‘What were you keeping us waiting for?’ demanded Bimam. ‘Where were you?’

‘Busy! Busy as a pig! No sleep for three nights.’ He looked up, and his small puckered face fairly flushed with delight. ‘Everything fell into place of a sudden. Like a jig-saw puzzle. Never saw anything like it. Kept us hopping, I tell you.’

‘You’ve gotten the dense force fields you’re after?’ asked Orloff in sudden excitement.

Prosser seemed annoyed. ‘No, not that. Something else. Come on.’ He glared at his watch and jumped out of his seat. ‘We’ve got half an hour. Let’s go.’

An electric-motored flivver waited outside and Prosser spoke excitedly as he sped the purring vehicle down the ramps into the depths of the Station.

‘Theory!’ he said. ‘Theory! Damned important, that. You set a technician on a problem. He’ll fool around. Waste lifetimes. Get nowhere. Just putter about at random. A true scientist works with theory. Lets math solve his problems.’ He overflowed with self-satisfaction.

The flivver stopped on a dime before a huge double door and Prosser tumbled out, followed by the other two at a more leisurely pace.

‘Through here! Through here!’ he said. He shoved the door open and led them down the corridor and up a narrow flight of stairs onto a wall-hugging passageway that circled a huge three-level room. Orloff recognized the gleaming’ quartz-and-steel pipe-sprouting ellipsoid two levels below as an atomic generator.

He adjusted his monocle and watched the scurrying activity below. An earphoned man on a high stool before a control board studded with dials looked up and waved. Prosser waved back and grinned.

Orloff said, ‘You create your force fields here?’

‘That’s right! Ever see one?’

‘No.’ The commissioner smiled, ruefully. ‘I don’t even know what one is, except that it can be used as a meteor shield.’

Prosser said, ‘It’s very simple. Elementary matter. All matter is composed of atoms. Atoms are held together by interatomic forces. Toke away atoms. Leave interatomic forces behind. That’s a force field.’

Orloff looked blank, and Bimam chuckled deep in his throat and scratched the back of his ear.

‘That explanation reminds me of our Ganymedan method of suspending an egg a mile high in the air. It goes like this. ou find a mountain just a mile high and put the egg on top. Then, keepmg the egg where it is, you take the mountain away. That’s all.’

The colonial commissioner threw his head back to laugh, and the irascible Dr Prosser puckered his lips in a pursed symbol of disapproval.

‘Come, come. No joke, you know. Force fields most important. Got to be ready for the Jovians when they come.’

A sudden rasping bur from below sent Prosser back from the railing.

‘Get behind screen here,’ he babbled. ‘The twenty-millimeter field 1s going up. Bad radiation.’

The bur muted almost into silence, and the three walked out onto the passageway again. There was no apparent change, but Prosser shoved his hand out over the railing and said, ‘Feel!’

Orloff extended a cautious finger, gasped, and slapped out with the palm of his hand. It was like pushing against very soft sponge rubber or superresilient steel springs.

Bimam tried, too. ‘That’s better than anything we’ve done yet, isn’t it?’ He explained to Orloff, ‘A twenty-millimeter screen is one that can hold an atmosphere of a pressure of twenty millimeters of mercury against a vacuum without appreciable leakage.’

The commissioner nodded, ‘I see! You’d need a seven-hundred-

sixty-millimeter screen to hold Earth’s atmosphere then.’

‘Yes! That would be a unit atmosphere screen. Well, Prosser, is this what got you excited?’

‘This twenty-millimeter screen? Of course not. I can go up t? rn:o hundred fifty millimeters using the activated vanadium pentasulph1de m the praseodymium breakdown. But it’s not necessary. Technician would do it and blow up the place. Scientist checks on theory and goes slow.’ He winked. ‘We’re hardening the field now. Watch!’

‘Shall we get behind the screen?’

‘Not necessary now. Radiation bad only at beginning.’

The burring waxed again, but not as loudly as before. Prosser shouted to the man at the control board, and a spreading wave of the hand was the only reply.

Then the control man waved a clenched fist and Prosser cried, ‘We’ve passed fifty millimeters! Feel the field!’

Orloff extended his hand and poked it curiously. The sponge rubber had hardened! He tried to pinch it between finger and thumb so perfect was the illusion, but here the ‘rubber’ faded to unresisting air.

Prosser tch-tched impatiently. ‘No resistance at right angles to force. Elementary mechanics, that is.’

The control man was gesturing again. ‘Past seventy,’ explained Prosser. ‘We’re slowing down now. Critical point is 83.42.’

He hung over the railing and kicked out with his feet at the other two. ‘Stay away! Dangerous!’

And then he yelled, ‘Careful! The generator’s bucking!’

The bur had risen to a hoarse maximum and the control man worked frantically at his switches. From within the quartz heart of the central atomic generator, the sullen red glow of the bursting atoms had brightened dangerously.

There was a break in the bur, a reverberant roar, and a blast of air that threw Orloff hard against the wall.

Prosser dashed up. There was a cut over his eye. ‘Hurt? No? Good, good! I was expecting something of the sort. Should have warned you. Let’s go down. Where’s Birnam?’

The tall Ganymedan picked himself up off the floor and brushed at his clothes. ‘Here I am. What blew up?’

‘Nothing blew up. Something buckled. Come on, down we go.’ He dabbed at his forehead with a handkerchief and led the way downward. The control man removed his earphones as he approached and got off his stool. He looked tired, and his dirt-smeared face was greasy with perspiration.

‘The damn thing started going at 82.8, boss. It almost caught me.’

‘It did, did it?’ growled Prosser. ‘Within limits of error, isn’t it? How’s the generator? Hey, Stoddard!’

The technician addressed replied from his station at the generator, ‘Tube 5 died. It’ll take two days to replace.’

Prosser turned in satisfaction and said, ‘It worked. Went exactly as presumed. Problem solved, gentlemen. Trouble over. Let’s get back to my office. I want to eat. And then I want to sleep.’

He did not refer to the subject again until once more behind the desk in his office, and then he spoke between huge bites of a liver-and-onion sandwich.

He addressed Birnam, ‘Remember the work on space strain last June. It flopped, but we kept at it. Finch got a lead last week and I developed it. Everything fell into place. Slick as goose grease. Never saw anything like it.’

‘Go ahead,’ said Birnam, calmly. He knew Prosser sufficiently well to avoid showing impatience.

‘You saw what happened. When a field tops 83.42 millimeters, it becomes unstable. Space won’t stand the strain. It buckles and the field blows. Boom!’

Birnam’s mouth dropped open and the arms of Orloff’s chair creaked under sudden pressure. Silence for a while, and then Birnam said unsteadily, ‘You mean force fields stronger than that are impossible?’

‘They’re possible. You can create them. But the denser they are, the more unstable they are. If I had turned on the two-hundred-and-fifty-millimeter field, it would have lasted one tenth of a second. Then, blooie! Would have blown up the Station! And myself! Technician would have done it. Scientist is warned by theory. Works carefully, the way I did. No harm done.’