cause. The grid-guard meant to allow no landing. He'd threatened to blow out their controls if they tried to use the grid on the Med ship, but they wanted it ready for use as a weapon against the space fleet. They couldn't use it against him. He couldn't damage it unless they tried. They wanted him away.

He went back to his work. From time to time, annoyedly, he looked up at the outside. Presently a young-warrior group moved toward the ship, carrying something very heavy. A larger charge of explosive, perhaps.

He waited until they were within yards of the ship. He stabbed the emergency-rocket button. A thin, pencillike rod of flame shot downward between the landing-fins. It was blue-white—the white of a sun's surface. For one instant it splashed out hungrily before it bored and melted a hole into the ground itself into which to flow. But in that instant it had ignited the covering of the burden the youths carried. They dropped it and fled. The pencil flame bored deeper and deeper into the ground. Clouds of smoke and steam arose.

There was a lurid flash. The burden that the young warriors had abandoned, vanished in a flare that looked like a lightning bolt. The ship quivered from the detonation. A crater appeared where the explosive had been.

Calhoun cut off the emergency rocket, which had burned for ten seconds at one-quarter thrust.

Sunset came and night fell for the second time. He noticed, abruptly, that some of the ground-cars from about the control building went racing away. But they did not pass close to the Med ship in their departure. He labored on. He'd spent nearly thirty hours making cultures from the specimens swabbed from children's throats, and injecting Murgatroyd, and waiting for his reaction, and then separating a tiny quantity of antibody—which would not total more than the dust from a butterfly's wing—from the serum he obtained.

Now he worked on, through the night. Far away—some tens or scores of millions of miles—the hospital ships of the Phaedrian fleet took off from the next outward planet. They would be coming at full speed toward Canis III. They would need the results of the work Calhoun was doing, if they were to prevent an appalling multiple plague which could wipe out all the sacrifice the building of the colony had entailed. But his work had to be exact.

It was tedious. It was exacting. It was exhaustingly time-consuming. He did have the help of previous experience, and the knowledge that 

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