He gave a violent start--not of panic, for he was long familiar now with that weird throaty chuckle and its cause, but of mere nervous repercussion--and, coming erect, found himself caught at the back. Some part of his dress had hitched in a projection of the wood-work, and held him prisoner. He pulled, and the object, whatever it was, appeared to come away. Suddenly it yielded, and fell heavily at his heels. He turned to see what it was, and discovered that he had drawn out from the circumference of the wheel one of the short boards or panels of which it was composed. For the moment, stooping to lift the thing, he imagined that its detachment was due to mere accident or decay; but suddenly, as he idly handled it, his fingers touched on something which set him oddly thinking. This was a notch or groove on the _underside_ of the board--representing the _outside_ of the wheel--which had been intentionally cut there, it would seem, to facilitate its removal. But for what purpose? A little excited, he made a quick examination of the gap exposed, and saw at once that chance had led him on a curious discovery. The board, a stout one, some eighteen inches in width, was furnished with a tongue on each side, the two being formed to slide into corresponding grooves cut in the adjacent sections, in the manner of a ‘match-joint,’ and it was fitted on its outer edge, like a box-lid, with a flange which, when the panel was pushed home, came up flush with the wheel-rim. Brion tried it in and out, and found that it ran quite easily. But why was it there, and what was the meaning of the notch? Now the lowest segment of the wheel, as already said, turned, for a foot or more below the surface, in a little oblong pit cut to receive it in the pavement, as if some miscalculation of the diameter had necessitated that stratagem. Brion had always so looked upon it as a makeshift, but now he began to wonder. After a minute of thrilling reflection, he slid out the panel, laid it aside, and, turning the wheel until the gap in it came over the pit, steadied the slowly rotating monster and looked down. And instantly, with a little shock of the blood, he understood. The pit was no shallow depression made to receive the wheel, but the entrance to an underground chamber of unknown depth and significance.