Though a chart in the form of Fig. 79 might be justified in the Sunday supplement of a newspaper where an untrained audience must be reached, it is much better to use a curve in the form of Fig. 80 Avhenever a trained audience is assured. The most interesting thing about Fig. 79 is the slanting line which gives an unusual optical illusion if observed under artificial hght, especially with a bare gas flame. The slanting line then appears blue, although it is printed black like the horizontal bars of the chart.

The dotted line in Fig. 80 corresponds to the slanting line of Fig. 79, and represents a progressive average of all the points on the curve

CURVE PLOTTING

above. The dotted line, of course, coincides with the soKd line at the first point where there is only one point to consider in the average. Figures for the dotted line are obtained by averaging the figures for the first two years, then the first three years, then the first four years, etc., until the last point on the dotted line represents an average for all the points on the solid line.

Fig. 80 is worthy of attention as a model of good practice which may be studied carefully by anyone just beginning to plot curves.

1879- I95TONS

PittsbuTgh and Lake Erie Railroad

Fig. 79. Yearly Average of Revenue Tons per Train Mile on the Pittsbxirgh and Lake Erie Railroad. The Slanting Line Shows a Progressive Average

If this illustration is observed with artificial light, an interesting optical illusion may be noticed in that the