A graph illustrating the increasing precision of timekeeping. This graph is based
on the chart devised by F.A.B. Ward, formerly of the Science Museum, London. It
illustrates the increasing rate of increase in the accuracy of timekeeping that has occurred
since the invention of the first mechanical clock about AD 1300. It has been estimated by
Joseph Needham that the accuracy of Su Sung's Chinese water-clock of about AD 1100
was such that, if it were represented by a point on this graph, its vertical ordinate would
be between 10 and 100. Two horologists whose names occur on this graph are not
mentioned in the main text: Robinson and Riefler. Thomas Robinson ( 1792- 1882) was
an Irish astronomer who became Director of the Armagh Observatory. In 1831 he
attached a small mercury barometer to a clock-pendulum in order to make it possible to
compensate for the rather complicated effect of barometric pressure on the pendulum's
rate of oscillation. In 1889 A. Riefler of Munich patented an escapement in which the
impulses of the pendulum were transmitted through its suspension spring, the
pendulum being otherwise free from interference. His clocks were so successful that they
were adopted as standard clocks in many observatories until they were replaced by Shortt
clocks.