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Inventory no. 31922 - Former Display Label

English DIALLING SPHERE
c.1700

Brass. Signed, "I: Rowley fec:"; undated.

This is an instrument for drawing sundials. There is a contemporary manuscript, possibly written by Rowley himself, which describes its use. The manuscript gives instructions for drawing "Horizontal", "Declining", "Declining Inclining", and "Declining Reclining" dials.

The three bands represent the Meridian, the Equinoctial and the Horizon. The thin plate, which tilts and rotates within the Horizon band, represents the plane of inclination of the dials to be drawn, and can be adjusted to the required declination.

For example, to draw a horizontal sundial, the equinoctial circle is first set to the complement of the latitude of the place where the dial is to be used. The slide on the equatorial circle, to which is attached a piece of cat-gut, passing through the centre of the plate and held taut by a weight, is moved to the latitude of the place. (Latitudes and their complements are marked on either side of the meridian circle.) The cat-gut now represents the position of the Earth's axis and the equinoctial circle the Earth's Equator. In drawing a horizontal dial, the thin plate within the horizon circle is not tilted, as in the horizontal position it represents the plane of the plate of a horizontal dial. Hours are marked on the equinoctial circle, and a lighted candle, fixed in the moveable holder on the equinoctial circle, is then brought to the position of each hour-line. The shadows cast by the cat-gut on to the horizontal plate represent the positions where hour lines are to be drawn for the dial.

In drawing reclining and declining dials the method is similar, but the thin horizontal plate is turned to the correct declination East or West of the meridian and is also tilted to the correct inclination, by means of the semicircular scale below it marked in degrees.

[Orrery Collection, no. 3]
Lent by Christ Church.

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