"of the knockers and the blue-caps who live down the mines"

Knockers live below ground and look like little miners. They knock on the walls of mines. They are Welsh and Cornish in origin, and some see them as mischief-makers where others consider them harmless practical jokers. English Brownies and Irish Leprechauns are comparable.

Blue-caps are similar to Knockers, but they are benevolent and forwarn people about cave-ins. They show miners where rich deposits can be found, as long as they are treated with respect.