“Now,” he said. “Here’s a drawing of the green arrow. Twenty-one green lights make the arrow. Thirteen in a row,” he pointed out, “two here, two there, and two more on each side at the other end. The last eight blink all at the same time, but the thirteen—only one at a time. By their blinking they are conveying messages. But what do they say? Here’s a set of papers with records of their blinking, all marked with numbers. If you can work that out, you go to the head of the class!”
“I see. Easy as that!” Mildred laughed, and promptly seated herself across from him.
After that, save for the lazy hum of bees or the sudden whir of humming birds’ wings, there was silence in the place....
Suddenly the girl sprang up. “Why, I—I’ve got it!” she cried, excitedly.
“Just like that!” Johnny smiled.
“Well, I certainly have! Listen! This is what that first message says:
“Keep a sharp lookout. There are counter-spies afloat.”
“WHAT! Gee willikens!” Johnny gazed at her, truly amazed. “How could you make it read like that?”
“Because that’s the way it does read!” she raced on. “It’s really easy. There are twenty-six letters in the alphabet. Having thirteen lights suggests that they have split that twenty-six in two. Each light must stand for two letters. But the question is—which two? Well, the top thirteen stand for A, B, C, etc. But what about the bottom ones?
“The simplest way,” she leaned forward, smiling, “would be to put the last thirteen letters under the first thirteen! Then, blinking one light for two letters, let the fellow receiving the message see which of the two letters makes sense.