The Mont Cenis Tunnel marks a transition period in tunnelling. During the four years that hand labour was used, the average rate of progress was but nine inches a-day on either side; but when the rock-drills worked by compressed air were introduced, the speed was five times as great. Still further, at the Arlberg Tunnel through the Tyrolese Alps the average rate of progress was 9·07 yards per day, and the cost £108 per lineal yard; while the cost of the Mont Cenis was £226 per lineal yard. These figures show immense progress in economy and in speed. The St. Gotthard Tunnel was begun in 1872, and the machine drills were used throughout. A heading was first cut about eight feet square, and the hollow thus gained was afterwards enlarged and finally sunk to the desired level. Several Ferroux drills were used, placed on a carriage, and an average charge of 1¾ lbs. of dynamite placed in the holes made. After firing, the compressed air was discharged and the shattered soil was cleared away. In the Arlberg Tunnel a chief heading was driven, and then shafts opened up enabling smaller headings to be driven on both hands. Drills worked by hydraulic power were used, as well as drills worked by air, and, after the explosions, water spray was thrown out to assist in clearing and purifying the air. Ventilators also were used, which injected air at the rate of more than 8000 cubic feet per minute. Speedy transit of the earth excavated and the materials for masonry were also effected, it being estimated that some 900 tons of earth had to be taken out of each end, and about 350 tons of masonry had to be brought in, every day. Tunnels through huge thicknesses of rock or under rivers can only be cut from the two opposite ends. Where possible, however, other shafts have been sunk along the line the subway was to take, and thus excavation might continue at several places along the line of route, the shafts being used for ventilation and for the conveyance of the excavated soil. But the use of machine drills and of blasting explosives, with improved appliances for ventilation, have, with possibly some rare exceptions, rendered these methods obsolete. According to Pliny the tunnel for draining Lake Fucino was the greatest work of his day. It was over 3½ miles long, and cut under Monte Salviano. Forty shafts were sunk in cutting it, also sloping galleries, and huge copper buckets were used to carry away the earth. It is stated that this tunnel—some ten feet high, by six wide—occupied 30,000 men eleven years. Compare this with the Arlberg, or even the Gotthard, double and treble the length, occupying much less time. Sir Benjamin Baker has calculated that the Fucino tunnel could now be cut in eleven months. Gunpowder gave some advance on old Roman methods of tunnelling. The improved explosives and rock-drills have gone further.