Page:Graphic methods for presenting facts (1914).djvu/181

 distances stepped off by the dividers locate the new combined curve so that it can be quickly drawn in. The combined curve in Fig. 138 shows at a glance that the locomotives and tugs together take water in such manner that the greatest rate of flow from the tank occurs between six and eight in the morning.

One problem involved in Fig. 138 was to determine the minimum steady rate of flow into the tank and the size of the tank so that there would always be sufficient water available. By drawing a line diagonally touching the hump on the curve between seven and eight in the morning and the hump on the curve between six and seven at night, we get at once the smallest size flow which will keep the tank full throughout the whole twenty-four hours if locomotives and tugs always draw out the water as the average curves shown in Fig. 138 would indicate. The actual rate per hour at which the water should flow is obtained most easily by assuming a horizontal distance of several hours on the scale, and then reading upward to the diagonal line that vertical distance which corresponds with the horizontal distance taken. Thus, if from the point on the line for 40,000 at which the diagonal line intersects the vertical line for five a. m., we count 10 blocks horizontally to the right, we see that it takes 9.1 blocks before we again reach the diagonal line. Nine and one-tenth vertical blocks correspond on the scale to 45,500 gallons of water flowing in 10 hours, so that the minimum steady rate of flow must be 4,550 gallons per hour.

The dimension marks at "C" and "D" indicate the great amount of water used between four and eight p. m. The diagonal line drawn parallel to the minimum-flow line shows the amount of water which flows into the tank between three and eight p. m., thus reducing the quantity to be supplied from the storage space of the tank to the amount indicated on the vertical scale by the dimension mark "C". Between six and eight a. m., when the amount which can flow into the tank through the regular flow-pipe is limited to the quantity 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