What weapon did Leonardo da Vinci propose using steam pressure to fire?

Leonardo da Vinci proposed a steam cannon, often associated with the Architonnerre.

The Architonnerre was described as a cannon concept that used heated water and steam pressure to propel a projectile. In the account associated with Leonardo, water in a heated chamber would create pressure, which could drive a cannon shot from the barrel.

The design appears in Renaissance discussions of powerful artillery and is usually linked to Leonardo’s military engineering interests. It is best treated as a proposed or conceptual weapon, not as a proven steam-powered cannon that Leonardo successfully manufactured and deployed.

The idea is sometimes confused with the steam engine. A steam engine is designed to provide sustained mechanical work through controlled pressure and motion. A steam cannon would instead release pressure in a single firing event, making it a weapon concept rather than an engine for continuous power.