What would it be like, to have built that? Daniel could only guess at it, by considering what he had built, and trying to appraise it in a similar spirit. But Daniel’s work was not finished yet. He was not that old-or so he felt, in present company. When Wren’s son had laid the last stone into its place in the lantern atop the dome of St. Paul’s, Sir Christopher had been ten years older than Daniel was today.

St. Paul’s had passed from view; they had turned onto Watling Street and come to a dead stop in the congestion; the tables had turned, and now Wren was looking at Daniel bemusedly. “I do not intend to make your business mine,” he said, “but it would help me to help you, if you would allow me to know what sort of Hooke-stuff you are looking for. Some of his artwork, to adorn your walls? Navigational instruments, for finding your way back to Boston? Architectural drawings? Astronomical observations? Schemes for flying machines? Samples of exotic plants and animals? Clock-work? Optical devices? Chymical Receipts? Cartographical innovations?”

“Forgive me, Sir Christopher, my affairs divide and multiply from one day to the next, I am compelled to pursue several errands at once, and so my answer is not as plain as it might be. Almost anything will serve the end I have already mentioned, viz. giving the Russian savants-in-training food for thought. As for my own purposes, I require anything to do with machines.”

“I have heard it mentioned that you are a member of the Court of Directors of the Proprietors-”

“No. It is not that. Mr. Newcomen’s Engine is a huge and beastly piece of ironmongery, and he needs no assistance from me to make it. I am thinking of small, precise, clever machines.”

“I suppose you mean, small, precise machines, made cleverly.”

“I meant what I said, Sir Christopher.”

“So it’s the Logic Mill again? I thought Leibniz gave up on it, what, forty years ago.”

“Leibniz only set it aside forty years ago, so that he could-” Here Daniel was struck dumb for a few moments out of sheer awe at the faux pas he had been about to commit; he was going to say, invent the calculus.

Sir Christopher’s face, as he regarded this narrowly averted conversational disaster, looked like the death-mask of a man who had died in his sleep while having a pleasant dream.

Finally Wren said, brightly, “I recall Oldenburg was furious. Never forgave him for not finishing it.”

A short pause. Daniel was thinking something unforgivable: perhaps Oldenburg had been right, Leibniz should have built the damned machine and never trespassed upon the holy ground that Isaac had discovered and walled round. He sighed.

Sir Christopher was regarding him with infinite patience. ’Twas like sharing a coach with a Corinthian column.

“I am serving two masters and one mistress,” Daniel began. “Just now, I don’t know what the mistress expects of me, and so let us leave her out of the discussion, and consider my masters. Both men of power. One, a prince of a faraway land, of an old style, but with new ideas. The other, a new sort of prince: a Parliamentary potentate. I can satisfy both, by achieving the same object: construction of a Logic Mill. I know how to build it, for I have been thinking about it, and making test-pieces, for twenty years. I shall soon have a place to build it in. There is even money. I want tools, and clever men who can work miracles with them.”

“Hooke devised machines for cutting tiny gears, and the like.”

“And he knew all of the watch-makers. Among his papers there might be names.”

Wren was amused. “Oh, you’ll have no difficulty getting watch-makers to talk to you, after my lord Ravenscar passes the Longitude Act.”

“That depends on whether they perceive me as a competitor.”

“Are you?”

“I believe that the way to find longitude is not to make better clocks, but to make certain astronomical observations-”

“The Method of Lunar Distances.”

“Indeed.”

“But there is so much arithmetick to be done, with that method.”

“And so let us equip every ship with an Arithmetickal Engine.”

Sir Christopher Wren pinkened-not because he was angry, or embarrassed, but because he was interested. His mind worked for a while. Daniel let it. Finally Wren said, “The most ingenious mechanics I have ever seen, have not been those who make clocks-though they are admittedly very clever-but the ones who make organs.”

“Pipe-organs, you mean?”

“Yes. For churches.”

Daniel felt something very strange happening to his face: he was smiling. “Sir Christopher, you must have employed more organ-makers than any man in history.”

Wren held up a steadying hand. “The furnishings are put in by the parish vestries-it is they who employ the organ-makers. But this much is true: I see them all the time.”

“London must be infested with them!”

“That was more true ten and twenty years ago than now. London’s churches are finished. Many of the organ-makers have gone back to the Continent, to rebuild instruments destroyed in the wars. But many are still here. I shall make inquiries, Daniel.”

They arrived at the church of St. Stephen on Walbrook. Walbrook had been a stream in Roman times, and was now assumed to be a sewer flowing somewhere beneath the street of the same name, though no one was volunteering to go down and verify this. It was a good omen for the day, because this was Daniel’s favorite church. (1) Wren had put it up early in his career-come to think of it, during the same years Leibniz had been toiling on the calculus. It was all domes and arches, as white and pure as an egg; and whatever uplifting thoughts its parishioners might think as they filed into it, Daniel knew it was Wren’s secret anthem to Mathematicks. (2) Thomas Ham, his goldsmith uncle, had lived and worked close enough to hear the hymns being sung in this church. His widow Mayflower-who late in life had converted to Anglicanism-had attended services there with her surviving son William. (3) When King Charles II had ennobled Thomas Ham by way of apology for absconding with all of his customers’ deposits, he had named him Viscount Walbrook, and so to Daniel the Church of St. Stephen Walbrook felt almost like a family chapel.

Wren had put up so many churches so quickly that he’d not had time to plant steeples on them. They all looked splendid on the inside. But steeples were essential to his vision of how London ought to look from the outside, and so now, in semi-retirement, he was going round to his old projects and banging out majestic yet tasteful steeples one after the other. From here Daniel could see another being finished at St. James Garlickhythe, a quarter-mile away, and yet another freshly completed one across the street from there at St. Michael Paternoster Royal. Apparently Sir Christopher’s steeple project was rolling through London a neighborhood at a time. Eminently practical, that. This one, at St. Stephen Walbrook, was just getting underway, using men and materiel being moved over from the other two.

They were taking over the near end of an anomalous open ground north of the church, which spanned a distance of a hundred yards or so between it and the riotous Poultry/Threadneedle/Cornhill/Lombard intersection. Formerly this had been the Stocks Market. It was impossible for so much uncovered dirt to exist in a city like London without becoming a breeding-ground for Crime or Commerce, and Daniel spied instances of both as soon as he got out of Wren’s carriage. At the nearer end, Wren’s workmen had set up, and were guarding, supply-dumps for the masons and carpenters who would spend the next year or two working here, and were erecting a tiny encampment of shacks and tents. Their dogs were parading around, solemn as doctors, urinating on anything that did not move fast enough. Amid this mess, Daniel spied one cart laden with parcels he’d packed with his own hands in the attic of the Royal Society.