During visits to London in the late 18th century, Benjamin Franklin became friendly with James Ferguson, the famous astronomer and designer of orreries and astronomical clocks. Few of Ferguson's designs were built, but his ideas led Franklin to devise a clock with as few parts as possible. His clock has a dial divided into four segments marked 0-60 minutes for each segment, the whole series of twelve hour figures lying between four concentric rings. A single hand indicates the minutes and hours simultaneously, but not unambiguously as three hour figures are indicated. A later version employed a spiral groove with a steel ball to overcome the difficulty. The ball rested in the spiral to show the hours, being gradually moved to a hole in the centre from which it fell by gravity through a tube behind the dial to the outer end of the spiral every twelve hours. Some of Franklin's clocks also indicated seconds on a dial in the arch of the clock. Ferguson made a number of variations based on Franklin's design, some going for one day and one going for a week, but without seconds indication.