Horology. Equation Clock by Enderlin.

Early C19th engraving of Swiss clock makers,  Henri Enderlin’s equation clock.

Equation clocks are mechanical clocks which include a mechanism that simulates the equation of time, so that the user can read or calculate solar time, as would be shown by a sundial. The first accurate clocks, controlled by pendulums, were patented by Christiaan Huyghens in 1657. For the next few decades, people were still accustomed to using sundials, and wanted to be able to use clocks to find solar time. Equation clocks were invented to fill this need.

From Abraham Rees’s Cyclopaedia, or, Universal Dictionary of Arts, Sciences, and Literature.