It will be obvious from the above that for these conditions storage is much more important than sedimentation. This brings us back to the old English idea of having storage-reservoirs large enough to carry water-works over flood periods without the use of flood-waters. Reservoirs of this kind were, and still are, considered necessary for the successful utilization of waters of many English rivers, although these waters do not approach in turbidity the waters of some American streams. This idea of storage has been but little used in the United States. In the above case, if we use our reservoir for storage instead of as a sedimentation-basin, the average quality of the water can be greatly improved. The reservoir should ordinarily be kept full, and pumping to it should be stopped whenever the turbidity exceeds a certain limit, to be determined by experience; and the reservoir is then to be drawn upon for the supply until the turbidity again falls to the normal. In the case assumed above, with a stream in which all of the water reaches the intake in 24 hours, a reservoir holding a 24-hours’ supply, or in practice, to be safe, a somewhat larger one, would yield a water having a very much lower average turbidity than would be obtained with water pumped constantly from the stream without a reservoir. With a river having a watershed so long that 48 hours are required to bring the water down from the most remote feeders, a reservoir twice as large would be required, and would result in a still greater reduction in the average turbidity. As the stream becomes larger, and the turbid periods longer, the size of a reservoir necessary to utilize this action rapidly becomes larger, and the times during which it can be filled are shortened, and thus the engineering difficulties of the problem are increased. For moderately short streams, cost for cost, storage is far more effective than sedimentation, and we must come back to the old English practice of stopping our pumps during periods of maximum turbidity. EFFECT OF MUD UPON SAND FILTERS. There are two aspects of the effect of mud upon the operation of sand filters which require particular consideration. The first relates to the rapidity of clogging, and consequently the frequency of scraping and the cost of operation; while the second relates to the ability of the filters to yield well-clarified effluents. EFFECT OF TURBIDITY UPON THE LENGTH OF PERIOD.