A 26-year (1868--1893) data series of daily ozone readings performed at Moncalieri, northern Italy, by the Sch¿nbein test paper technique has been analyzed. The availability of a series of simultaneous readings by the Sch¿nbein and a quantitative technique (Levy, 1877) and the conversion chart for humidity by Linvill et al. (1980) allowed us to develop a procedure to convert the Moncalieri data into parts per billion by volume values. The results seem to indicate that in comparison to one century ago, the ozone level in Europe has increased by more than twice not only at the surface but also in the free troposphere. ¿American Geophysical Union 1991