D O C U M E N T 2 7 J U LY 1 9 2 5 7 7 [1]Giorgi (1871–1950) was Docente of physics and mathematics at the University of Rome. [2]Here is the vertical gradient of the ether drift, i.e., the horizontal velocity u of the ether with respect to the surface of the earth. See also the second paragraph of the following note. [3]In Giorgi 1925, the manuscript of which was attached to Abs. 45, Giorgi referred to the conclu- sion drawn by Ludwik Silberstein that the ether drift observed by Dayton Miller (see Doc. 12 for more on these observations) “knocked out” relativity theory and could easily be explained on George G. Stokes’s ether theory, as modified by Max Planck and Hendrik A. Lorentz (see Silberstein 1925b). In its original form, Stokes’s ether moved irrotationally and was completely dragged along by the Earth at its surface. After Lorentz had pointed out that these two properties were irreconcilable for an incompressible ether, Planck had proposed a modified theory in which the ether is compressible and subject to gravity, so that it could “condense” at the surface of the Earth. This theory allows an ether with an irrotational motion that increases with distance to the Earth from almost zero at its surface, in accordance with Miller’s observations of an ether drift of close to zero in Cleveland and of 10 km/s at the summit of Mount Wilson (see Miller 1925a, 1925b, 1925c see also, e.g., Lorentz 1909, sections 147–149 for Stokes’s theory and Planck’s modifications). In his paper Giorgi also disputed an earlier objection by Arthur S. Eddington, who claimed that the drift observed by Miller implied a rotational motion of the ether, which had the consequence that light from a star that reaches the summit of Mount Wilson perpendicularly would have an inclination of 7″ when reaching sea level. Thus, measurements of absolute star positions at mountain observato- ries would differ by this value from those made at sea level (see Eddington 1925c). According to Giorgi, however, this objection became invalid if the irrotationality could be restored by postulating an additional vertical ether drift. He did admit, though, that the amount of ether “condensation” at the surface of the Earth and the magnitude of the vertical gradient of the drift that were needed posed problems. 27. To Wilhelm Westphal[1] [Berlin, after 18 July 1925][2] Sehr verehrter Herr Professor Westphal! In der Sitzung unseres Kuratoriums vom 18. d. Mts. wurde der Brief zur Kennt- nis gebracht, den Sie für das Kuratorium an Herrn Dr. Ruge[3] unter dem 12. d. Mts. gerichtet hatten. In diesem Briefe haben Sie Ihre Mitgliedschaft dem Kuratorium zur Verfügung gestellt, weil Ihnen eine Fortdauer dieser Mitgliedschaft mit Ihrer jetzigen Stellung als Hauptobservator am Astrophysikalischen Observatorium nicht vereinbar erscheint. Namens des Kuratoriums teile ich Ihnen mit, dass wir mit Bedauern von diesem Entschlusse Kenntnis genommen habe[n.] Wir werden uns stets gern der Zusam- menarbeit mit Ihnen erinnern. Es begrüsst Sie Ihr sehr ergebener A. Einstein. Als Privatmensch freue ich mich, dass Sie aus der Sphäre der Zigarrenrauchenden wichtig thuenden Götter in die Sphäre der wissenschaftlich thätigen Menschen wieder den Weg zurück gefunden haben. Sie und Ihre Frau grüsst herzlich Ihr A. Einstein.[4] ∂u ∂z -----