  During the summer of 1837 Christian Ludwig Gerling, a former student of Carl
Friedrich Gauss, organized the world wide first determination of a vertical
deflection in longitude. From a mobile observatory at the Frauenberg close to
Marburg (Hessen) he measured the astronomical longitude difference between the
observatory of C.F. Gauss at G\"ottingen and the observatory of F.G.B. Nicolai
at Mannheim. By comparing these astronomical results with the geodetic
determined longitude differences, which he just had measured as a part of the
triangulation of Kurhessen, he was able to extract a combined value of the
vertical deflection of G\"ottingen and Mannheim. His results are in very good
agreement with modern vertical deflection data.
