Page:Graphic methods for presenting facts (1914).djvu/179

 Fig. 138. Cumulative Curves Plotted to Determine the Minimum Size of Tank and the Minimum Steady Flow of Water Required for a Group of Locomotives and a Group of Tugboats Taking Boiler-Feed Water from the Same Source of Supply

Curves for locomotives and tugs are plotted separately. A third curve is then made by adding the vertical distances of points on the two curves to get the vertical distance for points on the third curve. The sloping straight line shows the minimum rate of steady flow. The greatest vertical distance between the sloping line and the combined curve shows the necessary minimum tank capacity

seven at night. The points are plotted in the middle of each space because the values are based on the average quantity of water taken between any two hours specified in the horizontal scale. The next curve above gives on a cumulative basis the average of the amount of water taken by the tugboats each day. It can be seen that the tugs take water between six and eight in the morning and between four and eight at night, the greatest quantity of water being taken between seven and eight. In order to see what would happen if the locomotives and the tugs should take water from the same tanks, the combined curve was made for both locomotives and tugboats by adding the quantities for each one-hour period. The easiest way to make a combined curve when only two curves are to be combined, is to use a pair of dividers, taking the vertical distance above zero for each point on one curve and stepping off that measured distance above each point on the other curve. The prick marks showing the