The full name of this instrument, which was used to tell the time, is universal equinoctial ringdial.
It comprises: an outer ring - the so-called "meridian" ring - against which the suspension mount is adjusted for local latitude; an inner ring - the so-called "hours" ring - at right angles, in the 12 hour position, with the first and divided into 24 hours; a central bar, hinged to the outer ring. The bar has a slit with a declination scale, the months being engraved on one side, the signs of the zodiac on the other. By rotating the instrument, a ray of light, projected through the small hole on the cursor that is situated in the slit of the bar - a part that is adjusted to compensate for the change in solar declination throughout the year - falls on the hour scale, indicating local apparent solar time.
The bigger ring models could tell the time to within a minute and were for a long time the only way to check whether a clock was working, without having recourse to an astronomical observatory.
Nicolas Bion, Engineer to the King for mathematical instruments, was one of the best known names of the time, even if he was not the author of certain important technical innovations - as Daumas tells us (op.cit.) - and few of his objects remain with us today. He did, however, have a very accurate technique and excellent methods of execution. He published several works describing different astronomical and mathematical instruments and how to use them, works that were read widely in his time and repeatedly reedited by his children after his death.
In the 1709 Traité de la construction et des principaux usages des instruments de mathématique, Bion includes the illustration and accurately describes the construction and use of what he calls Anneau ou Cadran Astronomique universel, an instrument identical to the one we have in our possession (op.cit.).
J.A. Bennett (1987), p.80.
N. Bion (1709), p.377 and table 32 of IVth edition.
M. Daumas (1953), p.109.
F. Farinelli (1979), p.190.
A.J. Turner ed. (1990), pp.105, 121, 139.
G.L’E. Turner (1991), p.130.