Probably the oldest working clock in existence, made of hand-wrought iron in or before 1386. The clock may be the work of three horologists from the Low Countries, Johannes and Williemus Vriemand and Johannes Jietuijt of Delft, who came to England at the invitation of Edward III. Bishop Erghum (1375-1388) consecrated at Bruges and may have known of these men.
As is usual of the period, the clock has no face, being designed only to strike hours. In two sections, the right-hand one being known as Going Train and the left-hand one as Striking Train, each driven by falling weights which have to be wound up once a day, though the clock will run for slightly over 24 hours. ‘Great’ or hour wheel is attached to the drum of Going Train regulated by ‘verge and foliot’ escapement (like horizontal pendulum) so that it makes one revolution per hour and so set up that a protruding pin on it makes contact with a catch (‘striking detent’) exactly on the hour. On the hour, therefore, the pin lifts striking detent, which is connected to ‘locking detent’ a little further to the left. Locking detent consequently lifts out of a gap hoopwheel (a thin metal ring attached to a toothed wheel), unlocking Striking Train.
Read more at : Medieval Clock – Salisbury