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Data of the New York Edison Company

Fig. 97. Number of Telephone Messages Each Hour for One Day in a Business District in New York City as Compared with a Residence District

Solid line, the "Broad" exchange -- a tj'pical business exchange Dotted line, the "Riverside" exchange -- a typical residence exchange Note the great number of business calls after mail has been opened in

the morning and after return from lunch. In the residence district

there is much morning shopping by telephone

written magazme article. Fig. 98 is taken from a commendable report by the health department of the city of Boston. In the report itself, different colors of ink were used for the various curves, with the effect of emphasizing the contrast.

Though the colored inks assisted in catching the eye of the reader, the chart with curves designated by letters is usually sufficient for all practical purposes whenever the curves do not cross over each other in such manner as to be confusing. As mentioned elsewhere, a certain slope of a curve plotted on rectangular co-ordinate paper does not in itself indicate a greater or less amount of increase or decrease than holds true for some other curve having a different slope. The slope of a curve plotted on paper with ordinary co-ordinate ruling depends largely on whether the data of the curve are in large figures, so as to bring the curve near the top of the chart, or in small figures, bringing the curve near the bottom of the chart. In Fig. 98 the fact that curve A slopes more than curve C is due to the fact that curve A is placed higher on the vertical scale of the chart than curve C.