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1750-1800 British Back Sight Octant


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ENGLISH BACK SIGHT OCTANT by Spencer, Browning & Rust

An octant is a portable instrument that uses a small mirror to bring two images together--those of the sun and the horizon, for instance--to determine latitude at sea by observing the altitude of celestial bodies. It has an arc of 45o or more that measures angles of 90o or more. John Hadley described an instrument of this sort to the Royal Society of London in 1731 and obtained a British patent in 1734, and so octants are sometimes known as Hadley quadrants.

This mariner's octant was made during the last half of the 18th Century and is the work of Spencer, Browning & Rust under private label for A. Johnson. It has two horizon mirrors and two peep sights. The lower assemblage is for taking “back sights“ and the upper is for traditional observations. The mark, SBR, Spencer, Browning & Rust, appears on the arc between 45°; and 50°. SBR is known to be one of England's foremost instrument makers. The sharpness of the arc's numerals shows that this instrument was made to the highest level of then existing technology by a company that stood at the top of its trade. You can clearly see the mark of SBR between 45 and 50 degree position on the arc. A. Johnson name is on the cross piece of the arc. The ivory arc measures from - 5 to plus 95 degrees on a ebony frame which is in excellent condition. It is known that mahogany went out of use in favor of ebony some time after 1750. The octant has a flat brass index arm without a stiffening brace that measures 14 inches from its top to the bottom of the index arm. Tangent screws began replacing finger clamps in the last quarter of the 18th Century. There are two horizon mirrors, each mounted on an adjustable turntable where the base can be rotated by loosening a screw which locks the mirror's position. There is a single set of three shades which may be moved from the upper to lower position. The ivory vernier scale reads from right to left from 20 to 0 arc minutes. There are two peep sights. The adjustable tangent screw followed the use of the index arm clamp which ended around the last quarter if the 1700's. All of these features date the instrument after 1750 and before 1800.

 

 

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Now that is super cool! How big is this? Would be interested in seeing you holding it or how tall it is. thanks for sharing! super amazing!

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Hi,

The total height of the octant is 14". There are very few navigational instruments marked to the Royal Navy since all naval officer's bought their own equipment. The exception would be the ship's master who was a warrant officer who would use the ships instruments.

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