D O C U M E N T S 3 0 5 , 3 0 6 N O V E M B E R 1 9 2 8 2 9 3 305. To K. Böhm [Berlin, after 6 November 1928][1] Dear Sir, Regarding your question I, I have not yet reached a firm conclusion.[2] The alternative “velocity or gravitational potential” appears to me to be not quite logi- cal, since in fact the same “-term” of the equations[3] produces not only potentials, but also potential gradients, and thus velocities (away from us), so that no station- ary state can be imagined so long as the -term is only partially compensated by matter. The accumulation of mass cannot be—as you yourself say—the cause of the phenomenon. II. Friedländer’s experiment must fail owing to the smallness of the expected effect.[4] A more promising method would be the investigation of the influence of the Earth’s rotation on the plane of oscillation of a Foucault pendulum, or on a gy- roscope, or on the motions of the Moon, or the influence of the Sun’s rotation on the motions of Mercury and Venus. However, all these are effects that cannot be detected today, because of their smallness. Respectfully yours, A. E. (P.S. To the secretary: Keep the original, send a copy.) 306. Recommendation for David Reichinstein to Heyl- Beringer-Farbenfabriken AG [Berlin,] 7 November 1928  Strictly confidential Professor Reichinstein is an extremely original thinker, and certainly not inex- perienced in coping with the practical problems of industry.[1] Whether Prof. Reichinstein is especially qualified to solve the problem that you have assigned to him I cannot, of course, judge. In any case, however, I would like to suggest that you try it with Prof. Reichinstein, because he is doubtless an inventive man and an indefatigable worker.— Respectfully yours,