D O C . 3 1 A P R I L 1 9 1 9 2 7 is in doubt about whether his way of approaching the matter doesn’t agree with Westphal’s (which you are challenging).[5] I write you the following on the advice of Epstein. Epstein’s idea is that between 2 walls, each at different but equalized temperatures, there is a constantly pulsed current that is equal to the radiometric force. The molecules, at some position in the gap at temperature τ, upon collision ac- quire an average velocity (momentum ). In the next collision, at the distance of the average length of path λ from the first, they give off the impulse , but receive a smaller one back, namely . One arrives at a rough theory, therefore, simply by using Boltzmann’s formula for the transfer of one quantity (Gas Theory, I, p. 78),[6] the quantity g would be for us the momentum in direction x. becomes directly equal to the sought after radiometric force T = K. Hence: now, , thus . In this connection the accommodation coeff. is set = 1.[7] Provided you approve of the fundamental concept, Epstein would be very glad to be of assistance to me in developing this further. Alternatively, I would be very grateful if you would let us know, perhaps a little more explicitly, your interpreta- tion of the matter. In the meantime, I’ll take a quick look at the literature related to this. I would be so happy if something came of this. Otherwise not much news. Just that Janka and I unfortunately had to leave our little apartment[8] (we were given notice for speculative reasons), and now I live in a lonely little room on Sonneggstrasse. I’ll be writing to Elsa soon. For now, send her my very affectionate greetings and thank you wholeheartedly for having come to my aid in my “employment need”! With warm regards, yours, Edith. My address is 70 Sonnegg St. vx mvx px px ∂x ∂pxλ⎞ + ⎝ ⎠ –⎛ T ∂g -=-----λxx∂Nv g mvx t ⋅ = K Nmλvx------- ∂vx ∂x - ------------mvx2x∂∂λ2N = = mvx 2 2 --------- kT 2 ----- -= K Nλk∂τ 2 --------------- ∂x NλkT1 2 --------------------------- T2 – l = =