To make a bar scale for a map of unknown scale that shows only a single meridian and parallel, or for a map on which no meridians or parallels are shown, first ascertain the distance between two points shown on the map by reference to other authentic maps. If, for example, the distance between two such points is 16.315 miles draw a horizontal line (_a_ in fig. 8) representing this distance on the map, and at its end, at right angles to it, draw another line (_b_) actually measuring 16.315 units of any convenient denomination. Draw a straight line (c) diagonally between the ends of lines _a_ and _b_. Then set off on line _b_ any convenient number of the units selected, say 5 or 10, and project from the points set off lines exactly parallel with line _c_ to line _a_. The distance and the number of the units thus marked on line a will indicate the number of miles covered by that distance on the map, as shown in figure 8. [Illustration: Figure 7.--Designs for bar scales.] [Illustration: Figure 8.--Method of making a bar scale for a map of unknown scale.] [Illustration: U. S. GEOLOGICAL SURVEY PREPARATION OF ILLUSTRATIONS PLATE VIII PATTERNS USED TO SHOW DISTINCTIONS BETWEEN AREAS ON BLACK AND WHITE MAPS Contrasts may be increased by varying the direction and spacing of Lines] SYMBOLS. Symbols should be drawn with as much care as letters, though to a critic they may not appear so bad as poor lettering unless he finds them glaringly large or so small that he can discover or identify them only with difficulty. The size of a symbol must depend on its importance on the map bearing it. On a map that shows numerous mines, for instance, the crossed hammers or the symbols for shafts should be not only visible but conspicuous. The draftsman who is to make such a map must know beforehand how much his drawing will be reduced in reproduction and must make the symbols in proportion to the reduction. The symbols shown in Plate II (p. 20) should be used in all the Survey's illustrations where they are appropriate. AREAL PATTERNS FOR BLACK AND WHITE MAPS.