A Coehorn was a lightweight mortar originally designed by Dutch military engineer Baron Menno van Coehoorn. The British model was cast in brass and was slightly longer and with a larger calibre then the Dutch model and manufactured by I & P Verbruggen and dated to 1779. The length of the coehorn mortars at the Convent is 13.5in (34cm) and weighed 84lb (38kg).

The projectile was a 7lb (3kg) hollow shell with a diameter of 4.6in (12cm). It was discharged using 8oz (230g) of black gun-powder.

During the Anglo-Spanish War of 1762–63 it was noted that Willis's Battery was equipped with six 8-inch mortars and twenty-two 4.66-inch Coehorn mortars. The use of this type of ordnance was increased during the Great Siege of 1779-1783 when sixty-nine Coehorns were recorded as in use on various batteries around the Rock.

Over the century and half of its existence several changes were made to the weight, calibre and dimensions. The Coehorn mortars were used in the American Wars, Peninsula Wars and other actions against Napoleon. And in many actions during the 1800s. Mortars lost favour in the late 1800’s and were not on general issue in the British Army. This lack of such a weapon saw the Coehorns actually being used in the early part of World War I.

Two of the four extant Coehorn Mortars in Gibraltar are located at the Convent, the Governor’s Residence. The remaining two can be found inside Grand Battery House. Another known mortar taken from Gibraltar, was presented to Lodge Ubique located at Lindfield, New South Wales, Australia by the Brethren of THE UBIQUE LODGE No 1789 EC to mark the Consecration of Lodge Ubique on the 5^th May 1956. Bro. Don Rossell, a member of both Lodges brought 'Cœthe Bombard' as this piece is called from London as part of his personal luggage. It was stated 'Cœthe Bombard' had once been an ornamental piece at one of the Messes in Gibraltar. This mortar is today recorded in the Australian National Artillery Register.

Lodge Ubiquecoehorn mortar.