II. On measuring the relative thermal intensity of the Sun, and on a self-registering instrument for that purpose

Abstract

The thermometric estimation of relative solar intensity, according to the best known means, requires first the determination of the temperature of the air—so-called shade temperature—and secondly, and simultaneously, that of a thermometer with a blackened bulb placed in vacuo in the sunshine—sun temperature: the difference between the two temperatures being taken as a measure of the sun’s radiant heat operating at the time and place of the two observations. The chief sources of error in this method are the difficulty of ascertaining the temperature of the air immediately surrounding the vacuous globe containing the blackened bulb, and the placing of this thermometer under exactly similar conditions at different meteorological stations.