This is a very sophisticated and complex sundial made by esteemed scientific instrument maker Thomas Heath (1698-1773), who worked in London from 1721 until the late 1760 s. It is finely engraved allover.
The sundial allows the viewer to calculate the difference between the apparent and the mean solar time, allowing him to accurately set his watch or clock. If you were wealthy enough to own a domestic clock you needed a sundial to establish time allowing you to set the clock.
This dial shows the Equation of Time (Natural Days). which describes the discrepancy between two kinds of solar time. The word equation is used in the medieval sense of "reconcile a difference". The two times that differ are the apparent solar time, which directly tracks the diurnal motion of the Sun, and mean solar time, which tracks a theoretical mean Sun ( as shown by a clock) with uniform motion along the celestial equator. Apparent solar time can be obtained by measurement of the current position (hour angle) of the Sun, as indicated (with limited accuracy) by a sundial. Mean solar time, for the same place, would be the time indicated by a steady clock set so that over the year its differences from apparent solar time would have a mean of zero.
Date: 1720-1750 (Maker was active 1721 to the 1760s)
Measurement: Height: 7.75" diameter: 11.75"
Condition: Very good, as expected... weatherworn displaying dark surface with verdigris