On Sep 5, 5:02 pm, Jim Kelley wrote:
....

I'd like to offer m = E/c^2 as a guess.

73, ac6xg

A link is worth a thousand words (perhaps 10k-100k of Richard's...):
http://www.newton.dep.anl.gov/askasc...0/phy00332.htm
(in particular the first paragraph of the second response).