The device is a cannonball oven, intended to heat the missiles in the hope that they would set enemy ships on fire. Designed in 1794, it was staffed by five men. The first was responsible for the fire which burned in the left-hand part of the structure. Wood was fed in through the upper opening, while the lower provided ventilation. A third opening in the side wall allowed ashes to be removed.
When the fire was going well, a load of 20 cannonballs would be sent down into the oven through an opening at the back. They rolled down three channels to a flat metal area where they would sit in a bowl for two-and-a-half hours until they turned red-hot. The process could be supervised from the upper right-hand opening.
Once ready, the cannonballs exited by the lower opening. They were quickly carried in a large, metal 'spoon' to the nearby cannon, already prepared with a charge of gunpowder. It would be fired, and the hope was that if it reached its target then it would not only damage it, but set it alight.
