A couple of months ago, Mike Cowham asked the SML for an explanation of the two small dots or holes seen on the dialplates of antique horizontal dials, just where the tip of the gnomon meets the dialplate. Several suggestions were made, such as their use as alignment marks to position the gnomon. Tony Moss made the suggestion that they acted as location points for a swivelling straight-edge which was used to mark out the hourlines.
I have recently come across a picture of a c.1700 declinatory by John Coggs which gives support to Tonys suggestion. This small instrument consists of a circular horizontal dial which is free to rotate on a larger square plate with a protractor scale. As befits a portable instrument, the gnomon has the facility of folding flat. The resulting hinge mechanism means that the southern tip of the gnomon is truncated and it thus stops just short of the VI-VI line. BUT, the two holes are clearly visible at the origins of delineation. Since the holes cannot, in this case, be used to position the gnomon correctly, it seems highly likely that they were part of t!
he
delineation process.
When were these holes first used? Very early (before 1600) horizontal dials usually had very thin gnomons and no evidence of the holes. Elias Allens double horizontal dials in the 1620-1650 period do have them.
Can anyone give other examples of early dials with these delineation origins.
Regards,
John Davis
