'How does he navigate?'

'Stars,' said Byrne. 'And experience. Now, I figure to get to where he'll be passing through before sundown, and I also figure that he'll be passing through some time during the night. Camels don't have headlights and tail lights, you know; and an azelai moves along goddamn quiet, like. At night a caravan could pass within two hundred yards of you and you wouldn't know it, even though in daylight it would be in plain sight. That's why I want to get there when I can see.'

'See what?'

'I'll figure that out when I get there. Now go to sleep.'

I was about to turn away when I thought of something. 'What happens if we miss the caravan?'

Then we walk to Bilma – that's why we brought all the water. Konti and I would make it You might. Billson wouldn't.'

That was plain enough. I dug out a trench in the side of the dune to lie in, and hoped it didn't look too much like a grave. Then I pulled the djellaba closer around me, and lay down. I looked up at the pock-marked moon for a long time before I went to sleep. It must have been all of three minutes.

We drank all of Billson's remaining water the following morning and abandoned his jerrican. 'Soak yourselves hi it,' advised Byrne. 'Get as much water into you as you can hold.'

Breakfast in the light of dawn was frugal and soon done with. I cleaned out the last few crumbs from my pocket, hoisted the jerrican on to my back with distaste, and was ready to go.

Billson said, 'Stafford, why don't we put half your water in here?' He kicked the discarded jerrican with a clang. 'I could carry it.'

' I looked at him in surprise. That was the first time he had offered to do anything for anyone so far. Maybe there was hope of reclaiming him for the human race, after all. I said, 'Better put that to Byrne; his can is full.'

Byrne stepped over to Billson. 'Let's see your shoulder.' He examined it and shook his head. 'You couldn't do it, Paul. More abrasion and more sand in there and you'll get gangrene. Keep it wrapped up. Let's move.'

And so we set off again – Tip the airy mountain, down the rushy glen' – the mountain bit was real enough and it was certainly airy on that cold morning, but there were no rushes and the valleys were anything but glen-like, although welcome enough because they gave a brief respite for level walking.

I pictured us as four ants toiling across a sand-box in a children's playground. At mid-morning, when we stopped to take on water, I said, 'To think I liked building sand castles when I was a kid.'

Byrne chuckled. 'I remember a drawing I once saw, a cartoon, you know. It was in a magazine Daisy Wakefield had up in Tarn. There was a detachment of the Foreign Legion doing a march across country like this, and one guy is saying to the other, 'I joined the Legion to forget her, but her name is Sandra.' I thought that was real funny.'

'I'm glad you're keeping your sense of humour.'

Billson was doing all right. He didn't say much, but kept up with us just in front of Konti. I had the idea that Byrne had detailed Konti as rearguard to keep an eye on Paul. Although he kept up with us I doubt if he'd have been able to if he still had water to carry. His wound was troubling him; not that he complained about it, but I noticed he favoured his right arm when he took a tumble and fell.

There wasn't much point in stopping long at midday because we had nothing to eat and only needed to drink water. Byrne said, 'Okay, Max; take off your pack.'

'We had drunk from my jerrican at the mid-morning stop, so I said, 'No; you take off yours.'

The wrinkles about his eyes deepened as he stared at me, but then he said obligingly, 'Okay.' So we lightened his load by a few pounds between us.

That day I found I was glad to be wearing the Tuareg veil and the rest of the fancy dress. I could see that Paul, apart from anything else, was beginning to suffer from exposure whereas I was protected.

The rest of the day until sunset passed in a blur of exhaustion. Up one side, down the other, and still another one to come. Against the grain of the land, Byrne had said. It was a good descriptive phrase and I was now really beginning to find how good it was.

I fell into a blind, mindless rhythm and a chant was created in my mind – what the Germans call an 'earworm' – something that goes round and round in your head and you can't get rid of it. One bloody foot before the next bloody foot.

One bloody dune after the next bloody dune. One bloody foot before the next bloody foot. One bloody dune after the next bloody dune. One bloody… It went on and on and on…

Maybe it helped me.

And so it went on for hour after hour until I staggered into Byrne who had stopped. So did my bloody foot. 'We made it just in time.' He looked at the sun. 'Three-quarters of an hour to nightfall.'

'We're there?' I said thickly, and looked down the side of the dune. The valley bottom didn't look much different than any of the others we'd crossed.

'Yeah. Mokhtar will be coming along there.'

I looked around. 'Where's Billson?'

'Maybe a quarter-mile back. Konti's looking after him. Let's go down.'

When we got to the bottom I looked up and saw Paul and Konti silhouetted against the sky at the top of the dune. 'You mean we can rest now?'

'No,' said Byrne relentlessly. He started to walk up the valley so I followed. I was tired but at least this was reasonably level ground and I didn't have to go up and down. The dunes began to close in on either side and then the valley widened. Byrne stopped. This is the place. How wide would you say this valley is?'

'Quarter of a mile.'

'More. Six hundred yards. I want three trenches dug going up and down the valley. Each maybe ten yards long but I'd like more.'

That sounded like work and I wasn't in the mood for it 'How deep?'

'Not much; just so that your feet can recognize it in the dark. We're all going to stand sentry tonight.'

The idea was simple and good. The trenches divided the width of the valley into four equal parts and the four of us would patrol back and forth, each on our 150-yard stretch. When our feet encountered a trench we'd know it was time to turn around smartly and go back, just like a sentry in front of Buck House. If the caravan was coming through, then statistically it was highly likely that one of us was going to run into a camel. And the walking would keep us awake.

I began to dig where Byrne indicated, heaping the soft sand aside with my hands because I had nothing else to dig with. But first I unloaded the jerrican and had some water.

Billson and Konti came down and were put to work, and by nightfall we had done all we could do, not to Byrne's satisfaction but that couldn't be helped.

Then came the patrolling back and forth across the width of the valley, each in our own sector. I was weary and the slow trudge in the thick sand didn't help. Every so often my feet encountered the edge of the trench and I turned to go back. Say three hundred yards for the round trip, about six to the mile. I wondered how many miles I was going to walk that night. Still, it was better than the bloody dunes.

On a couple of occasions at a trench I met Byrne and we exchanged a word before we turned and went our opposite ways. If we could have synchronized our speeds we could have met every time, but there was no way of doing that in the darkness.

The night wore on and my pace became slower. I was desperately tired and it was only because I had to walk that I kept awake, although sometimes I wonder if at times I wasn't walking in my sleep. But the walking and a few hunger pangs kept me going.

I encountered Byrne again, and he said, 'Have you seen Konti?'

'We met a couple of patrols ago. He's awake, if that's what's worrying you.'

'It's not I should be finding Billson, and I'm not.'