air apparatus of Professor Colladon, of Geneva, which had already been used at Mont Cenis. Water was obtained in great abundance and of strong fall at Goeschenen from the river Reuss. The supply of water at Airolo was not so abundant. The river (the Ticino) has not a rapid descent, and a very long canal would be required to procure a sufficient fall from it. A torrent, the Tremola, was chiefly relied upon, but the supply from this fell short at times, and had to be supplemented by an auxiliary supply from the Ticino. The force of the stream was applied to turn four turbine wheels which made three hundred revolutions in a minute, and exerted a force of about two hundred horse-power. These wheels were made to turn an horizontal axis with cranks revolving eighty-five times a minute, which kept the compressors in operation. The air, subjected to a compression of from four to eight atmospheres, became very hot, and had to be cooled by special applications of water circulating in cold currents around the pumps and in the pistons and piston-rods, and by injections of fine spray. After being further cooled and freed from water in suspension by passing it through reservoirs, it was conducted into the tunnel by tubes which were of considerable size at the beginning, but were diminished to correspond with the diminishing expenditure of air as the work was advanced.
The borings were begun by first cutting out a gallery about eight feet square at the top of the intended excavation. When this advance boring was completed, it was enlarged on the right and left. The arches of the roof were then built, and a trench nearly ten feet wide was dug to the level of the tunnel's base. This was called the "Cunette de Strosse." All that remained on the right and left of the
Fig. 2.—System of Excavation: 1, advance pillory; 2, side-workings; 3, 4, "Cunette de Strosse"; 5, "Strosse."
trench, called the "Strosse," was next removed. Thus, most of the digging was done downward. The work was interfered with at times by the infiltration of water, which, as it did not affect dynamite and soon stopped, was not considered serious; by rocks of exceptional hardness; and by a bed of loose material in the Goeschenen end, in which
