He listened carefully. The thumping noise was growing louder. Although distorted by the fog, it was maddeningly persistent. It was only as he listened for a further second or two, trying to identify which direction it was coming from, that he realized it was too rhythmic to be thunder. And the tone kept alternating, like waves dashing against a rocky shoreline, one after the other, in quick succession.

Hawkwood’s blood suddenly ran cold. His mind flashed back to the cross-Channel voyage he’d taken aboard the Royal Navy cutter Griffin, when, engulfed by mountainous seas during an appallingly violent storm, both the cutter and his mission had come close to foundering on the Pas de Calais coast.

“What the hell is that?” Lawrence whispered.

Hawkwood was too intent on trying to pierce the curtain of fog to reply. There was something profoundly unsettling about not being able to see what lay beyond their bow. And with each second the sound – whatever it was – was getting louder.

The helmsman gasped.

Hawkwood turned to see a dark and monstrous outline materializing at the edge of his vision. And then, just as quickly, it was gone, back into the murk, leaving him with no clear idea of what it was he’d glimpsed. His first thought was that it had been part of a rockface, like the one they’d passed the previous evening. Stagg had referred to the feature as the Palisades, an apt name for the huge granite wall that had risen sheer out of the water to a height of some seventy or eighty feet on the New York side of the lake.

Was that what he’d glimpsed? To add to his confusion, in the brief moment before the shape had melted back into the shadows, Hawkwood could have sworn that high above it he’d seen sparks and an ember-like glow, as if someone was stoking a fire, which made no sense at all.

He was on the point of bellowing uselessly at the helmsman to get them the hell out of there when abruptly the noise changed. Hawkwood knew then that he was not listening to a natural sound, but something else, something man-made, something mechanical.

Something familiar.

And as the realization hit him, the helmsman screamed.

The steamboat was not travelling at maximum speed, but given the Snake’s sluggish rate of progress it seemed to erupt from the fog bank with the force of a battering ram, smoke belching, paddles churning furiously. Bright tongues of orange flame shot from the vessel’s stack, creating demonic flashes within the swirling vortex of smoke and fog.