We learned that there were two types of cannons on board the US Brig Niagra. Cannons and cannonades. Cannons were mounted on a cart that the wheels allowed movement forward and backward. A connonade's wheels allowed movement sideways allowing the gunners to aim and shoot left or right. As you can see in this pictures at the front there is a swivel mount that made it easier to swing the cannonade and kept it centered in the gun port. Cannons had longer barrels and were for shooting for distance and cannonades had shorter barrels and were for close broadside shots at enemy ships. The rather large hemp rope or line was to help control recoil.
