If a stream of pure water is available at a sufficient height, it may be led directly to the reservoir, or its current may be used to drive a water-wheel sufficient to do the pumping. In a majority of cases there will be found at no great distance a stream capable of supplying the water needed throughout the dryest season of the year, but not entirely free from organic impurities. In such cases it is often feasible, by excavating a filtering sump or pump-well at a little distance from the side of the stream, and at a sufficient depth below the level of its bed to secure a supply tolerably purified by filtration through the intervening earth. The distance at which this sump should be placed from the bed of the stream will depend on the character of the soil. The more porous this is, the greater should the distance be. This question as to the source from which the water is to be taken is one which, more than any other, calls for experienced judgment. Frequently the conformation of the surrounding country is such that, even where there is no constant stream, it is possible by the construction of dams to pond an amount of water, to be furnished by surface washing, sufficient to supply the demands of the longest drought. In this case, as in all others where reservoirs are used, it is important to have a good depth of water, and not to allow, even toward the edges, any considerable shallow area. So far as possible, the depth should be everywhere great enough to prevent vegetation, and in all the shallower parts the surface soil should be entirely removed. As a rule, there should be a depth of at least fifteen feet of water, except near the very edges of the pond, and as much more than this as circumstances will allow.