The "cannon" was thunder. At the beginning of the chapter, the narration says, "Over the island, the buildup of clouds continued." The storm builds as the action builds in this chapter; it is a device used by writers and by film makers - weather mirroring the action of the story. In this chapter, Simon, having realized through his "conversation" with the Lord of the Flies in the previous chapter, understands that there is no physically separate beast on the island; it's only themselves they have to fear. He untangles the dead parachutist, freeing the body to drift away. He runs to tell the others what he's discovered and the storm reaches its peak of intensity with lots of lightening and thunder as he rushes into the frenzied circle of boys. They are so insensed by their own chanting around the fire and the fierce and noisy storm, that they don't really hear Simon and they pounce on him as if he was the beast and they kill him. Golding uses the term "cannon" to liken the storm to warfare and the thundering noise of war cannons that also kill.