A clock in which the mechanism is not immediately apparent or explicable. The idea originated very early, probably in Germany or France, but most mystery clocks date from the 19th century when the French particularly were keen on such devices. In the later 17th century Grollier de Servi閞e produced the amusing 'tortoise clock', a horizontal table clock on which the centre of the dial within the chapter ring was dished and filled with water. On this floated a small turtle which crept round the edge of the chapter ring pointing to the hours. The secret of the movement of the tortoise lay in a magnetic hour hand fixed under the chapter ring, which attracted to it the turtle body which was primed with a small amount of soft iron. Versions of this principle with, for example, a mouse inching along a flat arch marked with the hours, have been described. Later horological enigmas took several forms, one of the most popular having hands which appear to revolve of their own accord. The dial, in fact, is three layers of glass, the centre one, to which the hand is attached, having a toothed edge which is rotated by a movement concealed in the base. The commonest form of mystery clock, produced cheaply in great numbers from c. 1880, is in the form of a figure with outstretched arm holding a timepiece; an elephant holding a timepiece in his trunk was also popular. The clock movement and dial form an upper weight on a rod with a pendulum bob at the lower end. This compound pendulum is supported just above its centre of gravity so that it swings slowly from side to side. The pendulum movement releases the escapement, which causes a small weight to move in the opposite direction to that of the pendulum, thus keeping it swinging.