They had a calorimeter and a pressure-gauge communicating with the outer world to tell the temperature and the height of the water in the valley. The former, after rising for a few hours to over a thousand degrees, had now sunk back to normal, while the latter stood at thirty feet above the entrance doors to the cavern. The machinery for raising the sluice-gate was put into motion and they watched it with almost breathless anxiety lest the straining or shifting of the rocks, which had been very perceptible during the terrific convulsions which had apparently lasted for nearly ten hours, should have so dislocated the grooves that the gate could not be raised. There were a few preliminary creaks and groans, a hitch and an increased strain on the lifting chains, and then the great sheet of steel rose easily and smoothly to the top of the channel and the pent-up waters rushed forth in a black boiling flood through the narrow opening and roared away, foaming and tossing along the bottom of the crevasse, once more on their way to their unknown destination. Very soon after this it was discovered that the waters were subsiding much more rapidly than could be accounted for by the volume that escaped through the subterranean channel. It was therefore necessary to conclude that there must have been some convulsion in another part of the mountains which had opened a fresh channel from the lake to the outer world. The next step was to raise the two inner of the three doors which guarded the entrance to the caverns. The raising of the first one showed the ice still intact between it and the second, and this had to be broken up and removed before the second could be reached. Then the middle door was raised an inch or so and the water spurted out from beneath it. Was this the water of the melted ice or was it that which filled the valley? Had their outer door stood firm or had it cracked or shrivelled up under the heat of the furnace through which the earth had passed? It flowed for ten minutes and then slackened and stopped. The outer door had held fast. Then, in case of accidents, the middle one was lowered again and they waited until the waters should have sufficiently subsided to enable them to challenge the last hazard on which their fate depended. The sluice-gate had been raised at what would be four o’clock on the morning of the 26th of September, if the cataclysm through which the earth had passed had not so far affected the terrestrial economy as to alter the relations of day and night. Twelve hours later the pressure-gauge ceased to act, showing that the rapidly-sinking waters of the lake had reached the threshold of the outer door. The time had now come to ask the question on the answer to which the lives of the remnant of humanity depended--was the atmosphere breathable or not?