"A canary bird!"
"What kind of a machine is that?" asked Eric, thinking the other was referring to some name for a piece of rescue apparatus.
"A canary bird? It's a yellow machine with feathers, and sings," said Ed, laughing.
"You mean a real canary bird?"
"Yes, a live one."
"But what the crickets do they need a canary bird for?"
"To give them a pointer as to when the air is bad. You see, Eric, there's all sorts of different kinds of poisonous gases in coal mines. Some you can spot right off, but there's others you can't."
"I thought gas was just gas," Eric answered, "'damp,' don't they call it?"
"There's several different 'damps.' Take 'fire damp' or just plain 'gas' as the miners call it. That's really methane, marsh gas, the same stuff that makes the will-o'-the-wisp you can see dancing around over a marsh. It'll explode, all right, but there's got to be a lot of it around before much damage'll be done. 'Fire damp' is like a rattlesnake, he's a gentleman."
"How do you mean?" queried the boy.