size was built to receive the discharge of the high-pressure compressor, through a 4-in. pipe. The high-pressure water required for the shield was furnished by three Blake direct-acting, duplex pumps with outside packed plungers. The steam end was 16 × 18 ins., the water end 2¹⁄₁₆ × 18 ins. At 55 r.p.m. pumping against 5000 lbs. per sq. in., the capacity of each pump was 57 gals. per minute. Two of them, one on each tunnel, were sufficient to run the shields and the third was held in reserve. The high-pressure water was conveyed to the front by means of a 2-in. double, extra strong pipe which was buried between the engine room and the shaft, in a trench, to prevent freezing in cold weather. The electric current for light and power was supplied by two 100-K.W. 250-volt G.E. direct-current generators directly connected to Ball & Wood high-speed engines running at 250 r.p.m. The switchboard had two machine panels, two distributing panels and one panel carrying a circuit breaker for the traction circuit. =Illumination.=--The tunnel was lighted by electricity, there being two rows of lamps, one in the crown and one in the south axial fine. The lamps were 16-c.p., 240-volt, two-wire system, and were spaced 35 ft. apart in the crown and 12¹⁄₂ ft. apart on the axial line. In addition, five nests of 5 lamps each were used at the front. Candles were supplied for miscellaneous and emergency uses. The sockets for electric globes were fitted to a wooden reflector, coated with white enamel paint on the inside. CHAPTER XXI. SUBMARINE TUNNELING (Continued); TUNNELS AT VERY SHALLOW DEPTH. THE COFFERDAM METHOD. THE PNEUMATIC CAISSON METHOD. THE JOINING TOGETHER SECTIONS OF TUNNELS BUILT ON LAND. The tunnels on the river bed or at such a shallow depth that only a few feet of material will remain between the bottom of the river and the roof of the tunnel can be built in three different ways, viz., (1) by a cofferdam; (2) by pneumatic caissons; (3) by sinking and joining together whole sections of tunnels that were built on land.