"He says the fire should be attached to the bladder, then the bladder will keep going up," said the box. Thorinn opened his mouth, then closed it again. With a charred twig he sketched on the side of a stone, while the children clustered around to watch: here the bladder, here the ring to hold the neck open, and here, suspended from the ring, a basket for the rider. Now, inside the basket, a fire pot lined with clay or earth: the fire ascends with the bladder, and as long as it burns the smoke cannot cool and turn to water; therefore the bladder stays aloft. But the basket can carry only so much wood to burn; when that is gone, down comes the bladder.
Thorinn fed more twigs to the fire to keep it from going out in the light drizzle. The air shimmered over the embers; flakes of white ash rose, wavered, and fell, yet there was no wind. Thorinn struggled with a thought: suppose it was not the smoke at all, but the air heated by the fire, that made the bladder rise?
At all events, he must begin to plan now for a bladder big enough to carry him and his possessions: how big must it be? The box was of no use: "That depends on the weight of the bladder and the lifting power of the air." When Thorinn asked what the lifting power of air was, the box replied, "That depends on how hot it is, and how hot the air around the bladder is."
"How am I to find that out?"
The box showed him a picture of a slender rod of glass, with marks on it and a thread of silver inside.
"This is an engine for measuring how hot a thing is."
"But where am I to get such an engine?"
"I don't know, Thorinn."
So it was evident that he must do it himself, and in truth he was rather glad of that, for when he asked the box's advice it always told him more than he wanted to know and more than he could understand, whereas when he worked a thing out for himself, no matter how difficult it was, at least when he was done he understood it.
That afternoon he made weights by cutting a stick into little pieces of equal length, and by attaching one after another to the neck of the bladder, determined how much it could lift when it was filled with hot air. He also found out by accident that he could measure equally well and much more easily by attaching a long cord to the bladder: then it rose until the lifting power of the air inside it exactly matched the weight of the cord which it raised from the ground.
From these studies Thorinn concluded that the height of the bladder ought to be at least eight times his own height in order to bear him and his belongings aloft. That meant a bladder sixteen or seventeen ells tall, much bigger than he had imagined it would be. He was tempted to make it smaller and therefore easier, but if it did not raise him, the work would be all for nothing. Because the bladder must bear its own weight as well as his, he wanted to make it as light as possible, and for this reason he gave up his first intention of making it out of pieces of leather or cloth; instead, he pieced it together out of the thin, parchment-like stuff the wingpeople used for interior walls and screens. Aided by Sven and Ilge, he cleared a space in one of the largest workshops and took what he needed. Presently the wingpeople brought more wall-stuff to replace what he had taken; when he needed more, he took that as well.
One morning he came upon Sven and Ilge in the workshop trying clumsily to fit a whittled stick into the hollow of a reed. Between amusement and sympathy, Thorinn explained to them through the box that both shaft and plunger must be perfectly round, or the fire stick would not work. Their grey-furred little faces were so earnest and trusting that he could not leave it at that, although he was impatient to get on with his own task. He found a good rod of hardwood in the wingmen's stores, for a plunger, and showed them how the shaft must be made in two halves carved to fit around it, then slowly tightened as the plunger was turned between them with wet sand to grind the pieces to a perfect fit, and finally glued together with an end-piece shaped to leave a hollow for the tinder. He left them toiling earnestly and clumsily at the task; he doubted that they could accomplish it, but at least they were happy in trying. He made his bladder in six sections shaped like a fish, each twenty ells long and nine ells wide. Under his direction, the children joined the pieces that made up each section with fish-glue, and hung the sections up to dry in the well. When that was done, Thorinn brought the sections back into his workroom, which in the meantime the wingpeople had begun to use again for their own purposes: he cleared out their benches, jars, and other rubbish, spread the sections on the floor, and began gluing the edges together. It proved exasperatingly difficult to make the flat pieces form a round shape without wrinkling or buckling, but after many failures he got the whole bladder assembled. He painted it all over with fish-glue, dried it again, and at last carried it outside for trial. Children trooped out after him. It was a still gray day. Thorinn unfolded his bladder and hung it from a cord stretched between two trees, with the neck about three ells from the ground. He made a ring of a bent sapling and secured it inside the neck. Beneath it he laid dry brush and branches in criss-cross layers, and kindled a fire. Flames curled up; smoke poured into the open neck of the bladder, and presently it began to fill. To Thorinn's disgust, a moment later there came a patter of raindrops in the trees and on the suspended limp bulk of the bladder. A gust of wind blew sparks slanting away from the fire; then the patter increased to a stuttering roar; water swept across the clearing in veils and torrents. Thorinn retreated to shelter until the rain stopped; by that time the fire was out.
Thorinn sent the children for dry wood. They found it without delay; the ground was dry only a few hundred ells distant. Thorinn built another fire and lighted it. After about the same interval, it rained again and put the fire out.
Thorinn saw then that whoever ruled the world, whether it was gods, demons, or even engines as the box seemed to think, they did not want him to inflate his bladder today. But he was loath to take it down, dry it and fold it, and put it away indoors, only to take it out again tomorrow. What if there were a roof over the bladder and the fire, to keep the rain off while the bladder filled? Such a roof would take him the rest of the day to build, and then it would be in the way when the bladder rose... But why not use the bladder itself as a roof?
He cut a pole a little less than three times his height, measured the bladder with it, cut it again, and used it as a measure to cut another the same length. He trimmed and whittled the ends carefully to roundness, so that they should not puncture the bladder. Leaping up with the first pole through the open neck, he managed to get it crosswise at an angle; then hanging from it and pulling the fabric of the bladder down, he adjusted it until it was level. He did the same with the second pole, crosswise to the first. Now the bladder was spread at four points to its full width of about seventeen ells, although it was hollow between. Thorinn built a third fire.
As he had more than half expected, it rained again. Now the spread bulk of the bladder sheltered the fire, although the rain smoked and streamed all around it. The hollow places between the poles began to fill out; watching from his place under the tree, Thorinn thought he saw the bladder straining upward. Exultantly, he took a step forward. While he was still in midair, there came a shattering crack and a white glare. Thorinn tumbled into the underbrush, blinded, stunned, and deaf. When he picked himself up again, the children's cries were receding in the distance and the bladder was in flames. Yellowish smoke was pouring from the fire; presently it went out. The rain continued, turning the fire into a soggy pile of ash. What was left of the bladder hung by its cord from a single tree; the other was split and slivered at the base, as if it had been struck by a mighty hammer; bits of the white wood lay scattered all over the clearing.