The operation of this plant during the first year and a half offered an excellent opportunity for the study of sedimentation in the sand, and the data in Table 30 are presented to show that certain of the phenomena of filter operation observed during this period seem to be fairly explicable by the physical theory of purification. These data are given only for the period of operation before the summer of 1907. At that time the experiments in filter cleaning already described were begun. Before that time, whenever a filter had been cleaned, all the discolored sand had been removed, leaving for the following run a new sand surface substantially in the perfect condition of a newly-constructed filter. After that time the experimental methods of cleaning, and the new routine adopted as a result thereof, interfered with the tracing of the evidence as clearly as during the earlier periods. [Illustration: ~Figure 14--Periods of Service and Depths of Scraping for Runs Ending in Various Months Covering Entire Period Oct. 1, 1905, to Mar. 1 1907.~] Table 30 and the corresponding diagram, Figure 14, show the general variations in the length of runs and depth of penetration, with the seasonal temperature changes. The increase in length of runs and quantity of sand removed under low temperature conditions is very marked. There is, however, a secondary maximum which appears, as the diagram shows, where a minimum for the year would be expected. This may have been an irregularity occurring this one year, which will not appear in the average of several years, and caused by some factor which has escaped observation. A careful analysis of the data at hand fails to show any explanation for it. It may exist in some of the little-understood biological actions which have their maximum effect under warm-water conditions, or it may be due--in some obscure way--to the liberation of air under the surface of the sand, accumulating with pressure enough to break the surface at innumerable points, thereby reducing the loss of head and extending the period of service. Some evidence was observed pointing to this explanation, but it was never conclusively proven. The general effect of temperature changes on the rapidity of removal of the sediment and its consequent concentration in the sand layer, however, seems plainly evident. In corroboration of the third point mentioned in the theoretical consideration of turbidity removal in the filters, the daily turbidities of the filtered water have been classified and summarized for different turbidities in the applied water, and also for different temperatures. The average turbidities thus obtained are given in Table 31.