We reached there inside a minute. I pulled back the canvas screen, and a thick white opaque cloud formed almost immediately as the relatively warm air inside met the far sub-zero arctic air outside. I waved my hand to dispel it, and peered inside. They were all still drinking coffee – it was the one thing we had in plenty. It seemed difficult to realise that we'd been gone only a few minutes.

"Hurry up and finish off," I said abruptly. "We're on our way within five minutes. Jackstraw, would you start the engine, please, before she chills right down?"

"On our way!" The protest, almost inevitably, came from Mrs Dansby-Gregg. "My dear man, we've hardly stopped. And you promised us three hours' sleep only a few minutes ago."

"That was a few minutes ago. That was before I found out about Mr Mahler here." Quickly I told them all I thought they needed to know. "It sounds brutal to say it in Mr Mahler's presence," I went on, "but the facts themselves are brutal. Whoever crashed that plane – and, to a lesser extent, stole the sugar – put Mr Mahler's life in the greatest danger. Only two things, normally, could save Mr Mahler– a properly balanced high-calorie diet as a short term measure, insulin as a long term one. We have neither. All we can give Mr Mahler is the chance to get one or other of these things with all speed humanly possible. Between now and the coast that tractor engine is going to stop only if it packs in completely, if we run into an impassable blizzard – or if the last of the drivers collapses over the wheel. Are there any objections?"

It was a stupid, unnecessary, gratuitously truculent question to ask, but that's just the way I felt at that moment. I suppose, really, that I was inviting protest so that I could have some victim for working off the accumulated rage inside me, the anger that could find its proper outlet only against those responsible for this fresh infliction of suffering, the anger at the near certainty that no matter what effort we made to save Mahler it would be completely nullified when the time came, as it inevitably must come, that the killers showed their hand. For one wild moment I considered the idea of tying them all up, lashing them inside the tractor body so that they couldn't move, and had the conditions been right I believe I would have done just that. But the conditions couldn't have been more hopeless: a bound person wouldn't have lasted a couple of hours in that bitter cold.

There were no objections. For the most part, I suppose, they were too cold, too tired, too hungry and too thirsty – for with the rapid evaporation of moisture from the warm, relatively humid body thirst was always a problem in dry, intensely cold air – to raise any objections. To people unaccustomed to the Arctic, it must have seemed that they had reached the nadir of their sufferings, that things could get no worse than they were: I hoped as much time as possible would elapse before they found out how wrong they were.

There were no objections, but there were two suggestions. Both came from Nick Corazzini.

"Look, Doc, about this diet Mr Mahler must have. Maybe we can't balance it, but we can at least make sure that he gets a fair number of calories – not that I know how you count the damn' things. Why don't we double his rations – no, even that wouldn't keep a decent sparrow alive. What say each of the rest of us docks a quarter of his rations and hands them over? That way Mr Mahler would have about four times his normal – "

"No, no!" Mahler protested. "Thank you, Mr Corazzini, but I cannot permit – "

"An excellent idea," I interrupted. "I was thinking along the same lines myself."

"Good," Corazzini grinned. "Carried unanimously. I also suggest we'd get along farther and faster if, say, Mr Zagero and I were to spell you two on the tractor." He held up a hand as if to forestall protest. "Either of us may be the man you want, in fact, we might be the two men you want – if it is two men. But if I'm one of the killers, and I know nothing about the Arctic, navigation, the maintenance of this damned Citroen and wouldn't as much as recognise a crevasse if I fell down one, it's as plain as the nose on your face that I'm not going to make a break for it until I'm within shouting distance of the coast. Agreed?"

"Agreed," I said. Even as I spoke, there came a coughing clattering roar as Jackstraw coaxed the still-warm Citroen back into life, and I looked up at Corazzini. "All right," I went on. "Come on down. You can have your first driving lesson now."

We left at half-past seven that morning, in driving conditions that were just about perfect. Not the slightest breath of air stirred. across the ice-cap and the deep blue-black vault of the sky was unmarred by even the tiniest wisp of cloud. The stars were strangely remote, pale and shimmering and unreal through the gossamer gauze of the glittering ice needles that filled the sky and sifted soundlessly down on the frozen snow, but even so visibility was all that could have been desired: the powerful headlights of the Citroen, striking a million sparkling diamond points of light off the ice spicules, reached a clear three hundred yards ahead into the darkness, leaving the ground to either side of the twin interlocking beams shrouded in impenetrable darkness. The cold was intense, and deepening by the hour: but the Citroen seemed to thrive on it that morning.

Luck was with us almost right away. Within fifteen minutes of starting off, Balto, ranging free as always, appeared out of the darkness to the south-west and ran alongside the dog-sledge, barking to attract Jackstraw's attention. Jackstraw gave us the signal to stop – a rapid flickering of the red and green lights on the tractor dashboard – and in two or three minutes appeared out of the darkness, grinning, to tell us that Balto had picked up a standing trail flag. That was good news in itself, in that it meant that our navigation the previous night had been all that could have been wished for and that we were almost exactly on course: even more important, however, was the fact that if this flag was the first of a series we could dispense with the navigator on the dog-sledge and that Jackstraw and I could have some sleep – if sleep were possible in that miserably cold and lurching tractor body. And, indeed, that flag proved to the first of an almost unbroken series that was to guide us all the length of that interminable day, so that from eight o'clock onwards Jackstraw, Zagero, Corazzini and I took it in turns to drive, with the Senator, the Reverend Smallwood or Solly Levin up front as lookout. Theirs was probably the coldest, certainly the most unwelcome job of all: but all three bore up uncomplainingly, even to the extent of thawing out in silent agony at the end of their hour on duty.

Shortly after eight o'clock I left an obviously competent Corazzini to his own devices, dropped back to the shelter of the tractor body and asked the Senator to go up front. I then set about breaking the strictest rule of all, where these old tractors were concerned – that no fire should ever be lit inside when they were in motion. But even the most stringent rules are to be observed only until such time as the need for breaking them is paramount: and now both the need and the time were here. My concern was not for the warmth and comfort of the passengers, or even for the cooking of the food – we had little enough of that, heaven knew, though a constant supply of warm water would come in useful for dealing with the inevitable cases of frostbite – but purely and simply for the life of Theodore Mahler.

Even following Corazzini's suggestion we couldn't give him enough food, and what we could didn't, and wouldn't, even begin to resemble a balanced diet. His best chance of survival, and that was slender enough, lay in conserving his body reserves and his energy as far as lay within our power. To achieve that, work, or exercise of even the lightest kind, was out: he had to remain as immobile as possible, which was why I had him climb into a sleeping-bag and lie down on one of the bunks, wrapped in a pair of heavy blankets, as soon as I entered. But without work or exercise he would have no means to combat that numbing cold except by a constant shivering which would deplete his reserves just as quickly as the most violent exercise would. So he had to have heat: heat from the stove, heat from the warm fluids which I told Margaret Ross that he was to have at least every two hours. Mahler protested strongly against all these arrangements being made on his behalf, but at the same time he was sensible enough to realise that his only chance of survival depended on doing what I said: but I believe that the main factor which finally made him yield was not so much my medical explanations as the pressure of public opinion.