This serial number remains the same year after year, even though changes are made in the manner of assembling data or in any other minor details affecting the curve. The serial number is placed at the upper left-hand corner of the card, where it can easily be seen when the cards are filed in card-index fifing cabinets. The cards are identified by the number and year, as may be seen in Fig. 211 where

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the three cards are tlio ml till. When two curves appear on the same card, as in Fig. 215, they may be identified by the letters "A", "B",

"C", etc., placed on the chart near each curve, and any reference to the several curves may be made as4323A-1910,4323B -1910, etc. When five years are placed on one card the last year given is marked in the upper left-

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Fig. 2 1 6. Each Curve Card Plotted Periodically Has a Serial Number. A 4-incli by 6-uicli Record Card is Filed by the Curve Serial Number to Give the Complete Information Necessary so that Anyone May Know How to Collect the Data and Plot the Curve. The Card, Shown Above, Refers to Curve 2678 Shown for Three Years in Fig. 207 and in Fig. 208