26 A TREATISE ON DREDGES AND DREDGING 



movement is caused by suitable connections to throw the friction 

 clutch into gear. 



To break the rock the ram is dropped on the same spot time 

 after time till the desired depth is reached; but if a greater depth 

 of excavation is needed it is usual to execute the excavation in 

 installments, first breaking up a layer 3 ft. to 6 ft. thick, this thickness 

 depending on the character of the rock. The broken rock is then 

 dredged away, and the work resumed on the cleaned surface till 

 the desired depth is attained. In finishing off, this depth should 

 be about 9 inches more than the depth of the excavation so as to 

 make certain that the dredge buckets on removing the de*bris shall 

 meet with no obstruction. It is very important that the ram should 

 fall exactly on the same spot on repeating its blows, a movement 

 of even a few inches reducing the efficiency of the machine. Accurate 

 maneuvering is thus very important, and this is obtained by means 

 of sighting rods which are collimated with base lines placed on land. 

 During work, the man in charge constantly controls the position 

 of the barge by keeping his eye on the sight-rods. After the work 

 is finished in one spot it is necessary to move to a fresh one about 

 3 ft. away. This is done by means of six mooring chains operated 

 with steam winding gear. 



In practice it is found that it is best to space the blows 3 ft. 

 apart. If wider spacing is adopted a greater quantity of rock is 

 loosened, but it is not as completely broken up and therefore cannot 

 be dredged economically. With 3 ft. spacing, approximately 50 

 per cent of the whole mass is absolutely pulverized, and the sand 

 thus formed, if there be a strong current, is washed away. In hard 

 rock at least 2 cu.ft. are, on the average, broken per blow, so that, 

 with a single-cutter machine about 9 cu.yds. may be taken as effective 

 work. The machine can be built either with one or with two rnms. 

 The efficiency of the double-cutter machine is almost double that of 

 the single one. To handle the machine a crew of six men is required 

 in the case of a single cutter, and eight for a double cutter. The 

 former w r ill consume 1 ton of coal per ten-hour day and the latter 

 1J tons. 



Concerning the progress and cost of the work of the Lobnitz 

 machine Mr. W. Henry Hunter, chief engineer of the Manchester 

 Ship Canal, has giv^n the following report: 



