snooping around. But once he had landed, he was safe—he could always claim to be peacefully exploring, hunting leapers or some such thing.

The radioactive had a half-life of about four days, which meant that it would be unsafe to approach for some three weeks—two at the minimum. That was time enough, when the Martian was boxed in so small an area.

There was no danger that he would try to cross it. The owlies had learned what radioactivity meant, back when they fought the humans. And their vision, extending well into the ultra-violet, made it directly visible to them through its fluorescence—to say nothing of the wholly unhuman extra senses they had. No, Kreega would try to hide, and perhaps to fight, and eventually he'd be cornered.

Still, there was no use taking chances. Riordan set a timer on the boat's radio. If he didn't come back within two weeks to turn it off, it would emit a signal which Wisby would hear, and he'd be rescued.

He checked his other equipment. He had an airsuit designed for Martian conditions, with a small pump operated by a power-beam from the boat to compress the atmosphere sufficiently for him to breathe it. The same unit recovered enough water from his breath so that the weight of supplies for several days was, in Martian gravity, not too great for him to bear. He had a .45 rifle built to shoot in Martian air, that was heavy enough for his purposes. And, of course, compass and binoculars and sleeping bag. Pretty light equipment, but he preferred a minimum anyway.

For ultimate emergencies there was the little tank of suspensine. By turning a valve, he could release it into his air system. The gas didn't exactly induce suspended animation, but it paralyzed efferent nerves and slowed the overall metabolism to a point where a man could live for weeks on one lungful of air. It was useful in surgery, and had saved the life of more than one interplanetary explorer whose oxygen system went awry. But Riordan didn't expect to have to use it. He certainly hoped he wouldn't. It would be tedious to lie fully conscious for days waiting for the automatic signal to call Wisby.

He stepped out of the boat and locked it. No danger that the owlie would break in if he should double back; it would take tordenite to crack that hull.

He whistled to his animals. They were native beasts, long ago domesticated by the Martians and later by man. The rockhound was like a gaunt wolf, but huge-breasted and feathered, a tracker as good as any Terrestrial bloodhound. The "hawk" had less resemblance to its counterpart of Earth: it was a bird of prey, but in the tenuous atmosphere it needed a six-foot wingspread to lift its small body. Riordan was pleased with their training.

The hound bayed, a low quavering note which would have been muffled almost to inaudibility by the thin air and the man's plastic helmet had the suit not included microphones and amplifiers. It circled, sniffing, while the hawk rose into the alien sky.

Riordan did not look closely at the tower. It was a crumbling stump atop a rusty hill, unhuman and grotesque. Once, perhaps ten thousand years ago, the Martians had had a civilization of sorts, cities and agriculture and a neolithic technology. But according to their own traditions they had achieved a union or symbiosis with the wild life of the planet and had abandoned such mechanical aids as unnecessary. Riordan snorted.

The hound bayed again. The noise seemed to hang eerily in the still, cold air; to shiver from cliff and crag and die reluctantly under the enormous silence. But it was a bugle call, a haughty challenge to a world grown old—stand aside, make way, here comes the conqueror!

The animal suddenly loped forward. He had a scent. Riordan swung into a long, easy low-gravity stride. His eyes gleamed like green ice. The hunt was begun!

reath