minutes, and the amount of any one dose is thus made 1 about 1/280 of the total daily volume. With the coarser filling, as when a size as large as hen’s eggs is used, the interval would be cut down to about one minute. It should be added that the intervals last mentioned are characteristic only of some devices used for dosing sprinkling filters and that there is a wide divergence of practice among engineers when dealing with any particular size of sand or stone particles in all kinds of filter beds. The third requirement, namely, the occasional resting of the bed, is met by providing some additional area over that theoretically required, so that the flow may be diverted from part to part of the total area (which is usually divided into beds for this purpose), and in this way each part is allowed, in turn, a period for resting. For example, if the required area be divided into two beds and a third bed added equal in area to one of the two and a regular rotation of dosing be practised, each bed would rest not only the time between the regular twelve-hour period dosing, but might also be given a complete rest, occasionally, for an extended period. This third requirement is probably less imperative with the coarser particles and there are many examples of coarse-grained beds which have been continuously operated for a period of years. It is found, however, that with such treatment clogging is inevitable, and that such clogging is partially relieved by a period of rest somewhat proportional to the length of time the beds have been operated. It is, then, only shortsighted policy to economize at the beginning and attempt to save money by not building an additional area, since the clogging of the whole plant is bound to occur in the course of time, and then another plant must be built or the material forming the bed taken out, washed, and replaced. Otherwise the sewage must go unpurified to the outfall while the bed is recovering from the long period of overwork.