It was during these excursions that Harry was more particularly struck by certain phenomena, which he vainly sought to explain.

Several times, while walking along some narrow cross-alley, he seemed to hear sounds similar to those which would be produced by violent blows of a pickax against the wall.

Harry hastened to seek the cause of this mysterious work.

The tunnel was empty. The light from the young miner's lamp, thrown on the wall, revealed no trace of any recent work with pick or crowbar. Harry would then ask himself if it was not the effect of some acoustic illusion, or some strange and fantastic echo.

At other times, on suddenly throwing a bright light into a suspicious-looking cleft in the rock, he thought he saw a shadow.

He rushed forward. Nothing, and there was no opening to permit a human being to evade his pursuit!

Twice in one month, Harry, whilst visiting the west end of the pit, distinctly heard distant reports, as if some miner had exploded a charge of dynamite. The second time, after many careful researches, he found that a pillar had just been blown up.

By the light of his lamp, Harry carefully examined the place attacked by the explosion. It had not been made in a simple embankment of stones, but in a mass of schist, which had penetrated to this depth in the coal stratum.

Had the object of the explosion been to discover a new vein?

Or had someone wished simply to destroy this portion of the mine?

Thus he questioned, and when he made known this occurrence to his father, neither could the old overman nor he himself answer the question in a satisfactory way.

"It is very queer," Harry often repeated. "The presence of an unknown being in the mine seems impossible, and yet there can be no doubt about it. Does someone besides ourselves wish to find out if a seam yet exists? Or, rather, has he attempted to destroy what remains of the Aberfoyle mines? But for what reason?

I will find that out, if it should cost me my life!"

A fortnight before the day on which Harry Ford guided the engineer through the labyrinth of the Dochart pit, he had been on the point of attaining the object of his search.

He was going over the southwest end of the mine, with a large lantern in his hand. All at once, it seemed to him that a light was suddenly extinguished, some hundred feet before him, at the end of a narrow passage cut obliquely through the rock.

He darted forward.

His search was in vain. As Harry would not admit a supernatural explanation for a physical occurrence, he concluded that certainly some strange being prowled about in the pit.

But whatever he could do, searching with the greatest care, scrutinizing every crevice in the gallery, he found nothing for his trouble.

If Jack Ryan and the other superstitious fellows in the mine had seen these lights, they would, without fail, have called them supernatural, but Harry did not dream of doing so, nor did his father.

And when they talked over these phenomena, evidently due to a physical cause, "My lad," the old man would say, "we must wait.

It will all be explained some day."

However, it must be observed that, hitherto, neither Harry nor his father had ever been exposed to any act of violence.

If the stone which had fallen at the feet of James Starr had been thrown by the hand of some ill-disposed person, it was the first criminal act of that description.

James Starr was of opinion that the stone had become detached from the roof of the gallery; but Harry would not admit of such a simple explanation. According to him, the stone had not fallen, it had been thrown; for otherwise, without rebounding, it could never have described a trajectory as it did.

Harry saw in it a direct attempt against himself and his father, or even against the engineer.