Page:Graphic methods for presenting facts (1914).djvu/69

 method of propulsion. Dimension lines used in this way are an extremely valuable adjunct to chart work and should be used much more commonly than they are at present.

World's Work

Fig. 54. Wage-Scale Increase

This illustration is so drawn that wages appear to have increased much more rapidly than is proved by the actual data given in the chart itself. The chart should have been extended to show the zero line of the vertical scale so as to assist the reader in getting a correct interpretation

Tendencies for the future are frequently very accurately predicted by drawing a smooth curve through known points and then continuing this curve for future years in the manner shown by the broad line in Fig. 55. The trend of this curve indicates that by 1925, we are likely to have ships about 1,200 feet long. Though the method of prediction by extending a curve into the future is very valuable, care must always be used to apply the method with judgment. If in Fig. 55 a curve had been drawn from the data up to the year 1860 and extended from that point to show the probable length of vessels in succeeding years, the curve would have indicated a likelihood of 1,200-foot ships by 1870. We can see in the light of history that such a prediction would have been most erroneous. It would have assumed the continuance of side-wheel steamers and would have been based chiefly on the length of the Great Eastern alone. The Great Eastern was too far in advance of the age and was in reality an engineering failure. Hence, we see the error which would result from basing a prediction curve on too limited an amount of data.

Periodic photographs of any kind of construction work are one of the most striking forms of graphic presentation. Many large contractors and machine manufacturers now make a practice of having photographs taken of each job at least once each week. The