Astonishing Science. Spectacular museum.
John Shelton made five longcase clocks for the Royal Society to use for timing the transits of Venus in 1761 and 1769 from different parts of the world. These clocks travelled extensively and were used in various experiments to determine the shape and mass of the Earth. Captain Cook took one of the clocks on his first voyage and two on each of his other two voyages.
The clock in the Museum was used in 1828 to compare the strength of gravity at the top and bottom of a mine in an attempt to find the density of the Earth. From 1865 to 1873, it was used in the survey of India. In 1882, it was used to measure another transit of Venus from New Zealand. A few years later, it was back in London being used to compare the clocks at Greenwich and Kew Observatories.
Changes in air pressure can also affect the period of a pendulum. In the 19th century, complicated systems were invented for compensating for changes in pressure by using magnets controlled by barometers. This became unnecessary when pendulum clocks began to be wound electrically instead of by hand because they could then be kept in sealed vessels where the pressure was constant.
The last stage in the development of pendulum clocks was the Shortt free pendulum clock, whose timekeeping pendulum was temperature compensated, kept in a vacuum and only interfered with once every thirty seconds.
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