The almost invisible illumination being there, the Irishman wanted no philosopher to tell him that it was the sun striking the water as it reached the outside, and the outer world, which he was so desirous of re-entering, was close at hand.

Mickey was in high glee at the discovery, but when he regained his mental poise, he could not shut his eyes to the fact that if he attempted to reach the outer world by means of the stream, he ran a terrible risk of losing his life.  There was no vacancy between the water and the stone which shut down upon it.  The outlet was like an open faucet to a full barrel.  The escaping fluid filled up all the space at command.

No one can live long without air.  A few seconds of suspended respiration is fatal to the strongest swimmer.  If the distance traveled by Mickey, when he should attempt to dive or float through to the outer world, should prove a trifle too long, the stream would cast out a dead man instead of a live one.

But he was a person of thorough grit, and before he would consent to see himself and Fred imprisoned in this cavern, he would make the attempt, perilous as it was.

Was there no other way of escape?  Was there not some opening which had been used by those who had entered this cave ahead of him?  Or was it possible that the imprisoning walls were to thin and shell-like in some places that there was a means of forcing their way out?  Or was there no plan of climbing up the side of the prison and reaching an opening in the roof, through which they could clamber to safety?

These and other thoughts were surging through the mind of Mickey O’Rooney, when an exclamation from Fred caused him to turn his head.  The boy was running toward him, apparently in great excitement.

“What’s the matter, me laddy?” asked Mickey, cocking his rifle, which he had taken from him at the time of handing him the torch.  “Oh, Mickey, Mickey!  I saw a man just now!”

CHAPTER XXIX A MYSTERY

O’Rooney stood with rifle grasped, while young Munson ran toward him from the centre of the cave, exclaiming in his excited tones: 

“There’s another man back yonder!  I saw him and spoke to him!”

“Did ye ax him anything, and did he make a sensible reply?” demanded the Irishman, whose concern was by no means equal to that of the lad.

“He made no answer at all, nor did he seem to take any notice of me.”

“Maybe it’s a ghost walking round the cave, on the same errand as meself.  But whist now; where is he, that I may go and ax him the state of his health?”

The lad turned to lead the way, while Mickey followed close at his heels, his gun ready to be used at an instant’s warning, while Fred kept glancing over his shoulder, to make sure that his friend was not falling too far in the rear.

It seemed that, while the man was engaged in his exploration, the lad had ventured upon a little prowling expedition of his own.  During this he made the startling discovery that some one else was in the cave, and he dashed off at once:  to notify his friend and guide.