What heat-powered machine did Leonardo da Vinci sketch using expanding steam?

Leonardo da Vinci sketched a steam cannon, a heat-powered device that used expanding water vapor to propel a projectile.

The design is known as the architonnerre. It placed a heated chamber behind a projectile; when water entered the hot chamber, the resulting steam pressure was intended to force the projectile from the barrel. Leonardo’s notes show an interest in using steam explosively, but the historical evidence does not establish that he built a working battlefield cannon of this kind.

The architonnerre was not a steam engine in the later industrial sense. A steam engine continuously converts heat into mechanical work, while Leonardo’s concept used pressure in a single launch cycle.