Prom this projected a long limb or bar, which was now almost horizontal, but which the captain believed to be the thick rod which had stood upright when he clutched it, and which had yielded to his weight and had gone down with him.

He knew now what it was: it was a handle that had turned.
He hurried to the other end of the huge machine, where it rested against the rocky wall of the cavern.

There he saw in the shadow, but plain enough now that he was near it, a circular aperture, a yard or more in diameter.

Inside of this was something which looked like a solid wheel, very thick, and standing upright in the opening.

It was a valve.