What steam-powered cannon did Leonardo da Vinci design in his military studies?

Leonardo da Vinci designed a steam-powered cannon called the Architonnerre in his military studies.

The concept used heated water to create pressure that would propel a projectile from a cannon barrel. Leonardo’s notes describe a system in which a small amount of water was heated rapidly, producing steam and force. The idea belongs to the long history of proposals for heat-powered artillery.

The Architonnerre was a design concept rather than a weapon known to have entered regular service. Leonardo’s surviving military notebooks contain many inventions that were never built or tested under battlefield conditions.

It is sometimes described as an early steam cannon, but it should not be confused with later steam engines. Its purpose was a brief, explosive pressure release, not continuous rotary power for machinery.