book of this kind we can decipher most anything. The keying on a volume of physical constants is perfect and almost infinite in number. What do they use for Pi?"

"Circle with a double dot inside."

"And Planck's Constant?"

"Haven't hit that one yet. But we will. But to get back to the meat of this thing, the third section deals with something strange. It seems to have a bearing on this gadget from the top of the tower. I'd say that the volume was a technical volume on the construction, maintenance, and repair of the tower and its functions—whatever they are."

Barney spread the volume out for Jim to see. "That dingbat is some sort of electronic device. Or, perhaps subelectronic. Peel away that rusted side and we'll look inside."

Jim peeled a six-inch section from the side of the big metal tube, and they inspected the insides. Barney looked thoughtful for a minute and then flipped the pages of the book until he came to a diagram.

"Sure," he said exultantly, "this is she. Look, Jim, they draw a cathode like this, and the grids are made with a series of fine parallel lines. Different, but more like the real grid than our symbol of a zig-zag line. The plate is a round circle instead of a square, but that's so clearly defined that it comes out automatically. Here's your annular electrodes, and the ... call 'em deflection plates. I think we can hook this do-boodle up as soon as we get to our place in Lincoln Head."

"Let's go then. Not only would I like to see this thing work, but I'd give anything to know what it's for!"

"You run the crate," said Barney, "and I'll try to decipher this mess into voltages for the electrode-supply and so on. Then we'll be in shape to go ahead and hook her up."

The trip to Lincoln Head took almost an hour. Barney and Jim landed in their landing yards and took the book and the searchlight-thing inside. They went to their laboratory, and called for sandwiches and tea. Jim's sister brought in the food a little later and found them tinkering with the big beam tube.

"What have you got this time?" she groaned.

"Name it and it's yours," laughed Barney.

"A sort of gadget that we found on the Red Desert."

"What does it do?" asked Christine Baler.

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