The chase was growing decidedly mysterious.  The two men in the lead could be no others than Flemister and the chief clerk, presumably on their way to the carrying out of whatever plot they had agreed upon, with Lidgerwood for the potential victim.  But since this plot evidently turned upon the nearing approach of Lidgerwood’s special train, why were they plunging on blindly into the labyrinthine depths of the Wire-Silver mine?  This was an even half of the mystery, and the other half was quite as puzzling.  Who was the third man?  Was he a confederate in the plot, or was he also following to spy upon the conspirators?

Judson was puzzled, but he did not let his bewilderment tangle the feet of his principal purpose, which was to keep Flemister and his reluctant accomplice in sight.  This purpose was presently defeated in a most singular manner.  At the end of one of the longer tunnel levels, a black and dripping cavern, lighted only by a single incandescent shining like a star imprisoned in the dismal depths, the ex-engineer saw what appeared to be a wooden bulkhead built across the passage and effectively blocking it.  When the two men came to this bulkhead they passed through it and disappeared, and the shock of the confined air in the tunnel told of a door slammed behind them.

Judson broke into a stumbling run, and then stopped short in increasing bewilderment.  At the slamming of the door the third man had darted forward out of the shadows to fling himself upon the wooden barrier, beating upon it with his fists and cursing like a madman.  Judson saw, understood, and acted, all with the instinctive instantaneousness born of his trade of engine-driving.  The two men in advance were merely taking the short cut through the mountain to the old workings on the eastern slope, and the door in the bulkhead, which was doubtless one of the airlocks in the ventilating system of the mine, had fastened itself automatically after Flemister had released it.

Judson was a hundred yards down the tunnel, racing like a trained sprinter for the western exit, before he thought to ask himself why the third man was playing the madman before the locked door.  But that was a matter negligible to him; his affair was to get out of the mine with the loss of the fewest possible seconds of time—­to win out, to climb the ridge, and to descend the eastern slope to the old workings before the two plotters should disappear beyond the hope of rediscovery.

He did his best, flying down the long tunnel reaches with little regard for the precarious footing, tripping over the cross-ties of the miniature tramway and colliding with the walls, now and then, between the widely separated electric bulbs.  Far below, in the deeper levels, he could hear the drumming chatter of the power-drills and the purring of the compressed air, but the upper gangway was deserted, and it was not until he was stumbling through the timbered portal that a watchman