All porous earth-beds are reservoirs of water, and give out their supplies more or less copiously according to their states of engorgement; and at higher or lower levels, as they are more or less replenished by rain. Rain percolates through the chalk rapidly at all times, it being greatly fissured and cavernous, and finds vent at the bottom of the hills, in ordinary seasons, in the perennial springs which issue there, at the top of the chalk marl, or of the galt (the clay so called) which underlies the chalk. But when long-continued rains have filled the fissures and caverns, and the chinks and crannies of the ordinary vents below are unequal to the drainage, the reservoir as it were overflows, and the superfluity exudes from the valleys and gullies of the upper surface; and these occasional sources continue to flow till the equilibrium is restored, and the perennial vents suffice to carry off the annual supply. Some approach to the full engorgement here spoken of takes place annually in many parts of the chalk districts, where springs break out after the autumnal and winter rains, and run themselves dry again in the course of a few months, or maybe have intermissions of a year or two, when the average falls are short. Thence it is we have so many "Winterbournes" in the counties of Wilts, Hants, and Dorset; as Winterbourne-basset, Winterbourne-gunner, Winterbourne-stoke, &c. (Vide Lewis's _Topog. Dict._) The highest sources of the Test, Itchen, and some other of our southern rivers which take their rise in the chalk, are often dry for months, and their channels void of water for miles; failing altogether when the rains do not fill the neighbouring strata to repletion.