292 TUNNELING 



Sometimes to afford better ventilation shafts 8 or 12 in. in di- 

 ameter are sunk exclusively for the purpose of changing the 

 air. When the inside temperature is equal to that outside, 

 as often happens during the spring and autumn, there are no 

 drafts, and consequently the air in the excavation is not re- 

 newed and becomes foul ; then fires are lighted under the 

 shaft and a draft is artificially produced. The hot air going 

 out through the shaft, as through a chimney, allows the fresh 

 air to come in as in ordinary ventilation. 



When the head of the excavation is very far from the en- 

 trances, or when the mountain is too high to allow excavation 

 by shafts, it is quite impossible to secure good natural ventila- 

 tion, especially during the spring and autumn months, and the 

 engineer has to resort to some artificial means by which to 

 supply fresh air to the workmen. 



Artificial Ventilation. Artificial ventilation in tunnels may 

 be obtained in two different ways, known as the vacuum and 

 plenum methods. Their characteristic difference consists in 

 this, that in the vacuum method the air is drawn from the in- 

 side and the vacuum thus produced causes the fresh air from 

 the outside to rush in, while the plenum method consists in 

 forcing in the fresh air which dilutes the carbonic air produced 

 inside the tunnel by workingmen and explosives. In the vac- 

 uum method the pressure of the atmosphere inside the tunnel is 

 always less than the pressure outside, while in the plenum 

 method the pressure within is always greater than that outside. 

 Ventilation is the result of this difference of pressure, as the 

 tendency of the air toward equilibrium produces continuous 

 drafts. Both these methods have their advantages and disad- 

 vantages ; but in the presence of hard rock, when explosives are 

 continually required, the vacuum method is considered the best, 

 because the gases attracted to the exhaust pipes are expelled 

 without passing through the whole length of the tunnel, thus 

 avoiding the trouble that a draft of foul air will give to the 

 workmen who are within the tunnel. In both these methods it 



