19. What is the horse-power of a pump that can pump out a cellar full of water 40 ft. × 20 ft. by 10 ft. deep, in 30 minutes? 20. How many tons of coal can a 5 horse-power hoisting engine raise in 30 minutes from a barge to the coal pockets, a height of 50 ft.? (7) WATER POWER =136. Energy of Falling Water.=--The energy of falling and running water has been used from the earliest times for developing power and running machinery. The energy is derived from the action of the moving water in striking and turning some form of _water-wheel_, several varieties of which are described below. =The Overshot Wheel.=--The overshot wheel (Fig. 112) is turned by the weight of the water in the buckets. It was formerly much used in the hilly and mountainous sections of this country for running sawmills and grist mills as it is very easily made and requires only a small amount of water. Its efficiency is high, being from 80 to 90 per cent., the loss being due to friction and spilling of water from the buckets. To secure this high efficiency the overshot wheel must have a diameter equal to the height of the fall which may be as much as 80 or 90 ft. [Illustration: FIG. 112.--Overshot water wheel.] [Illustration: FIG. 113.--Undershot water wheel.] [Illustration: FIG. 114.--Diagram illustrating the principle of the Pelton wheel.] =The Undershot Wheel.=--The old style undershot wheel (Fig. 113) is used in level countries, where there is little fall, often to raise water for irrigation. Its efficiency is very low, seldom rising more than 25 per cent. The principle of the undershot wheel, however, is extensively used in the water motor and the Pelton wheel (Fig. 114). In these the water is delivered from a nozzle in a jet against the lower buckets of the wheel. They have an efficiency of about 80 per cent. and are much used in cities for running small machines, washing machines, pipe organ blowers, etc., and in mountainous districts where the head is great. [Illustration: FIG. 115.--Diagram of a hydro-electric power house showing a vertical turbine _A_ with penstock _B_ and tail race _C_.] [Illustration: FIG. 116.--The outer case of a turbine showing the mechanism for controlling the gates.] [Illustration: FIG. 117.--Inner case of a turbine showing the gates and the lower end of the runner within.] [Illustration: FIG. 118.--The runner of a turbine.] [Illustration: FIG. 119.--Turbine and generator of the Tacoma hydro-electric power plant.]