Each curve plotted regularly is assigned a serial number for convenient identification. 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 filing cabinets. The cards are identified by the number and year, as may be seen in Fig. 211 where the three cards are 4961/1910 4691/1911 4691/1912. 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 as 4323A-1910, 4323B-1910, etc. When five years are placed on one card the last year given is marked in the upper left-hand corner of the card, as in Fig. 215, to identify the card and show that it represents all the data for the half-decade ending in that year.
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Fig. 216. Each Curve Card Plotted Periodically Has a Serial Number. A 4-inch by 6-inch 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
It is not feasible or desirable to place on each curve card a title so complete that it gives all the information concerning the source of the data used in plotting the curve. It is better to use a fairly short title, and then have all supplementary information given on a separate card to which reference, if necessary, can easily be made. Fig. 216 shows a 4-by-6-inch card such as is used in conjunction with each curve card regularly plotted. The number on this card corresponds with the serial number on the curve card as will be seen by referring to Fig. 205 and Fig. 207. If two or more curves appear on the curve card, a card like that in Fig. 216 would have subheadings such as 2678A, 2678B, etc., all shown on the same card No. 2678. It will be seen that the informa-
