R. Blandford, and M. Begelman. (1998)cite arxiv:astro-ph/9809083
Comment: 5 pages, 2 figures, submitted to Monthly Notices of the Royal
Astronomical Society Letters.
M. Park, and J. Ostriker. (1998)cite arxiv:astro-ph/9811048
Comment: Figs 1 and 2 replaced with corrected figures. 9 pages, 3 Postscript
figures. Plain tex. Review talk presented at 31st COSPAR meeting (1996).
Revised version will appear in Adv. in Space Research.