Universal ring dial, French, 18th century.
© Science Museum / Science & Society Picture Library
Sundials of this type can be used to find the time of day anywhere in the Northern Hemisphere. It indicates the time by a spot of light from the sun pasing through a pin hole and falling upon a scale of hours. The bras slider must first be adjusted to the correct date. The main body of the dial is then turned within its supporting ring so that the hour circle is at an angle towards the horizon to suit the latitude in which the dial is being used. With the pinhole facing the sun, the whole dial is then rotated until the spot of light falls onto the hour ring, and the time can then be read. This sundial does not require a compas for setting, but will not give an accurate time reading around midday when the sun's altitude above the horizon changes only slowly.