"To-morrow," continued Captain Hardy, "we will get a piece of transparent celluloid and make a disc like their own. We can ink in the circles and the radius lines and our disc will be almost a duplicate of theirs, except that our disc will be solid while their discs have open spaces between the circles. But that is only a detail. We can read their cipher as well as if we had one of their own discs."
"Wait," cried Willie, as his comrades started to cheer again. "What is the scratch on the milled edge of the dollar for?"
"That," replied Captain Hardy, "is to indicate how the disc is to be placed on the dollar. That scratch is exactly between the Z and the A sectors. It shows where the alphabet begins. Now we have their entire secret."
CHAPTER XV
ANOTHER MYSTERY UNRAVELED
A piece of transparent celluloid, furnished by their host from a broken side curtain of his automobile, supplied Captain Hardy with the material needed for making the disc that was to be the key to future communications of the enemy. Carefully he cut the celluloid the size of a dollar, then marked the exact centre of it. Next he clamped the disc on the captured coin. Between the rows of letters he scratched in the straight radius lines—the spokes of a wheel. Then Captain Hardy put the end of one arm of his dividers in the dot at the centre of his disc, and swept the other arm around, scratching a circle just outside the first letter in the message—the innermost T. Examination showed that this circle fell just inside of the second letter in the message—the W. Adjusting his calipers, the draughtsman made a second circle, just outside of this second letter. A third circle fell between the first O and the second T of the message. So Captain Hardy continued, each succeeding circle falling just outside of the succeeding letter in the message. When he had finished, his disc contained twenty-three concentric circles, between which could plainly be seen the bright dots or scratches in the dingy dollar.
"Whew!" said Captain Hardy, as he laid down his dividers. "That's pretty fine work—twenty-three circles within a space of an inch and a half. I'll wager a watchmaker made their pattern for them. The solid parts of their metal discs can't be much larger than these lines I have scratched on the celluloid. You were right when you named it, Willie. The parts of it must be just about as thick as a spider-web."
The boys passed the dollar and its superimposed disc from hand to hand, examining them with eager interest.
"Suppose they wanted to send a message with more than twenty-four letters in it," said Roy. "How could they do it? I'm sure some of the messages we intercepted had more than twenty-four letters in them."