22 A TREATISE ON DREDGES AND DREDCJIXd 



These machines drill vertical holes which are charged and fired in 

 round as soon as a few holes have been drilled. 



The rock to be removed may form a large reef extending to the 

 surface. This may be located in the way of navigable channels 

 and form serious obstacles to navigation. Such rocks being 

 isolated, in the middle of large bodies of water, and far from impor- 

 tant buildings, are conveniently removed by a huge single blast. 

 Such a blast can be prepared by honeycombing the rock, by sinking 

 shafts and driving small radiating headings at different levels and 

 from these drilling holes in all directions. These numerous holes 

 are charged with explosives and are fired simultaneously. This 

 method was used in blasting the Flood Rock at Hell Gate, in New 

 York Harbor. Another method of removing isolated reefs consists 

 in building a coffer-dam all around the rock. Within the coffer-dam 

 the rock is removed in the usual way as in land work until the pit 

 has been excavated to the required depth. Then lift holes are driven 

 all around underneath the ring of rock supporting the coffer-dam. 

 The holes are charged and fired simultaneously and the ledges of 

 rock surrounding the pit together with the coffer-dam are then 

 broken and shattered by the explosion and reduced to small 

 fragments that can be easily picked up by dredges. This method, 

 called " lift holes," was used in removing Henderson's Point at 

 Portsmouth Navy Yard, New Hampshire. 



The following machines and methods have been used to break 

 subaqueous rock by hammering: 



Scott & Godsir Cutter. This machine (see Fig. 3) was designed 

 and built in 1897 by Messrs. R. M. Scott and A. Godsir in Sidney, 

 New South Wales, and described by Mr. Charles Graham Hepburn 

 in the Transactions of the Inst. of C. E. This rock-breaking machine 

 consists of a drilling tool connected directly to a piston, recipro- 

 cating in a steam cylinder of considerable length, which is carried 

 on a vertical slide or carrier, capable of vertical adjustment on a 

 tower erected on a barge, pontoon or other floating structure. 



The drill varies with the class of work to be done. For ordinary 

 rock excavation the head is formed of three drills forged from a 

 steel bar 6 in. by 4 in. in cross section. The drill is secured to the 

 end of the piston rod by means of a cotter. The cylinder is 14 in. 

 in diameter and 5 ft. in length; the piston rod is (> in. in diameter 

 at its top and increasing to 1\ i.n. in diameter at the lower end. and 

 is guided by three bearings. To obviate the danger of the piston 



