He need only glance at the figures above any point to learn the value for that point far more correctly than would ever be possible with even the most accurately plotted curve if the value of the point had to be interpolated from the vertical scale of the chart. The vertical scale of these cards on which the figures are given in the upper margin fulfills almost no purpose except that of giving a record of the scale to the man who must plot points in succeeding months, or that of giving the values of horizontal lines which are convenient in locating high points or low points on any curve.

When another point has to be added on a large number of curves for a succeeding month or week, the most convenient procedure consists in first copying upon all curve cards, immediately above the vertical line for the proper month, the figures from the typewritten reports, or

GRAPHIC METHODS

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other sources of information. This is straight clerical work and may be done with a pen on one card after another with great rapidity. Ordinary liquid drawing ink is the best for the figures, as the opaqueness of the drawing ink gives better blue prints than are obtainable from writing ink. This is often partly transparent and prints pale blue instead of clear white.