WATERSPOUT. 353 WATER SUPPLY. over a small region within an hour's time, and cause great destruction. The popular term 'cloudburst' should he applied in these eases, and not the term 'waterspout.' Consult: Fer- rel, Recent Advances (Washington, 188.5) ; id., Popular Treatise on the ^Vinds (New York, 1889). WATER-STRIDER. A .slender, long-legged water-bug of the family Hydrobatida'. They live upon the surface of quiet waters and dart about ■with great rapidity. They prey upon other in- sects and may even leap into the air in pursuit of their prey. The numerous species are found upon both salt and fresh water. They have prom- inent eyes, a stout beak, long antennir, aii<l an ■ abdomen which usually tapers to the tip. Shiny species are dimorphic, one form being wing- less and tlic other winged. Their eggs are at- tached to submerged parts of plants. A com- mon species upon ponds and in the United .States is Hi/grotrechus remigis, a moderately stout, dark brown insect about half an inch long. It hibernates in mud, beneath leaves upon the banks, or at the bottom of the water under stones. A very strange form common in cer- tain portions of the United States is lihcvmato- htttes Rileyi. While truly aquatic, watcr-striders are structurally more nearly related to the land- bugs than to the other water-bugs. WATER SUPPLY. Water for irrigation, r.avij;af inn, water-works, and other industrial and domestic purposes, derived primarily from the rainfall and secondarily from running streams and lakes, or surface water; and from springs, wells, tuimels, and infiltration galleries or under- ground sources. The quality of water supplied foi; irrigation, water power, and navigation is of comparatively small consequence, except that a badly polluted stream might be a general nui- sance, and that the mineral contents of certain waters are harmful to crops. The water made available by water-works should be above all question as to sewage pollution and should pos- sess other qualities, natural or artificial, dis- cussed under W.TER-WoRKs and W.tkr PrBiFi- CATION. A gravity flow is generally an essential in the case of supplies for navigation and water power. The same is often but by no means al- ways true of water for irrigation. These limita- tions are due to the cost of pumping and (io not apply to supplies for domestic and manufactur- ing purjioses. Besides volume, there is a further essential to water for power, namely, sutTieient fall to yield the requisite amount of energy to accomplish the desired work. (See Water Power. ) As has been stated, the common source of all water supplies is the rainfall. Some of this finds its way to the streams, ponds, and lakes; some sinks into the soil to be utilized by plants, or to percolate through the soil to lakes and streams: some is stored in the soil and the permeable layers below it ready to be tajiped by wells; and a large part is evaporated directly from the surface of the ground, only to fall to -the earth again, in the form of rain. dew. or snow. With a given rainfall in any locality the amount of water naturally available will de- pend upon the extent, surface slope, and geo- logical formation of the gathering -ground, or drainage area, and the nature of the vegetation on that surface. The steeper the slope and the greater the proportion of the rainfall reaching the streams, the less the amount available as underground sujiplies. With verj' stce]) and im- jiervious drain:ige are:is, the streams rise and fall suddenly, leaving comparatively little water available for any useful purpose, and threaten- ing the stability of such water-supply works as are built. Forested and other well-covered areas retain the rainfalls for longer periods, but, in common with all vegetation, they make their own demands ujjon the water stored in the soil. Nevertheless, such areas, and also (lat. sandy stretches of country, tend greatly to equalize the seasonal yield of drainage areas. Temperature and hiunidily are inqiortant factors in all water- »iip|)ly calculations, high temperattires and low humidities increasing the eva])oration from both land and water. When the ground is frozen water cannot percolate into it and the surface flow, or run-off, is .so rapid that but little of the rainfall is available, unless there is ample stor- .age. The percentage of water surface to total drainage area plays a very important part, since the evaporation from water surfaces, particu- larly in hot and dry climates, is much greater than that from land areas. The excellent maps and reports of the United States Geological Survey and some of the State geological suneys often afford sufllcient data relating to the extent, surface, and ' subsurface character of drainage ai-eas for at least pre- liminary studies of water supplies. Where the.se maps are lacking or insufficient, special topo- graphical surveys are necessary, but it may not be so easy to secure geological data. In like manner, meteorological statistics (see Rai.n) may be secured from the United States, State, and local weather bureaus or meteorological sta- tions. Still further aids are the observations on the flow of streams made for a number of years past by the Hydrographic Department of the United States Geological Survey, bj' some of the State engineering bureaus, by municipal water departments, and by owners of private water powers. The New .Jersey Geological Survey, the State Engineer of New York, the cities of Bos- ton and New York, the private corporations which control the water power of Lawrence and Lowell, Mass., the East .Jersey Water Company, in northeastern New Jersey, and the Spring Val- ley Water Company at and near San Francisco, C'al., afford the most notable examples of valuable records of this sort. Observations on the amount of water required for various crops, and various phases of ground-water investigation, have been conducted for years past by the United States and the various' State agricultural ex- periment stations, imder the control or leader- ship of the United States Department of Agri- culture. In developing a water supply for any purpose, the first step is to determine the approximate re- lation between the quantity needed and the vari- ous supplies that are or appear to be available. 'here the source is to be some large river or lake, or say an artesian well from a basin of known capabilities, and in general where the re- quired supply is obviously only a small part of the minimum yield of the source, no prelimi- nary investigation as to quantity available will be required. But in the majority of cases this point demands at least some consideration. The
