Editing Differential (mechanical device) (section)

===Clocks===
The earliest verified use of a differential was in a clock made by Joseph Williamson in 1720. It employed a differential to add the [[equation of time]] to [[local mean time]], as determined by the clock mechanism, to produce [[solar time]], which would have been the same as the reading of a [[sundial]]. During the 18th century, sundials were considered to show the "correct" time, so an ordinary clock would frequently have to be readjusted, even if it worked perfectly, because of seasonal variations in the equation of time. Williamson's and other equation clocks showed sundial time without needing readjustment. Nowadays, we consider clocks to be "correct" and sundials usually incorrect, so many sundials carry instructions about how to use their readings to obtain clock time.