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
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