“There is nothing in my life quite so irritating as to be riddled and teased with inane conundrums, meant to prove my wit, and to try my senility,” he answered. “Bernoulli-Leibniz’s pawn-sent me-”

“The brachistochrone problem, I recall it,” Daniel said, “and you solved it in hours. It took me rather longer.”

“But you did solve it,” Isaac commanded. “Because it was a problem of the calculus, meant to try whether I understood the calculus or not! Can you fathom the impertinence of it!? I was the first man who could ever have solved it, Daniel, and you the second, because you had the calculus from me first-hand. To be hectored thus, by the Baron’s lackeys, three decades after I had invented it-”

“In truth my riddle is another sort of thing altogether,” Daniel said. “I really am quite sorry to have wrong-footed you.”

Isaac blinked and heaved a sigh. He seemed inordinately relieved. Perhaps he had feared that Daniel would dispute what he had just said: You had the calculus from me first-hand. That was the key. In Daniel, Isaac saw a witness who could testify to Isaac’s priority in the discovery of the calculus. Whatever other annoying and inconvenient qualities Daniel might have, vanished when placed beside that. Daniel felt the muscles of his scalp and neck easing, felt his lungs filling with air. He was going to be all right. He’d make it out of the room whole, even if he said things that made Isaac a little angry. To Isaac, Daniel was more than a pawn; he was a rook, kept sequestered in the corner of the board until the end-game, then brought out at last to sweep inexorably down the board, driving the foe back to the last rank and forcing surrender. Isaac would put up with a lot, from a rook.

He wondered whether Isaac had, through some machinations, caused Daniel to be brought back to London. Perhaps he had exerted some action at a distance upon Princess Caroline in Hanover.

“What is your riddle, Daniel?”

“Earlier today, I was with a man who knows a good deal more than I do about money. This fellow was trying to judge the value of a guinea.”

“Of a coin that purported to be a guinea,” Isaac corrected him.

“Indeed-I say ‘guinea’ because that is what, in the end, it turned out to be.”

“He should have weighed it.”

“That is just what he did. And he could say nothing against the weight of the coin. Which would seem to settle the matter. But then he did something that to me was very odd. He put the coin in his mouth and he bit down on it.”

Isaac made no answer, but Daniel thought he pinkened again, slightly. Certainly he was interested in the story. He clasped his hands together on the table in front of him, composing himself, rather like a cat.

“Now,” Daniel said. “Even I know that coiners frequently make their counterfeits by joining two faces stamped from gold foil, and filling the void between them with solder. The solder is both lighter and softer than gold. This provides two means of detection: one may weigh the coin, or bite it. Either should suffice. In particular, if a coin has passed the test of weighing, its value should be confirmed beyond doubt! For nothing is heavier than gold. Any adulteration should be betrayed by a want of gravity. The weighing test ought to be infallible. And yet this chap-who really is extremely knowledgeable concerning coins-felt it necessary to make the additional test of biting. Is there any reason for it? Or was he being foolish?”

“He was not being foolish,” Isaac said, and stared at Daniel expectantly. His eyes were great luminous ice-balls hanging in space, like comets.

“Do you mean to say that I was, Isaac?”

“To associate with such a man? Foolish, or naive,” Isaac returned. “As you have wandered in the wilderness for two decades, I shall grant you the benefit of the doubt.”

“Then cure me of naivete, and tell me, what sort of man is he?”

“A weigher.”

“Well, he is obviously that, inasmuch as he weighs things, but you seem to invest the word with connotations that are lost on a back-woodsman such as I.”

“In spite of all my efforts to reform the practices of the Mint, and to make each newly coined guinea identical to the last, some variation in weights persists. Some guineas are slightly heavier than others. Such errors are reducible but not eradicable. I have reduced them to the degree that, where honest persons are concerned, no variation exists. That is, most men in London-and I include sophisticated men of commerce-would trade one guinea for another without hesitation, and certainly without bothering to take out a scale and weigh them.”

“I well remember when that was not the case,” Daniel remarked.

“You refer to our visits to Stourbridge Fair, before the Plague,” Isaac said immediately.

“Yes,” Daniel answered, after a moment of awkwardness.

He and Isaac had walked from Trinity to the Fair once, to buy prisms, and along the way, Isaac had made some remarks about fluxions-the beginnings of the calculus. During his recent sea-voyage from Massachusetts, Daniel had summoned that ancient memory to mind, and brought it back to life in his head, remembering certain queer details, like the shapes of the aquatic plants in the river Cam, bent downstream by the sluggish fluxion of the water. It was now obvious that Isaac had been thinking hard, and recently, about the same memory.

To go on prating of coins, when the true topic of the conversation was so close to breaking the surface, were faintly ridiculous. But Englishmen, given a choice, would always prefer the faintly ridiculous over the painfully direct. So, on with numismatics.

“It got even worse-the coinage did-later,” Isaac said.

“I remind you that I did not depart until the middle of the 1690s, when there were hardly any coins left in the country, and our ?conomy was a confetti of I.O.U.s.”

“Now England is awash in gold. The currency is as hard as adamant. Our commerce is the wonder of all the earth, and even Amsterdam is in our shade. It were vanity for me to take too much credit for this. But it is simple honesty to say, that it could not happen in the absence of this plain understanding, shared by all Englishmen, that a guinea may be exchanged for a guinea without a second thought. That all guineas are the same.”

Suddenly all that Daniel had observed of Mr. Threader rearranged, in his mind, into a novel, strange, but perfectly coherent picture; it was like watching a pile of rubble spontaneously assemble itself into a marble statue. “Allow me to hazard,” Daniel said, “that a weigher” (he almost said, “Mr. Threader”) “is a chap who to outward appearances believes what every honest, plain-dealing Englishman believes about the value of a guinea. But in secret, he takes every guinea that comes his way, and weighs it ’pon scales of the most exacting precision. Such as are light, or of the mean weight, he returns into circulation. But such as are heavy, he hoards. And when he has hoarded an hundred such-I am only making up numbers for the sake of argument-perhaps he has enough gold, in sum, to mint an hundred and one guineas. He has created a new guinea out of thin air.”

Isaac said yes by slowly blinking his pink eyelids. “Of course, what you have described is only the most elementary of their practices. Those who master it, move on quickly to more nefarious schemes.”

But Daniel was still new to all of this, and stuck on the elementary. “It would only be feasible,” he guessed, “if one were already in a line of work that involved handling large numbers of coins.”

“Naturally! And that is why the practice is so rife among the money-scriveners. I make guineas, and send them out into the country; they scurry about unraveling the tapestry I’ve so laboriously woven, and return the heaviest coins to London, where they invariably make their way to the coffers of the most vile and execrable traitors in the realm!”