'That depends on how much energy is required to heat the Cloud,' remarked Weichart.

'And on its opacity, and a hundred and one other factors,' added Kingsley. 'I must say it seems very unlikely to me that much heat will get through the gas. Let's work out the energy required to heat it to an ordinary sort of temperature.'

He went out to the blackboard, and wrote: Ma.s.s of Cloud 1.3 1030 grams. grams.

Composition of Cloud probably hydrogen gas, for the most part in neutral form.

Energy required to lift temperature of gas by T degrees is 15 13 1030 RT ergs RT ergs where R is the gas constant. Writing L for the total energy emitted by the Sun, the time required to raise the temperature is 15 13 1030 RT/L seconds RT/L seconds Put R = 83 107, T = 300, L = 4 1033 ergs per second gives a time of about 12 10 ergs per second gives a time of about 12 107 seconds, i.e. about 5 months. seconds, i.e. about 5 months.

'That looks sound enough,' commented Weichart. 'And I'd say that what you've got is very much a minimum estimate.'

'That's so,' nodded Kingsley. 'And my minimum is already very much longer than it will take the Cloud to pa.s.s us by. At a speed of 80 kilometres per second it'll sweep across the Earth's...o...b..t in about a month. So it looks to me pretty certain that if the Cloud does come between us and the Sun it'll cut out the heat from the Sun quite completely.'

'You say if if the Cloud comes between us and the Sun. Do you think there's a chance it may miss us?' asked Herrick. the Cloud comes between us and the Sun. Do you think there's a chance it may miss us?' asked Herrick.

'There's certainly a chance, quite a chance I'd say. Look here.'

Kingsley moved again to the blackboard.

'Here's the Earth's...o...b..t round the Sun. We're here at the moment. And the Cloud, to draw it to scale, is over here. If it's moving like this, dead set for the Sun, then it'll certainly block the Sun. But if it's moving this second way, then it could well miss us altogether.'

KINGSLEY'S DRAWING OF PRESENT SITUATION

KINGSLEY'S DRAWING OF SITUATION IN SIXTEEN MONTHS' TIME 'It looks to me as if we're rather lucky,' Barnett laughed uneasily. 'Because of the Earth's motion round the Sun, the Earth will be on the far side of the Sun sixteen months hence when the Cloud arrives.'

'That only means that the Cloud will reach the Sun before it reaches the Earth. It won't stop the sunlight being blocked out if the Sun gets covered, as in Kingsley's case (a),' Marlowe remarked.

'The point about your cases (a) and (b),' said Weichart, 'is that you only get case (a) if the Cloud has almost exactly zero angular momentum about the Sun. It only needs a very slight angular momentum and we have case (b).'

'That's exactly it. Of course my case (b) was only one example. The Cloud could equally well sweep past the Sun and the Earth on the other side, like this:'

'Do we have anything to say about whether the Cloud is coming dead at the Sun or not?' asked Herrick.

'Not on the observational side,' answered Marlowe. 'Look at Kingsley's drawing of the present situation. Only a very slight difference of velocity makes a big difference, all the difference between the Cloud hitting and missing. We can't say yet which it's to be, but we can find out as the Cloud comes in nearer.'

'So that's one of the important things to be done,' concluded Herrick.

'Can you say anything more from the theory?'

'No, I don't think we can; the calculations aren't accurate enough.'

'Astonishing to hear you distrusting calculations, Kingsley,' remarked the Astronomer Royal.

'My calculations were based on your observations, A.R.! Anyway I agree with Marlowe. The thing to do is to keep a close watch on the Cloud. It should be possible to see whether we're going to have a hit or a miss without too much trouble. A month or two should settle it, I suppose.'

'Right!' answered Marlowe. 'You can rely on us to watch this fellow from now on as carefully as if it was made of gold.'

After lunch Marlowe, Kingsley, and the Astronomer Royal were sitting in Herrick's office. Herrick had explained the plan of writing a joint report.

'And I think our conclusions are very clear. May I just outline them for you?

1. A cloud of gas has invaded the solar system from outer s.p.a.ce.

2. It is moving more or less directly towards us.

3. It will arrive in the vicinity of the Earth about sixteen months from now.

4. It will remain in our vicinity for a time of about a month.

'So if the material of the Cloud interposes itself between the Sun and the Earth, the Earth will be plunged into darkness. Observations are not yet sufficiently definitive to decide whether or not this will occur, but further observations should be capable of deciding this question.'

'And I think we can go a little further concerning future observations,' Herrick went on. 'Optical observations will be prosecuted here with all energy. And we feel that work by the Australian radio astronomers will be complementary to ours, particularly with regard to keeping a watch on the line of sight motion of the Cloud.'

'That seems to sum up the situation admirably,' agreed the Astronomer Royal.

'I propose that we proceed with the report at full speed, that we four sign it, and that it be communicated to our respective Governments forthwith. I hardly need say that the whole matter is highly secret, or at least that we should treat it as so. It is rather unfortunate that so many are aware of the position, but I believe that we can rely on everybody proceeding with great discretion.'

Kingsley did not agree with Herrick on this point. Also he was feeling very tired, which no doubt made him express his views rather more forcibly than he would otherwise have done.

'I'm sorry, Dr Herrick, but I don't follow you there. I see no reason why we scientists should go to the politicians like a lot of dogs thumping our tails, saying "Please, sir, here's our report. Please give us a pat on the back and perhaps even a biscuit if you feel so disposed." I can't see the slightest point in having to do with a crowd of people that can't even run society properly during normal times when there's no serious stress. Will the politicians pa.s.s statutes to stop the Cloud coming? Will they be able to prevent it cutting off the light of the Sun? If they can, then consult them by all means, but if they can't, let's leave them out of the picture altogether.'

Dr Herrick was quietly firm.

'I'm sorry, Kingsley, but as I see it the United States Government and the British Government are the democratically elected representatives of our respective peoples. I regard it as our obvious duty to make this report, and to maintain silence until our Governments have made a p.r.o.nouncement on it.'

Kingsley stood up.

'I'm sorry if I seem brusque. I'm tired. I want to go and get some sleep. Send your report if you wish, but please understand that if I decide to say nothing publicly for the time being, it will be because I wish to say nothing, not because I feel under any form of compulsion or duty. And now if you'll excuse me, I'd like to get round to my hotel.'

When Kingsley had gone, Herrick looked at the Astronomer Royal.

'Dr Kingsley seems a trifle ... er ...'

'A trifle unstable?' said the Astronomer Royal. He smiled and went on: 'That's not very easy to say. Whenever you can follow his reasoning, Kingsley is always very sound and often brilliantly deductive. And I am inclined to think this is always so. I think he seemed rather odd just now because he was arguing from unusual premises, rather than because his logic was faulty. Kingsley probably thinks about society in quite a different way from us.'

'Anyway I think that while we work on this report it would be a good idea if Marlowe were to look after him,' remarked Herrick.

'That's fine,' Marlowe agreed, still struggling with his pipe, 'we've got a lot of astronomy to talk about.'

When Kingsley came down to breakfast the following morning he found Marlowe waiting.

'Thought you might like to drive out for the day into the desert.'

'Spendid, there's nothing I'd like better. I'll be ready in a few minutes.'

They drove out of Pasadena, turned sharply right off Highway 118 at La Canada, then cut through the hills, past the side road to Mount Wilson, and so on to the Mohave Desert. Three more hours' driving brought them under the wall of the Sierra Nevada, and at last they could see Mount Whitney plastered with snow. The far desert stretching towards Death Valley was veiled in a blue haze.

'There are a hundred and one tales,' said Kingsley, 'of what a man feels like when he's told that he's only got a year to live incurable diseases, and so on. Well, it's odd to think that every one of us probably only has a little more than a year to live. A couple of years hence, the mountains and the desert will be much the same as they are now, but there'll be no you and me, no people at all to drive along through it.'

'Oh my G.o.d, you're much too pessimistic,' grunted Marlowe. 'As you said yourself, there's every chance that the Cloud will sweep to one side or the other of the sun, and give us a complete miss.'

'Look, Marlowe, I didn't want to press you too much yesterday, but if you've got a photograph going back a number of years you must have a pretty good idea of whether or not there's any proper motion. Did you find any?'

'None that I could swear to.'

'Then surely that's pretty good evidence that the Cloud is coming dead towards us, or at any rate dead towards the Sun.'

'You might say so, but I can't be certain.'

'So what you mean is that the Cloud is probably going to hit us, but there's still a chance that it might not.'

'I still think you're being unduly pessimistic. We'll just have to see what we can learn during the next month or two. And anyway, even if the Sun is is blotted out, don't you think we can see it through? After all it'll only be for about a month.' blotted out, don't you think we can see it through? After all it'll only be for about a month.'

'Well, let's go into it from scratch,' began Kingsley. 'After a normal sunset the temperature goes down. But the decline is limited by two effects. One is the heat stored in the atmosphere, which acts as a reservoir that keeps us warm. But I reckon that this reservoir would soon become exhausted, I calculate, in less than a week. You've only got to think how cold it gets at night out here in the desert.'

'How do you square that with the Arctic night, when the Sun may be invisible for a month or more? I suppose the point is that the Arctic is constantly receiving air from lower lat.i.tudes; and that this air has been heated by the Sun.'

'Of course. The Arctic is constantly warmed by air that flows up from tropical and temperate regions.'

'What was your other point?'

'Well, the water vapour in the atmosphere tends to hold in the heat of the Earth. In the desert, where there's very little water vapour, the temperature goes down a long way at night. But in places where there's lots of humidity, like New York in summer, there's very little cooling at night.'

'And what does that lead you to?'

'You can see what will happen,' continued Kingsley. 'For the first day or two after the Sun is hidden if it is is shut out, that's to say there won't be a great deal of cooling, partly because the air will be still warm and partly because of the water vapour. But as the air cools the water will gradually turn, first into rain, then into snow, which will fall to the ground. So the water vapour will be removed from the air. It may take four or five days for that to happen, perhaps even a week or ten days. But then the temperature will go racing down. Within a fortnight we shall have a hundred degrees of frost, and within a month there'll be two hundred and fifty or more.' shut out, that's to say there won't be a great deal of cooling, partly because the air will be still warm and partly because of the water vapour. But as the air cools the water will gradually turn, first into rain, then into snow, which will fall to the ground. So the water vapour will be removed from the air. It may take four or five days for that to happen, perhaps even a week or ten days. But then the temperature will go racing down. Within a fortnight we shall have a hundred degrees of frost, and within a month there'll be two hundred and fifty or more.'

'You mean it'll be as bad here as it is on the Moon?'

'Yes, we know that at sunset on the Moon the temperature declines by over three hundred degrees in a single hour. Well, it'll be much the same here except that it'll take longer because of our atmosphere. But it'll come to the same thing in the end. No, Marlowe, I don't think we can last out a month, even though it doesn't seem very long.'

'You reject the possibility that we might keep warm the same way as they keep warm in winter in the Canadian prairies, by efficient central heating?'

'It's just possible I suppose that some buildings are sufficiently well insulated to stand the tremendous temperature gradients that'll be set up. They'll have to be very exceptional, because when we build offices and houses, and so on, we don't build with these temperature conditions in mind. Still I'll grant you that a few people may survive, people that have specially well designed buildings in cold climates. But I think there's no chance at all for anyone else. The tropical peoples with their ramshackle houses will be in a very poor case.'

'Sounds very grim, doesn't it?'

'I suppose the best thing will be to find a cave where we can get deep underground.'

'But we need air to breathe. What should we do when that gets very cold?'

'Have a heating plant. That wouldn't be too difficult. Heat the air going into a deep cave. That's what all the Governments that Herrick and the A.R. are so keen on will do. They'll have nice warm caves, while you and me, Marlowe my boy, will get the icicle treatment.'

'I don't believe they're quite as bad as that,' Marlowe laughed.

Kingsley went on quite seriously: 'Oh, I agree they won't be blatant about it. There'll be good reasons for everything they do. When it becomes clear that only a tiny nucleus of people can be saved, then it'll be argued that the lucky fellows must be those who are most important to society; and that, when it's boiled down and distilled, will turn out to mean the political fraternity, field-marshals, kings, archbishops, and so on. Who are more important than these?'

Marlowe saw that he had better change the subject slightly.

'Let's forget about humans for the time being. How about other animals and plants?'

'All growing plants will be killed, of course. But plant seeds will probably be all right. They can stand intense cold and still be capable of germination as soon as normal temperatures return. There'll probably be sufficient seeds around to ensure that the flora of the planet remains essentially undamaged. The case is very different with the animals. I don't see any large land animal surviving at all, except a small number of men, and perhaps a few animals that men take into shelter with them. Small furry burrowing animals may be able to get deep enough into the ground to withstand the cold, and by hibernating they may save themselves from dying for lack of food.

'Sea animals will be very much better off. Just as the atmosphere is a reservoir of heat, the sea is a vastly greater reservoir. The temperature of the seas won't fall very much at all, so the fish will probably be all right.'

'Now isn't there a fallacy in your whole argument?' exclaimed Marlowe with considerable excitement. 'If the seas stay warm, then the air over the seas will stay warm. So that there'll always be a supply of warm air to replenish the cold air over the land!'

'I don't agree there,' answered Kingsley. 'It isn't even certain that the air over the seas will stay warm. The seas will cool enough for them to freeze up at the surface although the water lower down will stay quite warm. And once the seas freeze over, there won't be much difference between the air over the land and the sea. It'll all get extremely cold.'

'Unfortunately what you say sounds right. So it looks as if a submarine might be the right place to be!'

'Well, a sub wouldn't be able to surface because of the ice, so a complete air supply would be needed and that wouldn't be easy. Ships wouldn't be any good either because of the ice. And there's another objection to your argument. Even if the air over the sea did stay comparatively warm, it would not supply heat to the air over the land, which being cold and dense would form tremendous stable anticyclones. The cold air would stay on the land and the warm air on the sea.'

'Look here, Kingsley,' laughed Marlowe, 'I'm not going to have my optimism damped by your pessimism. Have you thought of this point? There may be quite an appreciable radiation temperature inside the Cloud itself. The Cloud may have an appreciable heat of its own, and this might compensate us for the loss of sunlight, always supposing as I keep saying that we do find ourselves inside the Cloud!'

'But I thought the temperature inside the interstellar clouds was always very low indeed?'

'That's the usual sort of cloud, but this one is so much denser and smaller that its temperature may be anything at all, so far as we know. Of course it can't be extremely high, otherwise the Cloud would be shining bright, but it can be high enough to give us all the heat we want.'

'Optimist, did you say? Then what's to stop the Cloud being so hot that it boils us up? I didn't realize there was so much uncertainty about the temperature. Frankly, I like this possibility even less. It'll be completely disastrous if the Cloud is too hot.'

'Then we shall have to go into caves and refrigerate our air supply!'

'But that isn't so good. Plant seeds can stand cold but they can't stand excessive heat. It wouldn't be much good for Man to survive if the whole flora was destroyed.'

'Seeds could be stored in the caves, along with men, animals, and refrigerators. My G.o.d, it puts old Noah to shame, doesn't it?'

'Yes, maybe some future Saint-Saens will write the music for it.'

'Well, Kingsley, even if this chat hasn't been exactly consoling, at least it's brought out one highly important point. We must find the temperature of that Cloud and without delay too. It's obviously another job for the radio boys.'

'Twenty-one centimetre?' asked Kingsley.

'Right! You have a team at Cambridge that could do it, haven't you?'

'They've started in on the twenty-one centimetre game quite recently, and I think they could give us an answer to this point pretty quickly. I'll get on to 'em as soon as I get back.'