Between six and eight a. m., when the amount which can flow into the tank through the regular flow-pipe is limited to the cpantity indicated by the dimension mark "B", the draught on the tank is so rapid that all of the water indicated by the dimension mark "A" must be furnished by the storage capacity of the tank itself. The water does not flow into the tank nearly so fast as it is taken out by the tugs and locomotives at that particular time of the day. If we count the squares included vertically in the dimension line "A" we find 7.6 squares. This shows that the amount of water which must be furnished by the tank during the rush hours cannot be less than

162 GRAPHIC METHODS

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.