Clearing some of the moss away reveals a piece of shining metal. I brush more of the moss away on either side; it is long, like a pipe, and fastened to the floor of the channel. In cross-section it has the shape of a bloated-looking I. Closer inspection proves that it runs under the moss to each side, raising the green-brown surface in a continuous, hardly-noticeable ridge. On the other side of the tunnel there is a similar raised line of moss, near the wall.

I jump up, hurriedly brushing the moss back over the section of rail I have uncovered.

As I do so, the thick, warm air starts to move slowly past me, and from far down the tunnel's narrow curve there comes the faint, thin sound of a siren, coming closer.

Slightly hungover, waiting for my kippers in the Inches breakfast bar, I wonder whether I ought to take that drawing by Miss Arrol down from my wall.

The dream disturbed me; I woke up sweating, and lay tossing and turning in my bed, still wet with sweat, until finally I had to get up. I had a bath, fell asleep in the warm water and woke up, freezing cold, terrified, jarred as though electrocuted, suddenly certain in my immediate confusion that I was trapped in some constricting tunnel: the bath a tunnel-canal, its cold waters my own sweat.

I read the morning paper and sip my coffee. The Administration is being criticised for not having prevented yesterday's flypast. Unspecified new measures are under evaluation with a view to preventing further violation of the bridge's airspace.

My kippers arrive; their filleted bones have left a pattern on the pale brown flesh. I recall my thoughts on the general topography of the bridge. I try to ignore my hangover.

There are three possibilities:

The bridge is just that, a link between two landmasses. They are very far apart, and the bridge leads an existence independent of them, but traffic crosses the bridge from one to the other.

The bridge is, effectively, a pier; there is land at one end but not the other.

The bridge has no connection with land whatsoever, save for the small islands beneath every third section.

In cases 2 and 3, it might be still under construction. It may only be a pier because it has not yet reached the farther landmass, or if it has no connection with land, it might still be being built not just at one end, but two.

There is one interesting sub-possibility in case 3. The bridge appears to be straight, but there is a horizon, and the sun rises, arcs, falls. So the bridge might eventually meet itself, form a closed circuit; a circle, vertically, ringing the globe; topographically closed.

Visiting the library on the way here to look at a braille book reminded me of the still-lost Third City Library. I feel quite recovered after my breakfast and decide to walk to the section which houses both Dr Joyce's office and the fabled library. I'll have another crack at finding the damn thing.

It is another fine day; a soft warm wind blows up-river, slanting the wires of the barrage balloons as the grey blimps try to drift towards the bridge. Extra balloons are being floated into the sky; large barges support the half-inflated shapes of even more balloons, and some of the trawlers have already been equipped with two balloons, producing a giant V of cables in the sky above them. Some of the balloons have been painted black.

I walk whistling across the linking span from one section to the next, swinging my stick. A plush but conventional elevator bears me up to the highest available floor, still a few levels down from the actual summit of the section. The dark, tall, musty-smelling corridors up here seem familiar now, in their general character at least; the exact lay-out remains a mystery.

I walk beneath the ancient, age-grimed flags, between the niches occupied by stone-remembered officials, past rooms full of whispering, smartly uniformed clerks. I cross dim, white-tiled lightwells on rickety cross-corridors, peer through keyholes into locked, dark, deserted passages whose floors are inches deep in dust and debris. I test the doors, but the hinges have rusted.

Finally, I come to a familiar corridor. A large round patch of light glows on the carpet ahead, where the corridor broadens out. The air smells damp; I'd swear the thick, dark carpet squelches with each footfall. I can see tall pot-plants now, and a length of wall which ought to hold the entrance to the L-shaped lift. The patch of light on the floor has a shadow in the centre of it which I don't recall. The shadow moves.

I reach the light. The great round window is there, still staring down-river like a huge handless clock-face. The shadow is cast by Mr Johnson, Dr Joyce's patient who refuses to leave the cradle. He is cleaning the window, polishing the glass at its centre with a rag, an expression of rapt concentration on his face.

Behind and a little below him, in mid air, well over a thousand feet above sea-level, floats a small trawler.

It is suspended on three cables, it is black-brown in colour, rust-streaked above the waterline and barnacle-encrusted below it. It floats slowly towards the bridge, rising as it nears.

I walk towards the window. High above the drifting trawler are three black barrage balloons. I look up at the still-polishing Mr Johnson. I knock on the glass. He takes no notice.

The trawler, still rising, heads directly for the great round window. I bang on the glass as high as I can reach; I wave my stick and hat and shout as loudly as I can, 'Mr Johnson! Look out! Behind you!' He stops polishing, but only to lean forward, still smiling beatifically, breathe on the glass, and then start polishing again. I hammer on the glass near Mr Johnson's knees; as far as I can reach, even with my stick. The trawler is twenty feet away. Mr Johnson polishes happily on. I beat on the thick glass with the brass knob on the tip of the stick. The glass chips, cracks. Fifteen feet; the trawler is level with Mr Johnson's feet. 'Mr Johnson!' I thrash at the cracking pane of glass; it finally smashes, spraying shards. I stagger back from the hail of fragments. Mr Johnson is scowling furiously in at me. Ten feet.

'Behind you!' I scream, thrusting my stick out to point, then running for cover.

Mr Johnson watches me run, and turns round. The trawler is a fathom-length away. He dives to the deck of his cradle as the trawler crunches into the centre of the great round window, its keel scraping the rail of Mr Johnson's cradle and showering him with barnacles. Panes shatter, glass showers down onto the broad landing; the sound of breaking glass competes with that of creaking, fracturing metal. The trawler's stem grinds through the centre of the window, its metal frame bends like a huge spider's web, making a terrible groaning, screaming sound. The structure around me shakes.

Then it stops. The trawler appears to bounce back a little, then scrapes and grates its way up over the top part of the great mandala, breaking more glass as it goes; barnacles and fragments of the shattered panes fall together onto the carpet, beating at the broad leaves of the nearby pot-plants like some hard, fierce rain.

Then, incredibly, it is gone. The trawler disappears from view. Glass stops falling. The sounds of the boat's scraping progress up the side of the remaining floors of the upper bridge quiver through the air.

Mr Johnson's cradle swings to and fro, gradually slowing. He stirs, looks about him, and stands up slowly, spears of fallen glass sloughing from his back like glittering snakeskin. He licks at a couple of cuts he has sustained on the backs of his hands, carefully brushes his shoulders free of some dusty grains of glass, then walks along his still slightly swaying platform and picks up a short-handled broom. He starts to brush the fragments of window-pane off the cradle, whistling to himself. Every now and again, as he sweeps, he looks, with an expression of sad concern, at the bowed-in shattered mess of the great circular window.