provided with ogival heads, having a center 

 harder than their circumference and consequently are self-sharpen- 

 ing, the rock wearing away the hard center less rapidly than the 

 softer surrounding metal. The other end of the ram is attached 

 to a wire rope which is wound around the drum of a hoisting engine. 

 The rope drum is loose on its shaft, being driven by a friction clutch, 

 which is automatically thrown into gear the instant the ram reaches 

 the rock. This is effected by a bell crank lever carrying a sheave 

 which is pressed against the lifting rope by a weight on the short 

 arm of the bell crank. When the ram reaches the rock the lifting 

 rope slackens, and as a consequence, the bell crank lever moves 

 forward under the influence of the aforementioned weight, and this 



