The following is the simplest and most accurate method of marking the reduction or enlargement of a map to a selected scale: Measure the distance between the extreme meridians along one of the parallels. (See fig. 10.) Convert this distance into miles by multiplying the number of degrees it covers (say 3) by the number of miles in a degree. A degree on the forty-third parallel, for example, is 50.669 miles,[10] which multiplied by 3 equals 152.007 miles. Then draw a line on the margin of the map, outside the border, the exact length of the 3 degrees, and just below this line draw another line representing the same number of miles (152.007) on the scale to which the map is to be reduced or enlarged. Then mark to reduce or enlarge the upper line to the lower line, as shown in figure 10. A long line will reduce error and give greater accuracy than a short one, and therefore as great a distance should be set off as possible. The number of miles represented by both lines and the fractional scale to which it is to be reduced should be stated on the drawing, for permanent record. [Footnote 10: See U. S. Geol. Survey Bull. 650, p. 37. 1916.] Maps that will bear reduction without affecting the clearness of the details they show may be reduced to fit the book in which they are to appear, regardless of definite scale. The reduction for such maps is best marked in fractions, as "1/2 off," "1/3 off," "2/3 off." If the size needed is not exactly represented by these fractions it should be indicated in inches, as "Reduce this line to 7-1/2 inches," or "Reduce to 4-3/8 inches in width." [Illustration: Figure 10.--Diagram showing method of marking maps for reduction or enlargement (for record).] DIAGRAMS. In preparing a diagram a draftsman should endeavor to make its parts and relations perfectly clear to the reader. He should study the drawing or material furnished by the author until he fully understands it and should endeavor to reproduce it simply and legibly. Any lettering that may be needed should generally be in plain upright or slanting gothic type (see Pl. IX), or it may be in roman. A diagram should generally be drawn on bristol board or on blue-lined section paper and should be marked for reduction to the minimum size. It should bear no title, as the title will be set up in type by the printer. SECTIONS. The sections used in geologic reports are of two widely different kinds. One shows only the broader relations of parts; the other shows details of structure as well as relations. One is diagrammatic; the other is more realistic and graphic. The draftsman should prepare all sections strictly according to the copy supplied by the author but should use proper symbols and make a more finished drawing. The various kinds of sections, most of them geologic, are described on pages 29-30, and the conventions used to express lithologic character are shown in Plate III. [Illustration: U. S. GEOLOGICAL SURVEY PREPARATION OF ILLUSTRATIONS PLATE VIII