Gerhard Dohrn-van Rossum:
The now-faded fame of Archbishop Pacificus if Verona (d. 884) as the inventor of the mechanical clock was promoted above all by his epitaph on the Cathedral of Verona which Du Cange quoted in his Latin glossary. The epitaph speaks of a never-before-seen horologium nocturnum, which, according to Du Cange, was a wheeled clock. In the same inscription, however, Pacificus is also described as the author of an argumentum that came with the clock; apparently the text of the argumentum and the illustrations that accompanied it were no longer known in the 17th century. The text and the mediaeval illustrations clearly show that the horologium nocturnum was an observation tube with a cross-shaped sighting device or scale. It was meant to be suitable for determining the night hours in monasteries even without the crow of the cock, but like the mediaeval sundials it was also to be used for calendrical purposes.
[Images courtesy of the Virtual Manuscript Library of Switzerland: Cod. Sang. 18, pp. 43-44, Stiftsbibliothek St. Gallen / Codices Electronici Sangallenses]