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 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