Fig. 55 is a commendable piece of work for popular presentation, as for instance in a magazine. Note the use of dimension marks in two independent horizontal rows so that the upper row indicates the material from which the ships are made while the lower row shows the

Reprodvced by Permission from Droege's " Freight Terminals and Trains", copyright, 1913, by the McGraw-Hill Book Company

Fig. 53. Changes after Ten Years in Costs of Railroad Materials and in Freight Rates for a Large Railroad System

A simple chart of this kind is often advantageous, as it neglects all temporary fluctuations and shows only the important changes over a period of time considered as a whole. The line for 100 on the scale should have been made a broad line to indicate the basis of comparison. A wavy line should have been put at the bottom of the chart to show that the scale does not begin at zero

COMPARISONS INVOLVING TIME

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. 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