Double horizontal pedestal dial for latitude 51° 30' North. This dial is made up of an octagonal base-plate with concentric circles to show (from the rim inwards) a minute circle, an hour circle, a compass circle and finally a degree circle. Within the degree circle is the horizontal projection and the equinoxes are marked at 10th March and 13th September. A very faint solar altitude scale runs from the North-East point of the degree circle to the centre. The dial has a simple brass gnomon with a central knife-edge and a conventional polar gnomon for use with the main dial. 'Elias Allen fecit' is engraved in the hour-circle.

This octagonal brass pedestal sundial is known as a double horizontal dial because it has two scales for reading the hours. The first is a standard scale, which is used with the polar edge of the gnomon. The second is formed by the vertical edge of the gnomon (set at the centre of the dial) and the lines of projection of the celestial sphere on to the plane of the horizon (the horizontal projection).

The double horizontal dial was designed by the 17th century English mathematician William Oughtred. Elias Allen was a friend of Oughtred and he produced several double horizontal dials. They were useful not only for telling the time but also for demonstrating the motion of the sun through the day and also through the year. A number of double horizontal dials survive from the 17th century but it appears that they were not produced much after 1700.

For more information regarding this dial please refer to the OUP & NMM catalogue, 'Sundials at Greenwich'.