=Excavation.=--The sequence of excavation is shown by Fig. 14, p. 36. First the top center heading, No. 1, whose dimensions varied from 8.25 × 8.6 ft. to 8.5 × 9 ft., according to the quality of the rock, was driven never less than 1000 ft. and sometimes over 3000 ft. in advance of parts No. 2. The excavation of parts No. 2 opened up the full top section, and parts Nos. 3, 4, 5, 6, and 7, were removed in the order numbered. =Strutting.=--Where regular strutting was required, the construction shown in Fig. 62 was adopted. =Masonry.=--The St. Gothard tunnel is lined throughout with masonry. After the upper portion of the section was fully excavated, the roof arch was built with its feet resting upon short planks on the top of the bench. Plank centers were used in constructing the arch. For the arch brick masonry was employed, but the side walls were built of rubble masonry. Shelter niches, about 3 ft. deep, were built into the side walls at intervals, and about every 3,000 ft. storage niches about 10 ft. deep, and closed with a door, were constructed. The culvert was of brick masonry. =Mechanical Installation.=--Water-power was used exclusively in driving the St. Gothard tunnel. At the north end, the Reuss, and at the south end, the Tessin and the Tremola, rivers or torrents were dammed, and their waters conducted to turbine plants at the opposite ends of the tunnel. The power thus furnished by the Reuss was about 1,500 H.P., and the power furnished by the combined supply of the Tessin and Tremola was 1,220 H.P. The turbine plant at both ends at first consisted of four horizontal impulse turbines, but later, two more turbines were added at the south end. Each of the two sets of four turbines first installed drove five groups of three compressors each, and the two supplementary turbines drove two groups of four compressors each. The compressors were of the Colladon type with water injection, and four groups of three compressors each were capable of furnishing 1,000 cu. yds. of air compressed to between seven and eight atmospheres every hour, or about 100 H.P. per hour, delivered to the drills at the front. This air when exhausted provided about 8,000 cu. yds. of fresh air per hour for ventilation. The compressors at each entrance discharged into a group of four cylindrical receivers of wrought-iron each 5.3 ft. in diameter by 29.5 ft. long, and having a capacity of 593 cu. ft. The cylinders were placed horizontally, the first one receiving the air at one end and discharging it at the other end into the next cylinder, and so on. By this arrangement the air was drained of its moisture, and the discharge from the end receiver into the tunnel delivery pipes was not affected by the pulsations of the compressors. The delivery pipe decreased from 8 in. in diameter at the receiver to 4 ins. in diameter, and finally to 2¹⁄₂ ins. in diameter, at the front.