The rates of filtration used in the experiments have ranged from less than 100 to about 130 million gallons per acre daily. To employ a rate much higher than this involves the use of a much coarser sand, or an increase in the height of water upon the filter to an impracticable extent. There would seem to be no material advantage in the use of lower rates within certain limits, while the cost of filters would be greatly increased. The sand used in the Warren filters has been crushed quartz. In the Jewell filters a silicious sand from Red Wing, Minn., with rounded grains has been used. These sands are somewhat coarser than are commonly used in sand filters, and the uniformity coefficients are very low. It is necessary to use sand with the very lowest uniformity coefficients to avoid the separation of sand particles according to sizes as mentioned above, and for this reason the sand must be selected with much greater care than is required for sand filters. [Illustration: PLAN JUST ABOVE COPPER. SECTION SHOWING FILTER DURING ORDINARY OPERATION. FIG. 23.—WARREN FILTER: PITTSBURG EXPERIMENTS. SECTION NO. 1.] [Illustration: PLAN OF AGITATOR, GUTTER CASTINGS, ETC. SECTION SHOWING FILTER DURING OPERATION OF WASHING. FIG. 24.—WARREN FILTER: PITTSBURG EXPERIMENTS. SECTION NO. 2.] The round-grained sand is more readily and completely washed than the angular crushed quartz. It has been claimed that the crushed quartz is more efficient as a filtering material, but the evidence of this is not very clear. The amount of water filtered by a filter between washings is, in a general way, about the same as that filtered by a sand filter between scrapings, in relation to its area. The amount of water required for washing is, on an average, about equal to a vertical column 5 or 6 feet high equal in area to the area of the filter, exclusive of water on the top of the filter wasted before the current is reversed. With clear waters, as for instance, the Allegheny at low water, the amount of washing is almost directly proportional to the amount of sulphate of alumina used. With muddy waters the sulphate of alumina required is proportional to the mud, and the frequency of washing and the amount of wash-water are proportional to both. The amount of wash-water required averages about five per cent; with very muddy waters more is required. At Louisville, with the worst waters, the per cents of wash-water rose at times to 30 per cent of the total quantity of water filtered.