Beschreibung:

A very rare example of a large universal equinoctial ring dial, unsigned and undated , but possibly by Elias Allen of London, mid 17th-Century -- 45.5cm. (17 7/8in.) diameter; 4.5kg (10lb) weight See Colour Illustrations The components are an outer, meridian ring, inner equinoctial ring, and a declination bridge; there is evidence of past gilding. Both rings are made from three brass bands riveted and soldered together. The outer edge of the meridian ring is encircled by a brass rod, 8mm (0.3in.) diameter, which has been soldered round it. This is gripped by the carriage of the suspension shackle. One side of this ring is divided into four quadrants 0-90°-0-90°-0. The degrees are divided down to a remarkable 10 seconds of arc. The other side has a very large quadrant 0-90°, also subdivided down to 10 seconds of arc. This quadrant, known as a sea ring or nautical ring, is used for measuring solar altitudes at midday to find latitude. It requires a peg to be inserted in the rim at the apex of the quadrant; two small locating holes are evident. The equinoctial ring swivels between the zero-degree divisions of the meridian ring. It is divided on one side into twice twelve hours, marked in roman numerals. The inner edge of the equinoctial ring is divided to one minute of arc. At both the six o'clock points on the rim of the equinoctial ring are pivot holes to hold axis pins, so that the instrument can be used on a fixed stand. The declination bridge swivels between two brackets attached to the meridian ring at the 90° positions. This has a long central slot, in which slides the index, with a small hole that casts a beam of sunlight onto the hour scale. The edges of the slit are divided on both sides, one to the calendar between the Solstices, labelled in months and divided in days, and the other to 23½°, divided to 15 miinutes, the Declination North or South. On this side one of the scales is continuous, while the other has a hiatus that equals the width of the hour ring (12mm). This is necessary when the reading is taken at noon since the Sun would be prevented from casting light through the hole in the index. The suspension unit is a modern replacement, the screws are machine-made roundheads, and there is no fiducial mark to set the degree of latitude. This very large dial, with its consequent accuracy, its ingenious declination bridge, and its weight, which is greater than the average mariner's astrolabe, was quite likely to have been made for use at sea. The quality of the brass founding, with a large number of rivets and blow-holes, strongly indicates that the dial was made in the seventeenth century rather than later. The workmanship and the calligraphy suggest that it may have been the work of Elias Allen. However, it is impossible to make an emphatic ascription. Elias Allen (c.1588-1653) was apprenticed to Charles Whitwell in the last year of the reign of Queen Elizabeth, 1602. He became a Freeman of the Grocer's Company in 1612, and was involved in setting up the Clockmakers' Company that was incorporated in 1631, becoming its Master in 1636. Allen was the leading instrument maker of his time. His engraving hand follows that of Whitwell, but his zero and 8 are customarily rough-edged. On the present ring dial these are perfectly round because they are punched: an anomaly. The workmanship may be compared with the 18-inch diameter double horizontal dial of 1635 signed by Allen at the Museum of the History of Science, Oxford. The degree scale is also to 10 seconds of arc, and the calligraphy similar.