His collarbone hurt, where Ann’s forehead or elbow had struck it. But they were flying, up and up in an awkward embrace. And as the boat was accelerated to something nearer the wind’s speed, the turbulence lessened greatly. The balloons seemed to be connected by rigging to the top of the mast. Then just when Sax was beginning to hope for some kind of zeppelinlike stability, even to expect it, the boat shot straight up and began its horrible tumbling again. Updraft no doubt. They were probably over land by now, and it was all too possible they were being sucked up into a thunderhead, like a hail ball. On Mars there were thunderheads ten kilometers tall, often powered by howlers from far to the south, and balls of hail flew up and down in these thunderheads for a long time. Sometimes hail the size of cannonballs had come crashing down, devastating crops and even killing people. And if they were pulled up too high they might die of altitude, like those early balloonists in France, was in the Montgolfiers themselves it had happened to? Sax couldn’t remember. Up and up, tearing through wind and red haze, no chance to see very far—

BOOM! He jumped and hurt himself against his seat belt, came down hard. Thunder. Thunder banging around them, at what had to be well over 130 decibels. Ann seemed limp against him, and he shifted sideways, reached up awkwardly and twisted her ear, trying to turn her head so he could see her face. “Hey!” she cried, though it sounded to him like a whisper in the roar of the wind. “Sorry,” he said, though he was sure she couldn’t hear him. It was too loud to talk. They were spinning again, but without much centrifugal force. The boat was shrieking as the wind pushed it up; then they dove, and his eardrums hurt to bursting, he wiggled his jaw back and forth, back and forth. Then up again and they popped, painfully. He wondered how high they would go; very possible they would die of thin air. Though maybe the Da Vinci techs had thought to pressurize the cockpit, who knew. It behooved him to try to understand the boat as blimp, or at least master the altitude adjustment system. Not that there was much to be done against the force of such updrafts and downdrafts. Sudden rattle of hail against the cockpit shell. There were small toggles on the emergency panel; in a moment of less violent tumbling he was able to put his face down near the bar and read the display terminal embedded in it. Altitude . . . not obvious. He tried to calculate how high the boat would go before its weight caused it to level off. Hard when he wasn’t actually sure of the boat’s weight, or the amount of helium deployed.