Heliograph

HELIOGRAPH, an instrument used for recording the number of hours of sunshine; it is applied for the study of climate, especially of therapeutic localities. The most commonly used are the heliographs of the Campbell and Velichko systems. The Campbell heliograph (see figure 1) consists of a glass sphere that collects the solar rays falling upon it into a single point (focus). As the sun moves, the focus also moves. Along the path of the focus's movement is placed a paper strip divided into hourly intervals; on this

strip, a trace remains in the form of a burn during those intervals of time when the sun was shining. When the sun was not shining, there is no burn. Thus, the number of hours of sunshine per day is calculated from the strip. The Velichko heliograph (see figure 2) consists of a brass cylinder containing photographic light-sensitive paper (ferroprussiate) inside, which is turned with the sensitive layer inward; the paper is divided by vertical lines into hourly intervals. The cylinder has three narrow, short slits facing East, South, and West. A sunbeam, passing through the corresponding slit, leaves a trace on the light-sensitive paper, which becomes visible after the paper is developed. From this trace, the number of hours of sunshine per day is calculated.