VKNTII. \.: - \N:> I.ICHTING 



is necessary t > sepanite the fresh air from the foul one ; and this 

 is done by means of pipes which will exhaust and expel the 

 foul air in the vacuum method, or force to the front a current 

 of fresh air when the plenum method is used. Artificial venti- 

 lation may also be obtained by compressed air which is set free 

 after it has driven the machines, especially in tunnels excavated 

 through rock, when rock drilling machines moved by com- 

 pressed air are employed. 



Vacuum Method Contrivances. The most common of the vac- 

 uum appliances consists in the simple arrangement of a pipe 

 leading from the head of the tunnel out through the fire of a 

 furnace. The air in the pipe is rarefied by the heat of the fur- 

 nace and then set free from the other end of the pipe, thus 

 creating a partial vacuum in the pipe, into which the foul air of 

 the head rushes, the fresh air from the entrance taking its place, 

 and thus ventilating the tunnel. A similar arrangement may 

 be used with shafts, and the foul air may be driven out by a 

 furnace which is placed either at the top or bottom of the shaft. 

 Such furnaces act the same as those commonly used for heating 

 purposes in the houses, with this difference, that, instead of fresh 

 air being forced in, foul air is expelled. Another simple 

 arrangement for producing a vacuum is by means of a steam 

 jet which is thrown into the pipe, and which helps the expul- 

 sion of the air by heating it, thus producing a different density 

 which originates a draft besides that mechanically originated l>y 

 the force of the steam jet, which tends to carry out the foul air 

 of the pipes. 



Foul air may also be expelled by means of exhaust fans 

 which are connected with pipes near the entrance of the tunnel. 

 The fan consists of a box containing a kind of a i Middle wheel 

 turned by steam or water power and arranged so as to revolve 

 at a high sjM-ed. The air inside the pipe is forced out by 

 blades attached to the wheel, and thus the foul air of the front 

 is driven away and fresh air from the entrance rushes in to tike 

 its place, and perfect ventilation is obtained. 



