“Oh, yes; that’s as clear as it can be.”
“That’s good. Now we’ll go up to the mine. Let’s get into this empty car. It’s not as clean as a Pullman, nor as big, but it’s the only kind we run on this road.”
They helped her in, and one sat on either side to steady her, as the tipple-hands coupled it to the cable and the trip up the steep grade began.
“You see, the loaded cars going down pull up the empty ones,” he said. “We make gravitation do all the work. It’s a simple way, and mighty convenient.”
The loaded car, heaped high with coal, passed them midway, and in a moment they were at the mouth of the mine. To her surprise, she saw that there were two openings, one much smaller than the other.
“That smaller one’s the airway,” said Lambert. “Just inside there’s a big wheel, or fan, made very much like the wheel of a windmill, going around about a hundred times a minute, and blowing about a thousand cubic feet of air out of the mine at every revolution.”
“Out of the mine!” exclaimed Miss Andrews.
“Yes. The airway is connected with the gangway there, away back at the farthest limit of the mine. So what happens?”
He was smiling down at her, relishing intensely this novel chance to test the wits of the school-teacher.
“Why,” she began slowly, “if so much air is pumped out, just so much more must rush in to take its place through the other opening.”