Any clock which is driven by its own weight. The earliest appeared in the mid 17th century. They had a bracket in the form of an arm attached to the wall, the hand holding a chain on which the clock, in the form of a globe, hung. To wind the clock it was lifed, a spring retracted the chain inside the clock and, when released, the clock descended. Maurice Wheeler described on of his devising in 1684, the clock descending an inclined plane. A pivoted weight inside the clock provided the power to the train as it tried to assume a vertical position, though the actual power came from the clock descending the inclined plane. Hours and minutes were indicated on the clock, the days of the week on the plane. Nowadays the term more often refers to the silent keyless clock made in the early 1900s. This has a base carrying two columns with a toothed rack in one which engages a wheel in the clock movement. The clock is raised to the top of the columns by hand, thereupon its own weight drives it; the escapement is visible and impulses a short pendulum. These clocks, also called rack clocks, were popular for a limited period only.