d.i.c.k 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 d.i.c.k'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 pa.s.sed, 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.

d.i.c.k, 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. d.i.c.k 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 d.i.c.k 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 d.i.c.k 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 d.i.c.k to rush in to battle. To do so might call off that deadly barrage that held the murderer in a ring of death. Hovering, d.i.c.k watched. And then, perhaps panic-stricken, perhaps rendered desperate, perhaps through sheer, wanton courage that might have commanded admiration under n.o.bler circ.u.mstances, 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 d.i.c.k'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 sh.e.l.ls 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.

d.i.c.k 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 sh.e.l.ls 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. d.i.c.k 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 d.i.c.k 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.

d.i.c.k sprang from his c.o.c.kpit as the old man rose in alarm. He ran to him and caught him by the arm.

"Luke Evans!" he cried. "Thank G.o.d 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 pa.s.ses 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."

"d.a.m.ned 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 quant.i.ties 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."