It was a university professor that telephoned Max Fagin telling him about the Cavendish experiment after he did a radio show podcast about Flat Earthism. Max subsequently introduced the Cavendish argument on the forums.
I
reintroduced the Cavendish experiment after the radio broadcast, but I wasn't by any stretch of the imagination the first person to bring it up on these forums. Erasmus predated me by a fair amount.
http://theflatearthsociety.org/forum/index.php?topic=7278.0The first thing that came to mind when hearing it briefly described was obviously magnetism, but I honestly have never looked into this matter so I'll have to get back to you because I honestly do not currently know much either for or against Cavendish. (I do understand that it is being used to try to prove the rotundity of the Earth with the main argument being gravity allegedly pulls to the centre of an object instead of downwards.)
Magnetism is highly unlikely, because the results of the experiment vary with the mass of the objects in question. Thus, it seems a reasonable assumption that mass is what is causing the torsion balance to rotate.
Also, if it were magnetism, you would expect the two masses to repel each other some of the time. Yet this is never observed. The observed force is always attractive. Gravity seems the likely conclusion.
However, having things "either for or against Cavendish" is utterly irrelevant in this case. This is not an experiment that was conducted once by a historical scientist in antiquity and then never repeated again.
In fact, I recently discovered that the modern lab physics class at my college regularly performs the Cavendish experiment as part of a series of historical experiments. And positive results are regularly achieved. So let it be made clear that the Cavendish experiment works, there is know way to deny this.