3 7 8 D O C U M E N T 3 9 0 O C T O B E R 1 9 2 6 estimate the quantity of this internal pressure in the following manner: I imagine the closed spherical world as divided into two halves by a “plane” the masses in these halves will exert forces upon each other of the order of magnitude , if M is the world mass, a is the world radius, and is the grav- itational constant. This force distributes itself over a surface (the “plane”) as a pres- sure force of quantity , so a pressure results as the internal pressure of gravitation. Now, however, according to your theory of the closed world, is a relation that one can, incidentally, find in a purely dimensional way hence, now results, if σ is the density of the ponderable mass. In your units, accordingly, it would simply be . The (gravi.) internal pressure of the world therefore became just the value of your cosmological term, and I cannot avoid the suspicion that this cosmological term should be addressed as the internal pressure of the world, and that therefore its dependence on the size of the world also becomes understandable. The entropy of the world then would become insensitive to small variations, provided not only , but also the latter is guaranteed if , that is, if gas pressure + radiation pressure + internal pressure = 0, and because is neg- ligible hence, if , i.e., one finds the same condition, up to numerical factors, that I obtained in my note in the Phys[ikalische] Z[eit]s[chrift]. I took the liberty to write this to you because I would very much like to hear your verdict, even just a superficial and hasty one, and hope that you can make some- thing sensible out of it. If it suited you, I could take my return trip to Hamburg via Berlin and permit myself to visit you in Berlin on Oct. 29th or 30th. Otherwise, I could also come to Berlin from Hamburg sometime later, if desirable and not su- perfluous, due to the general foolishness of my considerations. I would be very gratefully obliged to you for a couple of lines about these last questions. Yours sincerely, W. Lenz κ c2 M2 a2 ⁄ ⋅ ⋅ κc2 a2 ∼ π κc2M2 a4 ⁄∼ κM a ∼ π c2M a3 ⁄ σc2 ∼∼ π σ ∼ T S δ δU pdV += δU 0= pdV 0= p 0= pgas pS u[S] 3 -------- π σc2 ∼ = =