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Inventory no. 35515 - Former Display Label

English DOUBLE HORIZONTAL DIAL
c.1630

Signed 'Elias Allen fecit'. Brass. 315 mm. (almost 12 1/2 inches) diameter.

The hour scale shows the hours IIII-XII-VIII, subdivided to an accuracy of one minute. In a band within this are marked 36 points of the compass, conforming not to the actual orientation of the sundial plate, but to the horizon of the projection which occupies the centre of the plate. This is a version of the 'horizontal instrument' invented by William Oughtred. It is a form of astrolabe drawn by stereographic projection, but in which the plane of projection is that of the horizon instead of that of the equator. It includes an ecliptic line on which the months are named, and an hour scale. The sharp vertical edge of the gnomon casts a shadow on to this scale.

The angle of the principal edge of the gnomon is 51°, which is close to the latitude at which Oughtred himself lived, near Guildford. The gnomon may, however, be a replacement. The plate itself is made of two sheets of brass, rivetted together. Its engraved surface is very worn, which suggests that it has a long history of use.

Oughtred's 'double horizontal dial', combining a conventional garden sundial with his earlier 'horizontal instrument', was published in 1636, having presumably been devised in collaboration with the instrument maker Elias Allen. Two other examples of their collaboration can be seen in adjacent cases: an equinoctial ring dial; and the mathematical instrument containing the 'circles of proportion' on one side and the 'horizontal instrument' in its original form on the other.

[43-5]
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