It records the hours of bright sunshine by burning a hole through a card.
The Campbell-Stokes Recorder (sometimes called a Stokes sphere) is a type of sunshine recorder. It was invented by John Francis Campbell in 1853 and modified in 1879 by Sir George Gabriel Stokes. The original design by Campbell consisted of a glass sphere set into a wooden bowl with the sun burning a trace on the bowl. Stokes’s refinement was to make the housing out of metal and to have a card holder set behind the sphere.