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A dew pond is an artificial pond usually sited on the top of a hill, intended for watering livestock. Dew ponds are used in areas where a natural supply of surface water may not be readily available. The name dew pond (sometimes cloud pond or mist pond) is first found in the ''Journal of the Royal Agricultural Society'' in 1865.〔''Oxford English Dictionary'': dew-pond〕 Despite the name, their primary source of water is believed to be rainfall rather than dew or mist. ==Construction== They are usually shallow, saucer-shaped and lined with puddled clay, chalk or marl on an insulating straw layer over a bottom layer of chalk or lime. To deter earthworms from their natural tendency of burrowing upwards, which in a short while would make the clay lining porous, a layer of soot would be incorporated or lime mixed with the clay.〔Martin (1915: 84–85)〕 The clay is usually covered with straw to prevent cracking by the sun〔 and a final layer of chalk rubble or broken stone to protect the lining from the hoofs of sheep or cattle. A method of constructing the base layer using chalk puddle was described in ''The Field'' 14 December 1907. A Sussex farmer born in 1850 tells how he and his forefathers made dew ponds: If the pond's temperature is kept low, evaporation (a major water loss) may be significantly reduced, thus maintaining the collected rainwater. According to researcher Edward Martin, this may be attained by building the pond in a hollow, where cool air is likely to gather, or by keeping the surrounding grass long to enhance heat radiation.〔Martin (1915: 133-135; 159)〕 As the water level in the basin falls a well of cool, moist air tends to form over the surface, restricting evaporation.〔Martin (1915: 160)〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「dew pond」の詳細全文を読む スポンサード リンク
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