They hadn't heard the soft purr of an expensive automobile on the roadway beyond the fence, at the foot of the slope. But now the sounds of a brief, angry argument at the gate, some hundred yards away, drew their startled, nervous attention. With so much that was unknown and unhintable pending, this was hardly the time to receive visitors of any kind, certainly not hostile visitors with ideas of their own. Uneasily, Wilcox and Murgatroyd turned to face a group of people hurrying toward them across the intervening area of the fenced enclosure. One was a trusted workman, left to guard the gate. But the others--there were four men and a girl--had been able to overrule the guard's refusal to admit them. Of the four men, three were burly, massive specimens with the scars of many combats marking their coarse features. The fourth was slender and bent, maybe fifty. His head was entirely bald, his cheeks had withered lines in them, and his squinted piggish eyes held a look of secretive, hungry searching. Murgatroyd and Wilcox had no trouble recognizing this uninvited guest, who clearly was the master mind of the intruding group. All the world knew Lyman Kerwin, whose colossal fortune had thrust dominance-seeking tentacles into most of the key industries of America. Path of Progress, Rod's and Perry's outfit, had tangled with him once. They'd taken newsreel pictures of the collapse of one of the gigantic but poorly constructed power and irrigation dams which he had built in one of the western states. Hundreds of people had been killed, and thousands had been rendered homeless by a disaster traceable to materials and workmanship far less costly than specified. Only Kerwin's money, fixing a corrupt court, had enabled him to escape the consequences of criminal misrepresentation. Seeing Kerwin, and the inquiring speculative glances he cast about the enclosure, Doctor Murgatroyd's pointed red face suddenly darkened with fury, chagrin, and something like a nameless, nervous panic. "Thunder of Jupiter!" he whispered hoarsely. "That polecat would have to barge in now--now, of all times! We might have known it, Perry! But you just wait till I sail into him! The dirty--" Perry silenced the old scientist with a poke in the ribs. "You keep still," he ordered. "Just make believe you're bossing the drill crew." * * * * *