An example of the kind of information which should be noted on the curve-card margin came up in a large public-service company, where the manager was for several minutes unable to explain a very great fluctuation which had affected the earnings of a trolley company some two years before. After careful study to explain the drop in the curve, he finallj^ recalled that this trolley line was in a city where all cars must pass over a drawbridge between two sections of the town. At the time in question a steamer had collided with the drawbridge, making it impossible for about two weeks for any street car to cross. This accident caused the earnings of the trolley line to drop greatly during the whole of the two-weeks period. The cause of the unusual condition for the curve should have been recorded for future reference.

In Fig. 207 we have the curves for three succeeding years placed one above the other, so the eye can glance up and down the vertical lines for months and see instantly the changes which have occurred during the entire period. As, automobile sales are very greatly affected by the weather conditions of different seasons of the year, these curves are important. Though weather conditions have affected the curves quite largely, we can see, by comparing the curves for 1910 and 1911, that probably conditions of management as well as weather conditions caused smaller shipments in November and December, 1911, than in those same months of 1910, when shipments were quite good. The card for 1912 is shown with the curve incomplete, just as the manager might have seen it early in the month of February, 1912, after the January reports had been received, tabulated and plotted. As Fig. 207 shows curves which are true records of the real happenings in an automobile plant, they are worthy of study for practice in curve interpretation.