The first mechanical clocks (as opposed to other types of timekeeping devices) probably did not show the hour of day as water clocks did. Rather they sounded an alarm at a predetermined point in the day or struck a bell at the hour. Originally called a mechanical horloge, a mechanical clock that struck a bell at the hour eventually came to be called a clocca, after the Latin word for bell. As early as 1371 the spelling had changed to clok in Latin or clocke in English.
– from The History of Clocks & Watches by Eric Bruton