running at 125 revolutions per minute.

A 1500 kilowatt alternating

current generator of this type is shown in fig. 36.

A representative example of a slow speed Corliss engine direct coupled to a 1500 kilowatt continuous current dynamo is shown in fig. 37; this plant runs at 100 revolutions per minute, the engine is by Messrs Musgrave, and the dynamo by the Electric Construction Co.

A class of steam engine in which the motion is purely one of rotation without the use of any reciprocating parts, is the steam turbine. In this the steam issues from a number of orifices, and, either by its impulse upon a series of blades, or by its reaction on emergence, causes rotation of the shaft. Steam turbines, to be efficient, must run at an exceedingly high speed; the circumferential velocity necessary, for example, with steam at a pressure of 140 lbs. per square inch, being some 2000 feet per second. For this reason, the number of revolutions is usually 10,000 and upwards per minute. The chief two examples are the Parsons and the Laval.

In the Parsons steam turbine, the steam flows radially outwards through sets of revolving vanes, and on reaching the outer edge is guided back to the centre of another set and so on, a large number of sets being employed, all mounted on one shaft. The action is partly one of reaction and partly of impact.

The Laval steam turbine, unlike the Parsons, depends entirely upon reaction. The steam issues from a number of nozzles, and acquires, in expanding mouthpieces, the full velocity due to its head. It impinges on the vanes of a single simple impulse turbine which revolves at some 30,000 revolutions per minute.

Both types of turbine give excellent results as regards steam consumption, and they possess the advantages of requiring small floor space and having few working parts; above all, however, they give complete immunity from vibration, and it is this feature chiefly which has led to their use for central station work.

Leaving steam driven plant, gas engines next claim attention. Theoreti cally, an internal combustion engine, in which the energy in the fuel is directly transformed into kinetic energy without the intervention of the steam boiler, is highly attractive. Practically, however, there are many drawbacks. The first cost of the plant is high, the repairs are considerable, steadiness of driving is far from satisfactory, and the power of a single engine is very limited. These drawbacks have led to such gas-driven generators as have been put down meeting with but little success; but there is evidence that some at least of the disadvantages are being overcome, and, before long, engines of large power, giving very fairly steady turning, may be looked for. If these expectations are fulfilled, it is extremely likely that considerable changes in central station practice may take place, for a gas engine meeting the conditions of this work in a thoroughly practicable and

satisfactory manner would enable the advantages of gas producers and recovery plant indicated in Chapter XI. to be fully realised.

The results attained with a gas engine driving a dynamo developing about 90 H.P. when run, practically continuously, with Mond producer gas, are most interesting. The plant ran for 8356 hours in the year, i.e., for 954 per cent. of the possible number of hours; and under these favourable conditions the consumption of gas was 1144 cubic feet per unit, corresponding to 1.76 lbs. of slack. Taking slack at 6s. 10d. per ton, the cost per unit is given at 0.066d. for slack, 0·051d. for labour, and 0·030d. for oil and stores. These are, of course, three items only of the whole cost, and the conditions are totally different from those obtaining in a central station.

Practically all modern gas engines work upon the Otto cycle, which occupies two complete strokes. The first outward stroke draws in the explosive mixture, the first inward stroke compresses it, the second outward stroke is the working stroke, the mixture being then ignited, while the second inward stroke expels the products of combustion. There is thus only one impulse in two revolutions, and the driving is therefore very unsteady. This unsteadiness is still further aggravated by the usual method of governing, which consists in omitting a certain proportion of the explosions as the load diminishes.

In the most modern development of the gas engine, viz., that made by the Westinghouse Co., the governing is effected by varying the proportion of the gas in the explosive mixture, instead of the number of explosions. By this means, and by arranging a number of cylinders to actuate the shaft, the running has been made so steady that the dynamo can be directly coupled, as with steam plant, and generators up to 1500 H.P. have been made. A 650 H.P. direct-coupled continuous-current gas engine driven set is shown in fig. 38.

The remaining agent for actuating prime movers that has been mentioned is running water. For central station work, turbines are practically always used for rendering the power available.

A turbine consists essentially of a fixed casing, containing guides to direct the water in the desired direction, and a number of curved blades mounted on a shaft, which they cause to revolve by receiving the water, and causing it to so alter its direction that its energy is expended in causing them to move.

Turbines fall into two main classes, according as there is or is not a pressure of water in the space between the moving vanes and the fixed guides. The former are known as 'Pressure' turbines, and require to be continually filled with water, working best when 'drowned'. The second class, or 'Impulse' turbines, require that the passages between the vanes and the blades should not be filled with water, and the water need not therefore be admitted round the whole circumference. Such turbines cannot be drowned.