Many have been made. Some of these are being continually wound up by means of a fan placed in a tall chimney shaft, up which there is a natural draught that always keeps the fan revolving. The fan is connected to the winding shaft of the clock by suitable gearing of a speed-reducing nature. Other clocks are driven by electricity; an impulse is given direct to the pendulum at each vibration by the closing of an electrical circuit in which is a weak battery made by burying carbon and zinc plates in moist earth. Perhaps the most noteworthy perpetual clock is in the British Horological Institute, 06, Northampton Square, London, E.C. It was made more than a century ago, and is dependent for its motive power on the variations in the density of the atmosphere. A sort of barometer containing many pounds of mercury is suspended from a rocking bar, and the constant shifting of the mercury causes the suspending bar to rock and drive the winding arbor by a rack and pinion. This clock teas gone for many years,and has only been stopped to be cleaned.