Chris Lusby Taylor wrote: <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 <snip>
and Mac Oglesby: <snip> Chris, is this the same technique as used by Christopher Wren on the All Souls College Dial? <snip> Yes, this is similar in principle to the All Souls' dial; but there in effect the lines are dispensed with and the intersections marked by dots. There are at least two other related schemes: some dials (for example, one at Erddig, Clwyd, and one at I think Beddington Hall in Shropshire, both C18) have two scales divided into 2 minute divisions and offset by one minute between them, so that one scale has the lines for even and the other for odd minutes. In the same area is a small horizontal dial with rings which have 0, 30, 60; 5, 25, 35, 55; 10, 20, 40, 50; 15, 45 minutes marked, so achieving 5 minute reading with wider line spacing. These can be regarded as a sort of discontinuous nonius with stepped rather than straight diagonals. In my experience these are less common than diagonal scales (nonius), but perhaps sometimes are not noticed, particularly on a corroded dial. Andrew James
