1. Adams, W. S.: 1939,A Rep. Mt. Wilson and Palomer Obs.
2. Adams, W. S.: 1941,Astrophys. J. 93, 11.
3. Babcock, H. D.: 1945,Astrophys. J. 102, 154.
4. Ballik, E. A. and Ramsay, D. A.: 1963,Astrophys. J. 137, 84.
5. Barth, C. A., Hord, C. W., Pearce, J. B., Kelly, K. K., Anderson, G. P., and Stewart, A. I.: 1971,J. Geophys. Res. (in press).