He developed a pyrheliometer and made, between 1837 and 1838,[4] the first quantitative measurements of the solar constant. His estimate was 1228 W/m2, very close to the current estimate of 1367 W/m2. Using the Dulong-Petit law inappropriately, he estimated the temperature of the Sun's surface to be around 1800 °C. This value was corrected in 1879 to 5430 °C by Jožef Stefan (1835–1893).