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TUNNEL, in engineering, a horizontal or slightly inclined gallery beneath the surface of the ground; generally used for an aqueduct or for the passage of a railway, roadway, or canal. In the construction of railroads it is frequently necessary to pierce the hills, so as to preserve a line of road as nearly level as practicable. The method of proceeding with tunneling depends mainly upon the kind of material to be excavated. This having been generally ascertained by borings and trial shafts, the work is commenced by sinking the working shafts, which must be sufficiently capacious to admit readily of lowering men and materials, raising the material excavated, fixing pumps, and also for starting the heading of the intended tunnel when the required depth is reached. Besides the trial and working shafts, air shafts are sunk for the purpose of effecting ventilation in the works below. Tunnels when not driven through solid rock have usually an arched roof, and are lined with brickwork or masonry. In mining, a level passage driven across the measures or at right angles to the veins which it is its object to reach. Thus distinguished from the drift or gangway which is led along the vein when reached by the tunnel.

The Great Divide Tunnel.—A notable engineering feat was accomplished in 1893 in the completion of the boring of the Busk-Ivanhoe railway tunnel under the continental divide of the Rocky Mountains at Hagerman Pass, Col. The tunnel is almost two miles long—9,393 feet—and is through solid gray granite. It took three years and 20 days, of 20 hours' work each day, to make the excavation. It is 10,800 feet above sea-level, through the top ridge of the continent. The water draining from the one side of the mountain, under which it is driven, runs to the Atlantic Ocean, and from the other to the Pacific. Its construction cost $1,000,000 and 20 human lives. The tunnel substitutes two miles of track for 10 and does away with one of the most expensive railway climbs in the world. Among more recent tunnels are those under the Hudson and East rivers in New York City.