Irish, c.1812-1814, in brass, hand engraved and divided to half a degree, the cross bar engraved 'R Spear 27 College Green Dublin' with two upright bars, each inclined half a degree from the vertical, and divided to one arc-minute, theoretically enabling construction or reading of angles to that level of accuracy, diameter 14.6cm

Note, this is a variation of an early 18th century design by Thomas Heath.