The rapidity of the increase is accounted for by the fact that every field it passes whose surface inclines towards it is a watershed from which an unseen but considerable drainage takes place. When no brook passes through the fields the water stands and soaks downwards, or evaporates slowly: directly a ditch is opened it fills, and the effect of a stream is not only to collect water till then unseen, but to preserve it from evaporation or disappearance into the subsoil. Probably, if it were possible to start an artificial stream in many places, after a while it would almost keep itself going at times, provided, of course, that the bottom was not porous. Below the mouth of the coombe the water has worn itself a channel quite six feet deep in the chalk--washing out the flints that now lie at the bottom. Hawthorn bushes bend over it, and great briars uncut since their first shoot was put forth; the elder, too, grows luxuriously, whose white flowers, emitting a rich but sickly odour, the village girls still gather to make elder-water to remove freckles. These bushes hide the deep gully in which the current winds its way--so deep that no cattle can get down to drink. A cottage stands on the very edge a little further along; the residents do not dip their water from the running stream, but have made a small pool beside it, with which no doubt it communicates, for the pool, or `dipping-place,' is ever full of cool, clear, limpid water. The plan is not without its advantages, because the stream itself, though usually clear, is liable to become foul from various causes--such as a flood, when it is white from suspended chalk, or from cattle higher up above the gully coming to slake their thirst and stirring the sandy grit of the bottom. But the little pool long remains clear, because the water from the stream to enter it has to strain itself through the narrow partition of chalky rubble. So limpid is the current in general, that the idea of seeing trout presently when it shall widen out naturally arises. But before then the soil changes, and clay and loam spoil the clean, sandy, or gravelly bottom trout delight in. In one such stream hard by, however, the experiment of keeping trout has been tried, and with some success: it could be done without a doubt if it were not that after a short course all the streams upon this side of the downs enter the meadows, and immediately run over a mud bottom. With care, a few young fish were maintained in the upper waters, but it was only as an experiment; left to themselves they would speedily disappear, and of course no angling could be thought of.