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An equinoctial is also known as equatorial sundial as the dial plate is set to rest on a plane parallel with the earth’s equator. The dial sits upon three threaded feet that are adjusted until the compass is exactly horizontal as indicated by the two spirit levels set at right angles to each other. The whole compass next needs to be rotated so the dial is facing north. The chapter ring, which is hinged beneath the XII, is then set to the angle of latitude of the users current location via the inclinometer, which is hinged open and rests in a slot in the side of the chapter ring. The brass pointer or gnomon is sprung and can be flipped perpendicular to the chapter ring either above or below dial depending on the time of year. When set-up properly the dial plate now sits on a ring parallel to the equator and the gnomon (sometimes called a polestyle on an equatorial dial) points along a line parallel to the earth’s axis. Weather permitting a shadow is cast by the gnomon onto the chapters that are engraved in five-minute increments. Once used, the compass needle is locked and the dial, gnomon and degree scale are folded flat and returned to the case.
Cary were a family of fine instrument makers with businesses in the Strand and St James, London. This dial is an example of the high quality of accuracy and workmanship the brothers John & William achieved.
|Height||4.00 cm||(1.57 inches)|
|Width||14.00 cm||(5.51 inches)|
|Depth||14.00 cm||(5.51 inches)|