Prelude to solar energy: Pouillet, Herschel, Forbes and the solar constant

Abstract

Summary Inspired by early-nineteenth-century discoveries about heat transfer, the French physicist Claude Pouillet measured the influx of solar radiation at the earth and, in 1838, asked what these observations revealed about the temperature of the sun and of space itself. At about the same time, the British natural philosophers John Herschel and J. D. Forbes made similar measurements in order to better understand the sun's influence on climate. This paper tells how and why Pouillet, Herschel and Forbes made the first estimates of the solar constant, estimates which would acquire new importance with the discovery of the laws of thermodynamics by the mid century.