Both vehicles disappeared quickly into a valley to the northeast, between the ash cones and a thousand-foot spine which rose just south of them. All the tractors were in good radio contact; Zaino made sure of that before he abandoned the radio watch to Rowson, suited up and joined Hargedon at the remaining one. They climbed in, and Hargedon set it in motion. At about the same time, the first tractor came into view again, now traveling north on the farther side of the chasm. Hargedon took this as evidence that the route thus far was unchanged, and kicked in highest speed. The cabin was pretty cramped, even though some of the equipment had been attached outside. The men could not expect much comfort for the next week. Hargedon was used to the trips, however. He disapproved on principle of people who complained about minor inconveniences such as having to sleep in spacesuits; fortunately, Zaino's interest and excitement overrode any thought he might have had about discomfort. This lasted through the time they spent doubling the vast crack in Mercury's crust, driving on a little to the north of the ship on the other side and then turning west toward the dark hemisphere. The route was identical to that of Harmon's machine for some time, though no trace of its passage showed on the hard surface. Then Hargedon angled off toward the southwest. He had driven this run often enough to know it well even without the markers which had been set out with the seismographs. The photographic maps were also aboard. With them, even Zaino had no trouble keeping track of their progress while they remained in sunlight. However, the sun sank as they traveled west. In two hours its lower rim would have been on the horizon, had they been able to see the horizon; as it was, more of the "sea level" lava plain was in shadow than not even near the ship, and their route now lay in semi-darkness. The light came from peaks projecting into the sunlight, from scattered sky-light which was growing rapidly fainter and from the brighter celestial objects such as Earth. Even with the tractor's lights it was getting harder to spot crevasses and seismometer markers. Zaino quickly found the fun wearing off ... though his pride made him cover this fact as best he could. If Hargedon saw this, he said nothing. He set Zaino to picking up every other instrument, as any partner would have, making no allowance for the work the youngster was doing for Schlossberg. This might, of course, have had the purpose of keeping the radioman too busy to think about discomfort. Or it might merely have been Hargedon's idea of normal procedure. Whatever the cause, Zaino got little chance to use the radio once they had driven into the darkness. He managed only one or two brief talks with those left at the ship. * * * * *