The key scale at the lower left-hand corner of the illustration shows that the different shades do not become darker by any uniform increase in horse power. The range included in shade number eight is 50,000 -- from 50,000 to 100,000. For shading number two, however, the range is 3,000,000 -- from 4,000,000 to 7,000,000. This numerical scale was badly selected, for the steps vary so greatly in size that the increasing degrees of shad-

MAP PRESENTATIONS

Fig. 178. Yield of Com per Acre in the Eastern Part of the United States in 1900

Here the data have been recorded by counties rather than by States. This chart is an example of what any draftsman may do by hand shading. It would have been better to use the smaller numbers to represent the best rank, as was done in Fig. 177

GRAPHIC METHODS

MAP PRESENTATIONS 219

ing, as they appear to the eye, mean practically nothing. States having shading number two could vary from each other in the amount of horse power by more than the whole quantity of power in States having shadings from three to nine inclusive. If the steps in the shading scale had been so made that there were nine increasing classes of shading, each representing 1,000,000 horse power, it can be seen that all the States here numbered five to nine inclusively would have the same shading. Such a map made with a scale of uniform steps would appear so entirely different from the map shown here that no one would ever recognize the maps to have been made from the same data. The selection of scale for maps of this kind is important in order that the map may tell a truthful story. Wherever feasible, the numerical scale intervals for colorings or shadings should be uniform.