A moment later he noticed a second ptertha, then a third, and within a minute he had picked out eight of the globes, all forming a single cluster. They were riding out on a gentle north-west breeze, and they faded from his vision some distance to his right where parallax merged the vertical strands of the mesh into a seemingly close-woven fabric.

Ibbler, watchful but still unconcerned, waited for the ptertha, to reappear in his field of view. On encountering the outer screen the globes, obeying the dictates of the air current, would nuzzle their way southwards along the camp’s perimeter and eventually, having found no prey, would break free and float off towards the south-west coast and the Otollan Sea.

On this occasion, however, they seemed to be behaving unpredictably.

When minutes had passed without the globes becoming visible, Ibbler’s young companions noticed that he had dropped out of the conversation. They were amused when he explained what was in his thoughts, deciding that the ptertha — assuming they had existed outside Ibbler’s imagination — must have entered a rising air stream and gone over the camp’s netted roofs. Anxious to avoid being classed as a nervous old woman, Ibbler allowed the matter to rest, even though it was rare for the ptertha to fly high when they were near humans.

On the following morning five diggers were found dead of pterthacosis in their hut. The soldier who blundered in on them also died, as did two others he ran to in his panic before the isolation drills were brought into force and all those thought to be contaminated were despatched along the Bright Road by archers.

It was Ibbler who noticed that the diggers’ hut was close to and downwind of the point where the group of ptertha would have reached the perimeter on the night before. He secured an interview with his commanding officer and put forward the theory that the ptertha had destroyed themselves against the outer screen as a group, producing a cloud of toxic dust so concentrated that it was effective beyond the standard thirty-yard safety margin. His words were noted with considerable scepticism, but within days the phenomenon they described had actually been witnessed at several locations.

None of the subsequent outbreaks of the ptertha plague was as well-contained as at Trompha, and many hundreds had died before the authorities realised that the war between the people of Kolcorron and the ptertha had entered a new phase.

The general population of the empire felt the effect in two ways. Buffer zones were doubled in size, but there was no longer any guarantee of their efficacy. A light, steady breeze was the weather condition most feared, because it could carry invisible wisps of the ptertha toxin a long way into a community before the concentration fell below lethal levels. But even in gusty and variable wind a large enough cluster of ptertha could lay the stealthy hand of death on a sleeping child, and by morning an entire family or group household would be affected.

The second factor which accelerated the shrinkage of population was the further drop in agricultural output. Regions which had known food shortages began to experience outright famine. The traditional system of continuous harvesting now worked against the Kolcorronians because they had never developed any great expertise in the long-term storage of grain and other edible crops. Meagre reserves of food rotted or became pest-ridden in hastily improvised granaries, and diseases unconnected with the ptertha took their toll of human life.

The work of transferring huge quantities of power crystals from Chamteth to Ro-Atabri continued throughout the worsening crisis, but the military organisations did not go unscathed. Not only were the five armies stood down in Chamteth — they were denied transportation to Kolcorron and the home provinces, and were ordered to take up permanent residence in the Land of the Long Days, where the ptertha — as though sensing their vulnerability — swarmed in ever-increasing numbers. Only those units concerned with gutting the brakka forests and shipping out the cargoes of green and purple crystals remained under the protective umbrella of Leddravohr’s high command.

And Prince Leddravohr himself changed.

In the beginning he had accepted the responsibility for the Overland migration almost solely because of loyalty to his father, offsetting his private reservations against the opportunity to conduct an all-out war against Chamteth. Throughout all his preparation for the building of the fleet of skyships he had nourished deep within him the belief that the unappealing venture would never come to fruition, that some less radical solution to Kolcorron’s problems would be found, one which was more in keeping with the established patterns of human history.

But above all else he was a realist, a man who understood the vital importance of balancing ambition and ability, and when he foresaw the inevitable outcome of the war against the ptertha he shifted his ground.

The migration to Overland was now part of his personal future and those about him, sensing his new attitude, understood that nothing would be allowed to stand in its way.

“But today of all days!” Colonel Kartkang said forcibly. “I suppose you realise your take-off is scheduled for the tenth hour?”

He was lightly-built for a member of the military caste, with a round face and a mouth so wide that there was a visible gap between each of his smallish teeth. A talent for administration and an unfailing eye for detail had brought him his appointment as head of Skyship Experimental Squadron, and he clearly disliked the idea of permitting a test pilot to leave the base shortly before the most important proving flight in his programme.

“I’ll be back long before that time, sir,” Toller said. “You know I wouldn’t take the slightest risk in this matter.”

“Yes, but… Do you know that Prince Leddravohr plans to watch the ascent in person?”

“All the more reason for me to be back in good time, sir. I don’t want to risk high treason.”

Kartkang, still not easy in his mind, squared a sheaf of papers on his desk. “Was Lord Glo important to you?”

“I was prepared to risk my life in his service.”

“In that case I suppose you had better pay your last respects,” Kartgang said. “But keep it in mind about the prince.”

“Thank you, sir.” Toller saluted and left the office, his mind a battleground for incompatible emotions. It seemed cruelly ironic, almost proof of the existence of a malign deity, that Glo was to be buried on the very day that a skyship was setting out to prove the feasibility of flying to Overland. The project had been conceived in Glo’s brain and had brought him ridicule and disgrace at first, followed by ignominious retirement, and just as he was about to receive personal vindication his beleaguered body had failed him. There would be no plump-bellied statue in the grounds of the Great Palace, and it was doubtful if Glo’s name would even be remembered by the nation he had helped to establish on another world. Everything should have been very different.

Visions of the migration fleet touching down on Overland brought a resurgence of the icy excitement which Toller had been living with for days. He had been in the grip of his monomania for so long, working with total commitment towards selection for the first interplanetary mission, that he had somehow lost sight of its astonishing realities. His impatience had slowed the passage of time so much that he had unconsciously begun to believe his goal would forever remain ahead of him, flickering beyond reach like a mirage, and now — with shocking suddenness — the present had collided with the future.

The time of the great voyage was at hand, and during it many things would be learned, not all of them to do with the technicalities of interplanetary flight.