Designed by Robert Mallet, Mallet’s mortars were two large 42 ton siege mortars that could fire 2,400lb explosive shells up to one and a half miles. Mallet made his cannon design public, but it was only with the support of Prime Minister Lord Palmerston that it would be build. Two were ordered but from the beginning it was a disaster. Costing a whopping £4,500 a piece they were not cheap. The contract went to Thames Ironworks and Shipbuilding Company, but the company was already in dire financial straites and went bankrupt. Three other companies had to be contracted to build the mortars in separate pieces.
Work was finished on the mortars in early 1857 and testing was conducted in October of the same year. Using an 80 pound charge, Mallets mortars fired a 2,300 - 3,000 lb shell almost 2,800 yards. The two mortars were intended for use in the Crimean War, most notably to break the heavy Russian fortifications at Sevastopol. However, due to construction delays the mortars were delivered a year too late. Today both mortars are part of the Royal Armouries Collection.