Campbell Stokes sunshine recorder
A heliograph is an instrument which records the amount of sunshine in a given location. In the Campbell heliograph, the sun's rays pass through the glass orb and are focused, like a magnifying glass, onto a point. The paper, which has a specially designed coating, burns when the sun's rays are not obscured by the clouds. The apparent movement of the sun during the day, and the marks which the rays leave on the paper in different places, make it possible to work out the length of time when the intensity of the sun's rays reached a detection threshold. We can then calculate the amount of sunshine.