Ancient tunnel works were constructed by the ancients. The oldest of which there is record is probably that known to have been built by the Babylonians beneath the river Euphrates to connect two.palaces. Tunnels were frequently formed by the Romans as a part of their superb structures for supplying cities with water, and the aqueducts of the Peruvians and Mexicans included remarkable works of this descrip tion. A number of these are still in a state of fair preservation.
Iodern modern times great numbers of tunnels have been built, principally to facilitate communication by railways. The first Thames tunnel, for years the most notable structure of its kind, was begun in 1825 by Sir M. I. Brunel, a French engineer, who, after several failures, completed and opened it for foot-passeugers in 1843. It is 13oo feet in length, 35 feet in width, and 20 feet in height.
The Mont Cods Tunnel, commenced in 1857 -under the superintendence of Sommeiller, aided by the engineers Grandis and Grattoni, and com pleted in 1871, was the first upon which improved machines for drilling and powerful modern explosives were experimented with and their great advanta,ges over the older methods demonstrated. The engineers in charge of this work, confronted with the difficulty of conveying power for their rock-drilling apparatus to the location of the headings, thousands of yards distant, adopted the use of air compressed to six atmospheres by water-power that was available for the purpose, thus solving at once not only the difficulty of transmitting power to a distance without serious loss, but also to some extent the important problem of ventilation. Figure 22 lo-ives a 0-eneral idea of the method of advancino- t le headino-s a number of Ls 7 drilling-machines being carried upon a frame mounted on wheels.
Various explosives (gunpowder, nitro-glycerine, giant-powder, and dynamite) were used at different times for blasting. The charges were exploded by means of a magneto-elcctric apparatus. 'Phe blasting was done in front of a movable bulkhead which was advanced with the work. The enlarging and finishing of the tunnel were carried forward as expedi tiously as possible in the rear of the headings. In the later stage of this work, the mode of drilling above named was modified by the employment of a large carrier of tubular iron, upon which the atmospheric drills were mounted. As above noted, the ventilation was only partly effected by the
drills, and it was found necessary to employ also a ventilating-pipe which delivered several thousand feet of air per minute to the working headings. In this work the height of the mountain prevented the sinking of inter mediate shafts along the line. The completed tunnel unites France and Italy through Le Grand Vallon, in the Savoy Alps. Its total length from the northern entrance, at Modane, in France, to the southern entrance, at Bardonneche, in Italy, is 13,393 yards (or about seven 'and five-eighth miles)., The tunnel is 24 feet 7 inches high at the Modane end and ii74 inches hig,her at the Bardonueche end, 26 feet 23/t inches wide at the broadest part, and 25 feet 3Y inches wide at the base. The top is semi circular. The roof and walls are lined throughout with masonry.
St. Gothard ana' Arlberg Mont Cenis tunnel for ten years remained the longest structure of the kind in the world, until the completion, in ISSI, of the St. Gothard tunnel, through the Swiss Alps, relegated it to the second place in importance. The length of this tunnel is 16,217 yards (about nine and one-quarter miles). The same general plan of working was followed in this as in the construction of the Mont Cenis tunnel. The Arlberg tunnel, through the Tyrolese Alps, approximately six and three-eighth miles long, making a third Alpine passage, was com pleted and opened for traffic in ISS4.. These constitute the most important structures of their class in the world.
Hoosar America, the most important tunnel thus far con structed is the one through the Hoosac mountain, on the railway between Troy, New York, and Greenfield, Massachusetts. It is approximately four and three-quarter miles in length. It was commenced in 1856, and after several periods of cessation was completed some twenty years later. Rail way tunnels approximating a mile in length are quite numerous.
The Sutra Tunnel, constructed to cut the celebrated Comstock lode 2000 feet below its highest point, to facilitate the ventilation and drainage of the mines, and to afford a passage-way for the cheap and rapid transpor tation of the ores to a point on the Carson River, is approximately four miles long; and when all the branches contemplated are built, this length will be nearly doubled.