She’d brought a small bark satchel from the teepee; now she arranged it over her shoulder and scurried over the rough track to Hafen’s Hill. From the summit she could see the Atad river, a glistening track to the south; the Bridge looked like an indestructible toy, one of the few of the old structures not yet swallowed by the ice. Smoke blurred the scene. She wondered if that was a good sign.

She soon forgot the distant battle as she got to work. She opened her satchel and drew out a small lamp, a gourd filled with alcohol fermented from cow-tree fruit. She cut a length of wick with the big stone knife her grandfather had made for her. She held a flint to the wick; it curled and popped as black smoke seeped into the crisp air. Now she opened out a small bag, a rough globe. She held its narrow neck over the flame, and soon her fingers were coated with lamp-black—

— and the simple balloon filled up and lurched a few feet into the air. Then it turned belly-up and flopped to the ground. Allel bared her teeth at the Shell as if she owned it already; her heart beat as had that lost bird’s. Now then, a little more weight around the mouth…

A sandal stamped down, crushing the balloon. The bark of the sandal was crusted with blood and dust.

“Get up.” Boyd spat the words; blood leaked from a new wound over her eyes.

Allel stood, furious. Her anger collided with her mother’s contempt. Save for the scars of battle, the years had been easy on Boyd. Mother and daughter faced each other like twins, images in a dark mirror.

“Our attack on the Bridge failed,” Boyd ground out. “Those bastards holding it want to keep the whole bloody south to themselves. Good people died. And you — you won’t even help the old folk with their chores. What do you think you’re doing?”

Allel picked up the sputtering lamp. “I doubt if you’d understand,” she said haughtily.

Boyd slapped the lamp from her hands. It smashed against a rock; alcohol pooled and puffed into flame. “You waste your time on rubbish. Don’t dare to speak to me like that.”

Allel bit back her rage. “I fill the bag with smoke. It flies. Build one big enough and I could fly with it—”

“More rubbish.” Boyd hawked and spat out a ball of bloodstained phlegm; it sizzled in the alcohol fire. “If it’s ever left up to you, we’ll all die of rubbish.” She grabbed a handful of Allel’s tunic; her breath was sour. “Or I’ll kill you first. And that’s not rubbish.” She strode off down the Hill’s broken flank. “Come on. You’ll be grown soon. It’s time I put a stop to your questions.”

Allel didn’t move. “Where are we going?”

“North. To the place where our people once lived, before the cold drove them out. North to the City.”

“Why should I come?”

Without looking back, Boyd said simply: “Because if you don’t I’ll break your rubbish neck.”

Allel looked back ruefully at her home, where the fires of the recent night were still burning. Then she clutched her crumpled shirt closed against the wind and followed her mother.

The breeze lifted the abandoned balloon; its final flight ended in the ruins of the lamp, where it began to burn fitfully.

The Sun wove its helical web around the world.

When night fell Boyd and Allel sheltered beneath a wild cow-tree. In silence, they drank from its milk nipples and broiled slices of meat fruit over a small fire. Boyd slept sternly beneath her quilted coat. Allel shivered in her thin garments, and burrowed into a nest of leaves. She peered up sourly at the Shell’s seamless dark, picking out clustered fires.

In the morning she stuffed leaves inside her clothes and fashioned herself a rough cap of cow-tree bark.

After some days of this the frost grew more persistent, until their feet crunched over thin ice. Light snow fell. They passed a few abandoned settlements; even the hardy cow-trees grew sparse here.

A blizzard closed around them like a white mouth. They staggered up to the milkless corpse of a cow-tree. Allel stared at the shrunken nipples and withered fruit. Boyd laughed at her, her eyelids sprinkled with snowflakes. “Comes as a shock, doesn’t it? A dead cow-tree. We were given a world filled with beautiful buildings, and cow-trees to feed and clothe us like mothers. A home safe from the Xeelee.

“But the world’s old and falling apart. The Sun seems to be failing. Ice has covered the cities and frozen the milk in the cow-trees. We trudge through the snow.” She began digging into the snow packed against the dry wood. “Come on. We’ll let this lot blow itself out. The snow will keep you warm.”

As she worked, Allel considered a changeless life of endless summer. What would there be to do all day? Her bare fingers grew numb.

When the storm blew over they continued the journey. With the Shell like a map over them it was impossible to get lost, and at last they came to the lip of a great natural bowl. Snow pooled around the low buildings of the City, which were sprinkled in two matching crescents.

Allel, used to crude teepees of cow-tree bark, touched walls that were as smooth as skin. But the interiors were cold and jumbled, and snow drifted waist-deep in the avenues.

Lifting heavy legs out of the snow, they forced their way to the common center of the City’s twin crescents. Here was a small cylindrical building, no more than three paces across. Allel helped her mother scrape snow from the door. Boyd blew on damp fingers. “Go ahead,” she said slyly. “You first.” Allel pushed through the light door—

— and stared in astonishment at the far wall of the chamber, at least a hundred paces away. She stumbled backwards and landed in the snow, which soaked into her thin trousers. Boyd laughed, not unkindly, and hauled her to her feet. “A vast hall crumpled into a tiny hut. The people who built this had powers even you never imagined, eh?”

Allel stumbled around the tiny building. Where was all that space stored? If not sideways — or behind — or up, or down — what fourth direction was there? The puzzle settled behind her eyes like a spider.

The floor area was empty, but the paper-thin walls were covered with pictures, still lit and animated after uncounted generations. “The pictures tell our story,” Boyd said gruffly. “How we rose and fell.” She stamped snow from her sandals and led the way around the walls. Afterwards Allel thought they could have walked in the opposite direction and lost little of the sense, for the story of humankind had a symmetrical design.

The bright side of the symmetry was expansion. From a world without a Shell, tiny ships like streamlined fish swam out on hyperdrive to the stars…

“What was ‘hyperdrive’? And ‘stars’?”

They were just words, Boyd said, passed on by other mothers on other days. Allel wondered if her balloon had risen on hyperdrive. She looked closely at the ships but could see no sign of burners. She tried to touch the picture—

— and her hand passed into the depthless wall, in a direction she could not identify. She fingered a model ship; it was like a nut drawn on an invisible string. More mysteries…

At its peak humanity was a master of many stars — which were evidently places very far away. And then—

“And then we met the Xeelee,” said Boyd, and they inspected a harrowing battle scene. Elusive fingers snatched at the little ships. “Whoever they were, they were too big for us.”

After the Xeelee wars came the dark obverse of humanity’s conquest of the stars: its sad subsidence back to its home world, prodded by the dark fingers of the Xeelee.

They came to the last two panels. Boyd said: “Finally we returned to our home and rebuilt it as a place safe from the Xeelee.” The first panel showed a sphere, blue capped with fat brown poles. Painted onto the central cerulean band were clouds and a tiny Sun that twinkled along the equator. The fringes of the polar caps held a lot of detail: sideways-on pictures of trees and men, oriented as if the clouds were “up” and the poles were “down.” “I don’t understand this one,” Boyd admitted. “Maybe it was a stage in the Shell’s construction. But here’s the world as it is now.” The last image was crudely sketched on the surface of the wall, with no depth or animation. It showed a globe with a Shell around it. Allel picked at flaking paint. Boyd coughed self-consciously. “So, you understand now why I brought you here?”