Re: Gnomons and screws

[Mike Cowham]

Dear John, I believe that Melville made his gnomons such that they have two tangs that poke through the slate.  These are then each bent in opposite directions. I am sure that he would not have used screws.  I can send you photographs if that helps. Regards, Mike Cowham Cambridge UK I'm helping to restore a Richard Melville (worked 1832-1871) slate dial with multiple gnomons.  The gnomons are held in with 2BA brass screws using square nuts.  The gnomons themselves are nominally 1/4 inch thick but with considerable variation, i.e. not the modern rolled plate accuracy. Can anyone tell me when the BA (British Association) screwthread standard started?  I think it points to the gnomons being (old) replacements.