The two of them approached the thunder-mouth carrying a heavy pail of water between them. Ibrahim saw they were going to tip the water into a kind of funnel mounted over the brazier.

‘All that water is coming out of somebody’s ration,’ Ibrahim said weakly.

‘This will put an end to rationing,’ Subh said.

Peter pointed. ‘The water, poured in here, goes straight down into the hot barrel. It immediately flashes to steam. And steam, as you know, requires more space than water. The steam will roar up the tube and shoot the iron lump out over the walls, as a man spits out a pea, propelling it with his breath, spit it away and into the Christian lines. I am confident of the range. We have tested smaller models; the arithmetic is simple.’

‘It will seem a miracle,’ Subh said. ‘The explosion of the steam, the roar of it as it gushes out of the thunder-mouth - and the iron ball itself, a mass heavier than a man, flying miles through the air. A mouth of thunder indeed.’

‘But you haven’t tested it,’ Ibrahim said.

‘Only smaller models. What else could we do, in the conditions of the siege?’

‘And what better way to prove it,’ Subh said silkily, ‘than against live Christians?’

Peter stood straight. ‘Do it,’ he said to the scholars.

The cowering scholars tipped their great bucket. The water gurgled into the funnel, and through a length of copper pipe that fed it straight through the brazier’s coals and into the cylinder. The thunder-mouth shuddered. And in that last heartbeat Ibrahim snatched his mother’s arm and pulled her back, putting his body between her and the engine.

Ibrahim was slammed in the back as if by an immense hot fist, and he was thrown forward. An enormous noise crashed painfully into his head. Steam washed over him in a moment, scorching, gone.

He found himself sprawled over his mother. He pushed himself up. His back was tight and sore, burned. His mother seemed unharmed. Lying on the floor, looking up at him, her lips moved. But he could not hear a word she said.

In fact, he realised, he couldn’t hear anything at all. He noticed blood trickling from his mother’s ears and pooling in her throat. When he touched the sides of his own face, his fingers came away sticky with blood. He felt shocked. He had never heard such a noise, never in his life.

He stood up and turned around.

The thunder-mouth was destroyed, ripped open. The brazier was shattered, its hot coals scattered smoking on the platform. The two scholars lay on their backs, unmoving. He saw with wonder that misshapen bits of copper were embedded in the stone wall.