English: These are parts from balloon-borne pyrheliometer #2 designed by Charles Greeley Abbot and built by his craftsman Andrew Kramer at the original Smithsonian Astrophysical Observatory in about 1913. Following designs that had been developed for meteorographs, the recording medium was sensitized photographic paper on a slowly rotating drum. The drum was encased in a light-tight chamber save for an entrance slit parallel to the diameter. A thermometer filled the slight so that the mercury level would be recorded by the rotating paper as a function of time, and of altitude, as recorder by a barometric needle whose shadow was also superimposed. Other clockdriven elements periodically exposed the bulb of the thermometer to direct skylight and to ambient air temperature. This instrument stands as a milestone in sophistication and ingenuity in early self-registering automata. The balloon-sondes were lofted from Catalina Island off the coast of California and some reached 25 kilometers altitude. Data from these devices led to greatly refined values of Langley's solar constant, and helped to establish the modern range. A complete unit (#3) is preserved at the National Museum of American History. These parts were retrieved from the stores of the original SAO workshops, housed in the Garber facility, in the mid-1980s. Picture taken at the National Air and Space Museum's Steven F. Udvar-Hazy Center in Chantilly, Virginia, USA.