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're all three, Venus, Sol, and Venus Equilateral, at the corners of an equilateral triangle, and will forever remain, barring outside influences. So that means we're running around a common point, the common center of gravity--which can be construed to mean that we are circling Sol at Venus' speed, or twenty-one point seven five miles per second. Now our beam is curved because of the angular velocity, just like a swung hose. However, it hits the _Empress of Kolain_ at an angle as though we were a couple of thousand feet away. That's fine. But the reflected wave starts back at that angle, right back through the beam, remember?" "I get it!" shouted Don in glee. "Thirty-two seconds at twenty-one point seven miles per second gives us seven hundred and sixteen miles to the rear. Walt, get your mechanical gang to hitch us up a couple of mirrors--say a yard in diameter. Put 'em so that they can be used as a range finder. Set the angles for seven hundred and sixteen miles; a three-mile base line should do it, I'm sure; and then we'll shoot us a skeeter out there with a detector. Get carving!" "Shall Jim stop?" asked Walt. "How long will it take to rig us a range finder?" "Hour, God willing." "Jim, get a relief for a half-hour. We'll keep the beam centered. Then he can take over when the going gets critical again." * * * * * The mounting of two mirrors at either end of Venus Equilateral gave little trouble. It was the amount of detailed work that consumed the time. There were girders to be cut and welded together. The hundred-odd doorways that centered on the axis of Venus Equilateral had to be opened and the clear, light path had to be cleared of packing cases, supplies, and in a few cases machinery had to be partially dismantled to clear the way. A good portion of Venus Equilateral's personnel of three thousand were taken off of their jobs, haled out of bed for the emergency, or made to work through their play period, depending upon which shift they worked.