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 to 80 per cent., while the efficiency of the undershot wheel was much less. The above-named wheels are seldom used now, except in small powers and in old mills. The two kinds of wheels used to-day are turbine-wheels and impact wheels of the Pelton type. The turbine wheel was introduced into France by Fourneyron about 1827, and from there soon afterwards into England and the United States by Fairbain and by Boyden. The peculiarity of the turbine is that the water flows between curved channels, so that it impinges on the vanes or buckets of the turbine tangentially, and thus by force of reaction rotates the turbine. The vanes of a turbine are commonly warped surfaces, shaped for local conditions and according to the special ideas and experience of the designer. The older turbines revolved about a vertical axis, but many modern turbines have horizontal axes.



There are three methods by which the water has been led into the wheel: from the center outward, from the rim inward, and from above downward through the wheel. Turbines are smaller and have a higher speed of rotation than older kinds of wheels. They run with greater regularity, and their efficiency is equal to, if not greater than, most of the older wheels. An efficiency of from 85 to 90 per cent, is common. The most powerful turbines ever built are those for the (q. v.) electric plant. There are ten of these, each with a capacity of 5,000 H. P. The water has a mean head of 136 feet and is led through a steel penstock 7½ feet in diameter. The wheels are 63 inches in diameter and make 250 revolutions per minute. The shafts of the turbines carry directly the revolving parts of the generators for this great plant. Since about 1880 a special form of water wheel, called the impact wheel, has been developed and introduced in California. The Pelton water-wheel is the best known of the impact wheels. It is used where the head is very great. A jet of water is led against the steel buckets on the edge of the wheel, and these are so shaped as to throw the water off to the side. Pelton wheels are used in practically all the long distance electrical transmission lines in California where the head is high. In the Fresno plant the water-head is 1,400 feet, and the Pelton wheels have an output of 1,000 H. P. The use of water-power in the world has been increased by the development of long-distance power-transmission by electricity. It is estimated that 5,000,000 H. P. in water-power can be made available in the United States by electric transmission, and a recent writer in The Engineering Magazine places the water-power of Italy that can be made available by electric transmission as over 1,000,000 H. P. The use of water-wheels is thus likely to increase.  , a system of pipes and conduits connected with reservoirs or pumping-stations for the supply of water to communities. The ordinary supply of water to communities where the population is scattered is from shallow wells, but in crowded cities such wells become very dangerous on account of the contamination of the water by impurities from the soil. The water-supply of large cities often forms a very difficult problem, and large reservoirs and long conduits often have to be built to get an ample supply of pure water. The common sources of supply are streams, lakes, springs and wells, if the water is derived from streams or lakes, the great difficulty is to get a drainage-area which has not been rendered impure by the refuse of the 