Part 15 (1/2)

We'll soon see if you are right. Make me another disc.”

With a pocket rule he measured the diameter of the dollar.

”Practically an inch and a half,” he announced, putting down the figures 1.5 on paper. He multiplied those figures by 3.1416.

”That,” said he, pointing to the resulting figures, 4.71+, ”represents the circ.u.mference of a dollar. Now we'll divide the circ.u.mference by 26, the number of letters in the alphabet.”

He performed the division. ”Eighteen one-hundredths of an inch,” he announced. ”That's practically a scant fifth of an inch. We'll call it so, anyhow,” he continued as he marked off the s.p.a.ce on a sheet of paper with his rule. ”Each sector,” he said, ”gets exactly that amount of s.p.a.ce on the circ.u.mference.”

He pulled open the drawer of the desk and began to rummage through a tray full of pens, pencils, and other drawing materials. ”I wonder if there is such a thing as a pair of dividers here,” he remarked. And a moment later he exclaimed ”Good!” and drew forth the compa.s.ses he was looking for.

He set his dividers according to the s.p.a.ce he had marked off with his rule, then proceeded to divide the circ.u.mference of the new paper disc.

When he had gone completely round the disc, he seized pencil and ruler and began to draw lines from centre to circ.u.mference--the spokes of his wheel--each spoke running from the dot in the centre to one of the points indicated by the dividers. When he had finished, the disc was divided into twenty-six equal sectors, like tiny pieces of a pie.

”We shall soon know whether you are right or not in your guess, Roy,”

said Captain Hardy.

He laid the dollar beside the disc and began to copy on the disc the marks on the dollar. ”We'll put four marks in this sector,” he said, making four dots with his pencil. ”They are like those four scratches here,” and he pointed to the four marks in a row on the dollar. ”They must be four T's. At any rate we'll call this the T sector. On the dollar you notice this row of three scratches--the next sector to the left of the T sector. You remember we had three O's, three A's, and three S's. These three scratches must, therefore, be O's, A's, or S's.

Since they are next to the T's, they are doubtless S's. I'll mark the sector so anyway. That gives us the T sector and the S sector. If we are on the right track, then the sector to the left of the S s.p.a.ce is the R sector, and so on. I'll mark the disc that way, anyhow.”

Slowly he turned the disc around, putting a letter at the bottom of each sector. When he had finished, he had completed the alphabet.

About him cl.u.s.tered his four comrades, too deeply interested to speak.

They hardly even breathed.

”Take this paper, Roy,” said Captain Hardy, ”and tell me how many times each letter in the message appears.”

Roy took the paper on which Captain Hardy had made his numerical enumeration. ”Three A's,” he said.

Captain Hardy made three marks in the A sector.

”No B's, no C's, and two D's.”

The D's were scored. So they went through the alphabet. When they were done, the markings on the disc were practically a duplicate of those on the dollar, for Captain Hardy studied the dollar each time before marking the paper disc.

”That's it,” cried Willie. ”That's it exactly.”

”It's right so far as it goes, Willie,” said their leader, ”but we haven't all of it yet. Suppose I hand you a disc with four T's, three S's, two Z's, three L's, and so on. Could you make a message out of it?”

Willie studied the disc on the desk. ”No,” he said, ”I couldn't. I shouldn't know how to arrange the letters to make words out of them.”

”Neither would anybody else,” continued Captain Hardy. ”Those spies have some way of knowing how to tell the order in which to read these letters.”

For some time he sat studying the scratches on the dollar. The four boys were quiet as mice, each trying to solve the problem that stood between them and complete mastery of the cipher.

”You said that the metal disc resembled a spider's web,” began Captain Hardy, talking more to himself than to the boys. ”We know what the straight lines--the spokes--are for. The concentric circles must be to indicate the order of the letters. Let me see.” Again he studied the dollar closely. ”Some of these marks are near the centre of the disc, some half-way between centre and circ.u.mference, and some close to the outer edge. I believe the secret lies there.”

”Listen!” cried Willie of a sudden. ”When a spider spins a web, she begins at the centre and works outward. Maybe these spies write their messages in the same way.”