Fours à boulets des îles de Lérins, Military defense furnaces on Lérins Islands, Cannes, France

The Fours à boulets des îles de Lérins are four hot-shot furnaces, two on Île Sainte-Marguerite and two on Île Saint-Honorat in the Gulf of La Napoule off Cannes. Each is a low, thick-walled mass of rough light grey and tan stone blocks, its rounded front pierced by a dark arched opening ringed with shaped stones; inside, a flat grooved floor carried the cannonballs. A rectangular stone mass with a flat top extends to the right.

In 1793 General Bonaparte, arriving at the Army of the Alps, ordered batteries built on the Lérins Islands to defend the coast. Four hot-shot furnaces were added, two on Sainte-Marguerite for the Dragon and Vengeur batteries and two on Saint-Honorat for the Républicains and Braves Gens batteries. The two Saint-Honorat furnaces were classified as monuments historiques on 22 October 1908.

The name four à boulets refers to the red-hot cannonballs the furnaces prepared for the coastal batteries. Each furnace could deliver one incendiary ball every ten minutes, and a pair of tongs lifted the glowing ball into the cannon, where a wad of wet straw or clay separated it from the powder charge. The two furnaces on Saint-Honorat were classified as monuments historiques on 22 October 1908.

Visitors reach the islands by boat from Cannes, which takes about 15 minutes. The furnaces are located on the islands and can be seen with a guide, so wear sturdy shoes for uneven ground.

The furnace worked by gravity: cannonballs entered at the top, rolled down rails to a lower chamber heated by a side hearth, and emerged red-hot after about thirty-five minutes. A pair of tongs lifted the glowing ball into a cannon, where a wad of wet straw or clay separated it from the powder charge.