“We are in England. I have heard rumors of it. In France I saw it once. But not today.”

“And the moon?”

“She is full, and she set over Westminster as we were loading on Tower Wharf.”

“The moon’s behind the world, the sun’s behind clouds. Yet the water that buoys us is obeying the dictates of both, is it not?”

“I have it on good authority that the tides are operational today,” Barnes allowed, and checked his watch. “Sheerness expects a low tide at seven o’clock.”

“A spring tide?”

“Uncommon low. Why, feel how the river’s current bears us along, hastening to the sea.”

“Why does the tide rush out to sea?”

“The influence of the sun and the moon.”

“Yet you and I cannot see the sun or the moon. The water does not have senses to see, or a will to follow them. How then do the sun and moon, so far away, affect the water?”

“Gravity,” responded Colonel Barnes, lowering his voice like a priest intoning the name of God, and glancing about to see whether Sir Isaac Newton were in earshot.

“That’s what everyone says now. ’Twas not so when I was a lad. We used to parrot Aristotle and say it was in the nature of water to be drawn up by the moon. Now, thanks to our fellow-passenger, we say ‘gravity.’ It seems a great improvement. But is it really? Do you understand the tides, Colonel Barnes, simply because you know to say ‘gravity’?”

“I’ve never claimed to understand them.”

“Ah, that is very wise practice.”

“All that matters is, he does,” Barnes continued, glancing down, as if he could see through the deck-planks.

“Does he then?”

“That’s what you lot have been telling everyone.”

“Meaning the Royal Society?”

Barnes nodded. He was eyeing Daniel with some alarm. Daniel, cruelly, said nothing, and let Barnes simmer until he could stand it no more, and continued, “Sir Isaac’s working on Volume the Third, isn’t he, and that’s going to settle the lunar problem. Wrap it all up.”

“He is working out equations that ought to agree with Mr. Flamsteed’s observations.”

“From which it would follow that Gravity’s a solved problem; and if Gravity predicts what the moon does, why, it should apply as well to the sloshing back and forth of the water in the oceans.”

“But is to describe something to understand it?”

“I should think it were a good first step.”

“Yes. And it is a step that Sir Isaac has taken. The question now becomes, who shall take the second step?”

“You mean, is it to be he or Leibniz?”

“Yes.”

“Leibniz has not done any work with Gravity, has he?”

“You mean, it seems obvious that Sir Isaac, having taken the first step, should be better positioned to take the second.”

“Yes.”

“One would certainly think so,” Daniel said sympathetically. “On the other hand, sometimes he who goes first wanders into a cul-de-sac, and is passed by.”

“How can his theory be a cul-de-sac if it describes everything perfectly?”

“You heard him, a short time ago, expressing concern about Leibniz,” Daniel pointed out.

“Because Leibniz has Sophie’s ear! Not because Leibniz is the better philosopher.”

“I beg your pardon, Colonel Barnes, but I have known Sir Isaac since we were students, and I say to you, he does not strain at gnats. When he is at such pains to gird for battle, you may be sure that his foe is a Titan.”

“What weapon could Leibniz possibly have that would do injury to Sir Isaac?”

“To begin with, a refusal to be over-awed, and a willingness, not shared at this time by any Englishman, to ask awkward questions.”

“What sort of awkward questions?”

“Such as I’ve already asked: how does the water know where the moon is? How can it perceive the Moon through the entire thickness of the Earth?”

“Gravity goes through the earth, like light through a pane of glass.”

“And what form does Gravity take, that gives it this astonishing power of streaming through the solid earth?”

“I’ve no idea.”

“Neither does Sir Isaac.”

Barnes was stopped in his tracks for a few moments. “Does Leibniz?”

“Leibniz has a completely different way of thinking about it, so different as to seem perverse to some. It has the great advantage that it avoids having to talk rubbish about Gravity streaming through Earth like light through glass.”

“Then it must have as great disadvantages, or else he, and not Sir Isaac, would be the world’s foremost Natural Philosopher.”

“Perhaps he is, and no one knows it,” Daniel said. “But you are right. Leibniz’s philosophy has the disadvantage that no one knows, yet, how to express it mathematically. And so he cannot predict tides and eclipses, as Sir Isaac can.”

“Then what good is Leibniz’s philosophy?”

“It might be the truth,” Daniel answered.

THE TOWER MIGHT HAVE ENDURED forever with very little upkeep, had it not been for a nasty infestation of humans. From a janitorial standpoint, the problem with this particular race was not that they were de-, but avidly con-structive, and would on no account leave off bringing new building-stuff in through the all-too-numerous gates, and fashioning them into shelters. Left to the elements, such improvisations would break down naturally in decades or centuries, leaving the Tower as God and the Normans had intended it to be. But the difficulty with a human was that where he found a shelter he would occupy it, and when it broke, fix it, and if not prevented, build annexes onto it. To the management of the Tower, it was less an infestation of termites than a plague of mud-daubing wasps.

Every time the Constable brought in a surveyor, and compared his work to the plan his predecessor had drawn up some decades before, he would discover new nests that had insensibly grown in the corners, as dust-balls under a bed. If he went to eject the people who lived in them, so that he could tear them down, he would be confronted with documents and precedents, showing that those people were not squatters but tenants, and that they’d been paying rent for decades to some other squatter-cum-

tenant, who in turn paid rent or performed necessary services for some Corporation or Office or other sui generis queer ancient Entity that claimed long standing or warrant Royal.

Short of a concerted arson campaign, the only brake on this infestation was a lack of space within the walls that circumscribed the hive. It came down, then, to a question of how much crowding human beings could endure. The answer: not as much as wasps, but still rather a lot. In fact, there was a certain type of human who thrived on it, and those types gravitated naturally to London.

Dart the Barber lived in a garret above a storehouse in Cold Harbour. Most of the year, Cold Harbour was cold without a doubt. To Dart and his roommates-Pete the Sutler and Tom the Boot-black-it was also a sort of metaphorical harbour. But beyond that the name made no sense at all. It was nowhere near the water, and performed no harbour-like functions. Cold Harbour was a patch of turf and a few storehouses in the middle of Tower Green, just off the southwest corner of the ancient Conqueror’s keep called the White Tower.

A wee hole had been worried through the wattle near the vertex of the gable, just large enough to admit a pigeon, vent smoke from a rush-light, or frame a man’s face. At the moment it was giving Dart a sort of dove’s-eye-view of the Parade. Accounting for about half of the Inner Ward, the Parade was the largest open space in the Tower. A well-tended patch of English turf it was. But it was scarred, below Dart, with ridges of rain-worn stone: the exposed foundations of walls that had been thrown down, ?ons ago, by long-dead Constables. For perhaps the only thing that could stir a Constable to use force against the gnawing accretion of sheds, annexes, pop-outs, amp;c., was the combined awareness of (a) his own mortality and (b) the fact that there was no place left in the Tower complex to dig his grave. At any rate, there was evidence here that Cold Harbour had once been a bigger thing than it was now. From ground level, these ruins were a meaningless maze of tripping-hazards. From Dart’s privileged viewpoint they could be made out as a page of rectilinear glyphs stroked in gray and yellow paint on green baize.