In applying these figures to actual cases it must be borne in mind that the turbidity is only one of the several factors which control the length of period; and that the turbidity of a water of a given stream is never constant, but fluctuates within wide limits; and that raw water can be applied to filters for a short time without injurious results, even though it is so turbid that its continued application would be fatal. It is very likely also that the suspended matters in different streams differ in their natures to such an extent that equal turbidities would give quite different periods, although the Pittsburg results were so regular as to give confidence in their application to other conditions within reasonable limits, and when so applied they afford a most convenient method of computing the approximate cost of operation of filters for waters of known or estimated turbidities. POWER OF SAND FILTERS TO PRODUCE CLEAR EFFLUENTS FROM MUDDY WATER. When the turbidity of the applied water is not too great it is entirely removed in the course of filtration. With extremely muddy raw waters, however, turbid effluents are often produced with sand filters. The conditions which control the passage of the finest suspended matters through filters have been studied by Mr. Fuller at Cincinnati at considerable length. They are similar in a general way to the conditions which control the removal of bacteria. That is to say, the removal is more complete with fine filter sand than with coarse sand; with a deep sand layer than with a shallow sand layer; and with low rates of filtration than with high rates. The practicable limits to the size of sand grain, depth of sand layer, and rate of filtration are established by other conditions, and the question remains whether within these limits a clear effluent can be produced. At Pittsburg the turbidity of the effluent from a sand filter operated as mentioned above, which received water which had passed through a sedimentation-basin holding about a 24-hours’ supply, but without taking any advantage of storage to avoid the use of muddy water, was nearly always less than 0.02, which may be taken as the admissible limit of turbidity in a public water-supply. This limit was exceeded on less than 20 days out of 365, these days being during the winter and spring freshets, and on these days the excess was not such as would be likely to be particularly objectionable. For the water of the Allegheny River, then, sand filtration with one day’s sedimentation is capable of yielding a water not absolutely clear, but sufficiently clear to be quite satisfactory for the purpose of municipal water-supply.