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LEFT WATERLOO 315 WATER POWER floating leaves; it is the white lotus of the Nile, The blue lotus {N. coerulea) has fragrant blue flowers. Several species and numerous varieties are in cultivation as ornamental plants, includ- ing the above and N. odorata of North America. In India the seeds are used as food. In the allied genus Nuphar the flowers are yellow, and the common species (Nuphar luteum) is known as the yellow water lily. WATERLOO, a city and county-seat of Blackhawk co., la.; on the Cedar river, and on the Illinois Central, the Chicago Great Western and other rail- roads; 6 miles E. S. E. of Cedar Falls. Here are a court house, numerous churches, West High School, Collegiate Institute, Conservatory of Music, School for Stammerers, waterworks, street railroads, electric lights. National and other banks, and several daily, weekly and monthly periodicals. The city has flour mills, foundries, woolen mills, and manufactures of farming implements, furniture, electric cars, gasoline engines, locomotives, paper, machinery, etc. Pop. (1910) 26,693; (1920) 36,230. WATERLOO, BATTLE OF, an im- portant battle won by the allied forces over Napoleon, near Waterloo, a Belgian village 11 miles S, of Brussels, June 18, 1815. The preliminary battles had been at Ligny, June 16 (when Napoleon had defeated the Prussians under Bliicher), and at Quartre-Bras, on the same day (when the allies under Wellington com- pelled the French Marshal Ney to re- tire). At Waterloo the French num- bered about 72,000. The allies (British, Dutch, and Germans), under Welling- ton, had about 67,000; the Prussians (about 50,000 more), under Bliicher, came up in time to take part in the close of the battle, and in the pursuit. The battle began about 11:30 A. M. Briefly it may be said to have consisted of a series of brilliant, but unsuccessful charges made by the French, and dogged resistance on the part of the British; in the evening the French Old Guard charged, but unavailingly ; after which the allies advanced. The French lost about 35,000, and many prisoners; the allies about 22,000. Marshal Grouchy, though he defeated Bliicher at Wavre (q. v.), June 18, failed to prevent him from joining Wellington, and himself failed to come to Napoleon's aid, though but a few miles distant. The rout of the French was complete, and the dis- aster final to Napoleon, the result being his deposition and exile to St. Helena. WATER METER, a contrivance for measuring the amount of water received or discharged through an orifice. There are numerous varieties. Also an instru- ment for determining the amount of water evaporated in a given time, as from a steam boiler. WATER POWER, a general phrase applied to the various means by which the energy of moving water may be utilized. To make such a source of energy effectual, it is necessary and sufficient to have water falling from a higher to a lower level; in other words the water must have what is technically called a "head." Such conditions more or less favorable exist in all streams, though in many cases the fall is so slight and therefore the volicity of the water so small that practically no useful work can be obtained. In such cases, by the construction of dams or otherwise, the energy may be stored up as potential energy in a large mass of water to be transformed at will. Even where the stream is one which is directly available for performing useful work, it is usually advisable to collect the water in reser- voirs, since the quantity of flow of most streams varies considerably with the seasons. Even with such precaution! the supply of water may be far from uni- form, overflowing the dams in the wet season and hardly filling them in the dry weather. Given the available fall and the total quantity of water which flows in a given time, the potential energy which is lost is expresed in foot pounds by the prod- uct of the mass in pounds multiplied by the height through which the water has fallen. This energy is mainly trans- formed into the kinetic energy of run- ning water, while part is transformed into heat and sound. If the former por- tion could be wholly transformed into useful work, the water would be brought to rest; but as this is never practically realized, we see that the available energy of a fall of water is much smaller than the total transformable energy. Of the various machines by which the necessary transformation is usefully effected, the most common are what are known as water wheels, in their several forms of turbines, undershot wheels, breast wheels and overshot wheels. The last three are very similar in construc- tion, and are made to rotate round a horizontal axis by the action of the water on the floats which are fixed at equal intervals more or less radically round the circumference of the wheel. In the overshot wheel, which is appli- cable to falls of from 12 to 60 feet, the water is received in the bucket-shaped partitions in the circumference, and by its weight, aided in some measure by the