Sweet cannonball. Is it solid shot or does it have a fuse?
not sure how to tell exactly, but I'm guessing a hole and I don't see one...not sure how to keep it from rusting away as Beest mentioned! Thanks to all for sharing!
Here ya go. Check out this thread and scroll down to the pics I added. They should help you ID it. It there's no fuse, then clean it with electrolysis and preserve it with some sort of oil, wax, or clear coat (Boiled Linseed oil, Danish oil, etc).
http://metaldetectingforum.com/showthread.php?t=151918