With a speed and dexterity that showed their knowledge of the work, these men proceeded to build up, at the side of the gallery, close to the point where the last obstruction still held, a solid face of concrete, and rapidly cemented it to the somewhat elaborate construction that had been in process of making all the preceding day, and to which Otto had paid no heed.
It was not long before it became evident that a completely closed room was being made. Other gangs came along, carrying strange screw-doors of iron, and a multitude of devices new to the eyes of miners. Everything had been measured and prepared above-ground. It remained only to throw the material together, according to a prearranged plan.
By midnight, all was ready.
Three "sand hogs," with a gallant young doctor who had volunteered, prepared to enter.
A steady throbbing sound told that machinery connected with an outlet pipe—solidly embedded in the cement—had been set in motion. The newly made walls threatened to bulge inwards, and the signal was given to stop.
Then a rushing noise was heard in the inlet pipe, similarly embedded. The outer of the double doors was opened and the four men stepped in, entering a tiny ante-chamber. They closed the outer door, which was absolutely air-tight, opened the inner one, and passed into the chamber built against the coal face, made of solid cement except for a circle of coal a yard in diameter.
A minute or two later, could be heard, faintly, the high screech of some rapid-cutting machine.
When Otto came back on his next shift, at 2 o'clock on Friday morning, the sand hogs were still working.
Curiosity overcame the old miner's desire not to seem ignorant.
"Just what is that, sir?" he asked the Bureau official, who was still on watch.