Sundial made with paper on wood by Henry Robert, the horologer of the Palais Royal. Column, or pillar, dials work by measuring the altitude of the Sun above the horizon, rather than a horizontal angle from the zenith. The gnomon, or pointer, is fixed horizontally into a slot so that it lines up with the correct month and the pillar is turned so that the sun's shadow falls directly down the side on to the appropriate place on the hour scale. The tip will then indicate the time.
© Science Museum / Science & Society Picture Library