Jim began to make the beam roam around the invisible spot in the sky. He swept the beam in microscopic scans, up and down, and advancing the beam by one-half of its apparent width at the receiver for each sweep.
Two more hours went by. The receiver was still silent of reflected signals.
It was a terrific strain, this necessary wait of approximately a half-hour between each minor adjustment and the subsequent knowledge of failure. Jim gave up the 'scope because of eyestrain, and though Don and Walt had confidence that the beam-control man was competent to use the cross-ruled screen to keep Mars on the beam, Jim was none too sure of himself, and so he kept checking the screen against the 'scope.
At the end of the next hour of abject failure, Walt Franks began to scribble on a pad of paper. Don came over to peer over Franks' shoulder, and because he couldn't read Walt's mind, he was forced to ask what the engineer was calculating.
"I've been thinking," said Franks.
"Beginner's luck?" asked Don with a wry smile.
"I hope not. Look, Don, we're moving on the orbit of Venus, at Venus' orbital velocity. Oh, all right, say it scientifical: We are circling Sol at twenty-one point seven five miles per second. The reflected wave starts back right through the beam, remember?"
"I get it," shouted Channing in glee. "Thirty-two minutes' transmission time at twenty-one point seven five miles per second gives us—ah—"
Walt looked up from his slide rule. "Fifty-two thousand, two hundred and twenty-four miles," he said.
"Just what I was about to say," grinned Don.