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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.
By stretching to the full extent of his arm, he managed to plunge the stick into the water. Tasting the drops, he found that they were quite sweet. The sand and porous rock provided the best of filter-beds. He rose, wall pleased, and noted that on the opposite side the appearance of the shrubs and tufts of long grass indicated the existence of a grown-over path towards the cliff.
Moreover, so foul are many of our rivers and streams becoming in thickly settled regions that fish can no longer live in them, and it is hardly safe to bathe in them. Fortunately, however, a great deal of the worst contamination can be prevented by using modern methods of disposing of sewage, such as filter-beds and sewage farms.
There are two filter-beds of light loam over a gravelly subsoil thoroughly underdrained with tiles at a depth of six feet. One of these beds is cultivated with some crop like Italian rye-grass, which bears copious irrigation; and the other by some crop like wheat, which, in the absence of irrigation, will thrive on the fertility left over from the previous season.
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