YKNTILATION AM U<;HTIN<; DL'HINU CONSTRUCTION 291 



carried inside by ventilation has a very small percentage of car- 

 bonic acid, mixes with that which contains a greater quantity, 

 and dilutes it until the air reaches the standard of purity. We 

 have not here considered the gases developed from the decom- 

 position of carboniferous and sulphuric rocks, which may be 

 met with in some tunnels, and which render ventilation still 

 more necessary. Tunnels may be ventilated either by natu- 

 ral or artificial means. 



Natural Ventilation. It is well known that if two rooms of 

 different temperatures are put in communication with each 

 other, e.g., by opening a door, a draft from the colder room will 

 enter the other from the bottom, and a similar draft at the top, 

 but with a contrary direction, will carry the hot air into the 

 colder room, thus producing perfect ventilation, until the two 

 mmns have the same temperature. Now, during the construc- 

 tion of tunnels the temperature inside may be considered as 

 constant, or independent of the outside atmospheric variations; 

 hence during summer and winter, there will always be a draft 

 affording ventilation, owing to the difference of temperature in- 

 side and outside the tunnel. In winter time the cold air out- 

 side will enter at the bottom of the entrances and headings, or 

 along the sides of the shafts, and the hot air will jwss out near 

 the top of the headings or'nt ranees or the center of the shafts; 

 in summer the air currents will take the contrary direction. 



Natural ventilation in tunnels is improved when the exca- 

 vation of the heading reaches a shaft, because the interior air 

 can then communicate with the exterior at two joints, at dif- 

 ferent levels. In such cases a force equal to the difference in 

 weight between a column of air in the shaft and a similar one 

 of different density at the entrance of the tunnel, will act upon 

 the mass of air in the tunnel and keep it in movement, thus 

 producing ventilation. ( 'oiise<|iiently, during winter, \\hen the 

 outside air has greater weight than that inside, the air will 

 come in by the headings and go out by the shaft, and in the 

 summer it will enter at the shaft and pass out at the entrance. 



