They sailed forward, maintaining the same gravity, thus keeping an equal distance from the crust all round, returning to the spot from which they started, finding but a repetition of the previous experience; in some wonderful and unaccountable way the deadly nitrogen had taken to itself, and united with, oxygen, giving promise of becoming less deadly. Slowly sank the ship, samples of the air being taken every few miles, and though for several hours there was no change, they eventually came to a stratum where there was a greater percentage of oxygen. All knew what this portended and again everybody became almost distracted, and it required all their self-control to enable them to conduct their observations calmly and systematically, step by step, as they proceeded. All at once Dennis threw down some wires from an induction coil which he had been using, saying to Ross,—“I’m played out, Ross! Tell them all to go to sleep, and insist on it; what’s coming can wait! And let the ship stay where she is.” And he passed on into his cabin, where he flung himself down just as he was, falling asleep almost before he had settled in his hammock, without heeding Ross’s reply. Ross then spoke up: “I say, you fellows, we must look after our health, you know! For nearly fifty hours we have had no sleep, and all the time have been under full pressure of exciting work. We cannot continue it without being ill, and illness on board would be a dreadful thing. Let us all retire for at least twelve hours and then we can continue our observations and experiments in detail, as we sink down to that which appears to be below us. In the meantime, the ship is stationary and will not move a hair’s-breadth, so we shall lose nothing. Good-night.” And he also passed into his room and was soon fast asleep. Loth to leave their work, yet feeling the wisdom of reserving their energies, and finding there was no movement in the air around, the others gradually sought repose in their cabins, going off in driblets till the saloon, laboratory and observatory were empty, and throughout the ship there reigned silence, broken only by the heavy breathing of the sleepers. Gilbert had retired some hours before Dennis; for over two days he had been working feverishly, but though fagged out, he would not seek rest except on Ross’s solemn promise to wake him in order to take charge of the vessel. Consequently, he was the first up, and saw that for the first time during the voyage the ship was unwatched, and felt somewhat annoyed that Ross had not called him as promised before vacating his position. Evidently both he and Dennis had been too weary to waken their sleeping companion.