18th Century, 20 inch Hadley’s Quadrant (octant) – Adams, London

18th Century, 20 inch Hadley’s Quadrant (octant) – Adams, London

Stock Number: NM. 2-42


An historical interesting three mirror octant, made by George Adams (1704-1773), London in 1751.

Country of manufacture

UK and Ireland

Category: Navigational instruments


An historical interesting three mirror octant, made by George Adams (1704-1773), London in 1751.


The first octant was created by Headly in 1731. In Great-Britain and North-America it was called the Hadley’s quadrant to distinguish it from the Davis quadrant. In 1734 Headly got a patent for eleven years. Immediately after it came to an end Adams started with making octants.


This octant is made of mahogany. The graduated scale is made of pear wood just as with the Davis Quadrant. The nonius has two scales and a transversal nonius. The lower scale runs from 0 to 90 degrees and the complementary upper one runs from 90 to 0. In the lower one every degree is divided in parts of 20 minutes. On the nonius there are 10 concentric circle lines.

The accuracy of reading with the transversal nonius is two minutes (20:10).


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.


This beautiful instrument is complete as it is made and in a very good condition, with three mirrors, two peep sights and two sunshades. On the backside three wooden stays and the adjusing-screws. Both peep sigths with only one hole.


Date: 1751

HW: 51×42,5 cm   (20×16,7 in)

Radius (rotation axis till nonius): 42 cm   (16 in)


Origin: England

Condition: excellent


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NL ARCHIPEL - International Maritime Galery

Archipel – International Maritime Galery in the Netherlands, sets itself to maritime objects of before 1900. The managing director worked as officer with the mercantile marine and the Royal Navy. As curator he was connected to one of the university museums in the Netherlands. As seaman and curator he has a lot of knowledge of naval history.

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