Siege Mortar

Origins

Ordinary cannon fired almost straight, which was good for battering a wall but useless against troops sheltering behind one. Gunners of the fifteenth century found that a short, wide barrel set at a steep angle could throw a heavy stone over a wall, and they called it a mortar, from its likeness to the bowl used to grind spices. In the sixteenth century mortars began to fire explosive shells, hollow iron spheres packed with gunpowder, and by the seventeenth century they were a normal part of siege warfare. Menno van Coehoorn, an engineer of the Netherlands, designed a small, light mortar in the 1670s that could be carried by a few men and moved into forward trenches. It became known as the Coehorn. The French engineer Vauban made mortars a key part of his method of attacking fortresses, and they were still standard in the sieges of the Napoleonic Wars.

Design & Construction

A mortar's barrel was short and thick, often of bronze or cast iron. Its bore was measured in inches across: the British used 4.4-inch Coehorns, and 8-inch, 10-inch and 13-inch mortars. It sat on a solid wooden or iron bed rather than a wheeled carriage, and it was usually fixed at about 45 degrees. Instead of changing the angle, gunners set the range mainly by changing the powder charge. The shell had a wooden fuse filled with slow-burning powder, cut to length so it would burst over or just after landing. By the eighteenth century the flash of the firing charge usually lit the fuse. Mortars could also throw carcasses, incendiary shells meant to set buildings on fire. At sea, special bomb vessels carried heavy mortars for attacking forts and harbors.

Combat Use

In a siege the attacking engineers dug trenches toward the fortress, and the mortars were placed in batteries behind them. Their shells fell into the fortress, where they killed gun crews, wrecked buildings and wore down the defenders night and day. At the Siege of Yorktown in 1781, near the end of the American Revolution, French and American mortars pounded the British lines and the town without pause for more than a week. In 1814 British bomb vessels fired heavy mortar shells at Fort McHenry in Baltimore, the bombardment that gave the American national anthem its "bombs bursting in air". In the American Civil War Union forces at Petersburg used a 13-inch mortar mounted on a railway flatcar, nicknamed the Dictator.

Legacy

Rifled guns and new explosive shells made the old smoothbore mortars obsolete in the late nineteenth century. But the idea of high-angle fire did not die. In the trenches of World War I the British Stokes mortar and similar weapons brought the mortar back as a light infantry weapon. Modern infantry mortars fire the same way, dropping their bombs onto targets that direct fire cannot reach. Many old siege mortars survive in museums and on the grounds of historic forts, including the battlefields of Yorktown and Petersburg.