_The Mont Cenis Tunnel_ was constructed to establish railway communication between France and Italy through the Alps. It was begun in 1857, and after having been in progress of construction for thirteen years, was opened for traffic in 1871. This tunnel was commenced by hand borings, being for the most part through solid rock, and its progress up to 1862 was so slow that it was estimated that thirty years would be required for its construction. Its earlier completion was due to the introduction of rock drills operated by compressed air, which trebled the rate of advance, and which device made a new epoch in all rock-boring and mining operations. This tunnel was cut from both ends at the same time, and so accurate were the surveys in establishing the alignment of the two headings through the mountain mass, that, although the tunnel was more than 7½ miles long, when the two headings came together in the middle, only a difference of one foot in level existed between them. When it is remembered that most of the 7½ miles of tunnel was cut through solid rock, by boring and blasting, the immensity of the undertaking can be appreciated. As completed the tunnel is 8 miles long, and wide enough for a double track railway. _The St. Gothard Tunnel_ is another tunnel through the Alps, which involved even a longer and deeper cut through the mountains than the Mont Cenis Tunnel. This is 9¼ miles long, and it was begun in 1872, the headings joined in 1880, and the tunnel opened for traffic in 1882. Although by far the largest undertaking yet made, the improvement in rock-boring machinery enabled it to be constructed much more rapidly and at less expense. The Arlberg is still another Alpine tunnel. It is 6½ miles long, was commenced in 1880, and opened for traffic in 1884. Tunneling under rivers presents many more difficulties than driving through the hardest rock. This is so by reason of the inflow of water. Among successful tunnels of this kind may be named the Mersey and Severn tunnels in England, opened in 1886, and the St. Clair tunnel between the United States and Canada. The histories of the abandoned Detroit and Hudson river tunnels are object lessons of the difficulties encountered in this class of work.