Two of the commonest types of hour striking mechanism which have been used in clocks. The earlier locking-plate or 'count-wheel' system, shown on the left, is found in the Salisbury Cathedral clock of 1386, and has been widely used, especially in large clocks, ever since. In this system the number of strokes each hour is controlled by a slotted count-wheel, the spacing of the slots around the circumference corresponding to the number of strokes required. In the 'rack' type of striking mechanism, invented by Rev E Barlow in 1676 and shown on the right, the number of strokes is controlled by a stepped cam attached to the hour wheel. Shortly before the hour the toothed rack falls, controlled by the cam, and at the hour is raised, operating the bell hammer.
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