West side sundial.
St Margaret’s has four sundials , designed by C. St.J. Daniel, on its tower, and is a very good illustration of how the appearance of a vertical sundial depends on the direction in which the wall is facing. . The sundial on the south face has the gnomon (which casts the shadow) slanting out from sundial at the angle of the co-latitude (so that the gnomon points at the Celestial Pole, and is thus parallel with the axis of the earth), and the hour lines fanning out from the base of the gnomon. You can see that the hour lines are symmetrical about the vertical 12 o’clock line. This shows that this sundial is pointing due south.
The gnomons on the east and west faces also have to point directly at the Celestial Pole, and thus be parallel to the gnomon on the south-facing dial. The only way this can be done is by having the gnomon standing out from the surface of the dial. The hour lines are parallel lines marked on the dialplate. The east-facing dial records the morning hours, up to solar noon, and the west-facing dial records the afternoon hours.
taken from – http://www.sundials.co.uk/~thames.htm
A very interesting website to check out! If/When I ever get back
to London, I would love to see each one of these sundials.