1. Downes, D., and Martin, A. H. M., Nature, 233, 112 (1971).
2. Ekers, R. D., and Lynden-Bell, D., Astrophys. Lett., 9, 189 (1971).
3. Thompson, A. R., Riddle, A. C., and Lang, K. R., Astrophys. Lett., 3, 49 (1969).
4. Sandqvist, A., thesis, Univ. Maryland (1971).
5. Maxwell, A., and Taylor, J. H., Astrophys. Lett., 2, 191 (1968).