Southern Exposures

(right) had been Rosecrans’ chief topographical engineer prior to Merrill’s appointment, and his “quick process” of duplicating maps was critical to Rosecrans’] consisted of a light box containing chemically treated India rubber baths fitting inside one another. Printing was done by tracing a map on thin tissue paper, placing it over photographic paper treated with nitrate of silver, and exposing it to the sun. This produced a negative copy of the map with roads, rivers, and other landmarks appearing white on a dark background. Sections of the map could be stitched together on canvas or cloth, varnished, and then distributed each night to all corps, divisions, and brigades, as well as individual regiments operating alone. Margedant’s staff consolidated notes received daily from the topographical engineers, revised the master tissue, and, with little delay, maps and charts containing everything ascertained the day before were in the hands of all commanding officers.