- Museum number
This brass instrument is similar in shape to an astrolabe with a planisphere on the mater and an azimuth dial on the back. A simple throne with a suspension ring is fixed to the rim of the mater.
The RIM of the mater bears a double degree scale (on the outside) 0 to 90 to 0 to 90 to 0 and (on the inside) 0 to 360, numbered by 10 and divided to 5 and 1. The MATER itself is engraved with a Gemma Frisius universal projection. The ecliptic is marked with the symbols of the zodiacal signs. There is a rotatable, semi-circular PLATE with the same universal projection.
The ruler and fixing mechanism are replacements.
The REVERSE of the instrument is laid out as an azimuth dial. The tropics are labelled 'The Tropick of Capricorn' and 'The Tropick of Cancer'. The equator is indicated by a double line engraved with alternate shading. The solar declinations are indicated by lines marked with arrowheads every 5 degrees, divided to 1. The curved hour lines are numbered IIII to XII to VIII, divided to 20 minutes. Along the outside of the rim is a degree scale 0 to 90 to 0 to 90 to 0.
In the circle created by the markings for the tropic of Cancer is an inscription with the name of the supposed inventor of this type of instrument and date of its invention.
- Production date
- 1595-1605 ('invented' ca. 1600; taken to Florence by 1606?)
Diameter: 145 millimetres
Thickness: 3 millimetres
- Curator's comments
It is noted in the register when purchased: 'nut and index both modern'.
The correct index is shown on an engraving of a dial of this type printed n Dudley, 'Dell' Arcano del Mare' (1647), Vol. 3, Book V, p. 4, fig. 4.The use of such a dial is explained in Book V, p.13, chapter XI, fig. 21.
It is generally accepted that this form of dial was invented by William Oughtred (1575-1660), a mathematician of King's College, Cambridge, where he studied from 1592 to 1603, when he took an ecclesiastical living near Guildford. That he may have achieved the invention between 1597 and 1600 is revealed in the 'Epistle dedicatore' written by William Forster, the translator of Oughtred's book from the Latin: 'The Circles of Proportion and the Horizontall Instrument, Both inuented and the vses of both Written in Latine by Mr. W.O.', printed for Elias Allen, and sold at his shop opposite St Clements Church, in 1632. In his dedication, Forster said he often travelled to visit and learn from Oughtred, and in the summer of 1630 he was shown notes, written in Latin, on the circles and the horizontal instrument, which Oughtred told Forster had been projected about 30 years before (i.e., about 1600). Forster offered to translate Oughtred's work, which was done by May 1632. A priority dispute then arose between Oughtred and Richard Delamaine (d. 1645), this book having angered Delamaine. A second edition was published in the next year (Oughtred 1633), and to this was added what Oughtred called 'An Apologeticall Epistle', where he denounced Delamaine. What exactly the present dial has to do with Sir Robert Dudley is not at all clear. Perhaps Oughtred and Dudley, just seven months older, knew each other. Dudley published an engraving of such a dial, and there are resemblances between the two. It is noteworthy that both the engraving and the present dial do not have ecliptic arcs, whereas the earliest printed diagram (Gunter 1624, p. 65), and all known other examples of the dial, are provided with the ecliptic. Oughtred, in his 'An Apologeticall Epistle', says that in 1618 Gunter had seen a drawing of the Horizontal Instrument, which was then made into a copperplate that was used, years later, to produce the illustration in Gunter's book on the sector published in 1624.
Of the engravings in 'Dell'Arcano', several are copied from English Elizabethan texts, and many from the Elizabethan instruments in Dudley's possession. It is likely that the present dial was the model copied by his engraver. Whitwell made instruments for Dudley, who took a large group to Florence in 1606; this dial must have been one of them, so a terminal date is provided. The attribution to Whitwell rests on the calligraphy and on the layout of the coordinate net on both sides of the instrument, which resembles that on the signed and dated astrolabe held in the Museo di Storia della Scienza, Florence ((1095) IC 482).
Literature: A. J. Turner (1981). William Oughtred, Richard Delamaine and the Horizontal Instrument in seventeenth-century England, 'Annali dell' Istituto e Museo di Storia della Scienza di Firenze', 6, fasc. 2, pp. 115-16, 122).
Although the inscription attributes the invention of this instrument to Sir Robert Dudley (1573-1649, son of the Earl of Leicester) in 1598, Turner attributes it to Charles Whitwell, who 'made instruments for [Robert] Dudley'; his attribution 'rests on the calligraphy and on the layout of the coordinate net on both sides of the instrument, which resembles that on the signed and dated astrolabe [cat. no.] 43 [Florence MSS (1095) IC 482]'; see Turner 2000, p.210.
- Not on display
Latest: 2 (Mar 2017)
3 (Feb 1996)
- Acquisition date
- Britain, Europe and Prehistory
- Registration number