Renny, the engineer, picked a slide rule from the drawer of a desk, a pair of dividers, some paper, a length of string. He probed the angle at which the bullet had passed through the inner safe door, calculated expertly the slight amount the window had probably deflected it. In less than a minute, he had his string aligned from the safe to a spot midway in the window, and was sighting down it.

"Snap out of it, Long Tom!" he called impatiently.

"Just keep your shirt on!" Long Tom complained. He was doing his own share as rapidly as the engineer.

Long Tom had made a swift swing into the library and laboratory, collecting odds and ends of electrical material. With a couple of powerful light bulbs he unscrewed from sockets, some tin, a pocket mirror he borrowed from — of all people — Monk, Long Tom rigged an apparatus to project a thin, extremely powerful beam of light. He added a flashlight lens, and borrowed the magnifying half of Johhny's glasses before he got just the effect he desired.

Long Tom sighted his light beam down Renny's string, thus locating precisely in the gloomy mass of skyscrapers, the spot from whence the shots had come.

In the meantime, Johnny, with fingers and eye made expert by years of assembling bits of pottery from ancient ruins, and the bones of prehistoric monsters, was fitting the shattered windowpane together. A task that would have taken a layman hours, Johnny accomplished in minutes.

Johnny turned the black-light apparatus on the glass. The message in glowing blue sprang out. Intact!

Monk came waddling in from the laboratory. In the big furry hands that swung below his knees, he carried several bottles, tightly corked. They held a fluid of villainous color.

Monk, from the wealth of chemical formulas within his head, had compounded a gas with which to fight their opponents, should they succeed in cornering whoever had fired that shot. It was a gas that would instantly paralyze any one who inhaled it, but the effects were only temporary, and not harmful.

They all gathered around the table on which Johnny had assembled the fragments of glass. All but Renny, who was still calculating his angles. And as Doc flashed the light upon the glass, they read the message written there:

Important papers back of the red brick -

Before the message could mean anything to their minds, Renny shouted his discovery.

"It's from the observation tower, on that unfinished skyscraper," he cried. "That's where the shot came from — and the sharpshooter must still be somewhere up there!"

"Let's go!" Doc ordered, and the men surged out into the massive, shining corridor of the building, straight to the battery of elevators.

If they noticed that Doc tarried behind several seconds, none of them remarked the fact. Doc was always doing little things like that — little things that often turned out to have amazing consequences later.

The men piled into the opened elevator with a suddenness that startled the dozing operator. He wouldn't be able to sleep on the job the rest of the night!

With a whine like a lost pup, the cage sank.

Grimly silent, Doc and his five friends were a remarkable collection of men. They so impressed the elevator operator that he would have shot the lift past the first floor into the basement, had Doc not dropped a bronze, long-fingered hand on the control.

Doc led out through the lobby at a trot. A taxi was cocked in at the curb, driver dreaming over the wheel. Four of the six men piled into the machine. Doc and Renny rode the running board.

"Do a Barney Oldfield!" Doc directed the cab driver.

The hack jumped away from the curb as if stung.

Rain sheeted against Doc's strong, bronzed face, and his straight, close-lying bronze hair. An unusual fact was at once evident. Doc's bronze skin and bronze hair had the strange quality of seeming impervious to water. They didn't get appreciably wet; he shed water like the proverbial duck's back.

The streets were virtually deserted in this shopping region. Over toward the theater district, perhaps, there would be a crowd.

Brakes giving one long squawk, the taxi skidded sidewise to the curb and stopped. Doc and Renny were instantly running for the entrance of the new skyscraper. The four passengers came out of the cab door as if blown out. Ham still carried his plain black cane.

"My pay!" howled the taxi driver.

"Wait for us!" Doc flung back at him.

In the recently finished building lobby, Doc yelled for the watchman. He got no answer. He was puzzled. There should be one around.

They entered an elevator, sent it upward to the topmost floor. Still no watchman! They sprang up a staircase to where all construction but steel work ceased. There they found the watchmen.

The man, a big Irishman with cheeks so plump and red they looked like the halves of Christmas apples, was bound and gagged. He was indeed grateful when Doc turned him loose — but quite astounded. For Doc, not bothering with the knots, simply freed the Irishman by snapping the stout ropes with his fingers as easily as he would cords.

"Begorra, man!" muttered the Irishman. "'Tis not human yez can be, with a strength like that!"

"Who tied you up?" Doc asked compellingly. "What did he look like?"

"Faith, I dunno!" declared the son of Erin. "'Twas not a single look or a smell I got of him, except for one thing. The fingers of the man were red on the ends. Like he had dipped 'em in blood!"

On up into the wilderness of steel girders, the six men climbed. They left the Irishman behind, rubbing spots where the ropes had hurt him, and mumbling to himself about a man who broke ropes with his fingers, and another man who had red finger tips.

"This is about the right height!" said the gaunt Johnny, bounding at Doc's heels. "He was shooting from about here."

Johnny was hardly breathing rapidly. A tall, poorly looking man, Johnny nevertheless exceeded all the others, excepting Doc, in endurance. He had been known to go for three days and three nights steadily with only a slice of bread and a canteen of water.

Doc veered right. He had taken a flashlight from an inside pocket.

It was not like other flashlights, that one of Doc's. It employed no battery. A tiny, powerful generator, built into the handle and driven by a stout spring and clockwork, supplied the current. One twist of the flash handle would wind the spring and furnish light current for some minutes. A special receptacle held spare bulbs. There was not much chance of Doc's light playing out.

The flash spiked a white rod of luminance ahead. It picked up a workman's platform of heavy planks.

"The shot came from there!" Doc vouchsafed.

A steel girder, a few inches wide, slippery with moisture, offered a short cut to the platform. Doc ran along it, surefooted as a bronze spider on a web thread. His five men, knowing they would be flirting with death among the steel beams hundreds of feet below, decided to go around, and did it very carefully.

Doc had picked two empty cartridges off the platform, and was scrutinizing them when his five friends put relieved feet on the planks.

"A cannon!" Monk gulped, after one look at the great size of the cartridges.

"Not quite," Doc replied. "They are cartridges for the elephant rifle I told you about. And it was a double-barreled rifle the sniper used."

"What makes you so sure, Doc?" asked big, sober-faced Renny.

Doc pointed at the plank surface of the platform. Barely visible were two tiny marks, side by side. Now that Doc had called their attention to the marks, the others knew they had been made by the muzzle of a double-barreled elephant rifle rested for a moment on the boards.

"He was a short man," Doc added. "Shorter, even, than Long Tom, here. And much wider."

"Huh?" This was beyond even quick-thinking Ham.