"Yes. I wonder if -- ah. The burners! Quick!"

The balloon was collapsing.

They worked grimly, dragging themselves into the rigging and cutting away the burning wicks. The envelope crumpled over the doused lamps.

And Boyd was upside down.

Or Allel was.

Her harness was slack. The components of their balloon drifted in a jumble. Boyd thrashed in the air as if drowning -- but there was no up to kick towards. Fear showed beneath her pale scars.

But Allel understood.

"It's the middle of the Gap!" Allel yelled, exhilarated by her mother's discomfiture. "The Shell dwellers live upside down. Up for us is down for them. Did we think we'd fly up and bump against the Shell like a ceiling? This is the place where up and down cross over!" Warm air spilled from the balloon and brushed her face. Ground and Shell were enormous parallel plates that careened identically around her. She laughed and swooped.

But their equilibrium in the weightless zone was unstable, and soon invisible fingers clutched at them. Wind whistled in the tangled rigging and their harness grew taut again. "We're falling back!" Allel cried in disappointment. Boyd struggled to keep her good arm free.

Now air resistance roughly righted them. The balloon opened out like a parachute but scarcely slowed their fall.

Boyd roared above the wind: "We've got to light the burners!"

They hunted for flints and cupped their hands around the wicks to keep out the snatching breeze. Heat roared up. Boyd thrust at the fuel pumps while Allel scrambled precariously into the tangled rigging to drag at the neck of the envelope, trying to trap all the warmed air.

Their descent slowed a little. Allel's arms ached and her hair whipped at her forehead. The ground exploded into unwelcome details, rivers and hills and trees and pebbles--

She rolled on impossibly hard earth, grass blades clutching at her face. Her blood was loud in her ears. The balloon folded as if wounded.

In a sunlit meadow, mother and daughter lay amid the ruins of their bark spaceship.

Sunlight scoured her eyes. Allel sat up, blinking, pushing at the knotted remains of her harness. She was surrounded by cool grass and flowers; a brook led to a stand of cow-trees and the horizon was made up of heather-coated hills.

And, as it had always done, the Shell curved over it all like a great blue tent.

Boyd slept peacefully in a tatter of the balloon. Allel hesitated for some minutes, vaguely fearful of her mother's reaction. Then she found a remnant of a shattered burner and woke her mother with a cup of brook water. Boyd sat up clumsily, favoring her bad arm.

"We failed," Allel said.

"Huh?"

Allel pointed at the Shell above them. "Look. We must have fallen back. If we'd reached the Shell we'd see the world up there, a ball of rock, cupped by the Shell. And the land would tilt up at the horizon..."