Comparison between the daily mean of the number of prominences recorded at the Madrid Astronomical Observatory for the period 1906–1957 (black) and, from top to bottom, the number of prominences recorded at the Astronomical Observatory of the University of Coimbra for the period 1929–1944 (dark green), the sunspot number for the period 1906–1957 (blue), the sunspot area by Mandal et al. (2020) for the period 1906–1957 (yellow), and the solar radio flux at 10.7 cm for the period 1947–1957 (red).