By my Journal of May, 1775, it appears that 'the
small and even the considerable ponds in the vales are now dried up, but
the small ponds on the very tops of the hills are but little affected.'
Can this difference be accounted for by evaporation alone, which is
certainly more prevalent in the bottoms? Or, rather, have not these
elevated pools _some unnoticed recruits, which in the night time
counterbalance the waste of the day?_" These unnoticed recruits, though
it is now certain that they come in the form of those swimming vapours
from which little moisture seems to fall, are enlisted by means still not
certainly known. The common explanation was that the cool surface of the
water condensed the dew, just as the surface of a glass of iced water
condenses moisture. The ponds are always made artificially in the first
instance, and puddled with clay and chalk.
In the notes to a recent edition of "White's Selborne," edited by
Professor L.C. Miall, F.R.S., and Mr. W. Warde Fowler, a considerable
amount of information on dew ponds is appended to the passage quoted
above, but the source of supply still remains obscure. The best dew ponds
seem to be on the Sussex Downs, where far more fog and cooling cloud
accumulates than on the more inland chalk ranges, because of the nearness
of the sea.
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