All these processes belong to the incipient civilization of the ante-historical periods, but the construction of subterranean channels for the removal of infiltrated water marks ages and countries distinguished by a great advance in agricultural theory and practice, a great accumulation of pecuniary capital, and a density of population which creates a ready demand and a high price for all products of rural industry. Under-draining, too, would be most advantageous in damp and cool climates, where evaporation is slow, and upon soils where the natural inclination of surface does not promote a very rapid flow of the surface waters. All the conditions required to make this mode of rural improvement, if not absolutely necessary, at least apparently profitable, exist in Great Britain, and it is, therefore, very natural that the wealthy and intelligent farmers of England should have carried this practice farther, and reaped a more abundant pecuniary return from it, than those of any other country. Besides superficial and subsoil drains, there is another method of disposing of superfluous surface water, which, however, can rarely be practised, because the necessary conditions for its employment are not of frequent occurrence. Whenever a tenacious water-holding stratum rests on a loose, gravelly bed, so situated as to admit of a free discharge of water from or through it by means of the outcropping of the bed at a lower level, or of deep-lying conduits leading to distant points of discharge, superficial waters may be carried off by opening a passage for them through the impervious into the permeable stratum. Thus, according to Bischof, as early as the time of King Rene, in the first half of the fifteenth century, the plain of Paluns, near Marseilles, was laid dry by boring, and Wittwer informs us that drainage is effected at Munich by conducting the superfluous water into large excavations, from which it filters through into a lower stratum of pebble and gravel lying a little above the level of the river Isar.[326] So at Washington, in the western part of the city, which lies high above the rivers Potomac and Rock Creek, many houses are provided with dry wells for draining their cellars and foundations. These extend through hard tenacious earth to the depth of thirty or forty feet, when they strike a stratum of gravel, through which the water readily passes off.