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Brass sextant signed Imray & Son 89 & 102 Minories London mid-19th century

About the Item

Brass sextant signed Imray & Son 89 & 102 Minories London, datable to around mid-19th century, instrument housed in its original mahogany box shaped like the instrument with hinges, brass locking hooks. Brass frame with engraved silver goniometric scale embedded in the arch, with scale from 0 to + 150, silver flap and vernier, ebony handle, 3 colored glasses for fixed mirror and 4 for movable mirror, a microscope for vernier reading, index and horizon mirror. Imray's long and prestigious history dates back to the mid-1700s, a period during which merchant ships filled the port area of London, they were makers of nautical instruments as well as being publishers of pilot books, working alongside ship captains and crews for the purpose of improving ship supplies. Box size cm 32.5x27.5x13 - inches 12.8x10.8x5.1. Conservation status: very good, complete with stand base made of custom wood and brass. The last photo is the gift box. The sextant is an optical instrument used in astronomical navigation to measure the height of the stars on the horizon in order to derive geographical coordinates relative to the ship's point. It is in the shape of a circular sector of 60°, that is, one-sixth of a circumference, hence the name, at the apex of which is pivoted a movable alidade on which is fixed a mirror that rotates with it. On the back of the mirror is a stand with a telescope oriented toward a second mirror, only one half of which is silvered, making it possible to simultaneously observe the sea horizon, in alignment, and the pointed star, whose image is reflected by the mirror attached to the alidade and subsequently by the silvered part of the mirror. By adjusting the position of the index of the alidade, it is possible to collimate the image of the horizon with that of the star and to derive on the graduated scale of the 60° sector the angle between the horizon and the star. To make a sextant measurement of the height of a star (for example, the Sun), one places the instrument in a vertical plane and, looking through the sighting device, aims at the horizon line visible through the unsilvered half of the fixed mirror. Moving the alidade, with which the mirror is integral, causes the light rays coming from the star and subsequently reflected by the moving mirror and the silvered half of the fixed mirror to be sent back by the latter in the direction of observation: if you look through the aiming device, you see the image of the star, obtained by double reflection, coincide with the horizon line. The height of the astro is expressed by the angle whose value is read on the graduated scale. The filter is used when the star to be looked at is the Sun. It was Sir Isaac Newton who invented the principle of double reflection in navigation instruments, but this research was never published. Subsequently, two men, independently of each other, discovered the sextant around 1730 : John Hadley (1682-1744), an English mathematician, and Thomas Godfrey, (1704-1749), an American inventor. But it was not until 1758 that Admiral John Campbell carried out a series of offshore trials to test a new method that relied on lunar distance as a means of calculating longitude. This was how the sextant was developed. Initially made of brass, they had scales divided with great precision by mathematicians who made scientific instruments.

Dimensions:

Height: 5.1 in (12.96 cm)Width: 10.8 in (27.44 cm)Depth: 12.8 in (32.52 cm)

Materials and Techniques:

Brass

Period:

Mid-19th Century

Date of Manufacture:

1850

Condition:

Wear consistent with age and use.

Seller Location:

Milan, IT

Materials

Brass

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