Gustave-Adolphe Hirn (21 August 1815 – 14 January 1890) was a French physicist, astronomer, mathematician, and engineer who made important measurements of the mechanical equivalent of heat and contributions to the early development of thermodynamics.

Hirn carried out numerous experiments during his career, demonstrating the relevance of Carnot's principle in animated engines: the equivalence of thermal and mechanical energy.

He was made a member of the French Academy of Science in 1867; in 1880 founded a meteorological observatory near Colmar; and later devoted himself to astronomy.

[4] Hirn invented the pandynanometer[citation needed] in 1880 and published a theory of the origin and chemical composition of Saturn's rings, exchanging correspondence with Urbain Jean Joseph Leverrier.

His study of friction in journal bearings revealed all the essential features of fluid film lubrication, although it lacked theoretical justification.