1. Waldmeier, M., The Sunspot Activity in the Years 1610-1960 (Technische Hochschule, Zurich, 1961).
2. Waldmeier, M., Monthly Sunspot Bulletin from the Swiss Federal Observatory (Zurich, 1960–1972).
3. Jose, P. D., Astr. J., 70, 193–200 (1965).
4. Wood, K. D., Nature, 240, 91–93 (1972).
5. Ulrych, T. J., J. geophys. Res., 77, 1396–1400 (1972).