294 TUNNELING 



The best manner of expelling foul air from tunnels, accord- 

 ing to the vacuum method, is by means of bell exhausters. 

 This consists of two sets of bells connected by an oscillating 

 beam and balancing each other. Eacli set consists of a movable 

 bell, which covers and surrounds a fixed bell with a water joint. 

 In the central part of the fixed bell there are valves which open 

 upwards, and on the bottom of each movable bell there are 

 valves which open from the outside. When one bell ascends, 

 the valves at the bottom are closed, the air beneath is then 

 rarefied, and a vacuum is produced ; the valves in the central 

 part of the fixed bell filled with water are opened, arid there is 

 an aspiratory action from the pipe leading to the headings, and 

 the foul air is thus carried away. The apparatus makes about 

 ten oscillations per minute, and the dimensions of the bells 

 depend upon the quantity of air to be exhausted in a minute. 

 In the St. Gothard tunnel, where these bell exhausters were 

 used, they exhausted 16,500 cu. ft. of air per minute. 



Plenum Method Contrivances. Fresh air may be driven into 

 tunnels to dilute the carbonic acid by two different ways, viz., 

 by water blast and by fans. Water when running at a great 

 velocity produces a movement in the air which may be some- 

 times usefully and economically employed for ventilating 

 tunnels. Water falling vertically is let run into a large 

 horizontal zinc pipe having a funnel at the outer end ; into this 

 the air attracted by the velocity of the water is forced. - By an 

 opening at the bottom the water is afterward withdrawn from 

 the pipe, and there remains only the air which is pushed for- 

 ward by the air which is being continually sucked in by the 

 velocity of the water. 



The best and most common means of ventilation by the 

 plenum method is by fans. There are numerous varieties of 

 these fans in the market, but they all consist of a kind of fan 

 wheel which by rapid revolution forces the fresh air into the 

 pipe leading to the headings of the tunnel or to the working 

 places. Instead of a large single fan, such as is used for min- 



