Lot 157
157
A complex and possibly unique 18th-Century lacquered-brass longitude-finding instrument,
No VAT will be charged on the hammer price, but VA… Read more
A complex and possibly unique 18th-Century lacquered-brass longitude-finding instrument,

Details
A complex and possibly unique 18th-Century lacquered-brass longitude-finding instrument,
unsigned -- 38.4cm. (15 1/8in.) wide

See Colour Illustration

The instrument is formed from a solid flat piece semi-circle of brass. Engraved lines eradiating from the centre are graduated with seven scales at the circumference. From the inside these are: a [?]minute scale, beginning with subdivisions labelled 20, 40, 1, 20, 40, 2, 20, 40, 3 and so on up to 20, 40, 15; next is an identical scale of half the size, with the sequence repeated twice; next is a larger scale of roman numerals XII-I twice, in red; following this is a smaller scale of the XII-I four times; next is a degree scale in red 360°-0; this is followed by a halfsize degree scale of 360°-0 twice. These outer two degree scales have 1° subdivisions. Reading off these scales are several alidades: two are simple straight-arm alidades, each with two screw clamps and micrometer fine adjustment. Also mounted on the central nut are two complex alidades: one comprises a sliding bar which moves directly beneath the nut. On one end is a curved vernier showing minutes and degrees, to read off the inner circumference scale. On the bar itself there are also two scales, for minutes and degrees, which read off a vernier graduated 0-30 on the bar's copper housing. The bar itself also displays an engraved R. The other alidade is an outlined brass rectangle, with a semi-circular piece of brass around the central nut, to which the simple alidades can be clamped to move the three in tandem. At the outer edge of the rectangular alidade is a vernier, a clamp and a micrometer screw for fine adjustment. There is also a groove to house a sliding bar, at the end of which is yet another vernier, with a screw clamp. The whole instrument stands horizontally on three rod feet.
Special notice
No VAT will be charged on the hammer price, but VAT at 17.5% will be added to the buyer's premium which is invoiced on a VAT inclusive basis

Lot Essay

Although the exact purpose of this instrument is unknown, and no matching examples can be found in the literature, it appears from its engraving and construction to date from around the mid eighteenth century, and to have been used, perhaps, for finding longitude through linear distance, presumably at sea. This was of course the era of the search to find a practical method for determining longitude at sea, the result being the development of Harrison's famous chronometer.

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