[Illustration: Figure 4.--Sections of coal beds. The Figure shows the publications size and the arrangement at the sections. Each section should be drawn three-tenths or four-tenths of an inch wide and reduced one-half. Thicknesses can be indicated by numbers, as shown on sections 1 and 10, or by bar scale.]
Sections designed to show the relative thickness of beds of coal, arranged in groups for publication either as plates or figures, should be drawn in columns three or four tenths of an inch wide and reduced one-half, as shown in figure 4. These sections, whether correlated or not, should be drawn to some definite vertical scale and should show the thickness of the coal beds, preferably by numbers indicating feet and inches, the other material being symbolized and the symbols explained graphically, as shown in figure 4. The vertical scale should always be stated for the use of the draftsman. A bar scale may be used instead of figures showing the dimensions of the individual beds.
The symbols used to indicate the various kinds of rocks illustrated in sections and diagrams are shown in Plate III. The units or elements of these symbols may be spaced more openly in generalized diagrammatic sections than in sections that show great detail.
Symbols should be used consistently throughout a report, and in order to make them consistent a set showing the symbol to be used for each kind of rock to be indicated should be prepared before the original drawings are made. Some inconsistencies may be unavoidable on account of the small size of some areas shown and the contrast needed between others; but the deviations from the set of symbols adopted should be minimized.
The foundation of a good photographic print is a good negative, and the best prints for reproduction as illustrations are those made from negatives in which the illumination is evenly distributed and the details are sharp--such negatives as are obtainable only by the use of small stops and correct focusing. A good print should not present too sharp contrasts between its dark and its light parts; if it does, the printed reproduction will show a loss of detail in both. Sufficiency of detail depends largely on focus, stopping down, and timing; brilliancy is the direct result of ample illumination by sun or artificial light, without which a photograph will be dull or "flat" and generally unsatisfactory for reproduction. Bad weather may prevent good field exposures, yet even in bad weather acceptable negatives may be obtained by judicious focusing, stopping down, and timing. If a negative is overexposed it may be full of detail, but flat and too thin to print well. If underexposed it will show no details in its lighter parts and the shadows will be black; and a black shadow is nothing less than a blemish. Some detail should appear in all shadows and in the middle tones, and some should appear in the high lights; and a print in which these are evenly developed and in which the illumination is distributed uniformly is technically perfect.