In this paper “Duhamel published a solution for the temperature distribution in a solid with variable boundary temperature. He was considering the situations in which the surface radiates into a medium and in which the temperature of the medium changes according to a known law. His object was to reduce these cases to those of a surface at constant temperature. His method, based on the principle of superposition, generalized a solution by Fourier and substituted, in place of the original temperature function, the sum of the constant temperature term and an integral term (an integral of the rate of change of the temperature function). This method generalizes Duhamel’s pinciple” (DSB IV.222).