THE UNIVERSAL EQUINOCTIAL RING DIAL
The two ring equinoctial ring dial was invented by the Mathematician William Oughtred (1574-1660) and described in his 1652 book on sundials. Unlike horizontal garden sundials that are designed for use at a fixed latitude it can be used anywhere on earth, at any latitude. A list of prominent cities of the world (London, New York, Paris, St. Petersburg, Istanbul and others) are engraved on the reverse of the rings along with their latitudes.
To use the sundial: the sliding pinhole on the bridge is set to the day of year on the calendrical scale and the bridge set at 90°; then the outer equinoctial ring is rotated until the desired latitude is selected by the index pointer on the vernier scale at the base of the instrument; and finally the inner meridian ring is turned to rest on the braces at 90° to the equinoctial ring. The ingenious nature of the sundial means that it is self-orienting, so that when suspended by the handle, sunlight will only shine through the pin-hole and highlight the correct time on the hour scale of the meridian ring when the sundial is aligned north-south.
Most equinoctial ring dials can be used to within a few minutes of accuracy, but will not be able to tell the time at midday because the meridian ring will cast a shadow over the pin-hole at exactly this moment. However, the standing equinoctial ring dial is fitted with two pin-hole sights below the meridian ring that will only align with a ray of sunlight at noon exactly. The current instrument is also larger, and therefore more accurate than most ring dials, and is mounted on a base inset with compass, two bubble levels and three screw feet so that it can be accurately oriented.
A sundial with a high level of accuracy such as this would have often been used to set a clock or watch to local solar time. A clock can keep the time, of course, but not find it; so the use of an accurate sundial is required.
Using the compass set into the base of the instrument, if the local magnetic variation (the difference between magnetic north and true north) is known the alignment of the dial can be corrected. Alternatively, the self-orienting nature of the dial enables it to be used to determine the magnetic variation once the dial is correctly aligned. John Hammond in his book The Practical Surveyor (London: 1725) noted that with this type of scientific instrument “it is ... convenient in all places to find [the variation]; and this may be done several ways: but none more readily, more easily or more exactly, than by the Universal Dial”.
THE EARLS OF MACCLESFIELD AND 18TH CENTURY SCIENCE
The Earls of Macclesfield were leading patrons of the arts and sciences in the eighteenth century. The first three earls were fellows of the Royal Society, and all were sufficiently well versed in the latest scientific discoveries of the time to be published in the society’s journal the Philosophical Transactions of the Royal Society of London. Thomas Parker, the first Earl of Macclesfield (1667-1732), was a pallbearer at the funeral for the famed astronomer Isaac Newton in 1727. Before its dispersal by Sotheby’s in twelve sales from 2004-2008 the library of the Earls of Macclesfield was one of the greatest scientific libraries assembled in the 17th and 18th centuries.
The second Earl built an observatory at the ancestral home, Shirburn Castle, in 1739 and fitted it with the finest instruments of the day. The instrument maker Johnathan Sisson (1690-1747) supplied the 5-foot transit instrument (in use from 1740 to 1787) and a quadrant (1743-1793), the clocks were from Tompion and Graham, a 14-foot refracting telescope, and later a 3¾-foot achromatic were made by Dollond. None of these instruments have survived.
His son, Viscount Parker, later the third Earl, was elected a fellow of the Royal Society in 1747. He presented a paper (Phil. Trans., vol. 49, 1755 pp. 368-370) on 20 November 1755 on the earth tremors detected in the moat at Shirburn Castle on the first day of that month. This was, of course, the Great Lisbon earthquake of 1 November 1755, which was the first earthquake to be studied by the scientific community.
James Simons (fl. 1770-1794) was a freeman of the Worshipful Company of Stationers (scientific instrument makers did not have their own guild until 1956). Sundials by him are rare in collections, but his trade card does illustrate a standing equinoctial dial along with advertising ‘Mathematical, Philosophical & Optical instruments’.
Since none of the other astronomical instruments from the Earls of Macclesfield are extant, the present sundial represents an important survival from a great family of scientific patronage.