"That you, Otto? So you want to know, now, do you? Well, that's a sort of lightly made caisson, or air-tight chamber, with an air-lock or double door. It's used a good deal for working under water, but for the job we have here, it doesn't have to be very solidly built.
"It's simple enough, when you think it out. We just cemented it up, put in an air-pump to take out the gassy air that was in it, and then turned in compressed air, with a pressure of a little more than one atmosphere, just enough to keep any of the gas from entering the hole that is being dug through the coal pillar."
"Why can't gas get in? Gas'll go through coal."
"Because the pressure from inside is bigger than from outside. The compressed air is leaking through the coal and driving any gas away."
"Why didn't you let us get in there to finish the job, if that's all there is to it?" protested Otto, indignant that strangers should have the glory of the final rescue, after the miners had done so much.
"Because you couldn't stand it. Those men are sand hogs. They're used to working in compressed air. Just as soon as a man gets into a pressure of two or three atmospheres, unless he's mighty careful he's apt to get dangerously ill. His blood absorbs too much air. While he's under compression, he doesn't feel it so much, but if he comes out of the compression too quickly, the surplus air in his blood can't come out as slowly as it ought, and little bubbles form in the blood current. That's deadly. Sometimes these bubbles cause a terrible caisson disease known as the 'bends,' when all the muscles and joints are affected; or it may give a paralysis known as 'diver's palsy,' because divers working in compressed atmospheres suffer the same way; all too often, it causes sudden death. So you see, Otto, it's not a chance a man ought to take who knows nothing about it."
"An' the sand hogs are diggin' in there?"
"No, they're not digging. We put in a tunnelling machine driven by compressed air, which is sometimes used for making sewers and the like. It will bore an even, round hole, just big enough for a man to crawl through, comfortably.
"As soon as that hole is pierced through into the room where the imprisoned men are, the doctor will go in, taking food, wine and medical supplies, and three respirators as well. Then, when the survivors are protected against the possible results of a sudden inrush of gas, it'll be up to you men to get the rest of the wall down as quick as you can."
"So that's how it is! We'll be ready, sir, as soon as you give the word."