Daily entries from the 17th century London diary
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Double horizontal dial
“A double horizontal dial is a horizontal dial with 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 onto the plane of the horizon (the horizontal projection). This projection shows the lines of solar declination, the ecliptic and the right ascension of the sun…. 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. They could be used for calculating the altitude of the sun at its azimuth, as well as showing the position of the sun on the ecliptic and the time of day….” Scroll down for a link to an image of the Dial. http://www.nmm.ac.uk/collections/explore/index.cfm/category/90052
“The Sundials at Wadham College, Oxford” by Harriet James
The British Sundial Society
http://www.sundialsoc.org.uk/
Worth a visit.