TUNNELS THROUGH HARD ROCK 93 



conduit of square section. When the work was well advanced 

 tliis apparatus was deemed to be insufficient, and the exhaust- 

 ing bells described in the Chapter Ventilation were used, and 

 operated by a powerful turbine, whose motive-power was a 

 stream of 75 gallons of water per second with a head of 60 ft 



