Doctor Weger took a deep breath and spoke quickly. ‘It will rise in the air with at least one man in a basket below. He will take pictures of this wood. The pictures will show where our big guns are. The British will pour shells down on our gunners. They could kill fifty of our men and wreck a dozen guns.

Marius shivered and watched as the grey monster swelled. ‘What can we do?’

‘Shoot it down.’

‘I haven’t got a gun,’ the boy said.

‘I mean send a message to the aerodrome and get a fighter plane to shoot it down.’ The doctor turned to the boy and gripped his shoulders. ‘The nearest aerodrome is just off the Peronne road. A farmer’s field turned into an airstrip. You can run there faster than I could.’

‘How far?’

‘Five kilometres.’

‘I can run that in twenty minutes,’ Marius said.

‘It will take that long to get the balloon in position. You might just get there in time. Now go.’

Marius turned back on to the track through the wood. He leaped over roots and clambered over trunks of trees that had been shattered by shells. He tripped on a broken branch from time to time, rolled and picked himself up, always moving forward.

At the far side of the trees he saw a field sloping down to the road. It was crowded with trucks but they would be no use to him. They were all moving westwards towards Bray. His boots clattered on the road and he had to weave around the groaning lorries. His lungs started to burn and his legs grew weak from the lack of good food. He pictured British shells landing among his comrades and found new strength.

His boots were heavy as houses. He paused, took them off and threw them over his shoulder. Marius sprinted on. The road sloped downwards towards the river. At last he saw the aeroplanes on the grass and the pilots sitting outside their tents while mechanics worked on the planes, filling them with fuel and loading the guns.

The grass was soft under his feet. A guard inside the gate tried to stop him. Marius gasped, ‘Urgent message,’ and raced across the airfield.

One of the pilots looked up from a game of cards and his pale thin lips twisted into a smile. ‘What’s the hurry, soldier?’

Marius’s lungs ached and his mouth was so dry his voice was a crow croak. ‘There’s a balloon going up... farm field near Bray...’

The pilot jumped to his feet. ‘Then we’ll have to shoot it down,’ he laughed. He strode across to the rows of planes. Some were painted in camouflage green but others – the fast fighters – were painted bright colours as if to say: ‘Come and shoot me if you dare.’

The pilot walked towards a bright red plane with three wings. ‘Is number 477 ready to fly?’ he barked at a grease-covered mechanic.