"Archaeopteryx",' Franz read off one of the cage indicators. The bird, lean and mildewed, uttered a painful croak when he fed a handful of beans to it.

'Some of these birds have the remnants of a pectoral girdle,' Franz said. 'Minute fragments of bone embedded in the tissues around their rib cages.'

'Wings?'

'Dr McGhee thinks so.'

They walked out between the lines of cages.

'When does he think they were flying?'

'Before the Foundation,' Franz said. 'Three million years ago.'

When they were outside the museum they started down 859th Avenue. Halfway down the street a dense crowd had gathered and people were packed into the windows and balconies above the elevated, watching a squad of Fire Police break their way into a house.

The bulkheads at either end of the block had been closed and heavy steel traps sealed off the stairways from the levels above and below. The ventilator and exhaust shafts were silent and already the air was stale and soupy.

'Pyros,' Gregson murmured. 'We should have brought our masks.'

'It's only a scare,' Franz said. He pointed to the monoxide detectors which were out everywhere, their long snouts sucking at the air. The dial needles stood safely at zero. 'Let's wait in the restaurant opposite.'

They edged their way over to the restaurant, sat down in the window and ordered coffee. This, like everything else on the menu, was cold. All cooking appliances were thermostated to a maximum 95...F., and only in the more expensive restaurants and hotels was it possible to obtain food that was at most tepid.

Below them in the street a lot of shouting went up. The Fire Police seemed unable to penetrate beyond the ground floor of the house and had started to baton back the crowd. An electric winch was wheeled up and bolted to the girders running below the kerb, and half a dozen heavy steel grabs were carried into the house and hooked round the walls.

Gregson laughed. 'The owners are going to be surprised when they get home.'

Franz was watching the house. It was a narrow shabby dwelling sandwiched between a large wholesale furniture store and a new supermarket.

An old sign running across the front had been painted over and evidently the ownership had recently changed. The present tenants had made a halfhearted attempt to convert the ground floor room into a cheap standup diner. The Fire Police appeared to be doing their best to wreck everything, and pies and smashed crockery were strewn all over the pavement.

The noise died away and everyone waited as the winch began to revolve. The hawsers wound in and tautened, and the front wall of the house staggered outwards in rigid jerky movements.

Suddenly there was a yell from the crowd.

Franz raised his arm. 'Up there! Look!'

On the fourth floor a man and woman had come to the window and were looking down helplessly. The man lifted the woman on to the ledge and she crawled out and clung to one of the waste pipes. Bottles were lobbed up at them and bounced down among the police. A wide crack split the house from top to bottom and the floor on which the man was standing dropped and catapulted him backwards out of sight. Then one of the lintels in the first floor snapped and the entire house tipped over and collapsed.

Franz and Gregson stood up, almost knocking over the table.

The crowd surged forward through the cordon. When the dust had settled there was nothing left but a heap of masonry and twisted beams. Embedded in this was the battered figure of the man. Almost smothered by the dust he moved slowly, trying to free himself with one hand, and the crowd started roaring again as one of the grabs wound in and dragged him down under the rubble.

The manager of the restaurant pushed past Franz and leant out of the window, his eyes fixed on the dial of a portable detector. Its needle, like all the others, pointed to zero.

A dozen hoses were playing on the remains of the house and after a few minutes the crowd shifted and began to thin out.

The manager switched off the detector and left the window, nodding to Franz. 'Damn Pyros. You can relax now, boys.'

Franz pointed at the detector. 'Your dial was dead. There wasn't a trace of monoxide anywhere here. How do you know they were Pyros?'

'Don't worry, we know.' He smiled obliquely. 'We don't want that sort of element in this neighbourhood.'

Franz shrugged and sat down. 'I suppose that's one way of getting rid of them.'

The manager eyed Franz. 'That's right, boy. This is a good dollar five neighbourhood.' He smirked to himself. 'Maybe a dollar six now everybody knows about our safety record.'

'Careful, Franz,' Gregson warned him when the manager had gone. 'He may be right. Pyromaniacs do take over small cafs and food bars.'

Franz stirred his coffee. 'Dr McGhee estimates that at least fifteen per cent of the City's population are submerged Pyros. He's convinced the number's growing and that eventually the whole City will flameout.'

He pushed away his coffee. 'How much money have you got?'

'On me?'

'Altogether.'

'About thirty dollars.'

'I've saved fifteen,' Franz said. 'Fortyfive dollars; that should be enough for three or four weeks.'

'Where?' Gregson asked.

'On a Supersleeper.'

'Super !' Gregson broke off, alarmed. 'Three or four weeks! What do you mean?'

'There's only one way to find out,' Franz explained calmly. 'I can't just sit here thinking. Somewhere there's free space and I'll ride the Sleeper until I find it. Will you lend me your thirty dollars?'

'But Franz , 'If I don't find anything within a couple of weeks I'll change tracks and come back.'

'But the ticket will cost...' Gregson searched '... billions. Fortyfive dollars won't even get you out of the Sector.'

'That's just for coffee and sandwiches,' Franz said. 'The ticket will be free.' He looked up from the table. 'You know...'

Gregson shook his head doubtfully. 'Can you try that on the Supersleepers?'

'Why not? If they query it I'll say I'm going back the long way round. Greg, will you?'

'I don't know if I should.' Gregson played helplessly with his coffee. 'Franz, how can there be free space? How?'

'That's what I'm going to find out,' Franz said. 'Think of it as my first physics practical.'

Passenger distances on the transport system were measured point to point by the application of a i b + c2 + d2. The actual itinerary taken was the passenger's responsibility, and as long as he remained within the system he could choose any route he liked. Tickets were checked only at the station exits, where necessary surcharges were collected by an inspector. If the passenger was unable to pay the surcharge ten cents a mile he was sent back to his original destination.

Franz and Gregson entered the station on 984th Street and went over to the large console where tickets were automatically dispensed. Franz put in a penny and pressed the destination button marked 984. The machine rumbled, coughed out a ticket, and the change slot gave him back his coin.

'Well, Greg, goodbye,' Franz said as they moved towards the barrier. 'I'll see you in about two weeks. They're covering me down at the dormitory. Tell Sanger I'm on Fire Duty.'

'What if you don't get back, Franz?' Gregson asked. 'Suppose they take you off the Sleeper?'

'How can they? I've got my ticket.'

'And if you do find free space? Will you come back then?'

'If I can.'

Franz patted Gregson on the shoulder reassuringly, waved and disappeared among the commuters.

He took the local Suburban Green to the district junction in the next county. The Green Line train travelled at an interrupted 70 m.p.h. and the ride took two and a half hours.

At the junction he changed to an express elevator which lifted him out of the sector in ninety minutes, at 400 m.p.h. Another fifty minutes in a ThroughSector Special brought him to the Mainline Terminus which served the Union.

There he bought a coffee and gathered his determination together. Supersleepers ran east and west, halting at this and every tenth station. The next arrived in seventytwo hours time, westbound.

The Mainline Terminus was the largest station Franz had seen, a milelong cavern thirty levels in depth. Hundreds of elevator shafts sank through the station and the maze of platforms, escalators, restaurants, hotels and theatres seemed like an exaggerated replica of the City itself.

Getting his bearings from one of the information booths, Franz made his way up an escalator to Tier 15, where the Supersleepers berthed. Running the length of the station were two steel vacuum tunnels each three hundred feet in diameter, supported at thirtyfour intervals by huge concrete buttresses.

Franz walked along the platform and stopped by the telescopic gangway that plunged into one of the airlocks. Two hundred and seventy degrees true, he thought, gazing up at the curving underbelly of the tunnel. It must come out somewhere. He had fortyfive dollars in his pocket, sufficient coffee and sandwich money to last him three weeks, six if he needed it, time anyway to find the City's end.

He passed the next three days nursing cups of coffee in any of the thirty cafeterias in the station, reading discarded newspapers and sleeping in the local Red trains which ran fourhour journeys round the nearest sector.

When at last the Supersleeper came in he joined the small group of Fire Police and municipal officials waiting by the gangway, and followed them into the train. There were two cars; a sleeper which no one used, and a day coach.

Franz took an inconspicuous corner seat near one of the indicator panels in the day coach, and pulled out his notebook ready to make his first entry.

1st Day: West 2700. Union 4,350.

'Coming out for a drink?' a Fire Captain across the aisle asked. 'We have a tenminute break here.'

'No thanks,' Franz said. 'I'll hold your seat for you.'

Dollar five a cubic foot. Free space, he knew, would bring the price down. There was no need to leave the train or make too many inquiries. All he had to do was borrow a paper and watch the market averages.

2nd Day: West 2700. Union 7,550.

'They're slowly cutting down on these Sleepers,' someone told him. 'Everyone sits in the day coach. Look at this one. Seats sixty, and only four people in it. There's no need to move around. People are staying where they are. In a few years there'll be nothing left but the suburban services.'

97 cents.

At an average of a dollar a cubic foot, Franz calculated idly, it's so far worth about $4 x 1027.

'Going on to the next stop, are you? Well, goodbye, young fellow.'

Few of the passengers stayed on the Sleeper for more than three or four hours. By the end of the second day Franz's back and neck ached from the constant acceleration. He managed to take a little exercise walking up and down the narrow corridor in the deserted sleeping coach, but had to spend most of his time strapped to his seat as the train began its long braking runs into the next station.

3rd Day: West 2700. Federation 657.

'Interesting, but how could you demonstrate it?'

'It's just an odd idea of mine,' Franz said, screwing up the sketch and dropping it in the disposal chute. 'Hasn't any real application.'

'Curious, but it rings a bell somewhere.'

Franz sat up. 'Do you mean you've seen machines like this? In a newspaper or a book?'

'No, no. In a dream.'

Every half day's run the pilot signed the log, the crew handed over to their opposites on an Eastbound sleeper, crossed the platform and started back for home.

125 cents.

$8 x 1028.

4th Day: West 2700. Federation 1,225.

'Dollar a cubic foot. You in the estate business?'

'Starting up,' Franz said easily. 'I'm hoping to open a new office of my own.'

He played cards, bought coffee and rolls from the dispenser in the washroom, watched the indicator panel and listened to the talk around him.

'Believe me, a time will come when each union, each sector, almost I might say, each street and avenue will have achieved complete local independence. Equipped with its own power services, aerators, reservoirs, farm laboratories..

The car bore.

$6 x 10.

5th Day: West 270' .17th Greater Federation.

At a kiosk on the station Franz bought a clip of razor blades and glanced at the brochure put out by the local chamber of commerce.

'12,000 levels, 98 cents a foot, unique Elm Drive, fire safety records unequalled...'

He went back to the train, shaved, and counted the thirty dollars left. He was now ninetyfive million GreatMiles from the suburban station on 984th Street and he knew he could not delay his return much longer. Next time he would save up a couple of thousand.

$7 x 10127.

7th Day: West 2700. 212th Metropolitan Empire.

Franz peered at the indicator.

'Aren't we stopping here?' he asked a man three seats away. 'I wanted to find out the market average.'

'Varies. Anything from fifty cents a '

'Fifty!' Franz shot back, jumping up. 'When's the next stop? I've got to get off!'