The octant is a reflection instrument designed by John Hadley c. 1731. It is used to measure the altitude of the Sun or a celestial body above the horizon at sea. A mobile arm carrying a mirror and pivoting on a graduated arc provides a reflected image of the celestial body overlapping the image of the horizon, which is observed directly. This specimen was probably built by Alexander Wellington, a scientific-instrument maker active in London between the late eighteenth and early nineteenth centuries. Provenance: Lorraine collections.