A John Ismay clockmaker's brass quadrant, English, early 18th century,
The obverse is engraved along the limb with a degree scale reading by a double sub-division to 20 arc minutes and numbered by groups of ten degrees. Concentric with this is a calendar scale with five day divisions and the inscription 'Lat: 54° 48' for the Gunter quadrant diagram drawn above. In the apex is a decimal shadow square, reading to 50 parts by ten groups numbered 1-9 (ten is not engraved through lack of space), of five divisions each. On either side of this is the maker's name, and within the square a sphere carrying a universal projection is depicted mounted on a column.
The reverse of the instrument is engraved on the limb with a degree scale reading to one degree; on the left hand edge is a non-linear 0-90scale. One third of the way up the face is a second 90° scale concentric with that on the limb, and the space within this is occupied by a squared octogesimal diagram. Inclined lines arising from points on the degree scale on the limbs presumably enable the laying off of small angles. A small hole at the apex of the instrument originally carried a plumb-line. The two larger holes were fixing points for the sights that are now missing.
8in (20cm) radius