He shouted this in a wildly frantic way, and then watched eagerly while Gwyn tied the card in a handkerchief and secured it to the dog’s collar, Grip going off directly, as if he quite understood the business now.

This done, Joe and Hardock lay down close to the orifice and listened to the clinking of the hammers, trying the while to imagine what kind of passage existed beyond the wedge-like block of stone, and calculating how long it would be before they were rescued. But that was all imagination, too, for there was nothing to base their calculations upon.

Meanwhile Gwyn was more matter-of-fact; for he took the lanthorn and descended to where the water had risen, and there, clinging with one hand, he held the light down, to gaze with a feeling of awe at the bubbling surface, which was in a violent state of agitation, looking as if it were boiling. Every now and then it was heaved up and then fell back with a splash.

Gwyn’s object in descending the sharp slope had been to make a mark upon the rock with his knife just at the level of the water, and then try and scratch other marks at about a foot apart, so as to descend again and see how much higher the water had risen.

But this seemed to be impossible, for the level was always changing, the water running up several feet at times and then descending, playing up and down evidently as the pressure of the confined air increased or sank.

Still he made some marks, and then returned to the others to join them in listening.

But this proved weary work, for it was only now and then that they could hear the sound of the hammer, for the current of air seemed to bear it away; while, when by chance the sounds did reach their ears they were most tantalising, at one time seeming very near, and at others so faint that they felt that the work going on must be very distant.

The dog came back with food and lights and stayed with them, now trotting to the opening to bark at the sounds; and at times standing at the edge of the lower cavity to bark fiercely at those from below, his ears and the thick wolf frill about his neck being blown about by the fierce current of air.

And so the time went on, first one and then the other descending to find that the water was steadily rising, and after each examination there was a thrill of dread as the looker-on asked himself, Would they win the race?

How long was it? Was it night, now, or day?