290 TUNNELING 



CHAPTER XXIV. 



THE VENTILATION AND LIGHTING OF TUN- 

 NELS DURING CONSTRUCTION. 



VENTILATION. 



IN long tunnels, especially when excavated in hard rock, 

 proper ventilation is of great importance, because the air cannot 

 be easily renewed, and the amount of oxygen consumed by 

 miners' horses and lamps during construction is very large. 

 The gases produced by blasting also tend to fill the head of ex- 

 cavation with foul air. Pure atmospheric air contains about 

 21 % of oxygen and only 0.04 % of carbonic acid ; when the 

 latter gas reaches 0.1 %, the fact is indicated by the bad odor; 

 at 0.3 ffo the air is considered foul, and when it reaches 0.5 % it 

 is dangerous. It is generally admitted that the standard of 

 purity of the air is when it contains 0.08 / of carbonic acid. 



A large quantity of carbonic acid in the air is easily detected 

 by observing the lamps, which then give out a dim red light 

 and smoke perceptibly ; the workmen also suffer from headache 

 and pains in the eyes, and breathe with difficulty. Naturally, 

 miners cannot easily work in foul air and, therefore, make very 

 slow progress. It is, therefore, to the interest of the engineer to 

 afford good ventilation, not only because of his duty to care for 

 the safety and health of his men, but also for reasons of econ- 

 omy, so that the men may work with the greatest possible ease, 

 thus assuring the rapid progress of the work. 



It would be impossible to change completely the atmosphere 

 inside a tunnel, as the gases developed from blasting will pene- 

 trate into all the cavities and gather there, but the fresh air 



