A smooth curve line has been drawn through the ends of the pictured ships so as to approximate most closely the general law which seems to govern progress in ship building. The smooth curve has been extended into the future as a prediction of the length of the ships which will probably be built during the next ten years.

Note the excellent use of dimension-line arrows at the base of the chart showing the materials used in ship building and the methods of driving ships at different periods in history

Bars to represent different intervals of time as in Fig. 51 and Fig. 52 maj^ be compared to the progress photographs mentioned above. Though the bars and progress photographs are valuable, they may be said to give information only in spots. A moving-picture machine shows pictures so rapidly that the pictures blend into a continuous narrative in the eye and the brain of the observer. What the moving picture is to separate progress photographs, the curve is to detached bars representing time. In just so much as the moving picture is superior to separate pictures shown by lantern slides, in just that much is a curve superior to a series of horizontal or vertical bars for the same data. Unless a person knows thoroughly how to read and how to plot curves he cannot hope to understand the graphic presentation of facts. The use of curves will be covered in later chapters.

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