AT sunset on the 15th the sluice-door had been finally lowered into its place and the pent-up waters of the lake of Aeria had risen nearly forty feet by the next morning. Only the upper parts of the villas on its banks were visible and its area was so enormously increased that the whole appearance of the valley was altered.

Rising at first at the rate of three feet an hour, a rate which of course decreased as the area became greater, the waters would reach the entrance to the caverns soon after sunset on the evening of the fatal 23rd.

A little before midnight on the 21st the Orion, the sentinel ship that was on guard at the time, sank swiftly down with the news that she had made out by the light of the Fire-Cloud which, lurid and ghastly as it was, was as brilliant and penetrating as that of the sun at noonday, a large fleet of air-ships approaching from the northwards. The city was by this time almost entirely submerged. Only a few minarets and towers and the top of the great golden dome of the temple surmounted by its crystal-winged figure, showed above the surface.

The remnant of the people of Aeria, now reduced to less than seven thousand souls, including those chosen to take refuge in the caverns, were occupying the villas on the slopes of Mount Austral about the entrance to the caverns. Six thousand of them were men who had lived solely in the hope of such an attack as was now about to be made and which would enable them to die fighting the common enemy of mankind to the last in defence of their beloved native land.

Not even now, when the hand of Destiny had set a definite limit to all human hopes and fears, and when the remainder of their own lives could be counted by hours, could this faithful remnant of the Aerians endure the thought that what had been their paradise and their home should be violated and polluted by the appearance of their foes.

Therefore they had lived for this last battle, and five hundred air-ships were waiting to carry them into the air to engage in the last fight that ever would be fought on earth. All their friends and kindred, saving only the Children of Deliverance, as in fond fancy they had called the little band of the chosen ones, were now dead, and the few hours of life that were left to them had nothing more to give them.

So they received with a grim joy the summons to battle which had been so long expected. Four thousand of them manned the air-ships, the rest occupied the mountain batteries, and within a quarter of an hour of the bringing of the news the war-ships had mounted into the air, and the great guns of the batteries were ready to hurl their projectiles upon the advancing foe.

It was a spectacle to make angels weep and devils laugh, this last marshalling of the forces of human hate and hostility in the closing hours of the life of humanity and on the threshold of eternity. It seemed that the Tragedy of Man was to be played out to the bitter end, and that human strife was only to cease on earth with the destruction of the world. This, too, was the work of a single woman inspired by quenchless hatred and insatiable ambition and a pride of spirit which, in its haughty incredulity, still refused to believe that the end of her conquering career had come.

Pitiless and without scruple to the end, Olga, while she was recovering from her wound under the shelter of the Sultan’s roof, had managed, with the aid of her waiting-woman Anna, not only to poison the Grand Vizier Musa and Hakem the astronomer, but also to bring Khalid himself into the same state of moral slavery in which she had so long held Alan and Alexis.

It was she who had brought this fleet from Alexandria to Aeria. Once under the fatal spell of her will-poison, she had commanded Khalid to revoke the orders that he had given for peace, and he had obeyed. A fleet of more than five hundred air-ships had been collected, and, taking Khalid with her on board the Revenge, so that there should be no chance of his recovering his volition, she had come to fulfil the prophecy which Paul Romanoff uttered when in the last hour of his life he had declared that one day the Eagle of Russia should fly over the battlements of Aeria.

All the materials for constructing ten air-ships had been taken into the caverns, so that in the event of the remnant surviving the empire of the air should still be theirs, but the Alma and the Isma still lay outside the entrance when the other ships had risen into the air.

At the supreme moment a controversy had arisen as to whether or not Alan and Alexis—the latter of whom had been placed without question among the chosen, not only because of his unequalled engineering skill, but also because without him a daughter of the House of Arnold would have died of her own will—should or should not take part with their companions in the near approaching conflict.

This dispute was brought to a sudden close by Alan, who, with a sudden inspiration, cut short all the loving entreaties that were being made to him to take refuge in the caverns and avoid the chance which in the heat of the conflict might destroy with him the male line of the descendants of the first conqueror of the air.

“Do you not see,” he said, “that it is quite possible that their fleet may be twice as strong as ours, and that in spite of all our gallant forlorn hope can do they may cross the mountains and send their shells into the valley?

“What if one of them exploded here and wrecked the outworks and the entrance to the caverns? All hope, even for us, would then be lost, the doors could not be lowered, and we should either have to let the waters of the lake flow out or they would flow into the caverns by the upper entrance and ruin all our labours.

“We have proved that the Alma and the Isma are the two best air-ships in existence. They can soar higher and travel faster than any others. Would it not be madness to deprive our defending force of them, and would it not be cowardice in us not to do all we can to save all that is left for us to hope for on earth? I for one shall go, and I don’t believe that I shall go alone.”

“If the Alma goes the Isma goes too,” said Alexis. “Alan is right. We should be cowards to turn our backs on the enemy at the last moment.”

“And if you go, we go,” said Alma and Isma in a breath. “If you live we will live with you, but we will not live without you.”

There was no answer to such reasoning as this, nor was there any longer any law on earth save that of individual will. The first motive power that had swayed the world was the last that survived and would be the last to die. Those of the old crews of the two air-ships who were found among the chosen at once came forward to take their places, and with them came too those who had elected to take the hazard of life or death with them.

“There shall be no widows in the new world,” said they. And so every man who rose into the air on board the two great warships carried with him the woman without whom the one last chance of life would not have been worth taking.

As they left the earth the remainder of the little company retired into the caverns, leaving two sentinels posted at the outer door ready to give the alarm in case it should be necessary to lower the doors. As they did so a long, dull, distant roar came from the northward telling that the last battle of man with man had begun.

In accordance with a plan hastily arranged before they rose, the Alma was to guard the northern end of the valley, while the Isma kept watch over the southern. They soared up and up until the peaks of the mountains were a good five thousand feet below them.

From this elevation those on board the Alma could see the enemy’s fleet stretching out in a huge crescent, made up of tiny points of light which shone in the unnatural glare that illumined the earth and sky, and ever and anon they saw enormous spheres of flame blaze out along the line as the projectiles from the land batteries burst in front of them. The gunners were only trying their range and the enemy were still beyond it.

The explosion of the projectiles told the assailants that Aeria was on the alert, still prepared for battle and still, for all they knew, as impregnable as ever. Seeing this, they ceased their advance and a battle of tactics preceded the pitiless struggle which only the victors would survive.

Hour after hour the Moslem and Russian air-ships strove to out-soar the Aerians, or to make a rush in twos and threes that would bring them within range of the charmed circle of the mountains. But no sooner did one of them sweep up at full speed out of the distance and slow down sufficiently to train her guns than the atmosphere about her was convulsed with a mighty shock and changed instantly into a mist of fire, and when this vanished she had vanished too, shattered to fragments which dropped in a rain of molten metal thousands of feet to the earth below.

Morning came, the flaming arch of the Fire-Cloud sank lower and lower in the heavens until it stretched a broad band of lurid light round the western horizon, and an unclouded sun brought the last dawn but one that the terror-maddened myriads of earth would ever see. Still the fight went on at long ranges; still ship after ship of the hostile fleet made its desperate effort to cross the invisible barrier which was drawn all round Aeria by the range of its protecting guns, only to be overturned and hurled to the earth by the shock of an exploding projectile or to be fairly struck and dissolved to dust.

No matter how high they attempted to soar, the Alma and the Isma were still above them, and if the shells from the land batteries failed to do their work the guns of the air-ships did it for them and the result was the same—annihilation.

The night of the 22nd was spent in incessant attack and defence. The crews of the Aerian ships, grown desperate in their supreme despair, now left the mountains and sallied forth into the open, engaging the enemy ship for ship and gun for gun in a last determined effort to destroy them, or be destroyed, and far out from the still untouched battlements of Aeria the fight raged fast and furious.

There now was no thought of safety in the hearts of the Aerians. They had come forth to kill and be killed. The rules of aerial tactics were utterly neglected. They laid their guns alongside and, rushing through the air at their utmost speed, they hurled themselves with the ram upon every Moslem or Russian vessel that they could meet or overtake, crashing into her with irresistible force and going with her into annihilation as their two cargoes of shells exploded under the shock.

The last sun rose and saw the fight still going on. What had begun as the greatest battle in the history of war had now dwindled down to a series of single combats. At length the end came. It was a few minutes after midday that the last blow in the battle was struck. Ten Russian and Moslem air-ships, all that remained of the great fleet that Olga had brought against Aeria, formed in line ten miles from the Ridge and made a last attempt to break through the defences.

Flying through a storm of shells from the land batteries, seven of them were torn to pieces and the other three, just as they reached the Ridge, were met obliquely by the five remaining vessels of the Aerian fleet. The same moment the Alma’s broadside was discharged upon them, friend and foe vanished together in a mist of flame—and so ended the assault and defence of Aeria.

“We can go down now!” said Alan in a broken voice to Alma, who was standing white and speechless with horror at his side in the bows of the air-ship. “It is all over! God rest their gallant souls, for they left the world like brave men and true Aerians!”

“Amen!” sighed Alma. Then, after a brief pause, she said—“I wonder whether Olga Romanoff is alive or dead?”

The two air-ships now sank together and alighted close to the entrance to the caverns.

There the splendid fabrics were reluctantly abandoned, their crews disembarked, taking with them everything they wished to preserve, and a minute inspection was made for the last time of the triple doors and the machinery for lowering them and filling the spaces between them with water to be frozen as soon as they were in their places.

This occupied the time until the evening, and then all went once more into the open air to take what might be their last look at the sun. The waters of the lake were now within a few feet of the entrance, creeping more and more slowly upwards, and across the vast expanse of water, lying unruffled by the lightest breeze, fell the mingled rays of the sinking sun and the brightening Fire-Cloud.

There was not a cloud in the heavens and no breath of wind relieved the almost suffocating heat of the inert and sultry air. It seemed as though all terrestrial nature lay paralysed in a stupor of terror, waiting for the fire-blast that would wither it into death and ruin.

As the sun sank down behind the veil of flame his disc loomed redly and dully through it. Long streams of fire, blue and green and orange, darted across the disc and leapt and played round its circumference until it sank finally out of sight. The little group on the shore of the lake gazed at each other in silence as it disappeared.

Their faces looked wan and ghastly in the awful light that now reigned supreme in the heavens. Most of them turned away in grief and horror too deep for words, and with one last look at earth and sky, crept into the caverns, unable any longer to support the terror of the scene.

But a few remained, determined to see the fearful drama played out to the end, if they could, and among these were Alan and Alexis, whose duty kept them by the doors, the President and Francis Tremayne, and Alma and Isma, whom nothing could persuade to leave their husbands’ sides.

No human eyes had ever beheld so magnificent or so awful a display of celestial splendours as they beheld during the three hours that they stood in the doorway after sunset. The Fire-Cloud now covered almost the whole heavens, and its enormous nucleus blazed like a gigantic sun down out of the zenith with a heat and radiance that were almost insupportable.

Huge masses of flame leapt out continuously, as though hurled from its fiery heart, and were projected far beyond its circumference, while the incandescent cloud-mass which surrounded it was torn and convulsed by internal commotions which spread out and out in enormous waves of many-coloured fires until they disappeared below the horizon.

Still there was neither sound nor breath of wind upon earth, only the awful stillness in which the world waited for the hour of its doom to strike. At last, towards ten o’clock, the water began to lap the threshold of the entrance, and Alan, pointing to it, said—

“Come, we must take our last look at the world! It is time to lower the doors.”

The words were scarcely out of his mouth before a low dull booming sound came echoing down the gorges of Mount Austral. They looked up and saw huge masses of snow and ice loosened from its upper heights gliding, at first slowly and then more and more swiftly, down towards the valley beneath, a mighty avalanche which in a few minutes more would carry irresistible ruin in its path.

“In with you all!” cried Alan. “Quick! That is the beginning of the end; the snows are melting and the waters will be over us in another hour.”

All but he and Alexis hurried in, and they, grasping the levers on either side of the door, pulled them, and the enormous sheet of steel descended quickly along its grooves and shut them in from the outer world, upon which chaos was about to fall.