A mechanism for winding a clock automatically at regular intervals. Large turret clocks require a great deal of effort to wind manually, and are increasingly being converted to automatic winding. In one common type, a continuous loop of bicycle chain passes round a sprocket on the clock barrel or other convenient arbor, thence round a pulley carrying the driving weight, and then over a sprocket driven by the electric motor. The remainder of the loop is carried out of the way by pulleys and kept taut by a counterweight. The clock is driven by the gradually falling weight which, when it reaches a lower limit, actuates a switch starting the electric motor, which hauls in the chain on the other side of the main weight. The switch cuts out when the weight reaches the upper limit. This system closely resembles Christiaan Huygens's winding mechanism so often seen on 30-hour clocks, and is called by that name. In another system, a mechanical epicyclic differential is used. In this case, one connexion to the differential drives the clock through a roller chain, and the other two connect with a weight drum and the electric motor respectively. A third type incorporates the electric motor with the weight. It climbs the chain, which passes as a loop over the barrel or other arbour of the clock. The motor may be a synchronous type, which climbs the chain at the same rate as the clock lets it out. Some clockmakers couple electric motors directly to the striking and chiming trains, disconnect the escapement, and use a synchronous motor to drive the hands. Much damage has been done in this way. Turret clocks rewound by hydraulic power are also known. Mechanical domestic clocks, with their more modest power requirements, are rarely wound automatically, although some 30-hour clocks have been converted to automatic winding to avoid this daily chore. The Atmos clock, whihc utilises atmospheric changes in pressure and temperature to provide winding energy, is a novel domestic automatic-winding clock.