It seems to me that the vertical offset of the base of a vertical rod arising from starting at the upper edge of the ordinary pole gnomon could prevent the shadow falling on the central compass rose unless the sun were fairly high in the sky. You may be able to judge the minimum solar altitude from your photos, Frank; for how much of a winter day would it be useful? On the other hand enough open vertical pole might be seen through the gnomon frets to cast a useful shadow.
Is it possible that the central post holes were to take not a gnomon to show azimuth but a windvane? After all, windvanes were common enough on C16/17 portable dials especially the Nuremberg ivory diptychs. That also requires something fitting loosely and hence removably into (a pair of) bearing holes rather than a shadow casting vertical which could be fixed, and gives a plausible reason for double marking. Neither explains the Elias Allen dial, though, with a 180 degree rotated compass rose in the absence of such holes. Perhaps it WAS a mistake - engravers weren't immune from making them (I vaguely remember reading of a clock by a famous maker - even Tompion perhaps - with the signature upside down) and a major one like that is just the sort of thing that goes unnoticed until afterwards! Andrew James N 51 04' W 01 18' -----Original Message----- From: Frank Evans [mailto:[EMAIL PROTECTED] Sent: 05 August 2002 14:45 <snip> In one of the four photos I took I have now been able to see two small holes drilled in the gnomon. These are clearly for the reception of a removable vertical rod arising over the centre point. Thus the shadow of this rod would give the sun's bearing plus 16 points (180 degrees) Moreover, by listing the reciprocal the sun's actual bearing is recorded directly. <snip> -
