7.6 times 5,000 (the value for each square on the scale), or 38,000 gallons. The distance "A" is really the same as the vertical distance between the point representing the average for the hour from five to six a. m. and the diagonal line of minimum flow. The storage capacity necessary in any case of this sort is very simply determined by means of curves or mass diagrams on the general scheme of Fig. 138. The measurement of the greatest distance which shows between any depression in the consumption curve and the minimum-flow line which joins the peaks on either side of it gives the minimum steady rate of flow.
There is great practical value in charts like Fig. 138. In this case the minimum-flow line determines the size of the pipe, pumps, or other machinery which must be installed to provide the requisite quantity of water if the water is kept running steadily all the time. The tank capacity must be as great as the diagram demands or there will not always be sufficient water. In practice, it would, of course, be customary to put in a pump considerably larger than that needed to provide the minimum flow which the chart shows to be necessary, and the tank would also be of larger capacity than the minimum-storage determination of the curve would indicate. The extra capacity of both pump and tank are, however, only a safeguard against abnormal conditions. The graphic solution shows the exact rate of flow and the storage capacity which would be satisfactory if the conditions indicated by the data on the curves were to be constantly maintained.
Fig. 139 shows the application of the cumulative or mass curve to problems of municipal water-supply. In working up data for rainfall in different watersheds and determining the greatest possible amount of water which can be obtained from watersheds when different sizes of reservoirs are used, the cumulative curve is almost indispensable. In Fig. 139 the method is nearly identical with that used in Fig. 138, except that in Fig. 139 we are determining the greatest possible rate of uniform consumption from a fluctuating supply, instead of determining the smallest possible rate of uniform supply for a fluctuating consumption. In Fig. 139, the lines beginning at the hump in 1870 are drawn at different angles to touch the different humps and show various rates of possible consumption. These flow lines are also continued in the other sections of the curve just as if the curve had been shown continuously in one line instead of in three separate sections. The scale for Fig. 139 is selected to show "million
