As the train remains constant in length throughout one whole journey it is obvious that the two curve lines must be a constant vertical distance apart throughout the length of the chart. At stations, subway trains must stop in such a way that the whole length of the train will be opposite the platform so that passengers may get in and out of every car. The trains follow each other very rapidly, and it is essential that sufficient room be left between two trains for safety. If two curves on a timedistance chart should touch each other, it would indicate a collision. The distance between different train curves on a chart gives information regarding the kind of signal and brake systems that must be used to give the desired degree of safety in operating the road.

Chapter V CURVE PLOTTING

INFORMATION may be charted in many different ways. Under present conditions, if six men were given a set of figures and asked

to chart these figures, the six resulting charts would be widely divergent in method. Though variety in method of charting is sometimes desirable in large reports where numerous illustrations must follow each other closely, or in wall exhibits where there must be a great number of charts in rapid sequence, it is better in general to use a variety of effects simply to attract attention, and to present the data themselves according to standard well-known methods.

In Fig. 66 the attempt to give a spectacular scheme of presentation seems to have overshadowed everything else in the mind of the illustrator. Though a striking architectural design has undoubtedly been obtained, the chart means nothing, for it is impossible of interpretation. No scale has been used in either the horizontal or the vertical direction, as can be seen by comparing the figures on the block for 1830 with the figures on the block for 1840.