and progress on the railroad continued. The increasing necessity for tunneling as they proceeded up the mountains then began to slow progress of the line yet again. The first step of construction was to survey the route and determine the locations where large excavations, tunnels and bridges would be needed. Crews could then start work in advance of the railroad reaching these locations. Supplies and workers were brought up to the work locations by wagon teams and work on several different sections proceeded simultaneously. One advantage of working on tunnels in winter was that tunnel work could often proceed since the work was nearly all "inside". Unfortunately, living quarters would have to be built outside and getting new supplies was difficult. Working and living in winter in the presence of snow slides and avalanches caused some deaths. To carve a tunnel, one worker held a rock drill on the granite face while one to two other workers swung eighteen-pound sledgehammers to sequentially hit the drill which slowly advanced into the rock. Once the hole was about deep, it would be filled with black powder, a fuse set and then ignited from a safe distance. Nitroglycerin, which had been invented less than two decades before the construction of the first transcontinental railroad, was used in relatively large quantities during its construction. This was especially true on the Central Pacific Railroad, which owned its own nitroglycerin plant to ensure it had a steady supply of the volatile explosive. This plant was operated by Chinese laborers as they were willing workers even under the most trying and dangerous of conditions. Chinese laborers were also crucial in the construction of 15 tunnels along the railroad's line through the Sierra Nevada mountains. These were about 32 feet (10 m) high and 16 feet (5 m) wide. When tunnels with vertical shafts were dug to increase construction speed, and tunneling began in the middle of the tunnel and at both ends simultaneously. At first hand-powered derricks were used to help remove loose rocks up the vertical shafts. These derricks were later replaced with steam hoists as work progressed. By using vertical shafts, four faces of the tunnel could be worked at the same time, two in the middle and one at each end. The average daily progress in some tunnels was only 0.85 feet (26 cm) a day per face, which was very slow, or 1.18