It had holes in it with quickmatch that ignited the rosin when the shot was fired, according to this page (toward the bottom). Also describes the other major types of cannon shot.
The site I linked to in my previous post also mentions those (halfway down, under “think big”). They stayed around long enough to pound the Royal Navy in 1807!
I just want to add that I find it interesting that after 150 years or so, the world’s most modern armies have returned to using solid shots in smoothbore barrels for their largest direct fire cannons - namely, western 105mm and 120mm tank guns. Sure, tungsten-tupped saboted rounds are hardly iron cannonballs, but the principle remains the same.