I'm always amazed by man's ability to find new and fucked up ways to kill their fellow man. I had never heard of execution by cannon. Here is an eyewitness description of the event from Wikipedia.
The prisoner is generally tied to a gun with the upper part of the small of his back resting against the muzzle. When the gun is fired, his head is seen to go straight up into the air some forty or fifty feet; the arms fly off right and left, high up in the air, and fall at, perhaps, a hundred yards distance; the legs drop to the ground beneath the muzzle of the gun; and the body is literally blown away altogether, not a vestige being seen.