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WATER PURIFICATION. an average of G,4G3,000 gallons of water per day was treated, at a cost of $1.66 per 1,000,000 gal- lons for filtration alone, and $4.52 for pumping to the settling basins, cleaning the latter, filtra- tion and laboratory expenses. The $1.66 of ex- pense for filtration alone was divided as follows: Scraping sand from beds, 24 cents; wheeling out sand, 42 cents; replacing sand, 32 cents; inci- dentals, 13 cents; lost time, 10 cents. The bac- terial reduction was about 09 per cent. Prac- tically all of the turbidity was removed and the color was reduced by 24 per cent. The first mechanical filter connected with a municipal water supply was put in operation at Raritan, N. J., in 1882, by the Somerville Water Company, which supplies Raritan and Somer- ville. No coagulant was used. In JIarch, 1883, John W. Hyatt patented a filter of the same general tvpe as the one erected at Raritan. In February', 1884, Isaiah S. Hyatt, a brother of John, patented a system of coagulation and filtra- tion combined. These and other patents granted to the H}-att brothers formed the basis of the Hyatt and later on the New York mechanical filters. Rivals appeared in the market from time to time and much litigation ensued. The latter was confined chicfiy to the use of a coagulant. In 1897 the United" States Circuit Court of Ap- peals, notwithstanding much expert and other testimony designed to show the prior use of coagulating materials coincident with filtration, declared the Hyatt patent valid. The patent ex- pired early in 1901.

For a number of years the use of mechanical filters was confined almost wholly to the produc- tion of a bright, clear water. Very little dis- interested evidence was available to show their eflicieney in this respect and practically none bearingon their bacterial efficiency. In 1893-94 experiments on a small scale were made under the direction of Edmund B. Weston, at Provi- dence, R. I., which indicated that mechanical filters would remove a high percentage of bac- teria. In 1895-97 more extensive investigations, on a large scale, were made at Louisville, Ky., with Jlr. George. Fuller in charge. Mr. Ful- ler's report on these investigations (see bibliog- raphy) mai-ked an era in water purification. It showed that a water as turbid as the Ohio, and with the large number of bacteria sometimes carried by that stream, could be rendered thor- oughly acce))table by means of sedimentation, coagulation, and mechanical filtration combined. Ihox Removal is accomplished most frequently by siiiipltf ai-ration to precipitate the iron, fol- lowed by filtration for its removal. If aeration is not sullicieut to effect precipitation a chemical may be employed, like milk of lime w-here the iron is in the form of a sulphate. The few iron removal plants built in the United States up to the mi<ld!e of 1901 were located on or very near the North Atlantic seacoast, to treat water from wells.

Water Softening has been employed but rarely in the United States, and then chiclly for supplies to the boilers of manufacturing plants or locomotives. The pioneer water-softening process, and one that is still largely used, was invented and patented by Prof. Thomas Clark, of .bcrdeen. Scotland, about 1841. In the simplest form of this process, lime water, or milk of lime, is thoroughly mixed with the water to be treated. After twelve to twenty-four hours' subsidence the water is so drawn off as to leave the precipitate behind. Modifications of the Clark system include a variety of settling and filtering devices. The process as described will remove only temporary hardness, or car- bonate of lime or magnesia. Permanent hard- ness, caused by the sulphates of lime and mag- nesia, may be removed or reduced by using car- bonate of sod.i as a precipitate, but the process is too expensive for general use.

Distillation, owing to its high cost, is prac- ticable only for small quantities of water under special conditions. It is used largely at arti- ficial ice plants (seeREFHiGER.TiON) and is some- times used to freshen sea water. In addition, it is used in connection with various industrial arts. It consists simply in evaporating water in stills by means of heat and subsequently con- densing the steam. The heat given off in cool- ing, in the best large plants, is utilized to aid in raising the water to the boiling point. The proc- ess clarifies as well as sterilizes water. Boil'inij, in any convenient way, is a very cheap and thor- oughly effective way of sterilizing water used for household purposes. Both distilled and boiled water must be aerated, or it will be very flat and unpleasant to the taste.

Household Filters, with the exception of a very few kinds, had better be avoided whenever it is feared that there are pathogenic bacteria in the water. There are numerous household filters which are more or less useful as strainers for the removal of mud, and some of the very fine- grained filter tubes do good bacterial work if properly cleaned and sterilized at intervals. Bibliography. Fuertes, 'Water Filtration Works (New York, 1001), treats of the practical details of design and construc- tion; Hazen, The Filtration of Puhlic Water Supplies (3d ed.. New Y'ork, 1900), devoted chief- ly to slow sand filtration; Hill, The Purification of Ptihlir ^yater Supplies (New York, 1898), a comprehensive general discussion, giving the relation between w-ater pollution and typhoid fever; Collet, Water Softeninff and Purification (London, 1895) ; Kirkwcwd, Report on the Fil- tration of River Waters (New York, 1869), largely descriptive and chiefly of historic value; Fuller, Report on the Purification of the Ohio River Water at Lo-iiisvillc, Ky. (New Y'ork, 1898), a detailed description of exhaustive ex- ])eriments with mechanical filters: see also re- poi'ts of the Massachusetts State Board of Health, 1887 to date, and special reports on ex- periments at Providence, R. I.. Pittsburg, Pa.. Cincinnati, Ohio, Washington, D. C, and New Orleans, l.a. See also Filter and Filtr.tio. and Water- Works, ami the references to gen- eral works in latter article.

WATER RAM. See Hydraulic Ram.

WATER-RAT. A large vole {Microtis am- philiins), eight and one-half inches in length of head and body and redilish brown in general color, which is numerous throughout Europe. It is closely related in stnuture and habits to the voles and American meadow-mice, and has sim- ilar habits, except that it is more aquatic and diurnal than other members of the genus. It is one of the most familiar of British mammals and typical of the ATuri<he.