dashed him backward to the ground.

"Help!" screamed the spy. With a wild motion he hurled the package from him in a long arch.

Bang!

But the explosion was the crack of a detonating cap, not the thunderous roar of a heavy charge of powder.

Major Coggleston lunged forward. His fists beat a meaty tattoo on the spy's face.

The next instant the crackle of military commands and the thud of footsteps burst upon them. The four—Professor Paulsen, Major Coggleston and the spy, in a heap on the ground; and Henry Horn, wide-eyed and trembling, standing near at hand—were illumined in a powerful flashlight's beam. Half a dozen soldiers rushed up.

"Major! We heard that shot! Are you all right?"

The officer struggled to his feet, trying hard to preserve the dignity of his rank despite his nudity. In the light of the flash he looked even more than before like an overgrown kewpie doll.

"Of course I'm all right!" he puffed. "What's more, that red-headed rat on the ground is the spy and murderer we've been looking for. Take him away, men!"

He turned to Professor Paulsen.

"Joe, this is one time I don't know what to say. If it hadn't been for you that devil would have made a clean getaway."

"Forget it," retorted the gaunt scientist. "It's little enough I can do for my country at my age."

"Honestly, Joseph, I can't see how you got the nerve to do it!" marveled Henry, still wide-eyed. "Just think, we might all have been killed—"

The professor glared.

"What do you mean, we might all have been killed?"

"Why, the explosive in that package, and the detonator—really, Joseph, it was terribly dangerous—"

"Dangerous!" snorted the savant. "The only dangerous part was that he might have hit me over the head with it."

"But—the explosive—"

"Explosive, my eye!" And, again glaring: "Do you mean to tell me you can't understand why that stuff he had in the package didn't go off, you abbreviated atom?"

Henry's goatee waggled uncertainly. He adjusted the steel-rimmed spectacles which were his only garment.

"Well ... really, Joseph...."

"I'll admit right out I don't get it," broke in Major Coggleston. "You mean there wasn't any danger of that stuff going off?"

"Of course not." Professor Paulsen was distinctly snappish.

"But why—"

The scientist turned back to Henry. "Don't you remember what I said to you this morning about those devil's glasses of yours transposing letters instead of just reversing them? And that you told me it would take a special lens to straighten them out?"

"You mean—"

"Take any formula and transpose the symbols all the way through, and see what you get. Trinitrocresol, for instance. The formula is C₇H₅N₃O₇. Transpose it all the way through, and you have ₇O₃N₅H₇C. In that particular case, it wouldn't even make sense. But when our red-headed spy said he was a chemist and hadn't had any trouble compounding this new explosive, I figured the formula must be one that would be at least half-way logical, no matter which way you wrote it. Only the odds were a million to one that one way it would equal an explosive; the other, nothing at all. So I didn't hesitate to attack him."

"Joe," said Major Coggleston admiringly, "that's a lot faster thinking than I've ever done. And I don't need to tell you how grateful the Army will be."

"Really, Joseph, it was awfully clever!" Henry chimed in. "I'd never have thought of it—"

And then, changing thought in mid-sentence:

"Look! There's that pretty blonde girl with the—"

"Henry!" exploded Professor Paulsen. "You're old enough to behave like a grown man, not an inspectionistic schoolboy!" His hand shot out to grip his little partner's goatee and jerk his eyes from the luscious creature now parading her charms before them.

"Ouch!" squealed