"And you think there can't be more than five or six hours before the flow arrives?" "I'd say that was a very optimistic guess. I'll stop and get a better speed estimate if you want, but won't swear to it." Rowson thought for a moment. "No," he said finally, "don't bother. Get back here as soon as you can. We need the tractor and human muscles more than we need even expert guesses." He turned to the operator. "Zaino, tell all the tractors there'll be no answer from the ship for a while, because no one will be aboard. Then suit up and come outside." He was gone. * * * * * Ten minutes later, six human beings and a tractor were assembled in the flame-lit near-darkness outside the ship. The cloud had spread to the horizon, and the sun was gone. Burkett and Hargedon had arrived, but Rowson wasted no time on congratulations. "We have work to do. It will be easy enough to keep the lava from the ship, since there seems to be a foot or more of ash on the ground and a touch of main drive would push it into a ringwall around us; but that's not the main problem. We have to keep it from reaching the chasm anywhere south of us, since that's the way the others will be coming. If they're cut off, they're dead. It will be brute work. We'll use the tractor any way we can think of. Unfortunately it has no plow attachment, and I can't think of anything aboard which could be turned into one. You have shovels, such as they are. The ash is light, especially here, but there's a mile and a half of dam to be built. I don't see how it can possibly be done ... but it's going to be." "Come on, Arnie! You're young and strong," came the voice of the mineralogist. "You should be able to lift as much of this stuff as I can. I understand you were lucky enough to get hold of Eileen--have you asked for the bonus yet?--but your work isn't done." "It wasn't luck," Zaino retorted. Burkett, in spite of her voice, seemed much less of a schoolmistress when encased in a spacesuit and carrying a shovel, so he was able to talk back to her. "I was simply alert enough to make use of existing conditions, which I had to observe for myself in spite of all the scientists around. I'm charging the achievement to my regular salary. I saw--" He stopped suddenly, both with tongue and shovel. Then, "Captain!" "What is it?" "The only reason we're starting this wall here is to keep well ahead of the flow so we can work as long as possible, isn't it?" "Yes, I suppose so. I never thought of trying anywhere else. The valley would mean a much shorter dam, but if the flow isn't through it by now it would be before we could get there--oh! Wait a minute!" "Yes, sir. You can put the main switch anywhere in a D. C. circuit. Where are the seismology stores we never had to use?"