The other method, which has to be adopted by cities situated on level plains, or at the mouths of great rivers, is to take the water of some lake, or river, as far out in the former, or as high up the latter, as possible, and purify it by filtration.

This can be done at a moderate expense by preparing great settling-basins and filter-beds.

The first are great pools or small lakes, into which the water is run and held until most of the mud and coarser dirt has settled or sunk.

Then this clear water above the sediment is run on to great beds, first of gravel, then of coarse sand, then of fine sand; and if these beds are large enough, and frequently changed and cleaned, so that they do not become clogged, and the process is carried out slowly, the water, when it comes through the last bed, is pure enough to drink safely.[16] [Illustration: A RESERVOIR AND COSTLY DAM] One of these sources of a safe and wholesome water-supply--the deep flowing well, or spring; the water shut up in the mountains in its lake or reservoir; or the slow filter-bed--should be used by every intelligent and progressive town of more than a thousand inhabitants.
Sewage and its Disposal.

At the same time, while seeking a source of water-supply far removed from any possibility of contagion, we must not neglect the other end of the problem, the protecting of our rivers and lakes from pollution so far as possible; for the water from these must necessarily be used by thousands of people along their banks, either directly, or in the form of shallow wells, sunk not far from the water's edge.