100 TUNNELING 



from wandering over the face of the rock, but in many instances 

 the hole is commenced with a circular tool. The exhaust 

 water during this period flows away by the hose underneath 

 the motor. In the antigorio gneiss, ten to twelve holes are 

 drilled for each attack, three to four in the center to a depth of 

 3 ft. 3 ins., the remainder, disposed round the outside of the 

 face, having a depth of 4 ft. 7 in. The average time taken to 

 complete the holes is If hr. to 2 hrs. Instead of pulverizing 

 the rock, as do the diamond drills, it is found that the rock is 

 crushed, and that headway is gained somewhat in the manner 

 of a circular saw through wood. The core of rock inside the 

 tool breaks up into small pieces, and can be taken out if 

 necessary when the drill requires lengthening. 



The lowest holes, inclined downwards, are full of water ; 

 consequently two detonators and two fuses are inserted, but 

 apart from this, water has little effect on the charge. The 

 fuses of the central holes are brought together and cut off 

 shorter than those of the outer holes, in order that they may 

 explode first to increase the effect of the outer charges. All 

 portable objects, such as drills, pipe connections, tools, etc., have 

 meanwhile been carried back ; the steel flooring is covered over 

 with a layer of debris to prevent injury from falling rock, and 

 to the end of the hydraulic main is screwed a brass plug 

 pierced by five holes ; and immediately the explosions occur a 

 valve is opened in the tunnel, and five jets of water play upon 

 the rock, laying the dust and clearing the air. The necessity 

 for this was shown on one occasion when this nozzle was 

 broken by the explosion and the water had to be turned off 

 immediately to avoid useless waste ; on reaching the face, the 

 atmosphere was found to be so highly charged with dust and 

 smoke that it was impossible to distinguish the stones at the 

 feet, although a lamp had been placed on the ground; and 

 despite the fact that the air tube was in full blast, the men ex- 

 perienced great difficulty in breathing. A truck is now brought 

 up, and four men clear a passage in front, through the heap of 



