The knockers of the Welsh mines

Besides the fairies of the story books there was a kind said to live in the mines, where they were heard copying the work of the miners and were thought willing to help them, and never to harm them unless insulted. In Wales they were called knockers, and were believed to point out the rich veins of silver and lead. John Lewis, writing to the antiquary Baxter, described them as little creatures about half a yard long, and said two miners who had found a new vein were ready to swear they had heard the knockers in the daytime.