"What'th in that little houthe over there?" asked Dicky. "Theems to me I thee something whithing round."
"That's the fan that blows fresh air into the mine so that the miners can breathe, and drives out the poisonous and dangerous gases."
"What would happen if the fan stopped running?" asked Ethel Brown.
"Many things might happen," said the Superintendent gravely. "Men might suffocate for lack of air, or an explosion might follow from the collection of the dreaded 'fire damp' ignited by some miner's lamp."
"Fire damp?" repeated Mrs. Morton. "That is really natural gas, isn't it?"
"Yes, they're both 'marsh gas' caused by the decay of the huge ferns and plants of the carboniferous age. Some of them hardened into coal and others rotted when they were buried, and the gas was caught in huge pockets. It is gas from these great pockets that people use for heating and cooking all about here and even up into Canada."
Ethel Brown had been listening and the words "some of them hardened into coal" caught her ear. She went close to her grandfather's side.
"Tell me," she said, "exactly what is coal and how did it get here?"
"What I want to know," retorted Mr. Emerson, "is what brand of curiosity you have in your cranium, and how did it get there? Answer me that."
Ethel Brown laughed.