Science: Dew Ponds

• Share

In southern England 3,000 years ago, voracious packs of wolves roamed the moist lowlands. The savage Britons of that Stone Age period, who had learned the art of domesticating animals, had to keep their cattle on the uplands lest they be devoured. On the uplands there were few streams of water. With the eerie ingenuity which savages sometimes manifest, the herders built "dew ponds" which stayed full of water though the animals drank from them every day. Some modern authorities contend that rain contributes practically all of the ponds' water supply, but others disagree, claiming that dew—moisture condensed from the air— provides the important portion.

The latter view is taken by the editors of Arthur D. Little, Inc.'s Industrial Bulletin (chemical news and scientific miscellany), who discussed the British dew ponds in last week's issue and gave an explanation of the heat economy which makes them possible. "Recent research," said the Bulletin, "has shown that water is nearly perfect as a 'black body' or a body that easily gives off heat by radiation." The pond must keep cool so that dew will condense in it, and so that it will not lose much water by evaporation. If it is insulated, it will not absorb much heat from the earth. The heat it absorbs from the air and the sun can be lost by radiation. In principle, it is something like a thermos bottle for cold drinks.

After digging out the basin for a pond, the Stone Age people lined it with straw, then covered the straw with a layer of clay. This furnished the necessary insulation. Some present-day English builders are reputedly able to make successful dew ponds, but they generally use concrete instead of straw and clay. Moreover, after construction, these modern ponds have to be filled with water first in order to keep going. Whether the ponds of the ancients filled up by natural accumulation of water, starting from a dry basin, no one knows.