  During the summer of 1837 Christian Ludwig Gerling, a former student of Carl
Friedrich Gau{\ss}'s, organized the world wide first determination of the
deflection of the vertical in longitude. From a mobile observatory at the
Frauenberg near Marburg (Hesse) he measured the astronomical longitude
difference between C.F. Gau{\ss}'s observatory at G\"ottingen and F.G.B.
Nicolai's observatory at Mannheim within an error of 0.4". To achieve this
precision he first used a series of light signals for synchronizing the
observatory clocks and, second, he very carefully corrected for the varying
reaction time of the observers. By comparing these astronomical results with
the geodetic--determined longitude differences he had recently measured for the
triangulation of Kurhessen, he was able to extract a combined value of the
deflection of the vertical in longitude of G\"ottingen and Mannheim. His
results closely agree with modern vertical deflection data.
