The ship made a single circle above the desolate ice pack before she took up her southerly course along the 170th East Meridian. From the moment that the ship struck south from the pole Jason Gridley remained constantly with Hines and Zuppner eagerly and anxiously watching the instruments, or gazing down upon the bleak landscape ahead. It was Gridley's belief that the north polar opening lay in the vicinity of 85 north latitude and 170 east longitude. Before him were compass, aneroids, bubble statoscope, air speed indicator, inclinometers, rise and fall indicator, bearing plate, clock and thermometers; but the instrument that commanded his closest attention was the compass, for Jason Gridley held a theory and upon the correctness of it depended their success in finding the north polar opening. For five hours the ship flew steadily toward the south, when she developed an apparent tendency to fall off toward the west. "Hold her steady, Captain," cautioned Gridley, "for if I am correct we are now going over the lip of the polar opening, and the deviation is in the compass only and not in our course. The further we go along this course the more erratic the compass will become and if we were presently to move upward, or in other words, straight out across the polar opening toward its center, the needle would spin erratically in a circle. But we could not reach the center of the polar opening because of the tremendous altitude which this would require. I believe that we are now on the eastern verge of the opening and if whatever deviation from the present course you make is to the starboard we shall slowly spiral downward into Pellucidar, but your compass will be useless for the next four to six hundred miles." Zuppner shook his head, dubiously. "If this weather holds, we may be able to do it," he said, "but if it commences to blow I doubt my ability to keep any sort of a course if I am not to follow the compass." "Do the best you can," said Gridley, "and when in doubt put her to starboard." So great was the nervous strain upon all of them that for hours at a time scarcely a word was exchanged. "Look!" exclaimed Hines suddenly. "There is open water just ahead of us." "That, of course, we might expect," said Zuppner, "even if there is no polar opening, and you know that I have been skeptical about that ever since Gridley first explained his theory to me."