Page:Graphic methods for presenting facts (1914).djvu/236

 In making up copy for the line cut of Fig. 177 very little hand work was required. If an outline map of suitable size is available, the only hand work necessary for a cut of this kind is in drawing the small circles and placing inside them the figures serving as key numbers for the shading used. The actual shading is done by the engraver making use of the Ben Day process. For information regarding the preparation of engraver's copy for plates on which Ben Day shading is desired, see Fig. 233.

Fig. 177. Potential Water Power in the Different States of the United States

This is a splendid example of the contrasts in shading made possible by the Ben Day mechanical processes of engraving. Nine contrasting shades increasing in darkness are used here with absolute distinctness. The small number in the circle used to identify the shading is of great advantage

There is one serious error which should be avoided in the interpretation of any shaded map like Fig. 177. 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