Leonardo da Vinci’s steam cannon was a weapon concept also known as the Architonnerre.
The Architonnerre was described as a cannon that used heated water to create steam pressure and propel a projectile. Leonardo’s notes associate the idea with Archimedes, although the historical basis of that attribution is uncertain. The machine combined a metal tube, a heating source, and a projectile chamber in an attempt to replace or supplement conventional gunpowder propulsion.
Steam pressure can produce powerful expansion, but controlling it safely would have been extremely difficult with Renaissance materials and manufacturing methods. There is no reliable evidence that Leonardo built a working steam cannon or that the design saw battlefield use. It is best understood as an experimental proposal in his military notebooks.
The Architonnerre is sometimes confused with Leonardo’s multi-barrel cannon. The multi-barrel design depended on several gun barrels and gunpowder, whereas the Architonnerre was a separate pressure-based concept. Both demonstrate Leonardo’s interest in finding new ways to increase artillery power.