1 1 6 D O C U M E N T S 1 0 0 , 1 0 1 D E C E M B E R 1 9 2 7 100. From Alfred Einstein[1] Berlin SW, 19 Jerusalemer Straße 46–49, 9 December 1927 Dear Professor, Don’t be annoyed by the postal confusion, which was caused by a misunder- standing on the part of Breitkopf & Härtel’s[2] representative, and accept my thanks for being so kind as to forward the letter to me.[3] Once we are truly living at Heilbronnerstraße 6/I (which will be in mid- January),[4] I will try to thank you in person. That is, I know, an immense importu- nity but you and I have an old relationship that goes back to 1894 or 1895, when we were at the Luitpold high school, where you liked to pull the hair of your younger namesake in singing class.[5] Maybe some time at a concert I’ll work up the courage to speak to you—that would be an easier way that would not disturb you at home. Yours sincerely, Alfred Einstein 101. From Paul S. Epstein[1] Aachen, 9 December 1927 Dear Professor, My time in Aachen is coming to an end. I had a very attentive and grateful au- dience here. However, the number of hours was so limited that I hardly had a chance to dwell on the deeper problems. During my visit to Berlin, I had the pleasure of speaking to you about my radiometer theory. I have now completed it, and the article is nearly written.[2] I be- lieve that it will make a serious contribution to the elucidation of the relevant (but not very current) questions. For a disklike structure made of a material of very poor thermal conductivity, I obtain precisely the same formula as the one you gave.[3] The physical prerequisite for its validity is that the heat exchange between the front and back surfaces of the disk occurs not through the disk, but rather around its rim, through the gas. However, this is not an edge effect in the strict sense, because the tension is distributed over the whole area of the disk. For larger disks, and in gen- eral when one has to take the internal thermal conductivity into account, the formula is different (and is also given explicitly). The mathematical aspects are quite amusing e.g., in the determination of the air flow around the disk in terms of the sum of partial solutions of the equations, one coefficient remains undetermined.