1. Prelude to solar energy: Pouillet, Herschel, Forbes and the solar constant
2. Greeley Charles Abbot, The Earth and the stars (New York, 1925), 135.
3. Agnes Clerke identifies the importance of the solar constant, and reviews the related problem of the amount of absorption created by the Earth's atmosphere, its effect on knowledge of the amount and character of solar heat, and on calculations of the temperature of the solar surface, See Agnes Clerke, Problems in astrophysics (London, 1903), 63–69.