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Description: | 32/04 (painted in black and white on the lid of the box). 32 (written in pencil on a printed label inside the lid of the box). SPECIAL NOTICE- The PANES OF GLASS are intentionally Fitted LOOSE, in order to prevent distortion on expansion in a hot sun. N.S. Sta. 1537/02. G & S [783] 100 11/02d (printed on the paper label inside the lid of the box). CARY, LONDON 32/04 (engraved on the oxidised brass side plate of the instrument) 32 (stamped into the brass plate inside thew box). MANU Cary circa 1890 181 Strand, London, England. PUB DAVIS, John. 1594 London, England. This is a Davis artificial horizon, which was used with sextants and associated instrumrents when a marine or true horizon was not available either because the instrument was being used inland or when the marine horizon was obscured by fog or darkness. At sea they could only be used in a flat calm with no movement of the ship.
It consisted of a pool of mercury, which acted as an automatically self levelling mirror. Over this the canopy of two clear glass panes angled at forty five degrees was placed in order to prevent air currents causing ripples.
When an object such as the sun or a star was viewed through the telescope sight of a sextant and the image, reflected by the mercury was superimposed on it, half the angle observed on the sextant arc was used as the angle of altitude or azimuth of the object. There are two major drawbacks involved with this type of artificial horizon; it is difficult to use with stars and actual azimuth angles are limited to half the range of the instrument. So octants, sextants and quintants are limited to azimuth angles of 45, 60 and 72 degrees above the horizon respectively. For this reason quintants became the preferred instrument for land surveying. | License: | http://www.hmag.gla.ac.uk/spirit/rights/ | Publisher: | Hunterian Museum and Art Gallery, University of Glasgow | Rights holder: | Hunterian Museum and Art Gallery, University of Glasgow | Subjects: | SCIENTIFIC COLLECTION : | Source: | Hunterian Museum | Creator: | Hunterian Museum and Art Gallery, University of Glasgow | Identifier: | http://www.huntsearch.gla.ac.uk/cgi-bin/... | Language: | en-GB | Go to resource |
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