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.

CHAPTER VI SIMON FORD'S EXPERIMENT

THE old clock in the cottage struck one as James Starr and his two companions went out. A dim light penetrated through the ventilating shaft into the glade. Harry's lamp was not necessary here, but it would very soon be of use, for the old overman was about to conduct the engineer to the very end of the Dochart pit.

After following the principal gallery for a distance of two miles, the three explorers--for, as will be seen, this was a regular exploration-- arrived at the entrance of a narrow tunnel. It was like a nave, the roof of which rested on woodwork, covered with white moss. It followed very nearly the line traced by the course of the river Forth, fifteen hundred feet above.

"So we are going to the end of the last vein?" said James Starr.

"Ay! You know the mine well still."

"Well, Simon," returned the engineer, "it will be difficult to go further than that, if I don't mistake."

"Yes, indeed, Mr. Starr. That was where our picks tore out the last bit of coal in the seam. I remember it as if it were yesterday. I myself gave that last blow, and it re-echoed in my heart more dismally than on the rock. Only sandstone and schist were round us after that, and when the truck rolled towards the shaft, I followed, with my heart as full as though it were a funeral. It seemed to me that the soul of the mine was going with it."

The gravity with which the old man uttered these words impressed the engineer, who was not far from sharing his sentiments. They were those of the sailor who leaves his disabled vessel-- of the proprietor who sees the house of his ancestors pulled down. He pressed Ford's hand; but now the latter seized that of the engineer, and, wringing it:

"That day we were all of us mistaken," he exclaimed.