Universal equinoctial ring sundial, London, c 1800.
© Science Museum / Science & Society Picture Library
Sundial, signed 'Cary, London'. To find the altitude of the Sun, the shadow of the pin was made to fall on the angle or 'nautical' scale on one side of the instrument. To find the time, the suspension ring was set at the latitude, the small index with the pinhole was moved to coincide with the date on the central bridge, and the point of light from the pinhole gave the time on the hour ring.