In 1840, Louis-Clément Breguet developed the hourly thermometrograph, an instrument designed to automatically record temperature variations over extended periods. Combining a metallic thermometer with a horological mechanism, the device continuously measured and preserved data, addressing the growing needs of scientific observation. Capable of recording ambient temperatures for more than a year, including under extreme conditions, it illustrates the diversification of the House of Breguet’s activities during the 19th century and its enduring commitment to precision in the service of knowledge and scientific progress.