A clearer conception of the capacities of the filters under these different conditions may be obtained from the four diagrams, Figure 12, showing, for the four different groups, the average number of days of service of the successive runs. The diagram for Group _A_ shows that the variations in the period of service of the filters scraped each time to clean sand follow a more or less definite curve from year to year. For the period covered by this curve, the tendency seems to be toward a slight decrease in capacity from year to year, as shown by the lower average maximum and minimum in the second year than in the first. Group _B_ shows a sudden decrease in capacity following the first light scrapings and, since that time, a low but quite constant capacity. Group _C_ shows a constantly decreasing capacity with successive rakings. The only significance attaching to the curve after the first raking is the prohibitively low capacity indicated, and the ineffectiveness of the measures taken to restore the capacity after the sixth raking. Group _D_, after the first raking, shows a prohibitively low and constantly decreasing capacity. The diagrams for _C_ and _D_ indicate a dangerous reduction in capacity if long persisted in. The method followed with Group _C_ may be dismissed with the statement that it is entirely insufficient, and would be of use only in the rarest emergencies. As far as the question of capacity is concerned, these diagrams indicate that a filter in normal condition may safely be raked once. It is believed that the constantly decreasing capacity shown in Group _D_ is not due so much to the rakings as to the small quantities of sand removed at the alternate scrapings, and therefore it would not be proper to condemn this method of treatment without a further trial in which this defect was remedied. This view seems to be supported by the results of Group _B_. The low but approximately constant capacity there shown would undoubtedly have been higher if a greater depth of sand had been removed each time. [Illustration: ~Figure 12--Average Number of Days of Service of Successive Runs for Groups _A_, _B_, _C_, and _D_.~] _Quality of the Effluent._--The averages given in Table 29 show but little difference in the bacterial contents of the effluents from the four groups of filters. All are entirely satisfactory, and the differences in favor of one method or another are small. In looking for possible differences in the quality of the effluents from the four groups, it was thought that such differences might be most apparent at a time when the entire plant was working under the most adverse conditions. The bacterial counts, therefore, were summarized for the period from December 23d, 1907, to January 6th, 1908, inclusive, following a period of high turbidity and high bacteria in the raw water, with results as follows: