Page:Handbook of Meteorology.djvu/257

 The Campbell-Stokes recorder consists of a sphere of colorless or slightly yellow glass of a high degree of transparency. It is mounted in a frame in front of a concave surface set at focal distance from the glass sphere. The central line of the recording chart must lie in the plane of the true meridian. The frame itself is adjustable to the sun’s altitude. The recording chart is graduated to hour intervals. The focal rays, shifting with the position of the sun, char a line along the chart. The charred line represents the duration of the sunshine. When the sunshine has occurred at short and irregular intervals the aggregate duaration may be found most quickly by placing the edge of a