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Rh of 70% has been obtained, and when a dynamo is driven directly by them about 66% of the hydraulic energy has been converted into electric energy.

4.—External View of Rigg's Water-Engine.

Pelton wheels are very sensitive to variation of load, and considerable trouble was experienced at first in securing adequate governing when they were used to generate electric energy; but this difficulty has been overcome, and they have been rendered most efficient machines for use with high falls, where ordinary turbines would be difficult to manage owing to the excessive speed at which they would run. In a small installation in the United States water is brought in a 36-in. pipe a distance of 1800 ft., and supplies six 5.—Pelton Wheel. Pelton wheels each 28 in. in diameter, running at 135 revolutions a minute under a head of 130 ft. The total power developed is 600 H.P., and though the load factor varies very greatly in this case, the differential type of governor used secures perfect control of the running of the wheels.

Turbines.—The turbine has now become one of the most efficient of the prime movers employed by man, and in the United States of America and on the continent of Europe its use has enormously increased of recent years. Though no radical changes have been made in the design of turbines for some years, an immense amount of skill and ingenuity has been shown in perfecting and improving details, and such machines of great size and power are now constantly being made, and give every satisfaction when in use.

In the "Hercules" turbine, shown in fig. 6, the flow is what is called mixed, that is, it is partly a radial inward and partly an axial flow machine. On entering, the water flows at first in a radial direction, and then gradually, as it passes through the wheel, it receives a downward component which becomes more and more important. Professor Thurston has published the results of a test

of one of these, which gave an efficiency of 87% at full load and 70% at about three-fifths full load.

Another turbine of the mixed flow type is the "Victor," which consists of three parts the outer guide case, and, inside this, the register gate, and the wheel. The gate regulates the speed of the wheel by varying the quantity of water; when fully open it merely forms a continuation of the guide passages, and thus offers no obstruction to the flow of the water, but by giving it a movement through a part of a revolution the passages are partly blocked and the flow of the water is checked. This form of regulation is fairly efficient down to three-quarter opening. Turbines of this type may also be used on horizontal shafts, and are very useful in the case of low falls where there is a large amount of water and the head is fairly constant. At Massena, in New York State, 75,000 H.P. is to be developed from fifteen sets of these turbines working under a head of 40 ft. Each generator can develop 5000 H.P. at a potential of 2200 volts, and is driven by three horizontal double turbines on the same shaft; when working under a minimum head of 32 ft., at 150 revolutions, each turbine will have a nominal horse- power of 1000.

6.—"Hercules" Turbine.

Probably the most important application of turbines to the generation of power on a great scale is that at Niagara Falls. The water is tapped off from the river Niagara about 1 m. above the falls and brought by a canal to the power-house. The wheel-pit is 180 ft. in depth, and is connected with the river below the falls by a tail-race, consisting of a tunnel 21 ft. high and 18 ft. 10 in. wide at its largest section. The original turbines were of the "Fourneyron" type, and a pair were mounted on each vertical shaft, the two being capable of giving out 5000 H.P. with a fall of 136 ft. Each pair of wheels is built in three storeys, and the outflow of the water is controlled by a cylindrical gate or sluice, which is moved up and down by the action of the governor. As the pair of wheels and the big vertical shaft (which is of hollow steel 38 in. in diameter) with the revolving part of the dynamo mounted on the upper end of the shaft weigh about 152,000 ft., a special device, since adopted in other similar power plants, was designed to balance in part this dead weight. The water passes from the penstock through the guide blades of the upper wheel, and in doing so acts in an upward direction on a cover of the upper wheel, which thus becomes, as it were, a balance-piston. The total upward pressure on this piston is calculated to be equal to ; hence the shaft-bearings are practically relieved from pressure when the wheels are running. Another turbine which has come into extensive use is the "Francis," an exceedingly efficient turbine on a low fall with large quantities of water. At Schaffhausen two of them with a fall of 12 ft. developed 430 H.P., when the older turbines only gave 260 H.P., the