=Reservoirs.=--When water-power is employed, a reservoir has to be formed by damming some near-by mountain stream at a point as high as 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 supply power for ventilation and other necessary purposes. In the first case they are usually destroyed after construction is finished. In either case, it is almost unnecessary to say, they should be built amply safe and strong according to good engineering 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 pipe 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 admission of water to the turbines. =Turbines.=--There are numerous forms of turbines on the market, but they may all be classed either as impulse turbines or as reaction turbines. Impulse turbines are those in which the whole available energy of the water is converted into kinetic energy before the water acts on the moving part of the turbine. Reaction turbines are those in which only a part of the available energy of the water is converted into kinetic energy before the water acts on the moving vanes. Impulse turbines give efficient results with any head and quantity of water, but they give better results when the quantity of water varies and the head remains constant. Reaction turbines, on the contrary, give better results when the quantity of water remains constant and the head varies. These observations indicate in a general way the form of turbine which will best meet the particular conditions in each case. The number of turbines required, and their dimensions, will be determined in each case by the number of horse-power required and the quantity of water available. The power of the turbines is transmitted to the air compressors or pumps by shafting and gearing.