15-inch mahogany Octant owned by T. Saunders – 18th century
A mahogany three mirror octant from the second half of the 18th century, owned by T. Saunders.
The first octant was created by Hadley in 1731. In Great-Britain and North-America it was called the Hadley’s quadrant to distinguish it from the Davis Quadrant. The early octants of Hadley are provided with a transversal nonius, like the Davis Quadrant. Hadley got in 1734 a patent for eleven years. Immediately after it came to an end other instrumentmakers started with making octants. They are all equipped with a Vernier nonius, invented by the Frenchman Vernier.
This octant is made of mahogany. The graduated scale is made of ivory in two parts with one scale and a Vernier nonius. In the graduation every degree is divided in parts of 20 minutes. The Vernier nonius is divided in twenty equal parts, so the accuracy of reading with the transversal nonius is one minute. The nameplate, pencilholder and slate on the backside are also of ivory. The wooden feet at the backside are not rare but still striking. The movable sunshades can easily fall overboard or get lost. Peepsights and shades seem to be of a bit later date then the otant itself. One shade is broken.
The lower horizon mirror and peepsight were used for backward observation. As the horizon just below the sun or moon 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.
This beautiful instrument is complete and in a good condition, with three mirrors, two peep sights and three sunshades. On the backside the slate, three wooden legs and adjusting-screws.
Date: ca. 1780
HW: 45,5×37 cm (17.6×14.5 in)
Radius (rotation axis till nonius): 39 cm (15.3 in)
Signed: T. Saunders (owner)
Condition: very well, with some general signs of use and wear