Mike I asked this question (elsewhere) a few months ago in connection with a mid C17 dial. The holes are not just punch dots which would aid in placing a compass or marking the gnomon corners, but drilled right through the plate (neatly filled in this case, though not always).
I believe (purely as conjecture I must say) that they were part of the laying out and indeed the marking process and not only denoted the gnomon corners but took pins against which a rule was laid. Other punch marks can be seen where compasses were centred to draw the hour circles for example but these do not go through the plate. On a double horizontal dial there are a lot of compass centre dots to be found! If you were, say, a London maker, most of your stock dials would probably be for its conventional 51d 30' latitude and using a scale on the bench or a workboard with a pin sticking up at the centre, a ruler could be held against the pin and easily aligned against each gradation on the scale in turn and the hour or other line scribed along it. Anyone who has made a dial or anything similar involving accurately placed straight lines by hand engraving or cutting will know that adjusting one end of a ruler to a defined position is much easier than fiddling with two ends, and having it positively located against a pin greatly reduces the chance of slipping or a line being marked out of position. If the rule had a little notch in it then it would take account of the diameter of the pin but a scriber point running a pin's radius away from the rule would do the same. The engraving on a good dial is accurate enough to see whether the lines go through the middle of the pin hole rather than tangential to the edge. It would seem to me to make a lot of sense to have a single layout carefully drawn which would cover manufacture for your local customers and could easily be replicated in a semi-skilled way onto a variety of dial sizes. Unfortunately I think we know very little about how the dials were actually made. Perhaps if "country" dials by makers who may be supposed to have made a very few individual dials tend not to have the pin holes and ones by supposedly prolific makers tend to have them it might add credence to the theory but I suspect that a lot of exceptions will exist. In any case I could see clear benefits in drilling the holes and using pins as I surmise even for a "one-off" because of the certainty of the lines converging accurately to the same point for minimum effort in positioning a straight edge. I have used a corner of something clamped to the plate for the same purpose of locating a long ruler when drilling was not a preferred option. Regards Andrew James -----Original Message----- From: Mike Cowham [mailto:[EMAIL PROTECTED] Sent: 29 October 2004 10:54 To: SUNDIAL Subject: Gnomon Holes I have just been asked the question to which I thought I knew the answer, but now I am not so sure. On English horizontal dials there are frequently two small holes drilled in the dial plate at the point where the root of the gnomon contacts the plate. The question is, 'What were the holes used for?' I understood it to be an aid for alignment of the gnomon but now I believe that these holes could have been used to take a pin or point of a compass/dividers during marking out. Perhaps some of you who still make such dials will use a similar method? Regards, Mike Cowham Cambridge UK This message has been scanned for viruses by MailControl. This correspondence is confidential and is solely for the intended recipient(s). If you are not the intended recipient, you must not use, disclose, copy, distribute or retain this message or any part of it. If you are not the intended recipient please delete this correspondence from your system and notify the sender immediately. This message has been scanned for viruses by MailControl -
