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 economy, 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 penetrate into all the cavities and gather there, but the fresh air carried inside by ventilation has a very small percentage of carbonic 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 decomposition 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 natural 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 rooms have the same temperature. Now, during the construction 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 inside and outside the tunnel. In winter time the cold air outside will enter at the bottom of the entrances and headings, or along the sides of the shafts, and the hot air will pass out near the top of the headings or entrances or the center of the shafts; in summer the air currents will take the contrary direction.