In reality, the left side of Johnny's glasses was an extremely powerful magnifying lens. His work often required a magnifier, so he wore one over his left eye, which was virtually useless because of an injury received in the World War.

"I can find nothing!" Johnny declared. "There's nothing unusual about the window!"

"I hope you're wrong," Doc said, sobriety in his wondrously modulated voice. "But you could not see the writing on that window, should there be any. The substance my father perfected for leaving secret messages was absolutely invisible. But it glows under ultra-violet light."

"You mean — " hairy Monk rumbled.

"That my father and I often left each other notes written on that window," Doc explained. "Watch!"

Doc crossed the room, a big, dynamic man, light on his feet as a kitten for all his size, and turned out the lights. He came back to the black-light box. His hand, supple despite its enormous tendons, clicked the switch that shot current into the apparatus.

Instantly, written words sprang out on the darkened windowpane. Glowing with a dazzling, electric blue, the effect of their sudden appearance was uncanny.

A split second later came a terrific report! A bullet knocked the glass into hundreds of fragments, wiping out the sparkling blue message before they could read it. The bullet passed entirely through the steel-plate inner door of the safe! It embedded in the safe back.

The room reeked silence. One second, two! Nobody had moved.

And then a new sound was heard. It was a low, mellow, trilling sound, like the song of some strange bird of the jungle, or the sound of the wind filtering through a jungled forest. It was melodious, though it had no tune; and it was inspiring, though it was not awesome.

The amazing sound had the peculiar quality of seeming to come from everywhere within the room rather than from a definite spot, as though permeated with an eerie essence of ventriloquism.

A purposeful calm settled over Doc Savage's five men as they heard that sound. Their breathing became less rapid, their brains more alert.

For this weird sound was part of Doc — a small, unconscious thing which he did in moments of utter concentration. To his friends it was both the cry of battle and the song of triumph. It would come upon his lips when a plan of action was being arranged, precoursing a master stroke which made all things certain.

It would come again in the midst of some struggle, when the odds were all against his men, when everything seemed lost. And with the sound, new strength would come to all, and the tide would always turn.

And again, it might come when some beleaguered member of the group, alone and attacked, had almost given up all hope of survival. Then that sound would filter through, some way, and the victim knew that help was at hand.

The whistling sound was a sign of Doc, and of safety, of victory.

"Who got it?" asked Johnny, and he could be heard settling his glasses more firmly on his bony nose.

"No one," said Doc. "Let us crawl, brothers, crawl. That was no ordinary rifle bullet, from the sound of it!"

At that instant, a second bullet crashed into the room. It came, not through the window, but through some inches of brick and mortar which comprised the wall! Plaster sprayed across the thick carpet.

Doc Savage was the last of the six to enter the adjoining room. But he was inside the room in less than ten seconds. They moved with amazing speed, these men.

Doc flashed across the big library. The speed with which he traversed the darkness, never disturbing an article of furniture, showed the marvelous development of his senses. No jungle cat could have done better.

Expensive binoculars reposed in a desk drawer, a highpower hunting rifle in a corner cabinet. In splits of seconds, Doc had these, and was at the window.

He watched, waited. No more shots followed the first two.

Four minutes, five, Doc bored into the night with the binoculars. He peered into every office window within range, and there were hundreds. He scrutinized the spidery framework of the observation tower atop the skyscraper under construction. Darkness packed the labyrinth of girders, and he could discern no trace of the bushwhacker.

"He's gone!" Doc concluded aloud.

No sound of movement followed his words. Then the window shade ran down loudly in the room where they had been shot at. The five men stiffened, then relaxed at Doc's low call, Doc had moved soundlessly to the shade and drawn it.

Doc was beside the safe, the lights turned on, when they entered.

The window glass had been clouted completely out of the sash. It lay in glistening chunks and spears on the luxuriant carpet.

The glowing message which had been on it seemed destroyed forever.

"Somebody was laying for me outside," Doc said, no worry at all in his well-developed voice. "They evidently couldn't get just the aim they wanted at me through the window. When we turned out the light to look at the writing on the window, they thought we were leaving the building. So they took a couple of shots for wild luck."

"Next time, Doc, suppose we have bulletproof glass in these windows!" Renny suggested, the humor in his voice belying his dour look.

"Sure," said Doc. "Next time! We're on the eighty-sixth floor, and it's quite common to be shot at here!"

Ham interposed a sarcastic snort. He bounced over, waspish, quick-moving, and nearly managed to thrust his slender arm through the hole the bullet had tunneled in the brick wall.

"Even if you put in bulletproof windows, you'd have to be blame careful to set in front of them!" he clipped dryly.

Doc was studying the hole in the safe door, noting particularly the angle at which the powerful bullet had entered. He opened the safe. The big bullet, almost intact, was embedded in the safe rear wall.

Renny ran a great arm into the safe, grasped the bullet with his fingers. His giant arm muscles corded as he tried to pull it out. The fist that could drive bodily through inch-thick planing with perfect ease was defied by the embedded metal slug.

"Whew!" snorted Renny. "That's a job for a drill and cold chisels."

Saying nothing, merely as if he wanted to see if the bullet was stuck as tightly as Renny said, Doc reached into the safe.

Great muscles popping up along his arm suddenly split his coat sleeve wide open. He glanced at the ruined sleeve ruefully, and brought his arm out of the safe. The bullet lay loosely in his palm.

Renny could not have looked more astounded had a spike-tailed devil hopped out of the safe. The expression on his puritanical face was ludicrous.

Doc weighed the bullet in his palm. The lids were drawn over his golden eyes. He seemed to be giving his marvelous brain every chance to work — and he was. He was guessing the weight of that bullet within a few grains, almost as accurately as a chemist's scale could weigh it.

"Seven hundred and fifty grains," he decided, "That makes it a .577 caliber Nitro-Express rifle. Probably the gun that fired that shot was a double-barreled rifle."

"How d'you figure that?" asked Ham. Possibly the most astute of Doc's five friends, Doc's reasoning nevertheless got away from even Ham.

"There were only two shots," Doc clarified. "Also, cartridges of this tremendous size are usually fired from double-barreled elephant rifles."

"Let's do somethin' about this!" boomed Monk. "The bushwhacker may get away while we're jawin'!"

"He's probably fled already, since I could locate no trace of him with the binoculars," Doc replied. "But we'll do something about it, right enough!"

With exactly four terse sentences, one each directed at Renny, Long Tom, Johnny, and Monk, Doc gave all the orders he needed to. He did not explain in detail what they were to do. That wasn't necessary. He merely gave them the idea of what he wanted, and they set to work and got it in short order. They were clever, these men of Doc's.