104 TUNNELING 



drill is exerted by a cylinder and hollow ram (/), which revolves 

 about the differential piston (), which is fixed to the envelope 

 holding the shell (.#) This envelope is rigidly connected to 

 the bed-plate of the motor, and, by means of the vertical hinge 

 and pin (T), is held by the clamp (F) embracing the rack-bar. 

 When water is admitted to the space in front of the differential 

 piston the ram carrying the drilling-tool is thrust forward, and 

 when admitted to the annular space behind the piston, the ram 

 recedes, withdrawing the tool from the blast-hole. The drill 

 proper is a hollow tube of tough steel 2j ins. in external diame- 

 ter, armed with three or four sharp and hardened teeth, and 

 makes from five to ten revolutions per minute, according to the 

 nature of the rock. When the ram has reached the end of its 

 stroke of 2 ft 2 ins., the tool is quickly withdrawn from the 

 hole and unscrewed from the ram ; an extension rod is then 

 screwed into the tool and into the ram, and the boring is con- 

 tinued, additional lengths being added as the tool grinds for- 

 ward; each change of tool or rod takes about 15 sees, to 25 

 sees, to perform. The extension rods are forged steel tubes, 

 fitted with four-threaded screws, and having the same external 

 diameter as the drill. They are made in standard lengths of 

 2 ft. 8 ins., 1 ft. 10 ins., and 11 J ins. The total weight of the 

 drilling-machine is 264 Ibs., and that of the rack-bar when full 

 of water is 308 Ibs. The exhaust water from the two motor 

 cylinders escapes through a tube in the center of the ram and 

 along the bore of the extension rods and drill, thereby scouring 

 away the debris and keeping the drill cool ; any superfluous 

 water finds an exit through a hose below the motors and thence 

 away down the heading. The distributor, already mentioned, 

 supplies each boring-machine and the rack-bar with hydraulic 

 pressure from the mains, with which connection is effected by 

 means of flexible or articulated pipe connections, allowing free- 

 dom in all directions. The area of the piston for advancing 

 the tool is 15^ sq. ins., which under a pressure of 1470 Ibs. per 

 sq. in. gives a pressure of over 10 tons on the tool, while for 



