There's a small instrument board set on one of the posts. Turn the upper of the two dials until the hand of the meter beside it moves up to 2700 exactly. Wait a moment, until you're sure you have the exact reading. Then turn the second dial until the two red lines coincide, and as you do so, mark the time. The thing is set to operate the reverse cycle at three-hour intervals exactly. When you come down, you'll start a new cycle, and it might be important for us to know at just what minute we can get back to our own plane. If you decide to try it, tell Perrin to do nothing for at least a week. If the law started experimenting on this equipment, we never could climb back. And leave word with them for Hope; tell her I'll scramble out somehow--that _we_ will, if you decide to try your luck. Vic Underneath, in Hope's clear, purposeful hand, was this: Peter dear: Not knowing when you'll arrive, I'm going on ahead. We must give Vic a hand--mustn't we? H. * * * * * Naturally, I didn't understand Vic's jargon about frequencies and light-rays, for I thought more about football than physics in college, but two things were clear to me. One was that Vic had plunged into some sort of wild experiment, and the other was that Hope had followed him. The rest didn't matter very much. "Perrin! Mr. Butler and Miss Hope are safe. Everything is explained in this note. You and Mrs. Perrin are to leave me here, and not disturb anything. Do nothing at all for at least a week. If we aren't all back here before that time ... take any action you see fit. Understand?" "No-no, sir. Where--" "You understand the orders, anyway. That's all that's necessary. Close the door--and keep it closed at least a week!" I glared at him, and Perrin closed the door. The apparatus Vic had mentioned was my first thought. It consisted primarily of four tall, slim posts, set in the form of a square, about a yard apart, and supported by heavy copper brackets mounted on a thick base of insulating material, and each post bore at its top, like a stalk with a single drooping flower, a deep, highly polished reflector, pointing inward and downward. The whole effect was not unlike the skeleton of a miniature skyscraper. I strode between two of the high, slim black pillars and glanced upward. All four of the reflectors seemed pointed directly at my face, and I could see that each held, not the bulb I had expected, but a crudely shaped blob of fused quartz. * * * * *