If now we assume a watershed so long that three days are required for the water from the most remote points to reach the intake, with computations as above, water taken for water-works purposes will have an average turbidity of 0.32; and with still longer watersheds this amount will increase, until with a watershed so long that ten days, or the interval between rains, are required for the water from the upper portions to reach the intake, the average turbidity of the water taken for water-works purposes will reach the average turbidity of the run-off, namely, 0.57. In the above computations the numbers taken are round ones, and of course do not represent closely actual conditions. They do serve, however, to illustrate clearly the principle that the larger the watershed, other things being equal, the more muddy will be the water obtained from it for water-works purposes, and the longer will be the periods of muddy water, and the shorter the periods of clear water between them. It cannot be too strongly emphasized that the period of duration of muddy water is, in general, dependent upon the length of time necessary for the muddy water to run out of the stream system after it is once in it, and be replaced by clear water; and that the settling out of the mud in the river has very little to do with it. Muddy waters result principally from the action of rains upon the surface of ground capable of being washed, and the turbidities of the stream at any point below will occur at the times when the muddy waters reach it in the natural course of flow, and will disappear again when the muddy waters present in the stream system at the end of the rain have run out, and have been replaced with clear water from underground sources, or from clearer surface sources. THE AMOUNTS OF SUSPENDED MATTERS IN WATER. There is a large class of waters, including most lake and reservoir waters, and surface-waters from certain geological formations, which are almost free from suspended matters and turbidities. That is to say, the average turbidities are less than 0.10, and the average suspended matters are less than 2 parts in 100,000, and are often only small fractions of these figures. This class includes the raw waters of the supplies of many English cities drawn from impounding reservoirs, and also the waters of the rivers Thames and Lea at London, and the raw waters used by both of the Berlin water-works, and in the United States the waters of the great lakes except at special points near the mouths of rivers, nearly all New England waters, and many other waters along the Atlantic coast and elsewhere where the geological formations are favorable.