The harpies ring a dinner bell for the sleepless dragon in order for him to come from deep within the cave to come eat Launchpad. However, the dragon had been chained next to the Golden Fleece deep within the maze of the cave. While Scrooge and the boys had tricked the dragon into melting its own fettered chains, the harpies could not have known that. They could not have reasonably expected the dragon to come to them when they rang the dinner bell.