Pocket Watch with Tourbillon

Mechanical watches did not generally keep time as well as clocks because they were subject to constant movement, and unless their balance was perfectly poised, their performance varied when they were held in different positions. To eliminate this problem, Abraham-Louis Breguet invented the tourbillon (literally, whirlwind), a device that mounted the escapement and balance on a small carriage that averaged out the errors by rotating at regular intervals.

Source: Vignon, Charlotte. The Frick Collection Decorative Arts Handbook. New York: The Frick Collection/Scala, 2015.