Large Mahogany Octant, 20 inch – Williamson, England, 18th century
An interesting three mirror octant with Vernier nonius, made by or for Robert Williamson in 1768.
The first octant was created by Headly in 1731. In Great-Britain and North-America the instrument was called the Hadley’s quadrant to distinguish it from the Davis quadrant. Headly got in 1734 a patent for eleven years. Immediately after it came to an end other instrumentmakers started with making octants.
This octant is made of mahogany. The graduated scale is made of ivory with a Vernier nonius. The engraving is made by hand. At the graduation every degree is divided in parts of 20 minutes, The Vernier nonius is divided in twenty equal parts. The accuracy of reading with this nonius is one minute. A mistake is made on the vernier. Where 15 is engraved it should be 5.
The lower horizon mirror and peepsight were used for backward observation. As the horizon just below the sun was invisible, in this way it was possible to use the opposite horizon. The instrument was used back to front. The observer had the sun in his back. The sunshades moved to the lower position.
The front peepsight has two holes. The central ones for direct observation of the sun, the other one, just outside the path of rays, could be used in stead of the sunshades to weaken the sunligth.
This beautiful instrument with nice patin, is complete and in a good condition, with three mirrors, two peep sights and two sunshades. On the backside three brass legs and the adjusing-screws.
The mirror table of this octant is also made of mahogany. Octants of a later date have brass ones. The nonius does not have a double scale anymore, but still has the wide of a double scale.
HW: 50×42 cm (19.8×16.5 in)
Radius (rotation axis till nonius): 43,5 cm (17.2 in)
Signed: * Robert Williamson 1768 *
Condition: for its age, very good, minor crack in the graduation left under, two cracks in one of the sunshades