A Mid 18th Century Brass Universal Equinoctial Ring Dial. This is a horological instrument for time telling. The dial consists of two rings and a bridging bar inside the outer ring. The outer ring is known as the meridian ring to represent a circle through the North and South celestial poles. It carries a 90-0-90 degree latitude scale on one side and a 0-90 degree latitude scale on the other, for measuring the height of the sun. The inner-hinged ring represents the celestial equator and is engraved with a double twelve-hour scale. The rotating bridging bar engraved with a calendar for the day of the month, is set at the poles of the meridian to represent the axis of the world and a slider with a pinhole is set between the bars of the bridge to act as the shadow marker. 
This dial shows that 11 March aligns approximately with 11 September, and thus it was laid out and crafted before the Gregorian calendar was adopted in England, and thus pre-1752.  The corresponding scale on the other side of the bridge is that of the solar declination, running from -23-1/2 to +23-1/2 degrees throughout the year.
Reference: Hester Higton. Sundial. An Illustrated History of Portable Dials. Philip Wilson Pub. London. [2001].
This universal ring dial, or more correctly Universal Equinoctial Ring Dial, is a portable sundial used for navigation. It was used for determining time during daylight hours and in use from the 1600s to mid-nineteenth century. 
To use the ring dial on first finds the altitude of the Sun, the shadow of the pin was made to fall on the angle or ‘nautical’ scale on one side of the instrument. To find the time, the suspension ring was set at the latitude, the small index with the pinhole was moved to coincide with the date on the central bridge, and the point of light from the pinhole gave the time on the hour ring. (I-0978)
Dimensions: Diameter with ring up 45, Diameter without ring 4 1/4, Diameter without ring holder 4, Inches
Condition: The ring dial is in excellent condition with typical age staining.