Mortars were designed to shoot an exploding shell at a very high angle, 45 degrees or more. They were used in the siege and defence of fortifications. An explosive shell was fired up into the air and arced downwards to drop within the enemy defences. When the shell's fuse burned down, it exploded. These projectiles are the 'bombs bursting in air' mentioned in the American national anthem, where they were being fired from a British fleet attacking Baltimore.