212 TUNNELING 



made to pass through it, it was finally decided to abandon the 

 heading and return to the shaft, sinking some 150 ft. deeper. 

 On this new level nothing but rock was encountered. In the 

 East River tunnel, however, the soft material was clearly a 

 decomposed vein, and to what depth this decomposition might 

 extend was unknown ; so that as there were no well-founded 

 reasons, in this case, for expecting any better conditions at a 

 lower level, it was decided to first attempt to drive the present 

 heading, in compressed air, leaving the sinking as a later ex- 

 pedient should the proposed means fail. An arrangement was 

 made with the contractors by which the company was to share 

 the expense of the work in soft ground. 



It was at this time that the writer became connected with 

 the work, having charge of installing and conducting the com- 

 pressed air operations for the company. To form the com- 

 pressed air-working chamber, a solid brick wall or bulkhead 

 8 ft. thick was built across the tunnel into gains in the rock 

 about 40 ft. back from the heading, and containing a cylindrical 

 steel air-lock 6 ft. in diameter and 10 ft. long. 



In the engine room, the 18 X 24-in. Ingersoll piston-inlet 

 compressor, used heretofore for running the rock-drills, was 

 supplemented by a small Rand compressor, and both arranged 

 to supply, independently, compressed air to the working cham- 

 ber below. Incandescent electric lighting was introduced into 

 the tunnel, which is almost a necessity in compressed air opera- 

 tions, as common illuminants produce an enormous quantity of 

 smoke when burning in compressed air. A telephone was also 

 taken into the working chamber, by which instant communica- 

 tion could be had with the engine room in case any sudden 

 increase of air pressure should be desired. 



On Feb. 25, 1893, operations were commenced, in the 

 heading, under 35 Ibs. of air pressure. The previous work 

 here had greatly increased the difficulties, and it was not long 

 before the air pressure had to be raised to 42 Ibs. to control 

 the water. The excavation was advanced under a cylindrical 



