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?"

Four minutes later the tractor set out from the Albireo, carrying Rowson and Zaino. Six minutes after that it stopped at the base of the ash cone which formed the north side of the valley from which the lava was coming. They parked a quarter of the way around the cone's base from the emerging flood and started to climb on foot, both carrying burdens.

Forty-seven minutes later they returned empty-handed to the vehicle, to find that it had been engulfed by the spreading liquid.

With noticeable haste they floundered through the loose ash a few yards above the base until they had outdistanced the glowing menace, descended and started back across the plain to where they knew the ship to be, though she was invisible through the falling detritus. Once they had to detour around a crack. Once they encountered one which widened toward the chasm on their right, and they knew a detour would be impossible. Leaping it seemed impossible, too, but they did it. Thirty seconds after this, forty minutes after finding the tractor destroyed, the landscape was bathed in a magnesium-white glare as the two one-and-a-half kiloton charges planted just inside the crater rim let go.

"Should we go back and see if it worked?" asked Zaino.

"What's the use? The only other charges we had were in the tractor. Thank goodness they were nuclear instead of H. E. If it didn't work we'd have more trouble to get back than we're having now."

"If it didn't work, is there any point in going back?"

"Stop quibbling and keep walking. Dr. Burkett, are you listening?"

"Yes, Captain."

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