Mechanical equinoctial sundial, mid 18th century.
© Science Museum / Science & Society Picture Library
The dial was made by Jeremiah Sison, before 1752. The clock face was turned approximately south, the first pinhole was moved to the date, and the orientation of the dial was altered until the spot of light from the first pinhole coincided with the second. Turning the dial turned the hands on the clock, which then showed the time.