'Tortoise' clock, probably 19th century.
© Science Museum / Science & Society Picture Library
In this clock the time is indicated by a floating 'tortoise' attached to a magnet. This magnet is acted on by another magnet beneath the plate dial which is rotated by the clock movement. In 1632, F Linus invented a mystery clock in which the hour was indicated by the rotation of a floating globe. Galileo (1564-1642) aserted that he had invented a similar clock many years before, the rotation being effected by a magnet carried by a concealed clock movement.