1. Drummond, A. J. andM. K. Kirsten: South Afr. J. Sci.48 (3), 103 (1951).
2. Smithsonian Institution, Annals of the Astrophysical Observatory, vol. 5, 6 and 7.
3. Pretoria, Weather Bureau. Annual Report 1951.
4. Drummond, A. J.: South Afr. J. Sci. (in press).
5. Pretoria, Weather Bureau. Annual Reports for South-West Africa 1941 ... 1950.