<snip>
Have you ever tried to squeeze one minute timelines onto an eight inch dial
face? It's almost impossible. <snip>
If I remember correctly, Tony said that his fine photoetching process could
just barely do this on an eight inch (20 cm) bronze face. But with my carved
stone sundials, I must have at least a 40 inch (100) cm face for one minute
lines because if carve lines too close together, it weakens the stone. (and
doesn't look good)
<snip>
An alternative approach doesn't require thin or closely spaced
lines. This is to use a nonius (after its inventor Pedro Nunes)
which uses diagonal lines at an oblique angle to the radial hour
lines. The position where the shadow crosses the diagonal line
indicates the exact minute. This is seen on old quadrants and some
sundials such as the Tompion dial at Hampton Court (and the replica
in Kew). I'm sure I have a book (Cousins, perhaps?) that illustrates
a slate dial with minute gradations using a nonius. It cannot be
more than 8 inches or so across, I think.
There are illustrations of nonii on Epact. Go to
<<http://www.mhs.ox.ac.uk/epact/>http://www.mhs.ox.ac.uk/epact/> and
search for nonius, or seach for 'minute' in 'Horizontal Dials', or
find catalog number 308 or 451.
Hope this helps
Regards to all
Chris
Newbury, England
51.4N, 1.3W
Chris, is this the same technique as used by Christopher Wren on the
All Souls College Dial? A picture of this dial appears on the front
cover of the June 2001 BSS Bulletin.
Mac
