In order to see the general shape of a frequency curve when plotted with flat tops instead of peaked tops, the book may be turned so that the illustration is seen from the left-hand edge. This chart was drawn primarily as a wall exhibit, to be used later as an illustration in a printed report. The general scheme is excellent and it could scarcely be improved upon, even though the independent variable has here been made the vertical scale instead of the horizontal scale. Putting these data in the form of a curve such as is used in Fig. 143 would probably not be as effective for untrained readers as the black

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bars of Fig. 142, placed against a field in the general shape of a New York manufacturing building. Fig. 143 shows a frequency diagram of the kind found most useful in ordinary work. The vertical scale here represents percentage, and the total of all the figures shown at the upper part of the chart added together is 100 per cent. Frequencj^ curves are very often used, however, with numbers rather than percentages represented on the vertical scale, and the vertical scale then shows the actual number in each class. To assist the reader, the total number of observations made would usually be recorded, perhaps in the title of the illustration. In biological work observations are usually made in vast number, to permit making a very accurate conclusion regarding the general laws of frequency for any particular subject under consideration. For a great many problems of everyday life, however, the observations are not