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

GEAFHIC METHODS

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CUMULATIVE CURVES 161

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.