We have reassessed measurements of the concentration of atmospheric carbon dioxide made during the 19th century by examining original data published between 1880 and 1905 and an interpretation of these data carried out during the 1930's by C. S. Callendar, a British engineer. We have perused an unpublished notebook of Callendar which contains a detailed analysis of air masses and wind velocities attending some of these 19th century analyses. We find new evidence to support Calendar's contention that the concentration of CO2 in the late nineteenth century was close to 290 ppm. Of particular interest are the observations of J. A. Reiset made on the coast of France from 1871 to 1880 which we show to be consistent with the seasonal cycle of CO2 as known from modern measurements. From Reiset's data, we deduce that the mean annual concentration of atmospheric CO2 circa 1880 in uncontaminated air at 50°N latitude was 292 ppm.