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Lot Essay
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 England and the World 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.
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”.
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.
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”.