Open sand filters, with masonry walls, with reasonably favorable conditions of construction, and not too small in area, have averaged to cost in the United States within the last few years perhaps about thirty thousand dollars per acre. The relative cost of small plants is somewhat greater, and with embankments instead of masonry walls, the cost is somewhat reduced. The cost is less where natural deposits of sand can be made use of practically in their original condition, and is increased where the filtering materials have to be transported by rail for long distances, or where the sites are difficult to build upon. Covered filters cost about a half more than open filters. Mechanical filters at current prices cost about $20 per square foot of filtering area, to which must be added the cost of foundations and buildings, which perhaps average to cost half as much more, but are dependent upon local conditions and the character of the buildings. To these figures must be added the costs of pumps, reservoirs, sedimentation-basins, and pipe-connections, which are often greater than the costs of the filters, but which differ so widely in different cases as to make any general estimate impossible. Filters must be provided sufficient to meet the maximum and not the average consumption. The excess of maximum over average requirements varies greatly in different cities, and depends largely upon reservoir capacities and arrangements. As a result of a considerable number of estimates made by the author for average American conditions, the cost of installing filters may be taken very roughly as five dollars per inhabitant, but the amounts differ widely in various cases. The cost of operation of sand filters in England probably averages about one dollar per million gallons of water filtered. The following table shows the costs of operation of the filters of the seven London companies for fifteen years, compiled in the office of Mr. W. B. Bryan, Chief Engineer of the East London Water Company. The results have been computed to dollars per million U. S. gallons, and include the cost of all labor, sand, and supplies for the filters, but do not include any pumping or interest costs. COST OF FILTRATION, LONDON WATER COMPANIES. (Computed from data furnished Wm. B. Bryan, C.E., East London Water Works.) Dollars per Million U. S. Gallons.