Monk felt as if he were on the witness stand. It was more uncomfortable than he had anticipated. It would be impossible to escape with inaccurate or incomplete answers. "Alan Argyll himself, or perhaps Toby," he answered. "Alan has taken great care to account for all his own time before and after Havilland's death, but Toby was several years younger and spent more time on the sites and knew some of the tougher navvies."

"According to whom?" Rathbone said quickly.

Monk smiled, but without pleasure. "According to Sixsmith. But it can be easily verified."

"You'll need to do it," Rathbone warned. "The money came from Argyll, you say?"

"Yes."

"If he says it was for wages, or a new machine, and that Sixsmith misappropriated it, can you prove he's lying?"

Monk felt his muscles tighten defensively. "No, not beyond a doubt."

"Reasonable doubt?"

"I don't know what doubt is reasonable. I'm certain myself."

"Not exactly relevant," Rathbone said dryly. "Why would Argyll want Havilland dead so much that he would be prepared to use Sixsmith to hire an assassin?"

"Knowledge that the tunnels were dangerous and work should be stopped," Monk replied.

"Isn't all such work dangerous? The Fleet sewer collapse was appalling."

"That's cut-and-cover," Monk told him. "Imagine that underground, possibly collapsing at both ends, with water, or worse-gas."

"Is gas worse? I would have thought water would be pretty dreadful."

"The gas would be methane. That's flammable. It would only need one spark and the whole thing would be ablaze. If it came up through the sewers, it could start another Great Fire of London."

Rathbone paled. "Yes, I have the idea, Monk. Why do you think that is anything more than a madman's nightmare? Surely Argyll wouldn't want that any more than Havilland or anyone else. If it were a real danger, he'd stop the work himself. What was he afraid of-that Havilland would frighten the workforce and they'd strike? Why not just bar him from the site? Isn't murder excessive, not to mention dangerous and expensive?"

"If it wasn't the navvies Havilland was going to, but the authorities, that would be different. He couldn't stop that so easily. And even an unfounded fear could close the excavations for enough time to delay the work seriously and cost a great deal of money. To a ruthless man, one perhaps running rather close to the edge of profit and loss, or with an over-large investment, that could be motive for murder."

Rathbone frowned. "But motive is not enough, Monk, which you know as well as I. Why not suppose it was Sixsmith, exactly as it appears to be?"

"Because it was Argyll's wife who sent the letter to her father asking him to be in the stable after midnight," Monk answered decisively. "At Argyll's request."

"And if Argyll says he did not ask her to write it?" Rathbone asked. "You cannot force her to incriminate him. It would be profoundly against her interest."