34 A TREATISE OX DREDGES AM) DREDGING 



by a larger number of drills. These machines are similar to 

 those described above, the only difference being that three or 

 four drills are mounted either on the port or starboard sides of the 

 pontoon and their distance apart being equal to the required dis- 

 tance of the drill holes. Fig. 8 shows one of these machines as built 

 by the Rand Drill Company. 



A uniform bottom cannot be obtained by blasting the rock in 

 the manner just described, on account of irregular ruptures caused 

 by lack of uniformity in the quality of stone. When it is desired 

 to have a level bottom the drilling must be made from a shaft or 



FIG. 8. Rand Drilling Boat. 



caisson. The shaft can be constructed of different materials, but 

 it is more convenient to build it in the shape of a metallic caisson 

 suspended from a boat, or better, between two boats close together 

 with a space for the caisson between them. After the boat or boats 

 have been anchored and made firm to the bottom the caisson is let 

 down to the bed of the river and the water forced out by means 

 of compressed air, which is also used for operating the drills. The 

 holes are drilled about a yard below the desired level of the bottom. 

 After the dynamite; cartridges have been put in place the boat is 

 pushed off to one side for a distance of about 130 to 160 ft., all the 

 cartridges being exploded simultaneously by electricity; then the 



