UNNLLo THUOUUH HARD KOCK 81 



compressors are replaced by high-pressure pumps ; and when 

 electric hauling is employed, one or more dynamos may be re- 

 quired to generate electricity for power purposes, as well as fur 

 lighting. Ill addition to the power generating machines proper, 

 there must be the necessary piping and wiring for transmitting 

 this power, and, of course, the equipment of drills and other 

 machines for doing the actual excavating, hauling, etc. 



Reservoirs. When water-power is employed, a reservoir 

 has to be formed by damming some near-by mountain stream at 

 a point as high au practicable above the tunnel. The provision 

 of a reservoir, instead of drawing the water directly from the 

 stream, serves two important purposes. It insures a continuous 

 supply and constant head of water in case of drought, and also 

 permits the water to deposit its sediment before it is delivered 

 to the turbines. The construction of these reservoirs may be 

 of a temporary character, or they may be made permanent 

 structures, and utilized after construction is completed to sup- 

 ply power for ventilation and other necessary purposes. In the 

 first case they are usually destroyed after construction is fin- 

 ished. In either case, it is almost unnecessary to say, they 

 should be built amply safe and strong according to good engi- 

 neering practice in such works, for the duration of time which 

 they are expected to exist. 



Canals and Pipe Lines. For conveying the water from the 

 reservoirs to the turbines, canals or pipe lines are employed. 

 The latter form of conduit is generally preferable, it being 

 both less expensive and more easily constructed than the 

 former. It is advisable also to have duplicate lines of pi|>e to 

 reduce the possibility of delay by accident or while necessary 

 repairs are being made to one of the pipes. The pipe lines 

 terminate in a penstock leading into the turbine chamber, and 

 provided with the necessary valves for controlling the admis- 

 sion of water to the turbines. 



Turbines. There are numerous forms of turbines on the 

 t. but they may all be classed either as impulse turbines 



