Carefully he eased the master reactor control to the “operate” position, and then even more gingerly started the motors forward. They responded properly; the treads on either side of him moved forward, and a clattering of small, hard objects against each other became audible for a moment. Then this ceased, and the treads began to race. Beetchermarlf instantly cut off the power, and crawled off the truck to see what had happened.
The plan had worked, just as a computer program with a logic error works — there is an answer forthcoming, but not the one desired. As the helmsman had planned, the treads had scuffed the rocks under them backward; but he had forgotten the effect of the pneumatic mattress above. The truck had settled under its own weight and the downward thrust of the gas pressure until the chassis between the treads had met the bottom. Looking up, Beetchermarlf could see the bulge in the mattress where the entire drive unit had been let down some four inches.
Takoorch appeared from his shelter and looked the situation over, but said nothing. There was nothing useful to say.
Neither of them could guess how much more give there was to the mattress, and how much further the truck would have to be let down before it would really hang free, though, of course, they knew the details of the Kwembly’s construction. The mattress was not a single gas bag but was divided into thirty separate cells, having two trucks in tandem attached to each. The helmsman knew the details of the attachment, of course — both had just spent many hours repairing the assemblies — but even the recent display of the Kwembly’s underside with the weight off nearly all the trucks left them very doubtful about how far any one of them could extend by itself.
“Well, back to the stone lugging,” remarked Takoorch as he worked his nippers under a pebble. Maybe these have been jarred loose now; but it’s going to be awkward, getting at them only from the ends.”
“There isn’t enough time for the job. The ice is still growing toward us, and we might have to get the treads a whole body-length deeper before they’d run free. Leave the trucks alone, Tak. We’ll have to try something else.”
“All I ask is to know what.”
Beetchermarlf showed him. Taking a light with him this time, he climbed once more to the top of the truck. Takoorch followed, mystified. The younger sailor reared up against the shaft which formed the swiveling support of the truck, and attacked the mattress with his knife.
“But you can’t hurt the ship!” Takoorch objected.
“We can fix it later. I don’t like it any better than you, and I’d gladly let the air out by the regular bleeder valve if we could reach it; but we can’t, if we don’t get the load off this truck very soon we won’t do it at all.” He continued slashing as he spoke.
It was little easier than moving the stones. The mattress fabric was extremely thick and tough; to support the Kwembly it had to hold in a pressure more than a hundred pounds per square inch. One of the nuisances of the long trips was the need to pump the cells up manually, or to bleed off the excess pressure, when the height of the ground they were traversing changed more than a few feet. At the moment the mattress was a little flat, since no pumping had been done after the run down the river, but the inner pressure was, of course, that much higher.
Again and again Beetchermarlf sliced at the same point on the taut-stretched surface. Each time the blade went just a little deeper. Takoorch, convinced at last of the necessity, joined him; the second blade’s path crossed that of the first, the two flashing alternately in a rhythm almost too fast for a human eye to follow — a human witness, had one been possible, would have expected them to sever each other’s nippers at any moment.
Even so, it took many minutes to get through. The first warning of success was a fine stream of bubbles which spread in all directions up the slope of the bulging gas cell. A few more slashes and then cross-shaped hole with its inch long arms was gushing Dhrawnian air in a flood of bubbles that made the work invisible. The prisoners ceased their efforts.
Slowly but visibly the stretched fabric was collapsing. The bubbles fled more slowly across its surface, gathering at the high point near the wall of ice. For a few moments Beetchermarlf thought the fabric would go entirely flat, but the weight of the suspended trick prevented that. The center of the cell — or at least, the point at which the truck was attached; neither of them knew just where the cell boundaries were — was straining downward, but it was now pull instead of push.
“I’ll started the engine again and see what happens,” said Beetchermarlf. “Get forward again for a minute.” Takoorch obeyed. The younger helmsman deliberately wedged a number of pebbles under the front ends of the treads, climbed the truck once more and settled between them. He had kept the light with him this time, not to help him but to make it easier to tell how and whether the unit moved. He looked at the point of attachment a few inches above him as he started the engine once more.
The pebbles had provided some traction; the fabric wrinkled and the swivel tilted slightly as the truck strained forward. An upper socket, inaccessible inside the cell, into which the shaft telescoped prevented the tilt from exceeding a few degrees — the trucks, of course, could not be allowed to touch each other — but the strain could be seen. AS the motion reached its limit the tracks continued moving, but this time they did not race free. Sound and tactile vibrations both indicated that they were slipping on pebbles, and after a few seconds the feel of swirling, eddying water became perceptible against Beetchermarlf’s airsuit. He started to climb down from the truck, and was nearly swept under one of the treads as he shifted grips; he barely stopped the motor in time with a hasty snatch at the control. He needed several seconds to regain his composure after that; even his resilient physique could hardly have survived being worked through the space between the treads and rocks. At the very least, his airsuit would have been ruined.
Then he took time to trace very carefully the control cords leading from the reactor to the upper guides along the bottom of the mattress, following them by eye to the point above the next truck forward where he could reach them. A few seconds later he was on top of the other truck, starting the motor up again from a safe distance and mentally kicking himself for not having done it that way from the beginning.
Takoorch reappeared beside him and remarked, “Well, we’ll soon know whether stirring water up does any warming.”
“It will,” replied Beetchermarlf. “Besides, the treads are rubbing against the stones on the bottom instead of kicking them out of the way this time. Whether or not you believe that stirring makes heat, you certainly know friction does. Watch the ice, or tell me if the neighborhood is getting too hot. I’m at the lowest power setting but that’s still a lot of energy.”
Takoorch rather pessimistically went over to a point where the cairn should be visible if it were ever freed of ice, and settled himself to wait. The currents weren’t too bad here, though he could feel them tugging at his not-too-well ballasted body. He anchored himself to a couple of medium sized rocks and stopped worrying about being washed under the treads.