A harpoon cannon is a whaling implement developed in the late 19th century and most used in the 20th century. It would be mounted on the bow of a whale catcher, where it could be easily aimed with a wide field of view at the target. Powered by black powder and later, smokeless powder, it would generally fire a large steel harpoon, either solid steel (cold harpoon) or fitted with an exploding black powder, or later, penthrite (PETN) grenade.

Harpoon cannons are still used today in whaling nations, but usually guns of a smaller caliber with the exception of Iceland, which hunts large whales regularly. (source)