Rodin was only moderately interested in the view, until the coast began to curve gently westward. Then he began to make careful checks, using one of the maps he had obtained at the weather station. Several times he lowered the ship to the water, checking its depth and temperature; frequently he cruised as low as was safe among the hills and above the trees, examining Vickers knew not what characteristic of the planet’s surface. The meteorologist’s pile of notes and computations grew in thickness, while Vickers did little save look on and enjoy himself.
Southward they drove, breaking away from the coast and moving far out over a broad stretch of sea, until the geodet told them they were nearly above the equator; then westward, still dropping occasionally for Rodin’s perpetual measurements, over more water, interrupted at times by islands. Twice they saw what were evidently Heklan communities; each time they were small, but each boasted a landing strip similar to, but much longer than, the one on Observatory Hill. Several winged aircraft were parked in the open near each strip, and a single machine, similar in exterior design to the terrestrian lifeboat. Vickers was curious about its method of propulsion, since the Heklans were without atomic power, but he did not bother to descend to investigate.
For ninety hours they chased the sun, veering far enough to right and left to examine the near shores of most of the continental masses.
Each time they did so, Rodin expressed greater confidence in his plan; and as the geodet told them that they were again approaching the longitude of Observatory Hill, he swung the ship northward, prepared to argue its merits to the limit.
Vickers took over the controls for a time, to let the meteorologist straighten out the last of his paper work. It was a token job, since the automatic controls were holding the craft on course and at a constant pressure altitude. They were cruising at a very moderate speed, since Rodin wanted time for his work; they were, Vickers calculated, about an hour and a half from the observatory. The usual layer of haze was overhead — thicker than normal, Vickers decided; the red sunlight pouring through the upper ports seemed less intense than usual.
He did not see the clouds until they were less than twenty miles ahead. It was the first extensive cumulus development he had seen on Hekla, and he debated calling Rodin; but he decided such clouds could not be too unusual, and failed to do so. He simply sat and watched the wall of vapor grow more distinct as the little ship approached it. It extended as far as he could see on either side and — up. An airplane pilot of an earlier century would not have come within miles of that angry black barrier; Rodin might have decided to go over it, but Vickers let the automatic controls carry the tiny machine straight into its heart. Even then, if the altitude control had been connected to the radio altimeter, no harm might have been done; unfortunately, Vickers had tied it in to the atmospheric pressure gauge, in anticipation of reaching land.
The initial turbulence made no impression on ship or occupants; but five seconds after the sun had faded from sight the ship stuck its nose into the low pressure of an updraft, and Vickers left his seat. For several seconds he was dazed by the force with which his head struck the ceiling. In those few seconds the ship lost six thousand feet of altitude as the automatic controls sought a level of pressure equal to that at which they had been set. Before they succeeded, and before Vickers could regain his feet and the manual controls, the updraft was passed; and he was pressed helplessly against the deck as the ship plunged upward again. As it slowed, he seized the back of his chair and tried to brace himself against the sickening motion. For a moment he was partially successful, and he dared to let go with one hand in order to reach once more for the controls. As he touched them, there was a violent sideward lurch; and his hand, instead of striking the toggle controlling the altitude mechanism, opened the bar switch handling the sensation currents from the attitude gyros on the automatic pilot.
The ship could not have been out of control more than three or four minutes altogether; but those minutes were more than enough. Without the gyros, she no longer held an even keel, but pitched, yawed, rolled completely over again and again, still striving to follow the dictates of the altitude control. That barometer was sensitive enough for control in the upper stratosphere of planets like Earth and Thanno; and in the tremendous pressure changes accompanying turbulence in Hekla’s dense atmosphere the little device went mad. Vickers, dazed and bleeding, bouncing from floor to ceiling and wall to wall of the control room, finally managed to hold on to the board long enough and firmly enough to set the selector at zero pressure. Still bucking and rolling, the ship went shooting upwards, and at last broke out into the crimson sunlight---more than thirty kilometers above the ocean, if the radio altimeter could be believed. The air was calmer here, and the ship quieted down enough for Vickers to level it by manual control, reset the toppled attitude gyros, and cut them in again.
With a steady deck once more under his feet, he staggered back to the library where Rodin had been working. The meteorologist had taken a beating, but had suffered less damage than Vickers, owing chiefly to the fact that the library furniture was for the most part heavily upholstered. He made acrid inquiry into the cause of the disturbance, and was not particularly sympathetic with Vickers’ injuries. They went forward to the control room together, and Vickers gazed through the port at the innocent-looking, fluffy pink mass below them while Rodin applied antiseptic and dressings to his contusions. When he had finished this job, the meteorologist began to observe, too.
Vickers had halted the ship when he had regained control, and they were hanging motionless above the wall of vapor. They were still in sight of the edge where they had entered it; and when Rodin set the ship in motion again, they ran within a few minutes into an almost equally sharp termination on the other side. The front was only thirty or forty miles wide; and this, together with the altitude of the cumulus barrier, indicated a frontal slope that made Rodin whistle. Then he stopped to think; and the more he thought the less he was able to understand how a mass of cold air of such size and, apparently, extreme low temperature could have wandered so far from the pole in midsummer. Then he remembered the violence which had resulted from a very slight temperature change, during the warm front he had watched at Observatory Hill; and he took the ship down on the cold side of the front to the altitude at which they had been flying when they ran into trouble, and compared temperatures. The difference was not great, but it was far greater than had been the case on the other occasion; and considering the density and other peculiarities of Hekla’s atmosphere, it could account for such a violent front. It remained to account for the air mass. Rodin began to think out loud, as he considered this problem.
“This stuff appears to be of polar continental origin, judging by its temperature and dryness,” he said. “It’s not extremely cold, but in Hekla’s atmosphere it could still have formed over the polar ice cap, and probably did. On Earth, such a mass couldn’t come anything like this far south in summer. The normal surface circulation is too strong for it, and remains too strong as long as the ground is receiving much solar energy. However, it could be forced down like this if we supposed another, still colder, mass to the east of its source region, against which it was carried by the normal trade circulation and thence deflected southward. Also, a general cooling of the continental areas to the south of the source region might permit it to be carried down here around a normal cyclone.