Stern was not long in carrying out his plan.
Even before Frumnos had returned, with the seventeen men still able to bear arms, he was at work.
In Cliff Villa he hastily lashed up half a dozen fireballs, of coarse cloth, thoroughly soaked them in oil, and, with a blazing torch, brought them out to the terrace. Old Gesafam, at his command, bolted the door behind him. At all hazards, Beta and the child must be protected from any possibility of peril.
"Here, Frumnos!" cried Stern.
"Yes, master?"
"Run quickly! Fetch the strongest bow in the colony and many arrows!"
"I go, master!"
Once more the man departed, running.
"Gad! If I only had my oxygen-containing bullets ready!" thought Stern, his mind reverting to an unfinished experiment down there in his laboratory in the Rapids power-house. "They would turn the trick, sure enough! They'd burst and rain fire everywhere. But they aren't ready yet; and even if they were, nobody could venture down there now!"
For already, plainly visible on the farther edge of the canyon, scores and hundreds of the hideous little beast-men were beginning to swarm. Their cries, despite the contrary stiff wind, carried across the river; and here and there a dart broke against the cliff.
Already a few of the Anthropoids were beginning to scramble down the opposite wall of stone.
"Men!" cried Allan commandingly, "not one of those creatures must ever reach this terrace! Take good aim. Waste no single shot. Every bullet must do its work!"
Choosing six of the best marksmen, he stationed them along the parapet with rifles. The firing began at once.
Irregularly the shots barked from the line of sharpshooters; and the little stabs of smoke, drifting out across the river, blent in a thin blue haze. Every moment or two, one of the Horde would writhe, scream, fall--or hang there twitching, to the cliff, with terrible, wild yells.
Stern greeted the return of Frumuos with eagerness.
"Here!" he exclaimed, scattering the arrows among half a dozen men. "Bind these fireballs fast to the arrowheads!"
He dealt out cord. In a moment the task was done.
"Sivad!" he called a man by name. "You, the best bowman of all! Here quickly!"
Even as Sivad fitted the first arrow to the string, and Stern was about to apply the torch, a rattling crash from above caused all to cringe and leap aside.
Down, leaping, ricochetting, thundering, hurtled a great boulder, spurning the cliff-face with a tremendous uproar.
It struck the parapet like a thirteen-inch shell, smashed out two yards of wall, and vanished in the depths. And after it, sliding, rattling and bouncing down, followed a rain of pebbles, fragments and detritus.