Then he knew it was over, and staggered to his feet. The force of the explosion had passed along the main road, and so up the shaft, and he owed his life to the fact that he had been in the road off the course. He returned into the main road, but near the bottom of the shaft he was brought to a standstill. The roof had fallen, and the passage was blocked with fragments of rock and broken waggons. He knew that the bottom of the shaft must be partly filled up, that his comrades were killed, and that there was no hope of escape in that direction. For a moment he paused to consider; then, turning up the side road to the left, he ran at full speed from the shaft. He knew that the danger now was not so much from the fire-damp—the explosive gas—as from the even more dreaded choke-damp, which surely follows after an explosion and the cessation of ventilation.
Many more miners are killed by this choke-damp, as they hasten to the bottom of the shaft after an explosion, than by the fire itself. Choke-damp, which is carbonic acid gas, is heavier than ordinary air, and thus the lowest parts of a colliery become first filled with it, as they would with water. In all coal-mines there is a slight, sometimes a considerable, inclination, or "dip" as it is called, of the otherwise flat bed of coal. The shaft is almost always sunk at the lower end of the area owned by the proprietors of the mine, as by this means the whole pit naturally drains to the "sump," or well, at the bottom of the shaft, whence it is pumped up by the engine above; the loaded waggons, too, are run down from the workings to the bottom of the shaft with comparative ease.
The explosion had, as Jack well knew, destroyed all the doors which direct the currents of the air, and the ventilation had entirely ceased. The lower part of the mine, where the explosion had been strongest, would soon be filled with choke-damp, the product of the explosion, and Jack was making for the old workings, near the upper boundary line of the pit. There the air would remain pure long after it had been vitiated elsewhere.
It was in this quarter of the mine that Bill Haden and some twenty other colliers worked.
Presently Jack saw lights ahead, and heard a clattering of steps. It was clear that, as he had hoped, the miners working there had escaped the force of the explosion, which had, without doubt, played awful havoc in the parts of the mine where the greater part of the men were at work.
"Stop! stop!" Jack shouted, as they came up to him.
"Is it fire, Jack?" Bill Haden, who was one of the first, asked.
"Yes, Bill; didn't you feel it?"
"Some of us thought we felt a suck of air a quarter hour since, but we weren't sure; and then came another, which blew out the lights. Come along, lad; there is no time for talking."
"It's of no use going on," Jack said; "the shaft's choked up. I came down after the first blow, and I fear there's no living soul in the new workings. By this time they must be full of the choke-damp."