Gregson nodded.

Franz closed the atlas. ‘Two hundred and fifty counties, including KNI, together form the 493rd Sector, and an association of 1,500 adjacent sectors comprise the 298th Local Union.’ He broke off and looked at Gregson. ‘As a matter of interest, ever heard of it?’

Gregson shook his head. ‘No. How did—’

Franz slapped the atlas on to the table. ‘Roughly 4 x 10’s cubic Great-Miles.’ He leaned on the window-ledge. ‘Now tell me: what lies beyond the 298th Local Union?’

‘Other unions, I suppose,’ Gregson said. ‘I don’t see your difficulty.’

‘And beyond those?’

‘Farther ones. Why not?’

‘For ever?’ Franz pressed.

‘Well, as far as for ever is.’

‘The great street directory in the old Treasury Library on 247th Street is the largest in the county,’ Franz said. ‘I went down there this morning. It occupies three complete levels. Millions of volumes. But it doesn’t extend beyond the 598th Local Union. No one there had any idea what lay farther out. Why not?’

‘Why should they?’ Gregson asked. ‘Franz, what are you driving at?’

Franz walked across to the door. ‘Come down to the Bio-History Museum. I’ll show you.’

The birds perched on humps of rock or waddled about the sandy paths between the water pools.

"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 half-hearted attempt to convert the ground floor room into a cheap stand-up 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. ‘Forty-five 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. Forty-five 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 ib + 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.