Hi Billy T, et al.
In 1851, Hippolyte Fizeau (1819-1896) measured how the speed of a moving liquid affects the propagation of light in it.
Here are some links
http://home.att.net/~numericana/answer/relativity.htm
http://laser.phys.ualberta.ca/~egerton/c&ether.htm

What a great reference for many interesting things. thanks. I was close in my memory (at least had first letter of name right and all or the "U" tube water flow / two path interferometer correct.)

PS by edit - have now looked briefly at your second ref. - It is great too.

There are no physical entities which could satisfy the equation x-ct=0 apart from light signals.

Thomas

I think recent studies of the flux from sun tends to indicate they may have small mass, but if they do not, then can not neutrinos go at speed of light? How sure are you that "dark matter" can not?