For falls not exceeding six feet, the most effective wheel is the l'oncelet. In it the water act' in a manner very similar to its action in a turbine, and although nut quite as efficient as turbines working under normal conditions, it is superior when working under a reduced water supply. The bucket. arc curved, and the water Cow mg along their concave sides presses upon them without impact and with greater effect than that of water impinging at right angles against buckets with plane surfaces. The effect at the wheels with the depth of the %% a:cr. and the %a-loco) of rotation is equal to about one half the %eloeity of the flowing water. The 1orm of the "impact' wheel con sists of a series of rectangular buckets fixed upon a wheel at an angle varying from 50 w 70 degrees to the horizontal. The water is con ducted to it through apyramidal flume set at an angle varying from 30 to 40 degrees, so that it strikes the blades nearly at right angles. The normal effect is about 5 per cent of the total mechanical power of the flow, but this may be increased somewhat by increasing the length of the paddles, so that the water exerts a combined impinging and pressing force.
In another form, known as the •impulse' wheel, a series of cup-shaped buckets attached to the periphery of the wheel are acted upon by a jet of water conducted by a pipe and delivered through a nozzle tangentially against the cups. More power is obtained by increasing the num ber of nozzles to two, or even four. Of this form the most notable are those of the Pelton type, which arc extensively employed to work air compressors in mining operations. At first limited in usefulness to the locality of the power, the development of electric power trans mission brought them into a greatly enlarged field of service. Such wheels are now operating under heads up to nearly 2,000 feet. A Pelton wheel in the service of the South California Edison Company, working under 1,900 feet head at 430 revolutions per minute, develops 1,600 horse power. This wheel has been running con tinuously since 17 March 1903. At a plant re cently erected in Europe, a jet of water one and one-half inches in diameter under a head of 5,250 feet develops 3,000 horse power. The Rio Janeiro Traction, Light and Power Company o. rates a Pelton wheel with four nozzles under
.1 i feet of head. The power developed is equivalent to 20,000 horse power.
The efficiency of the turbine being so much greater than that of any type of vertical water wheel they are universally preferred except in the smallest water power developments. This statement, however, does not apply to the Pelton wheel whose efficiency is equal to the turbine, and is available where the head Is too great for economical use in turbines. The largest vertical water wheel ever built in America was the Burden wheel, located on the bank of ‘%'yn ant's Kill, which enters the Hudson River near Troy, N. Y. This wheel was of tension con struction, the 264 spokes being one and one-half inch rods bolted to rosettes on the 12 inch shaft. Its diameter was 60 feet, and its face width was 20 feet. The water was brought to the wheel through a steel penstock which crossed the wheel transversely at its summit. Four gates at intervals in this penstock were in the constant charge of the wheelman who opened or closed them so as to keep the supply such that it would turn the wheel at a constant speed of two and one-half revolutions per minute. On the outer sides of the rim were circular cogged racks which engaged with pinions seven feet in diameter and nine inches face keyed to the shaft, which projected 31 feet into the mill. This wheel was in constam operation 45 years, much of the time running night and day for long periods. It was abandoned in 1898 as of insufficient power for the needs of the business, but remained intact for 15 years thereafter be fore it began to fall into decay. The largest water wheel in the world is that which supplies the ancient city of llama with water for dlt ing and irrigation from the river Orontes 'boot 120 miles north of Damascus. This wheel is of the undershot type, 70 feet in diameter and of comparatively narrow face. It is built of wood uppoo an iron axle.
Good descriptions of water wheels are scattered through the various en gineering periodicals. Especially valuable data may be obtained from 'Transactions of the In ternational Engineering Congress' (San Fran WC° 1916). See also TURBINI; WATIR Powm; %% arm MOMS.