"He went to this restaurant, sir?"

"He walked in, just before the place was surrounded, engaged a table, and ordered a sumptuous meal. He told the waiter his name, said he expected a friend to join him, walked into the wash-room—and vanished! Two minutes later Anstruther and his men were on the job. Von Kettler never came out of the wash-room, so far as anybody knows.

"In the midst of the hue and cry somebody pointed to the table that Von Kettler had engaged. There was a twenty-dollar bill upon it, and a scrap of paper reading: 'I've kept my word. Von K.'"

Colonel Stopford looked at Dick fixedly. "Rennell, we may be fools," he said, "but we realize what we're up against. It's a big thing, and we're going to need all our fighting grit to overcome it. You're one of the four men we're depending on. We're counting on you because of your record, and because of your degree in science at Heidelberg. The President wishes you to take charge of the whole Eastern Intelligence District, covering the entire south-eastern seaboard of the United States. You are to have complete freedom of action, and all civil, military, and naval officials have received instructions to co-operate with you."

"There goes Mrs. Wansleigh's ball," thought Dick, but he said nothing.

* * * * *

"We're not the hunters, Dick Rennell," went on Colonel Stopford. "We're hiding under cover, and I'm counting on you to turn the tables. They even know my office is here. I had a long distance call from Savannah this morning in mocking vein. They advised me to have the White House watched to-night. I warned the President, and we've posted guards all round it."

"They held the wire while you called up the President?" asked Dick.

"Damn it, no! They called me up from Scranton the instant he'd finished speaking. They have the power of the devil, Rennell, with that infernal invisibility invention of theirs. Rennell, we're fighting unknown forces. Who this Invisible Emperor is, we don't even know. But one thing we've found out. He has his headquarters somewhere in your district. Somewhere along the south Atlantic seaboard. The greater part of his activities emanate from there. But we're fighting in the dark. The clue, the master clue that will enable us to locate him—that's what we lack."

The sun had set, it was beginning to grow dark. Colonel Stopford switched on the electric lamp beside his desk.

"What have you to say, Rennell?" he asked; and Dick was aware that the two other men were regarding him attentively.

"It's evident," said Dick, "that Von Kettler possessed this means of invisibility in his cell, and wasn't detected. He simply slipped out when the guard came to fetch him."

"Invisibility? Yes! But invisible's not the same thing as transparent," cried Stopford. "These folks have operated in broad daylight. They're transparent, damn them! Not even a shadow! You know what I mean, Rennell! What I'm thinking of! That crazy man you were in touch with six months ago, who prophesied this! We turned him down! He showed me a watch and said the salvation of the world was inside the case! I thought him insane!"

* * * * *

"You mean Luke Evans, sir. That watch was his pocket model. He went off in a huff, saying the time would come when we'd want him and not be able to find him."

"But, damn him, he wanted to produce universal darkness, or some such nonsense, Rennell, and I told him that we wanted light, not darkness."

"It wasn't exactly that, sir." Colonel Stopford was a man of the old school: he had been an artillery officer in the Great War, and was characteristically impatient of new notions. Dick began carefully: "You'll remember, sir, old Evans claimed to have been the inventor of that shadow-breaking device that was stolen from him and sold in England."

"To a moving picture company!" snorted Stopford. "I asked him what moving pictures had to do with war."

"Evans was convinced that the invention would be applied to war. He claimed that it made the modern methods of military camouflage out of date completely. He said that by destroying shadows one could produce invisibility, since visibility consists in the refraction of wave lengths by material objects.

"When they stole his invention, he foresaw that it would be used in war. He set to work to nullify his own invention. He told me that he had unintentionally given to the enemies of the United States a means of bringing us to our knees, since he believed that British motion picture company was actually a subsidiary of Krupp's. He worked out a method of counteracting it."

"You must get him, Rennell. Even if it's all nonsense, we can't afford to let any chance go. If Evans's invention will counteract this damned invisibility business—"

The telephone on the Colonel's desk rang. He picked it up, and his face assumed an expression of incredulity. He looked about him, like a man bewildered. He beckoned to the police official, who hurried to his side, and thrust the receiver into his hand. The official listened.

"All right," he said. He turned to Dick and the Civil Service representative.

"Gentlemen," he said, "the President has disappeared from his office in the White House, and there are grave fears that he has been kidnapped!"

CHAPTER III

In the White House

Colonel Stopford's car had been parked around the corner of the building, and within a minute the four men were inside it, Stopford at the wheel, and racing in the direction of the White House. A nod to the guard at the gate, and they were inside the grounds. At the entrance a single guard, in place of the four who should have been posted there, challenged sharply, and attempted to bar the way, not recognizing Dick or Stopford in their civilian clothes.

"Where's your officer?" demanded Stopford sharply.

Half-cowed by the Colonel's manner, the young recruit hesitated, and the four swept him out of the way and hurried on. The scene outside the main entrance to the White House was one of indescribable confusion. Soldiers were swarming in confused groups, some trying to force an entrance, others pouring out. Every moment civilians, streaming over the lawn, added to the number. Discipline seemed almost abandoned. From inside the building came outbursts of screams and cursing, the scuffling of a mob.

"Roscoe! Roscoe!" shouted Stopford. "Where's the President's secretary? Who's seen him? Let us pass immediately!"

No one paid the least attention to him. But a short, bare-headed civilian, who was struggling in the crowd, heard, and shouted in answer, waved his arms, and began to force his way toward the four. It was Roscoe, the secretary of President Hargreaves. The President was a childless widower, and Roscoe lived in the White House with him and was intimately in his confidence.

Roscoe gained Stopford's side. "Say—they've got him!" he panted. "They've got him somewhere—inside the building. They're trying to get him out! We've got to save him—but we can't see them—or him. They've made him invisible too, curse them! I heard him crying, 'Help me, Roscoe!' He saw me, I tell you—and I didn't know where he was!"

* * * * *

The little secretary was almost incoherent with fear and anger. The five men, forming a wedge, hurled themselves forward. Out of the White House entrance appeared a tall officer, revolver in hand. It was Colonel Simpson, of the President's staff. Half beside himself, he swept the weapon menacingly about him, shouting incoherently, and clearing a passage, into which the five hurled themselves.

Stopford seized his revolver hand, and after a brief struggle Simpson recognized him.

"He's in the building!" he shouted wildly. "Somewhere upstairs! I'm trying to form a cordon, but this damned mob's in the way. Kick those civilians out!" he cried to the soldiers. "Shoot them if they don't go! Guard the windows!"

Stopford and Dick, at the head of the wedge, pushed past into the White House. The interior was packed, men were struggling frantically on the staircase; it seemed hopeless to try to do anything.

Suddenly renewed yells sounded from above, a scream of anguish, howls of terror. There came a downward surge, then a forward and upward one, which carried the two men up the stairs and into the President's private apartments above.

In the large reception-room a mob was struggling at a window, beneath a blaze of electric light. A soldier was standing there like a statue, his face fixed with a leer of horror. In his hands was a rifle, with a blood-stained bayonet, dripping upon the hardwood floor at the edge of the rug. Upon the rug itself a stream of blood was spouting out of the air.

Dick looked at the sight and choked. There was something appalling in the sight: it was the quintessence of horror, that widening pool of blood, staining the rug, and flowing from an invisible body that writhed and twisted, while moans of anguish came from unseen lips.

Colonel Stopford leaped back, livid and staring. "God, it's got eyes—two eyes!" he shouted.

Dick saw them too. The eyes, which alone were visible, were about six inches from the floor, and they were appearing and disappearing, as they opened and shut alternately. It was a man lying there, a dying man, pierced by the soldier's bayonet by pure accident, dying and yet invisible.

* * * * *

The mob had scattered with shrieks of terror, but a few bolder spirits remained in a thin circle about that fearful thing on the rug. Dick bent over the man, and felt the outlines of the writhing body. It was a man, apparently dressed in some sort of uniform, but this was covered, from the top of the head to the feet, with a sort of sheer silken garment, bifurcating below the waist, and resembling a cocoon. It seemed to appear and alternately to vanish.

Dick seized the filmy stuff in his fingers, rent it, and stripped it away. Yells of terror and amazement broke from the throats of all. Instantly the thin circle of spectators had become reinforced by a struggling mass of men.

The half-visible cocoon clung to Dick's body like spider webs. But the man who had been wearing it had sprung instantly into view beneath the cluster of electric lights. He was a fair-haired young fellow of about thirty years, his features white and set in the agony of death.

He was dressed in a trim uniform of black, with silver braid, and on his shoulders were the insignia of a lieutenant. He opened his eyes, blue as the skies, and stared about him. He seemed to understand what had happened to him.

"Dogs!" he muttered.

Shrieks of fury answered him. The mob surged toward him as if to grind his face to pieces under their feet—and then recoiled, mouthing and gibbering. But it was at Dick that they were looking, not at the dying man.

He raised himself upon one elbow with a mighty effort. "His Majesty the Invisible Emperor! Long be his reign triumphant!" he chanted. It was his last credo. The words broke from his lips accompanied by a torrent of red foam. His head dropped back, his body slipped down; he was gone. And no one seemed to observe his passing. They were all screaming and gibbering at Dick.

"Rennell! Rennell!" yelled Stopford. "Where are you, Rennell? God, man, what's happened to your legs?"

Dick looked down at himself. For a moment he had the illusion that he was a head and a trunk, floating in the air. His lower limbs had become invisible, except for patches of trousering that seemed to drift through space. The mob in the room had fallen back gaping at him in horror.

Then Dick understood. It was the invisible garment that had coiled itself about him. He tore it from him and became visibly a man once more.

Shouts from another room! A surging movement of the crowd toward it. The muffled sounds of an automatic pistol, fitted with a silencer! Then screams:

"The devils are in there! They're murdering the soldiers!"

There followed a panic-stricken rush, more muffled firing, and then the sharp roar of rifles, and the fall of plaster. Some one was bawling the President's name. The rooms became a mass of milling human beings, lost to all self-control.

A bedlam of noise and struggle. Men fought with one another blindly, cursing soldiers fired promiscuously among the mob, riddling the walls, stabbing at the air. The plaster was falling in great chunks everywhere, filling the rooms with a heavy white cloud, in which all choked and struggled. The yells of the civilian mob below, struggling helplessly in the packed crowd that wedged the great stairway, made babel. Outside the White House a dense mob that filled the lawns was yelling back, and struggling to gain admittance. Suddenly the lights went out.

"They've cut the wires!" rose a wild, wailing voice. "The devils have cut the wires! Kill them! Kill everybody!"

His cry ended in a gurgle. Somewhere in that dark hell a struggle was going on, a well defined struggle, different from the random, aimless battling of the half-crazed soldiers and the civilians. President Hargreaves was still within the walls of the White House, it was known; it was physically impossible for him to have been carried away when every foot of space was packed. And through that darkness the invisible assailants were edging him, foot by foot, toward the outside.

* * * * *

Dick was on the edge of this silent battle. He sensed it. Bracing himself against a bureau, while the mob surged past him, he tried to pierce the gloom, to reinforce with his perceptions what his instinct told him. A soldier, crazed with fear, came leaping at him, bayonet leveled. He thrust with a grunt. Dick avoided the glancing steel by a hand's breadth, and, as the impetus of the man's attack carried him forward, caught him beneath the chin with a stiff right-hand jolt that sent him sprawling.

From below the cries broke out again, with renewed violence: "They've got the President! Get them! Get them! Close all doors and windows!"

But a door went crashing down somewhere, to the tune of savage yells. The mob was pouring down the stairs. It was growing less packed above. Dick heard Stopford's voice calling his name.

"Here, sir" he shouted back, and the two men collided.

"For God's sake do what you can, Rennell!" shouted the Colonel. "They've got the President downstairs. They had him in this very room, in the thick of it all. I heard him cry out, as if under a gag. They put one of those damned cloths over him. God, Rennell, I'm going crazy!"

The upper floor of the White House was almost empty now. Dick thrust himself into the crowd that still jammed the stairs. He reached the ground floor. It was lighter here, but a glance showed him that it was impossible to attempt to restore any semblance of order. The big East Room was jammed with a fighting, cursing throng. Dick stumbled over the bodies of those who had fallen in the press, or had been shot down. Outside the mob was thickening, swarming through the grounds and screeching like madmen.

* * * * *

Nothing that could be done! Dick found himself caught once more in the human torrent. Presently he was wedged up against a broken window. He precipitated himself through the frame, dropped to the ground, stopped for an instant to catch breath.

The yelling mob was congregated about the main entrance of the White House, and on this side the grounds were comparatively empty. As Dick stopped, trying desperately to form some plan of action, he heard footsteps and low voices near him. Then two men came toward him, followed by three or four others.

The men—but, though the light was faint, Dick realized instantly that they were wearing invisible garments. He could see nothing of them; he could see through where they seemed to be—the trees, the buildings of the streets. Yet they were at his elbow. And they saw him. He heard one of them leap, and sprang aside as the butt of a pistol descended through the air and dropped where his head had been.

Yet no hand had seemed to hold it. It had been a pistol, reversed, and flashing downward, to be arrested in mid-air six inches from his face. But the men were not wholly invisible. Nearly six feet above the ground, three or four pairs of eyes were staring malevolently into Dick's. Only the eyes were there.

The two foremost men were breathing heavily. They were carrying something. Grotesquely through a rent in the invisible garment Dick saw a patch of trouser. He heard a muffled sigh. President Hargreaves, in the hands of his abductors!

Dick's actions were reflex. As the pistol hung beside his face, he snatched at it, wrested it away, struck with it, and heard a curse and felt the yielding impact of bone and flesh. He had missed the head but struck the shoulder. Next moment hands gripped the weapon, and a desperate struggle began.

* * * * *

It was torn from Dick's grasp. He struck out at random, and his fist collided with the chin of a substantial flesh and blood human being. Invisible arms grasped him. He fought free. The pistol slashed his face sidewise, the sight ripping a strip of flesh from the cheek. He was surrounded, he was being beaten down, though he was fighting gamely.

"Kill the swine! Shoot! Shoot!" Dick heard one of his assailants muttering.

Out of the void appeared the blue muzzle of another automatic, with a silencer on it. Dick ducked as a flame spurted from it. He felt the bullet stir his hair. He grasped at the hand that held it, and missed. Then he was held fast, and the muzzle swung implacably toward his head again. Helpless, he watched it describe that arc of death. It was only later that he wondered why he had fought all the while in silence, instead of crying for help.

But of a sudden the pistol was dashed aside. A woman's voice spoke peremptorily, in some language Dick did not understand. And he saw her eyes among the eyes that glared at him. Dark eyes that he knew, even if the voice had not revealed her identity. The eyes and voice of Fredegonde Valmy!

Dick cried her name. He put forth all his strength in a final struggle. Suddenly he felt a stunning impact on the back of the head. He slipped, reeled, threw out his hands, and sank down unconscious on the grass at the side of the path.

CHAPTER IV

The Invisible Ambassador

Fredegonde Valmy implicated in the conspiracy! That was the first thought that flashed into Dick's mind as he recovered consciousness. He might have suspected it! But the idea that the girl he loved was bound up with the murderous gang that was attacking the very foundations of civilization chilled him to the soul.

Dick had been picked up a few minutes after he had been struck down, identified by Colonel Stopford as he was about to be removed to a hospital, and carried into the White House. Order had been restored by the arrival of a detachment of troops from Fort Myers, the severed cables located and mended, and by midnight the interior of the Presidential home had been made habitable again.

President Hargreaves was gone—kidnapped despite the utmost efforts to protect him; and it was impossible to conceal that fact from the world. But the wheels of government still revolved. All night an emergency council sat in the White House, and, deciding that in a time of such grave danger heroic means must be adopted, with the consent of such of the Congressional leaders as could be summoned, a Council of Defence was organized.

The whole country east of the Mississippi was placed under martial law. The fleet and army were put on a war footing. Flights of airplanes were assembled at numerous points along the eastern seaboard. To this Council Donald was attached as head of Intelligence for the Eastern Division. Yet all this availed little unless the location of the Invisible Empire could be ascertained, and, despite telegraphic reports that came in hourly, alleging to have discovered its headquarters, nothing had been achieved in this direction.

* * * * *

The garment taken from the slain soldier had been examined by a half-dozen of the leading chemists of the East. Pending the arrival from New York of the celebrated Professor Hosmeyer, it was deposited under military guard in a dark closet. The result was unfortunate. The garment exhibited to the assembled scientists was a mere bifurcated silken bag.

The gas with which it had been impregnated, though it had been heavy enough to adhere to the fabric for hours, had also been volatile enough to have disappeared completely, leaving a residue which was identified as a magnesium isotope.

Equally spectacular had been the disappearance of Mademoiselle Fredegonde Valmy. A cable from the Slovakian Ambassador had arrived a few hours later, denying her authenticity. And with her disappearance came the discovery that she had been at the head of an espionage system with ramifications in every state department, and in every statesman's home.

Three days passed with no sign from the enemy. The Council sat all day. In the executive offices of the White House Dick toiled ceaselessly, planning, receiving reports, organizing the flights of airplanes at strategic points throughout his district. From time to time he would be summoned to the Council. At night he threw himself upon a cot in his office and slept a sleep broken by the constant arrival of messengers. And still there was no clue to the location of the headquarters of the marauders.

But in those three days there had been no sign of them. Hope had succeeded despair; in the rebound of confidence the populace was beginning to ridicule the nation-wide precautions against what were coming to be considered merely a gang of super-criminals. It was even whispered that President Hargreaves had not been kidnapped at all. The Freemen's Party accused the Government of a plot to subvert popular liberties.

* * * * *

Dick received a summons on the third evening. Utterly worn out with his work, he pulled himself together and made his way into the Blue Room, where the Council was assembled. Vice-president Tomlinson, an elderly man, was in the chair. A non-entity, pushed into a post it had been thought he would adorn innocuously, he had been overwhelmed by his succession to the chief office of State.

Tomlinson did not like Dick, or any of the hustling younger officers who, unlike himself, realized the real significance of the danger that overhung the country. He sat pompously in his leather chair, regarding Dick as he entered in obedience to the summons.

"Well, Captain Rennell, what have you to report to us this evening?" he inquired, as Dick saluted and stood to attention at the table.

"We're improving our concentrations, Mr. Vice-president. We've eight flights of seaplanes scouring the coast in the hope of locating the stronghold of the Invisible Emperor. We've—"

"I'm sick and tired of that title," shouted Tomlinson. He sprang to his feet, his face flushed with anger. His nerves had broken under the continuous strain. "I'll give you my opinion, Captain Rennell," he said. "And that is that this so-called Invisible Emperor is a myth.

"A gang of thieves has invented a paint that renders them inconspicuous, has created a panic, and is taking advantage of it to terrorize the country. The whole business is poppycock, in my opinion, and the sooner this bubble bursts the better. Well, sir, what have you to say to that?"

"Have you ever seen any of these men in their invisible clothing, if I may ask, Mr. Vice-president?" inquired Dick, trying to keep down his anger. His nerves, too, were badly frazzled.

"No, sir, I have not, but my opinion is that this story is grossly exaggerated, and that the persons responsible are the reporters of our sensational press!" thundered Tomlinson.

* * * * *

He looked about him, a weak man proud of having asserted his authority. Somebody laughed.

Tomlinson glared at Dick, his rubicund visage purpling. But it was not Dick who had laughed. Nor any one at the council table.

That laugh had come from the wall beside the door. Again it broke forth, high-pitched, cold, derisive. All heads turned as if upon pivots to see who had uttered it.

"Good God!" exclaimed Secretary Norris, of the War Department, and slumped in his chair.

Five feet eight inches from the floor a pair of grey eyes looked at the Council members out of emptiness. Grey eyes, a man's eyes, cool, contemptuous, and filled with authority, with a contemptuous sense of superiority that left every man there dumb.

Dick was the first to recover himself. He stepped forward, not to where the invisible man was standing, but to a point between him and the door.

That cold laugh broke forth again. "Gentlemen, I am an ambassador from my sovereign, who chooses to be known as the Invisible Emperor," came the words. "As such, I claim immunity. Not that I greatly care, should you wish to violate the laws of nations and put me to death. But, believe me, in such case the retribution will be a terrible one."

Suddenly the envoy peeled off the gas-impregnated garments that covered him. He stood before the Council, a fair-haired young man, clad in the same fashion of trim black uniform as the bayonetted soldier had worn upstairs three nights before.

He bowed disdainfully, and it was Tomlinson who shouted:

"Arrest that man! I know his face! I've seen it in the papers. He's Von Kettler, the murderer who escaped from jail in an invisible suit."

"Oh, come, Mr. Vice-president," laughed Von Kettler, "are you sure this isn't all very much exaggerated?"

Tomlinson sank back in his chair, his ruddy face covered with sweat. Dick stared at Von Kettler. A suspicion was forming in his mind. He had seen eyes like those before, dark instead of grey, and yet with the same look of pride and breeding in them; the look of the face, too, impossible to mistake—he knew!

Fredegonde Valmy was Von Kettler's sister!

* * * * *

"Well, gentlemen, am I to receive the courtesies of an ambassador?" inquired Van Kettler, advancing.

"You shall have the privileges of the gallows rope!" shouted Tomlinson. "Arrest that man at once, Captain Rennell!"

"Pardon me, Mr. Vice-president," suggested the Secretary for the Navy blandly, "but perhaps it would be more desirable to hear what he has to say."

"Immunity for thieves, robbers, murderers!"

"Might I suggest," said Von Kettler suavely, "that, since the United States has honored my master by placing itself upon a war footing, it has accorded him the rights of a belligerent?"

"We'll hear you, Mr. Von Kettler," said the Secretary of State, glancing along the table. Three or four nodded, two shook their heads: Tomlinson only glared speechlessly at the intruder. Von Kettler advanced to the table and laid a paper upon it.

"You recognize that signature, gentlemen?" he asked.

At the bottom of the paper Dick saw scrawled the bold and unmistakable signature of President Hargreaves.

"An order signed by the President of your country," purred Von Kettler, "ordering your military forces replaced upon a peace footing, and the acceptance of our conditions. They are not onerous, and will not interfere with the daily life of the country. Merely a little change in that outworn document, the Constitution. My master rules America henceforward."

Somebody laughed: another laughed: but it was the Secretary of State who did the fine thing. He took up the paper bearing what purported to be President Hargreaves's signature, and tore it in two.

"The people of this country are her rulers," he said, "not an old man dragooned into signing a proclamation while in captivity—if indeed that is President Hargreaves's signature."

* * * * *

There came a sudden burst of applause. Von Kettler's face became the mask of a savage beast. He shook his fist furiously.

"You call my master a forger?" he shouted. "You yourselves repudiate your own Constitution, which places the control of army and navy in the hands of your President? You refuse to honor his signature?"

"Listen to me, Mr. Von Kettler!" The voice of the Secretary of State cut like a steel edge. "You totally mistake the temper of the people of this country. We don't surrender, even to worthy adversaries, much less to a gang of common thieves, murderers, and criminals like yourselves. You have been accorded the privilege you sought, that of an envoy, and that was straining the point. Show yourself here again after two minutes have elapsed, and you'll go to the gallows—for keeps."

"Dogs!" shouted Von Kettler, beside himself with fury. "Your doom is upon you even at this moment. I have but to wave my arm, and Washington shall be destroyed, and with her a score of other cities. I tell you you are at our mercy. Thousands of lives shall pay for this insult to my master. I warn you, such a catastrophe is coming as shall show you the Invisible Emperor does not threaten in vain!"

With complete nonchalance the Secretary of State took out his watch. "One minute and fifteen seconds remaining. Captain Rennell," he said. "At the expiration of that time, put Mr. Von Kettler under arrest. I advise you to go back to your master quickly, Mr. Von Kettler," he added, "and tell him that we'll have no dealings with him, now or ever."

* * * * *

For a moment longer Von Kettler stood glaring; then, with a laugh of derision and a gesture of the hands he vanished from view. And, though they might have expected that denouement, the members of the Council leaped to their feet, staring incredulously at the place where he had been. Nothing of Von Kettler was visible, not even the eyes, and there sounded not the slightest footfall.

Dick sprang forward to the door, but his outstretched arms encountered only emptiness. In spite of the Secretary of State's instructions, he was almost minded to apprehend the man. If he could get him!

The corridor was empty. A guard of soldiers was at the entrance, but they did not block the entrance. Even now Von Kettler might be passing them! Why didn't his feet sound upon the floor? How could a bulky man glide so smoothly?

Perhaps because Dick was undecided what to do, Von Kettler escaped him. By the time he reached the guards he knew he had escaped. Suddenly there came an unexpected denouement. Somewhere on the White House lawn a guard challenged, fired. The snap of one of the silenced automatics answered him.

When Dick and the guards reached the spot, the man was lying in a crumpled heap.

"An airplane," he gasped. "Invisible airplane. I—bumped into it. Men—in it. The damned dogs!"

He died. Dick stared around him. There was no sign of any airplane on the lawn, nothing but the tents of the guards, white in the moonlight, and the grim array of anti-aircraft guns that Dick had placed there.

But behind the White House, in hastily constructed hangars, were a half-dozen of the latest pursuit airships—beautiful slim hulls, heavily armored, with armored turrets containing each a quick-firer with the new armor-piercing bullets. One of these ships, Dick's own, was kept perpetually warmed and ready to take the air.

* * * * *

Dick raced across the lawn, yelled to the startled guard in charge. The mechanics came running from their quarters. Almost by the time Dick reached it the ship was ready.

He twirled the helicopter starter, and she roared and zoomed, taking an angle of a hundred and twenty-five degrees upward off a runway of twenty yards. Into the air she soared, into the moonlight, up like an arrow for five hundred feet.

Dick pulled the soaring lever, and she hung there, buzzing like a bee as her helicopters, counteracting the pull of gravity, held her comparatively stable. He scanned the air all about him.

Washington lay below, her myriad lights gleaming. Immediately beneath him Dick saw the guns and the tents of the soldiers, and the little group that was removing the body of the murdered soldier on a stretcher. But there were no signs of any hostile craft.

Had the murdered man really bumped into an invisible airship, or had he only thought he had? Had those devils learned to apply the gas to the surfaces of airplanes? There was no reason why they should not have done so.

But surely the utmost ingenuity of man had not contrived to render a modern plane, with its metalwork and machinery, absolutely transparent?

* * * * *

And, again, how was it possible to have silenced the sound of engines, the whir of a propeller, so that there should be no auditory indication whatever of a plane's presence?

Dick looked all about him. Nothing was in the air—he could have sworn it. He replaced the soaring lever and banked in a close circle, his glance piercing the night. No, there was nothing.

Crash! Boom! The plane rocked violently, tossing upon gusts of air. A huge, gaping hole of blackness had suddenly appeared in the middle of the White House lawn. The tents were flat upon the ground. Through the rising smoke clouds Dick saw tongues of flame.

No shell that, but a bomb, and dropped from the skies less than five hundred feet from where Dick hovered. Yet there was nothing visible in the skies save the round orb of the moon.

A rush of wind past Dick's face! One of the vanes of the helicopter crumpled and fluttered away into the night. Dick needed no further persuasion. The dead soldier had not lied.

Von Kettler had begun the fulfillment of his threat!

CHAPTER V

The Enemy Strikes

As Dick's airship veered and side-slipped, he kicked hard on the left rudder and brought the nose around. Furiously he sprayed the air with a leaden hail from his quick-firer. He heard a rush of wind go past him, and realized that his unseen antagonist had all but rammed him.

Yet nothing was visible at all, save the moon and the empty sky. He had heard the rush of the prop-wash, but he had seen nothing, heard nothing else. Incredible as it seemed, the pilot was flying a plane that had attained not merely invisibility but complete absence of all sound.

Dick side-slipped down, pancaked, and crashed. He emerged from a plane wrecked beyond hope of early repair, yet luckily with no injury beyond a few minor bruises. He rushed toward the hangar, to encounter a bevy of scared mechanics.

"Another plane! Rev one up quick!" he shouted.

Planes were already being wheeled out, pilots in flying suits and goggles were striding beside them. Dick ordered one of them away, stepped into his plane, and in a moment was in the air again.

In the minute or two that had elapsed since the encounter, the enemy had been active. Crash after crash was resounding from various parts of Washington. Buildings were rocking and toppling, debris strewed the streets, fires were springing up everywhere. A thousand feet aloft, Dick could see the holocaust of destruction that was being wrought by the infernal missiles.

Bombs of such power had been the unattained ambition of every government of the world—and it had been left to the men of the Invisible Emperor to attain to them. Whole streets went into ruin at each discharge and from everywhere within the city the wailing cry of the injured went up, in a resonant moan of pain.

In the central part of the city, the district about F Street and the government buildings, nothing was standing, except those buildings fashioned of structural steel, and these showed twisted girders like the skeletons of primeval monsters, supporting sections of sagging floors. Houses, hotels had melted into shapeless heaps of rubble, which filled the streets to a depth of a dozen yards, burying everything beneath them. Yet here and there could be seen the forms of dead pedestrians, motor-cars emerging out of the debris, lying in every conceivable position; horses, horribly mangled, were shrieking as they tried to free themselves. And yet, despite this ruin, the general impression upon Dick's mind, as he beat to and fro, signaling to his flight to spread, was that of a vast, empty desolation.

* * * * *

Further away: where the ruin had not yet fallen, thousands of human beings were milling in a mass, those upon the fringes of the crowd perpetually breaking away, other swarms approaching them, so that the entire agglomeration resembled a seething whirlpool turning slowly upon itself.

Then of a sudden the strains of the national anthem floated up to Dick's ears. A band was playing in the White House grounds. The tune was ragged, and the drum came in a fraction of a second late, but an immense pride and elation filled Dick's soul.

"They'll never beat us!" he thought, intensely, "with such a spirit as that!"

He had signaled his flight to spread, and search the air. He could see the individual ships darting here and there over the immensity of the city, but none knew better than he how fruitless their effort was. And the marauders had not ceased their deadly work.

A bomb dropped near the Washington Monument, sending up a huge spout of dust that veiled it from his eyes. Instinctively Dick shot toward the scene. Slowly the dust subsided, and then a yell of exultation broke from Dick's lips. The noble shaft still stood, a slim taper pointing to the skies.

It was an omen of ultimate success, and Dick took heart. No, they'd never beat the grim, unconquerable tenacity of the American people.

Yet the damage was proceeding at a frightful rate. A bomb dropped squarely on the Corcoran Gallery and resolved it into a heap of silly stones. A bomb fell in the middle of Pennsylvania Avenue, and the houses on either side collapsed like houses of cards, falling into a sulphurous, fiery pit. And still there was nothing visible but the sky and the moon.

* * * * *

Dick gritted his teeth and swore as he circled over the site of destruction, out of which tiny figures were struggling. He heard the clang of the fire bells as the motor trucks came roaring toward the scene. Then crash! again. Five blocks northward another dense cloud of dust arose, and the new area of destruction, spreading as swiftly as ripples over a pond, joined the former one, leaving a huge, irregular open space, piled up with masonry and brick in a number of flat-topped pyramids.

Into this, houses went crashing every moment, with a sound like the clatter of falling crockery, but infinitely magnified.

"The devils! The swine!" shouted Dick. "And we gave Von Kettler the privileges of an ambassador!"

And Fredegonde was the sister of this devil! The remembrance of that struck a cold chill to Dick's heart again. He tried to blot out her picture from his mind, but he still saw her as she had appeared that day after the air ride, flushed, smiling, radiant in her dark beauty.

A murderess and a spy! He cursed her as he banked and circled back. He was helpless. He could do nothing. And all Washington would be destroyed by morning, if the supply of bombs kept up. But there was more to come. Suddenly Dick became aware that two of his flight, at widely separated distances, were going down in flames. Flaming comets, they dropped plump into the destruction below. Another caught fire and was going down. No need to question what was happening.

The invisible enemy was attacking his flight and picking off his men one by one!

He drove furiously toward two of his planes whose erratic movements showed that they were being attacked. As he neared them he saw one catch fire and begin its earthward swoop. Then the fuselage crackled beside him, and his instrument board dissolved into ruin. Instinctively he went round in a tight bank and loosed his machine-gun. Nothing there! Nothing at all! Yet his right wing went ragged, and his own furious blasts into the sky, their echoes drowned by the roar of his propeller, were productive of nothing.

* * * * *

He shot past the uninjured plane, signalling it to descend. He wasn't going to let his men ride aloft to helpless butchery. Nothing could be done until some means was discovered of counteracting the enemy's terrific advantage.

He darted across the heart of the city to where another of the flight was circling, waggling his wings to indicate to it to descend. Then on to the next plane and the next, shepherding them. Thank God they understood! They were bunching toward the hangar. Yet another took fire and dropped, a burning wreck. Half his flight out of commission, and not an enemy visible!

He was aloft alone now, courting death—instant, invisible death. He wouldn't descend until that carnival of murder was at an end. But it was not at an end. Another crash, far up Pennsylvania Avenue, showed an attempt upon the Capitol. Again—again, and a smoking hell wreathed the noble buildings so that it was no longer possible to see them. A lull, and then a crash nearer the city's heart. Crash! Crash!

Invisible though the enemy was, it was easy to trace the movements of this particular plane by the successive areas of destruction that it left behind it. It was coming back over Pennsylvania Avenue, dropping its bombs at intervals. It was methodically wiping out an entire section of Washington.

Dick drove his plane toward it. There was one chance in a thousand that, if he could accurately gauge the progress of his invisible antagonist, he could crash him and go down with him to death. If he could get close enough to feel his prop-wash! A wild chance, but Dick's mind was keyed up to desperation. He shot like an arrow toward the scene, with a view to intercepting the murderer.

Then of a sudden he became aware of a curious phenomenon. A black beam was shooting across the sky. A black searchlight! It came from the flat top of a large hotel that had somehow escaped the universal destruction, and, with its gaunt skeleton of structural steel showing in squares, towered out of the ruin all about it like an island.

* * * * *

It was from here that the black beam started. It spread fanwise across the sky. But it was not merely blackness. It was utter and impenetrable darkness, cleaving the sky like a knife. Where it passed, the rays of the moon were extinguished as fire is extinguished by water.

A beam of absolute blackness, that pierced the air like a widening cone, and made the night seem, by contrast, of dazzling brightness along either dark border.

High into the air that dark beam shot, moving to and fro in the sky. Dick, darting toward the spot where he hoped to find his invisible enemy, found himself caught in it.

In utter, inextinguishable darkness! Like a trapped bird he fluttered, hurling himself this way and that till suddenly he found himself blinking in the dazzling light of the moon again, and the black beam was overhead.

Crash! Another widening sphere of ruin as the invisible marauder dropped a bomb. Dick cursed bitterly. Trapped in that black beam, he had lost his direction. The invisible plane had shot past the point where he had hoped to intercept it.

He flung his soaring lever, and hung suspended in the air. An easy mark for the enemy, if he chose to take the opportunity. No matter. Death was all that Dick craved. He had seen half his flight wiped out, and a hundred thousand human beings hurled to destruction. He wanted to die.

Then suddenly a wild shout came to his ears, as if all Washington had gone mad with triumph. And Dick heard himself shouting too, before he knew it, almost before he knew why.

* * * * *

For overhead, where the inky finger searched the sky, a luminous shape appeared, a silvery cigar, riding in the void. The finger missed it, and again there was only the moonlight. It caught it again—and again the whole devastated city rang with yells of derision, hate, and anger as the black beam held it.

It held it! To and fro that silvery cigar scurried in a frantic attempt to avoid detection, and remorselessly the black beam held it down.

It held it down, and it outlined it as clearly as a figure on the moving picture screen. Then suddenly there came a flash, followed by a dull detonation, and a black cloud appeared, spreading into a flower of death near the cigar, and at the edge of the black beam. The cheers grew frantic. The anti-aircraft battery in the White House grounds had grasped the situation, and was opening fire.

To and fro, like a trapped beast, the cigar-shaped airplane fled. Once it seemed to escape. It faded from the edge of the black finger—faded into nothingness amid a roar of execretion. Then it was caught and held.

Truncated, bounded by an arc of sky, the black finger followed the murderer in his flight remorselessly. And all around him the anti-aircraft guns were placing a barrage of death.

He was trapped. No need for Dick to rush in to battle. To do so might call off that deadly barrage that held the murderer in a ring of death. Hovering, Dick watched. And then, perhaps panic-stricken, perhaps rendered desperate, perhaps through sheer, wanton courage that might have commanded admiration under nobler circumstances, the airship turned and drove straight in the direction of the battery, dropping another bomb as she did so.

* * * * *

It fell in a crowded street, swarming with spectators who had clambered upon the fallen debris, and it wrought hideous destruction. But this time there was hardly a cry—no unison of despair such as had come to Dick's ears before. The suspense was too tense. All eyes watched the airship as, seeming to bear a charmed life, she drove for the White House itself, through a ring of shells that widened and contracted alternately, with the object of placing a last bomb squarely upon the building before going down in death. And all the while the black searchlight held it.

Dick Rennell was to experience many thrilling moments afterward, but there was never a period, measurable by seconds, yet seeming to extend through all eternity—never a period quite so fraught with suspense as, hovering there, he watched the flight of that silvery plane speeding straight toward the executive mansion while all around it the shells bloomed and spread. It was over the White House grounds. The archies had failed; they were being outmaneuvered, they could not be swung in time to follow the trajectory of the plane. Dick held his breath.

Then suddenly the silvery ship dissolved in a blaze of fire, a shower of golden sparks such as fly from a rocket, and simultaneously the last bomb that she was to drop broke upon the ground below.

Down she plunged, instantly invisible as she escaped the finger of the black beam; but she dropped into the vortex of ruin that she herself had created. Into a pit of blazing fire, criss-crossed by falling trees, that had engulfed the battery and a score of men.

Then suddenly Dick understood. He flung home the soaring lever, banked, and headed, not for the White House, but for the flat roof of the hotel from which the black searchlight was still projecting itself through the skies. He hovered above, and dropped, light as a feather, upon the rooftop.

* * * * *

There was only one person there—an old man dressed in a shabby suit, kneeling before a great block of stone that had been dislodged upward from the parapet and formed a sort of table. Upon this table the old man had placed a large, square box, resembling an exaggerated kodak, and it was from the lens of this box that the black beam was projecting.

Dick sprang from his cockpit as the old man rose in alarm. He ran to him and caught him by the arm.

"Luke Evans!" he cried. "Thank God you've come back in time to save America!"

CHAPTER VI

The Gas

In the Blue Room of the White House the Council listened to old Luke Evans's exposition of his invention with feelings ranging from incredulity to hope.

"I've been at work all the time," said the old man, "not far from here. I knew the day would come when you'd need me. I put my pride aside for the sake of my country."

"Tell us in a few words about this discovery of yours, Mr. Evans," said Colonel Stopford.

Luke Evans placed the square black case upon the table. "It's simple, like all big things, sir," he answered. "The original shadow-breaking device that I invented was a heavy, inert gas, invisible, but almost as viscous as paint. Applied to textiles, to inorganic matter, to animal bodies, it adheres for hours. Its property is to render such substances invisible by absorbing all the visible light rays that fall upon it, from red to violet. Light passes through all substances that are coated with this paint as if they did not exist."

"And this antidote of yours?" asked Colonel Stopford.

"Darkness," replied Luke Evans. "A beam of darkness that means absolute invisibility. It can be shot from this apparatus"—he indicated the box upon the table. "This box contains a minute portion of a gas which exists in nature in the form of a black, crystalline powder. The peculiar property of this powder is that it is the solidified form of a gas more volatile than any that is known. So volatile is it that, when the ordinary atmospheric pressure of fifteen pounds to the square inch is removed, the powder instantly changes to the gaseous condition."

"By pressing this lever"—Evans pointed at the box—"a vacuum is created. Instantly the powder becomes a gas, which shoots forth through this aperture with the speed of a projectile, taking the form of a beam of absolute blackness. Or it can be discharged from cylinders in such a way as to extend over a large area within a few minutes."

"But how does this darkness make the invisible airships luminous?" asked Stopford. "Why does not your darkness destroy all light?"

"In this way, sir," replied the old inventor. "The shadow-breaking gas with which the airships are painted confers invisibility because it absorbs sunlight. But it does not absorb the still more rapid waves, or oscillations which manifest themselves as radio-activity. On the contrary, it gathers and reflects these.

"Now Roentgen, the discoverer of the X-ray, observed that if X-rays are allowed to enter the eye of an observer who is in complete darkness, the retina receives a stimulus, and light is perceived, due to the fluorescent action of the X-rays upon the eyeball.

"Consequently, by creating a beam of complete darkness, I bring into clear visibility the fluorescent gas that coats the airships; in other words, the airships become visible."

"If a light ray is nullified upon entering the field of darkness, will it emerge at the other edge as a perfect light ray again?" asked Stopford.

"It will emerge unchanged, since the black beam destroys light by slightly slowing down the vibrations to a point where they are not perceived as light by the human eye. On emerging from the beam, however, these vibrations immediately resume their natural frequency. To give you a homely parallel, the telephone changes sound waves to electric waves, and re-converts them into sound waves at the other end, without any appreciable interruption."

"Then," said Stopford, "the logical application of your method is to plunge every city in the land into darkness by means of this gas?"

"That is so, sir, and then we shall have the advantage of invisibility, and the enemy ships will be in fluorescence."

"Damned impracticable!" muttered Stopford.

"You seriously propose to darken the greater part of eastern North America?" asked the Secretary for War.

"The gas can be produced in large quantities from coal tar besides existing in crystalline deposits," replied Luke Evans. "It is so volatile that I estimate that a single ton will darken all eastern North America for five days. Whereas the concentration would be made only in specific areas liable to attack. The gas is distilled with great facility from one of the tri-phenyl-carbinol coal-tar derivatives."

Vice-president Tomlinson was a pompous, irascible old man, but it was he who hit the nail on the head.

"That's all very well as an emergency measure, but we've got to find the haunt of that gang and smash it!"

An orderly brought in a telegraphic dispatch and handed it to him. The Vice-president opened it, glanced through it, and tried to hand it to the Secretary of State. Instead, it fluttered from his nerveless fingers, and he sank back with a groan. The Secretary picked it up and glanced at it.

"Gentlemen," he said, trying to control his voice, "New York was bombed out of the blue at sunrise this morning, and the whole lower part of the city is a heap of ruins."

* * * * *

In the days that followed it became clear that all the resources of America would be needed to cope with the Invisible Empire. Not a day passed without some blow being struck. Boston, Charleston, Baltimore, Pittsburg in turn were devastated. Three cruisers and a score of minor craft were sunk in the harbor of Newport News, where they were concentrating, and thenceforward the fleet became a fugitive force, seeking concealment rather than an offensive. Trans-Atlantic sea-traffic ceased.

Meanwhile the black gas was being hurriedly manufactured. From cylinders placed in central positions in a score of cities it was discharged continuously, covering these centers with an impenetrable pall of night that no light would penetrate. Only by the glow of radium paint, which commanded fabulous prices, could official business be transacted, and that only to a very small degree.

Courts were closed, business suspended, prisoners released, perforce, from jails. Famine ruled. The remedy was proving worse than the disease. Within a week the use of the dark gas had had to be discontinued. And a temporary suspension of the raids served only to accentuate the general terror.

There were food riots everywhere, demands that the Government come to terms, and counter-demands that the war be fought out to the bitter end.

Fought out, when everything was disorganized? Stocks of food congested all the terminals, mobs rioted and battled and plundered all through the east.

"It means surrender," was voiced at the Council meeting by one of the members. And nobody answered him.

Three days of respite, then, instead of bombs, proclamations fluttering down from a cloudless sky. Unless the white flag of surrender was hoisted from the summit of the battered Capitol, the Invisible Emperor would strike such a blow as should bring America to her knees!

* * * * *

It was a twelve-hour ultimatum, and before three hours had passed thousands of citizens had taken possession of the Capitol and filled all the approaches. Over their heads floated banners—the Stars and Stripes, and, blazoned across them the words, "No Surrender."

It was a spontaneous uprising of the people of Washington. Hungry, homeless in the sharpening autumn weather, and nearly all bereft of members of their families, too often of the breadwinner, now lying deep beneath the rubble that littered the streets, they had gathered in their thousands to protest against any attempt to yield.

Dick, flying overhead at the apex of his squadron, felt his heart swell with elation as he watched the orderly crowds. This was at three in the afternoon: at six the ultimatum ended, the new frightfulness was to begin.

At five, Vice-president Tomlinson was to address the crowds. The old man had risen to the occasion. He had cast off his pompousness and vanity, and was known to favor war to the bitter end. Dick and his squadron circled above the broken dome as the car that carried the Vice-president and the secretaries of State and for War approached along the Avenue.

Rat-tat, rat-a-tat-tat!

Out of the blue sky streams of lead were poured into the assembled multitudes. Instantly they had become converted into a panic-stricken mob, turning this way and that.

Rat-a-tat-tat. Swaths of dead and dying men rolled in the dust, and, as wheat falls under the reaper's blade, the mob melted away in lines and by battalions. Within thirty seconds the whole terrain was piled with dead and dying.

"My God, it's massacre! It's murder!" shouted Dick.

* * * * *

They had not even waited for the twelve hours to expire. To and fro the invisible airplanes shot through the blue evening sky, till the last fugitives were streaming away in all directions like hunted deer, and the dead lay piled in ghastly heaps everywhere.

Out of these heaps wounded and dying men would stagger to their feet to shake their fists impotently at their murderers.

In vain Dick and his squadron strove to dash themselves into the invisible airships. The pilots eluded them with ease, sometimes sending a contemptuous round of machine-gun bullets in their direction, but not troubling to shoot them down.

Two small boys, carrying a huge banner with "No Surrender" across it, were walking off the ghastly field. Twelve or fourteen years old at most, they disdained to run. They were singing, singing the National Anthem, though their voices were inaudible through the turmoil.

Rat-tat! Rat-tat-a-tat! The fiends above loosed a storm of lead upon them. Both fell. One rose, still clutching the banner in his hand and waved it aloft. In a sudden silence his childish treble could be heard:

My country, 'tis of thee Sweet land of lib-er-ty—

The guns rattled again. Clutching the blood-stained banner, he dropped across the body of his companion.

Suddenly a broad band of black soared upward from the earth. Those in charge of the cylinders placed about the Capitol had released the gas.

A band of darkness, rising into the blue, cutting off the earth, making the summit of the ruined Capitol a floating dome. But, fast as it rose, the invisible airships rose faster above it.

A last vicious volley! Two of Dick's flight crashing down upon the piles of dead men underneath! And nothing was visible, though the darkness rose till it obliterated the blue above.

* * * * *

At dawn the Council sat, after an all-night meeting. Vice-president Tomlinson, one arm shattered by a machine-gun bullet, still occupied the chair at the head of the table.

Outside, immediately about the White House, there was not a sound. Washington might have been a city of the dead. The railroad terminals, however, were occupied by a mob of people, busily looting. There was great disorder. Organized government had simply disappeared.

Each man was occupied only with obtaining as much food as he could carry, and taking his family into rural districts where the Terror would not be likely to pursue. All the roads leading out of Washington—into Virginia, into Maryland, were congested with columns of fugitives that stretched for miles.

Some, who were fortunate enough to possess automobiles, and—what was rarer—a few gallons of gas, were trying to force their way through the masses ahead of them; here and there a family trudged beside a pack-horse, or a big dog drew an improvised sled on wheels, loaded with flour, bacon, blankets, pillows. Old men and young children trudged on uncomplaining.

The telegraph wires were still, for the most part, working. All the world knew what was happening. From all the big cities of the East a similar exodus was proceeding. There was little bitterness and little disorder.

It was not the airship raids from which these crowds were fleeing. Something grimmer was happening. The murderous attack upon the populace about the Capitol had been merely an incident. This later development was the fulfilment of the Invisible Emperor's ultimatum.

Death was afield, death, invisible, instantaneous, and inevitable. Death blown on the winds, in the form of the deadliest of unknown gases.

* * * * *

In the Blue Room of the White House a score of experts had gathered. Dick, too, with the chiefs of his staff, Stopford, and the army and naval heads. Among them was the chief of the Meteorological Bureau, and it was to him primarily that Tomlinson was reading a telegraphic dispatch from Wilmington, South Carolina:

"The Invisible Death has reached this point and is working havoc throughout the city, spreading from street to street. Men are dropping dead everywhere. A few have fled, but—"

The sudden ending of the dispatch was significant enough. Tomlinson picked up another dispatch from Columbia, in the same State:

"Invisible Death now circling city," he read. "Business section already invaded. All other telegraphists have left posts. Can't say how long—"

And this, too, ended in the same way. There were piles of such communications, and they had been coming in for eighteen hours. At that moment an orderly brought in a dozen more.

Tomlinson showed the head of the Meteorological Bureau the chart upon the table. "We've plotted out a map as the wires came in, Mr. Graves," he said. "The Invisible Death struck the southeast shore of the United States yesterday afternoon near Charleston. It has spread approximately at a steady rate. The wind velocity—?"

"Remains constant. Seventy miles an hour. Dying down a little," answered Graves.

"The death line now runs from Wilmington, South Carolina, straight to Augusta, Georgia," the Vice-president pursued. "Every living thing that this gas has encountered has been instantly destroyed. Men, cattle, birds, vermin, wild beasts. The gas is invisible and inodorous. These gentlemen believe it may be a form of hydrocyanic acid, but of a concentration beyond anything known to chemistry, so deadly that a billionth part of it to one of air must be fatal, otherwise it could not have traveled as it has done. Warnings have been broadcasted, but there are no stocks of chemicals that might counteract it. Flight is the only hope—flight at seventy miles an hour!"

* * * * *

His voice shook. "This gas has been loosed, as you told us, upon the wings of the hurricane that came through the Florida Strait. What are the chances of its reaching Washington?"

"Mr. Vice-president, if the wind continues, and this gas has sufficient concentration, it should be in Washington within the next eight hours." Graves replied. "If the wind changes direction, however, this gas will probably be blown out to sea, or into the Alleghanies, where it will probably be dissipated among the hills, or by the foliage on the mountains. I'm not a chemist—"

"No, sir, and I am not consulting you as one," answered old Tomlinson. "A death belt several hundred miles in length and three or four hundred deep has already been cut across this continent. We are faced with wholesale, unmitigated murder, on such a scale as was never known before. But we are an integral part of America, and Washington has no more right to expect immunity than our devastated Southern States. The question we wish to put to you is, can you trace the exact course taken by the hurricane?"

"I can, Mr. Vice-president," answered Graves. "It originated somewhere in the West Indian seas, like all these storms. We've been getting our reports almost as usual. Our first one came from Nassau, which was badly damaged. The storm missed the Florida coast, as many of them do, and struck the coast of South Carolina—in fact, we received a report from Charleston, which must have almost coincided with your first report of the gas."

"If the storm missed the Florida coast, it follows that the gas was not discharged from any point on the American continent," said Tomlinson. "From some point off Florida—from some island, or from a plane or from a ship at sea."

"Not from a ship at sea, Mr. Vice-president," interposed the head of the Chemical Bureau. "To discharge gas on such an extensive scale would require more space than could be furnished by the largest vessel, in my opinion."

"In all probability the gas was 'loaded,' so to say, onto the gale somewhere in the Bahamas," said Graves. "That seems to me the most likely explanation."

* * * * *

Vice-president Tomlinson nodded, and picked up one of the latest telegraphic dispatches, as if absently.

"Gentlemen," he said, "the Invisible Death has already reached Charlotte."

He picked up another. "Reported Abaco Island, Bahamas, totally wrecked by storm. All communication has ceased," he read. He turned to Dick and spoke as if inspired. "Captain Rennell, there is your destination," he thundered. "They've betrayed themselves. We've got them now. You understand?"

"By God, sir! It's from Abaco Island, then, that those devils have been carrying on their game of wholesale murder!"

Suddenly a contagion of enthusiasm seemed to sweep the whole assemblage. Every man was upon his feet in an instant, white, quivering, lips opened for speech that trembled there and did not come.

It was Secretary Norris spoke. "The Vice-president has hit the mark," he said, with a dramatic gesture of his arm. "Yes, they've betrayed themselves. Their headquarters are on Abaco Island. It's one of the largest in the Bahamas." He turned to the Secretary for the Navy. "You can rush the fleet there, sir?" he asked.

"Within forty-eight hours I'll have every vessel that can float off Abaco Island."

"I'll concentrate all airplanes. Take your flight, Captain Rennell. We'll stamp out that nest of murderers if we blow Abaco Island to the bottom of the sea. It can be done!"

"It can be done, sir—with Luke Evans and his invention," answered Dick.

CHAPTER VII

On the Trail

Three hours later, about the time when the war council rose after completing its plans, a sudden shift of the wind blew the poison gas out to sea, just when it appeared certain that it would reach the capital of the nation.

The southern half of Virginia had been swept over. Operators, telegraph and telephone, staying at their posts had sent in constant messages that had terminated with an abruptness which told of the tragic sequel. Yet, at that distance from its source, the intensity of the gas had been to some extent dissipated.

Poisonous beyond any gas known, so deadly as to make hydrocyanic gas innocuous in comparison, still as it was swept northward on the wings of the wind, there had been an increasing number of non-fatal casualties. The most northernly point reached by the gas was Richmond, and here some fifty per cent of those stricken had suffered paralysis instead of death.

But a new element had been injected into the situation. Even the heroic courage shown by the populace in the beginning had had its limits. The morning after the news of the Invisible Death's advent was made public mobs had gathered in all the large cities of the East, demanding surrender.

The submerged elements of crime and disorder had come to the surface at last. Committees were formed, with the avowed object of yielding to the Invisible Emperor, and averting further disaster. In Washington, a city of the dead, half the members of Congress and the Senators had gathered in the ruined Capitol, to debate the situation.

There were rumors of an impending march on the White House, of a coup d'etat.

* * * * *

The action of the Government was prompt. Five hundred loyalists were enrolled, armed, and posted round the White House: every avenue of approach was commanded by machine-guns. Meanwhile the news was spread by radio that the headquarters of the Invisible Emperor had been located, and that a strong bombing squadron was being dispatched to destroy it.

The entire fleet was to follow, and it was confidently anticipated that within a little while the Terror would be at an end.

Those at the white House were less sanguine. There was none but realized the diabolical strength of their antagonists.

"Everything depends upon the outcome of the next forty-eight hours, and everything depends on you, Rennell," said Secretary Norris to Dick, as he stood beside his plane. Behind him his flight of a dozen airships was drawn up.

"Find them," added the Secretary; "cover Abaco Island with the black gas, and the navy and the marines will wipe up the mess that you leave behind you. God help you—and all of us, Rennell!"

He gripped Dick's hand and turned away. Dick was very sober-minded as he climbed into his cockpit. He knew to the full how much depended upon himself and Luke Evans. Already the shouts of the insurgents were to be heard at the ends of the barriers, commanded by the machine-guns, and patrolled by the enlisted volunteers.

Negro mobs were building counter-barricades of their own with rubble from the fallen edifices. Civil war might be postponed for eight-and-forty hours, but after that unless there was news of victory, the whole structure of civilization would be smashed irreparably.

It was up to Dick and Luke Evans, and they had assumed such a responsibility as rarely falls to the lot of man in war.

* * * * *

Dick was to lead the flight in a two-seater Barwell plane. This was one of the latest types, and had been hurriedly adapted to the purpose for which it was to be used. Dick himself occupied the rear seat, with its dual controls, and the gun in its armored casing. In front sat old Luke Evans, in charge of the black gas projector.

His famous camera box, containing a minute quantity of gas in slow combustion, and projecting the black searchlight, had been built into the plane. In the rack beside him were a number of the black gas bombs, each of which, dropped to earth, would release enough gas to cover a considerable area with darkness. Both Luke and Dick wore respirators filled with charcoal and sodium thio-sulphate, and beside Dick a cage containing three guinea-pigs rested.

These little rodents were so sensitive to atmospheric changes that a quantity of hydrocyanic acid too minute to affect a man would produce instantaneous death on them.

From its hiding-place off the Virginia coast the American fleet was steaming hotly southward toward Abaco Island, cruisers, destroyers, submarines. That Abaco was British territory had simply not been considered in this crisis of history.

The twelve airships that followed Dick's contained enough bombs to put the headquarters of the Invisible Empire out of business for good. The naval guns would complete the same business.

All day Dick and Luke Evans flew southwestward. At first glance, everything appeared normal. The catastrophe that had fallen upon the land was visible only in the shape of the lines of tiny figures, extending for miles, that choked all the roads radiating out of the principal cities. It was only when they were over the southern portion of Virginia that the ravages of deadly gas became apparent.

Flying low, Dick could see the fields strewn with the bodies of dead cattle. Here and there, at the doors of farmhouses, the inmates could be seen, lying together in gruesome heaps, caught and killed instantaneously as they attempted flight. Here, too, were figures on the roads. But they were figures of dead men and women.

* * * * *

They strewed the roads for miles, lying as they had been trapped—men, women, children, horses, mules, and dogs. The spectacle was an appalling one. Dick set his jaws grimly. He was thinking that the Council had let Von Kettler escape. He was thinking of Fredegonde. But he would not let himself think of her. She deserved no more pity than the rest of the murderous crew.

Over the Carolinas the conditions were still more appalling. Here deadly gas had struck with all its concentrated power. A city materialized out of the blue distance, a factory town with all chimneys spiring upward into the blue, a section of tall buildings intersected by canyonlike streets, around it a rim of trim houses, bungalows, indicative of prosperity and comfort. And it was a city of the dead.

For everywhere around it, on all the roads, the dead lay piled on top of one another. For miles—all the inhabitants, rich and poor, business men, factory hands, negroes. There had been a mad rush as the fatal gas drove onward upon its lethal way, and all the fugitives had been overwhelmed simultaneously.

Here were golf links, with little groups strewn on the grass and fairways; here, at one of the holes, four men, their putters still in their hands, crouched in death. Here was the wreckage of a train that had collided with a string of freight cars at an untended switch, and from the shattered windows the heads and bodies of the dead protruded in serried ranks.

Dick looked back. His flight was driving on behind him. He guessed their feelings. They had sworn, as he had sworn, that none of them would return without stamping out that abomination from the earth forever.

* * * * *

He signaled to the flight to rise, and zoomed upward to twelve thousand feet. He did not want to look upon any more of those horrors. At that height, the peaceful landscape lay extended underneath, in a checker-board of farms and woodlands. One could pretend that it was all a vile dream.

He avoided Charleston, and winged out above the Atlantic, striking a straight course along the coast toward the Bahamas. The shores of Georgia vanished in the west. Dick began to breathe more freely. His mind shook off its weight of horror. Only the blue sea and the blue sky were visible The aftermath of the gale remained in the shape of a strong head breeze and white crests below.

Dick glanced at the guinea-pigs. They were busily gnawing their cabbage and carrots. The gas had evidently been entirely dissipated by the wind.

Toward sunset the low jutting fore-land of Canaveral on the east coast of Florida, came into view. Dick shifted course a little. Three hours more should see them over Abaco.

His flight had explicit instructions. As soon as the black gas had rendered visible the headquarters of the Invisible Emperor, they were to circle above, dropping their bombs. When these were exhausted, the machine guns would come into play. There was to be no attention paid to signals of surrender. They were to wipe out the headquarters, to kill every living thing that showed itself—and the navy and the marines would mop up anything left over.

The sun went down in a blaze of gold and crimson. Night fell. The moon began to climb the east. The black sea, stretching beneath, was as empty as on the day when it was created. Nothing in the shape of navigation appeared.

Two hours, three hours, and old Evans turned round in his cockpit and pointed. On the horizon a black thread was beginning to stretch against the sky. It was Abaco Island, in the Bahama group. They were nearly at their destination. An hour more—perhaps two hours, and the deadly menace that threatened America might be removed forever. Dick breathed a silent prayer for success.

* * * * *

They were over Abaco. A long, flat island, seventy miles or so in extreme length, and fairly wide, covered with a dense growth of tropical brush and forest, with here and there open spaces, near the seacoast an occasional farm-house. Dick dropped to five thousand, to three, to one. The moon made the whole land underneath as bright as day.

There were no evidence of destruction by the hurricane. The farmhouses stood substantial and well roofed. If death had struck Abaco Island, it had been the work of man, not Nature.

Dick zoomed almost to his ceiling, until, in the brilliant moonlight, he could see Abaco Island from side to side. For the most part it was heavily wooded with mahogany and lignum vitae: toward the central portion there was open land, but there was not the least sign of any construction work.

Again he swooped, indicating to his flight to follow him. At a thousand feet he examined the open district intently. Here, if anywhere upon the island, the Invisible Emperor had his headquarters. Was it conceivable that a gas factory, hangars, ammunition depots could exist here invisibly, when he could look straight down upon the ground?

Dick's heart sank. The hideous fear came to him that Graves had been mistaken, that he had come on a wild-goose chase. This could not be the place. It was quite incredible.

Again and again he circled, studying the ground beneath. Now he could see that the tough grass and undergrowth marked curious geometrical patterns. Here, for example, was an oblong of bare earth around which the vegetation grew, and it was obviously the work of man.

Here were four squares of bare ground set side by side, with thin strips of vegetation growing between them.

Then of a sudden Dick knew! Those squares and parallelograms of bare ground indicated the foundations of buildings. He was looking down on the very site of the Invisible Emperor's stronghold!

He shouted, and pointed downward. Luke Evans looked round and nodded. He understood. He patted the camera-box with a grim smile on his old face.

CHAPTER VIII

The Magnetic Trap

Upon those squares and oblongs of bare earth, incredible as it seemed, rose the structures of the Invisible Empire, themselves both invisible and transparent, so that one looked straight down through them and saw only the ground beneath them.

Every interior floor and girder must have been treated with the gas. They had been cunning. They must have discovered some permanent means of charging paint with the shadow-breaking gas, so that the buildings would remain invisible for months and years instead of hours.

But they had not been cunning enough. It had not occurred to them that the foundations would still be visible underneath, for the simple reason that grass does not grow without sunlight.

Dick saw old Luke Evans nodding and pointing downward. The old man picked up his end of the speaking-tube, but Dick ignored the gesture. He signaled to his flight to rise, and zoomed up, circling, and studying the land beneath.

That oblong was evidently the central building. Those four squares probably housed airplanes, and each would hold half a dozen. That elliptical building might contain a dirigible. That round patch was probably the gas factory.

Now Dick could see more patches of bare ground, extending in the direction of the sea. He gunned his ship and followed the gap among the trees to the ocean, a few miles distant. Yes, there were more evidence of activity here. Beside the water, in what looked like a deep natural harbor, was what seemed to be the foundations of a dock. Perhaps even vessels of war floated on the phosphorescent Bahama sea.

* * * * *

He circled back, his flock wheeling like a flight of birds and following him. He signaled to them to scatter. They had certainly been observed; at any moment a hail of lead might assail them invisibly out of the air. They must get to work quickly. But had they understood the significance of those bare patches?

Dick saw Luke Evans still fidgeting impatiently with his end of the speaking-tube, and picked it up.

"I'm thinking, Captain Rennell, we've got no time to lose if we want to keep the upper hand of those devils," called the old man.

"Yes, you're right," Dick answered. "Lay a trail of gas bombs all around those hangars and buildings, enough to hold them dark for some time. And keep a bomb or two in reserve."

Luke Evans shouted back. The plane was again above the structures. The old man dropped a bomb over the side, and Dick zoomed again, his flight wheeling up behind him.

Higher and higher, banking and going round in a succession of tight spirals, Dick flew. Every moment he expected the blow to fall. As he rose, Luke Evans dropped bomb after bomb. A thousand feet beneath the flight was taking up positions, hovering with the helicopters, looking up to Dick for the signal, and waiting.

Then from beneath the cloud of black gas began to rise, as Luke Evans dropped his bombs. It filled the lower spaces of the sky, blotting out the land in impenetrable darkness. That darkness, above which Dick and his flight were soaring, rose like a solid wall, built by some prehistoric race that aimed to fling a tower into the heavens.

* * * * *

And then—the miracle! Dick gasped in sheer delight as he realized that he had made no mistake.

At first all he could see was a number of criss-crossing phosphorescent lines that appeared shimmering through the blackness underneath. They ran luminously here and there, forming no particular pattern, much like the figures on the radium dial of a watch when first they come into wavering visibility at night.

Then the lines began to intersect one another, to assume geometric patterns and curves. And bit by bit they took meaning and significance.

And suddenly the whole invisible stronghold lay revealed upon the ground beneath, a shining, dazzling play of weaving light.

Buildings and hangars stood out, clearly revealed; the rounded vault of a dirigible hangar, and the shining ribbon of a road that ran through a pitch-dark tarmac, and was evidently constructed from some gas-impregnated materials. On this tarmac was a flight of shining airplanes, ready to take off. There were the odd, ovoid figures of the aviators in their silken overalls. More figures appeared, running out from the buildings. It was clear that the sudden raid had taken them all by surprise.

Luke Evans yelled and pointed. "We've got them now, sir!" Dick heard above the whine of the helicopter engine. "We've—"

But of a sudden the old man's voice died away, though his mouth was still moving.

Dick leaned out of his cockpit and fired a single red Very light, the signal for the attack. And from each plane of his flight, beneath him, a bomb slid from its rack and went hurtling down upon the gang below, while the airplanes circled and hovered, each taking up its station.

* * * * *

Dick was too late. By a whole minute he had missed his chance. He realized that immediately, for before the red light had flared from his pistol, the hostile planes were in the air. He had flown too low, and given the alarm.

It meant a fight now, instead of a mad dog destruction, and Dick did not underestimate the power of the enemy. But he felt a thrill of furious satisfaction at the prospect of battle. From every plane the bombs were falling. Underneath, ruin and destruction, and leaping flames—and yet darkness, save for the phosphorescent outlines of the buildings.

And the lines of these were broken, converging into strange criss-crosses of luminosity, as the beams fell in shapeless heaps. Dark fire, sweeping through the headquarters of the Invisible Emperor, a veritable hell for those below! A taste of the hell that they had made for others!

Then a strange phenomenon obtruded itself upon Dick's notice. Nothing was audible! The bombs were falling, but they were falling silently. No sound came up from beneath. And, except for the throbbing of his engine, Dick would have thought it had stopped. He could no longer hear it.

That terrific holocaust of death and destruction was inaudible. Skimming the upper reach of the air, high above that wall of darkness, Dick saw old Luke Evans pick up his end of the speaking-tube, and mechanically followed suit. He could see the old man's lips moving. But he heard nothing!

And now another phenomenon was borne in on his notice. His flight were perhaps five hundred feet beneath him, hovering a little above the barrage of black gas. But they were converging oddly. And there was no sight of the airplanes that Dick had just seen taking off from the invisible tarmac.

* * * * *

Dick fired two Very lights as a signal to his flight to scatter. What were they doing, bunching together like a flock of sheep, when at any moment the enemy planes might come swooping in, riddling them with bullets? He thrust the stick forward—and then realized that his controls had gone dead!

He thought for a moment that a wire had snapped. But the stick responded perfectly to his hand, only it had no longer control over his plane. He kicked right rudder, and the plane remained motionless. He pushed home the soaring lever, to neutralize the helicopter and the plane still soared.

Then he noticed that the needle of his earth-inductor compass-indicator was oscillating madly, and realized that it was not his plane that was at fault.

Underneath him, his flight seemed to be milling wildly as the ships turned in every direction of the compass. But not for long. They were nosing in, until the whole flight resembled an enormous airplane engine, with twelve radial points, corresponding to their propellers, and the noses pointing symmetrically inward, like a herd of game, yarding in winter time.

And now the true significance came home to Dick. A vertical line of magnetic force, an invisible mast, had been shot upward from the ground. The airplanes were moored to it by their noses, as effectively as if they had been fastened with steel wires.

And he, too, was struggling against that magnetic force that was slowly drawing him, despite his utmost efforts, to a fixed position five hundred feet above his flight.

* * * * *

For a few moments, by feeding his engine gas to the limit, Dick thought he might have a chance of escaping. Her nose a fixed point, Dick whirled round and round in a dizzy maze, attempting to break that invisible mooring-chain. Then suddenly the engine went dead. He was trapped helplessly.

He saw old Evans gesticulating wildly in the front cockpit. The old man hoisted himself, leaned over the cowling gibbered in Dick's ear. The silent engine had ceased to throb, and the old man's shouts were simply not translated into sound.

Suddenly the flight beneath jerked downward, just as a flag jerks when it is hauled down a pole. They vanished into the dark cloud beneath. At the same time there came a jerk that dropped Dick's plane a hundred feet, and flung him violently against the rim of the cockpit.

Another followed. By drops of a hundred feet at a time, Dick was being hauled down into the darkness underneath him.

It rushed up at him. One moment he was suspended upon the rim of it, seeing the moon and stars above him; the next he had been plunged into utter blackness. Blackness more intense than anything that could be conceived—soundless blackness, that was the added horror of it. Blackness of Luke Evans's contriving, but none the less fearful on that account!

And yet, as Dick was jerked slowly downward, slowly a pale visibility began to diffuse itself underneath. The black cloud was beginning to roll away. The luminous lines began to fade, and in place of them appeared little leaping tongues of fire. In front of him Dick saw Luke Evans's form begin to pattern itself upon the darkness. He saw the form move sidewise, and caught at Luke's arm as he was about to hurl another gas bomb. "No!" he shouted—and heard no sound come from his lips.

* * * * *

Luke understood. He seemed to be replacing the bomb in the rack. Beneath them now, as they were jerked downward, were fantastic swirls of black mist, and, at the bottom, a pit of fire that was slowly coming into visibility.

Dick uttered a cry of horror! Five hundred feet below his plane he saw the dim forms of his flight, still bunched together, noses almost touching. And they were dropping straight into that flaming furnace of ruin underneath, which was growing clearer every instant.

Down, jerk by jerk. Down! The black cloud was fast dispersing from the ground. The flight were hardly a thousand feet above the fire. Down—a long jerk that one! Once more! The flames leaped up hungrily about the doomed airships. Cries of mad horror broke from Dick's lips as he witnessed the destruction of ships and men.

He could see almost clearly now. The twelve ships, still retaining their nose-to-nose formation, were in the very heart of the fire. Spurts of exploding gasoline thrust their white tongues upward. There was only one consolation: for the doomed men, death must have come practically instantaneously.

From where he hung, Dick could feel the fierce heat of the flames below. In front of him, old Luke Evans sat in his cockpit like one petrified. He was feebly fumbling at his camera-box, as if he had some idea of using it, and had forgotten that it was fixed to the plane, but the old man seemed temporarily to have lost his wits.

Rushing flames surrounded the burning airships, reducing them to a solid, welded mass of incandescent metal. Dick looked down, waiting for the next jerk that would summon him to join his men. At the moment he was not conscious of either fear or horror, only intense rage against the murderers and regret that he could never bring back the news of victory.

* * * * *

The cloud had almost dissipated. In place of the phosphorescence, electric lights had appeared, making the ground beneath perfectly visible. Dick could see a number of men grouped together at the entrance to a large building, part of which had been wrecked by a bomb, though there were no evidences of fire. Other structures had been dismantled and knocked about, but what remained of them had not been charred by fire. Evidently they had been fireproofed. Perhaps the gas itself was incombustible. Only in the middle of the tarmac, where the remnants of the airplanes blazed, was there any sign of fire.

There were three machines resembling dynamos, placed one at each corner of the tarmac, equidistant from the central holocaust. A half-dozen men were grouped about each of them, and by the light from the huge reflector over each Dick saw that they were whirring busily. At the time it did not occur to him that these were the machines that were sending out the electrical force that had held the airplanes powerless.

But as he looked, his mind still a turmoil of hate and hopeless anger, he saw one of the three machines cease whirring. The group about it dispersed, the light above went out. And now his plane, as if drawn by the power of the two remaining machines, began to move jerkily again, not down toward the burning wreckage, but sidewise, away from it.