110 TUNNELING 



flanges embedded in the masonry, constitute a rigid fixed point. 

 Straw mats are thrown over the pipe where it is exposed to the 

 sun. The temperature of the conduit is not, however, found to 

 vary greatly, since the pipe is kept full of water. To supply 

 the rock-drills with water at a maximum pressure of 100 

 atmospheres, or 1,470 Ibs. per sq. in., a plant of four pairs of 

 high-pressure pumps has been laid down, and a still larger 

 addition is in course of erection. At present, two Pelton 

 turbines of 250 H.P. each, running at 170 revolutions per 

 minute, drive the pumps, by means of toothed gearing, at 63 

 revolutions per minute. These pumps are of very simple but 

 strong construction, single suction and double delivery, entail- 

 ing one suction and one deli very- valve, both heavy and both of 

 small lift. The larger portion of the plunger has exactly 

 double the cross-sectional area of the smaller portion, so that in 

 the forward stroke half of the water taken in at the last 

 admission is pumped into the high-pressure mains, and at the 

 same time a fresh supply of water is sucked in. During the 

 backward stroke half of this new supply is pumped into 

 the mains, and the remainder enters the second chamber, to 

 be pumped during the next forward stroke. Thus the work 

 done in the two strokes is practically the same. The pumps 

 are in pairs, and are set at an angle of 90, to insure uniform 

 pressure and uniform delivery in the mains. Their size varies ; 

 but at Iselle there are three pairs, with a stroke of 2 ft. 2 ins., 

 and the plungers of 2 {& in. and Ij ins. (approximately) in 

 diameter, supplying 1.32 gallons per second. 



To avoid injury to the valves, the water to be pumped is 

 taken from a stream up the mountain side, and is passed 

 through filter screens. The high-pressure water, after passing 

 an accumulator, enters the tunnel in solid drawn wrought-iroii 

 tubes, 3^ ins. in internal diameter, f s in. thick, and in lengths 

 of 26 ft. The diameter of these mains varies with their length, 

 so as to avoid loss of pressure. With the 1,250 yds. of tunnel 

 now driven 10 atmospheres are lost. 



