by Ken Alder
After years of quest, time had suddenly contracted. Delambre hurried to measure the northern baseline, a straight stretch of the king’s highway near Melun, known today as the N6. In the company of Laplace and the nation’s chief engineer, he had scouted the terrain and requested the construction of two masonry markers, each containing a copper plug that would define the exact terminal point. Above each stone foundation, he now erected a sixty-one-foot wooden tower. Yet even from this high platform, the beautiful twin rows of plane trees along the highway blocked his line of sight. Over the next six weeks he had workmen prune back some six hundred trees, while he triangulated the two ends of the baseline from the roof of the farmhouse at nearby Malvoisine. Six years earlier, the proprietors there had let him raise their chimney so that he might sight the d’Assy country château, thereby weaving his own life story into the making of the impersonal meter. Since then his patron had been executed, and the world turned upside down. Now, back on the same farmhouse roof, Delambre and Bellet took their final geodesic measurements.
At eleven o’clock on the morning of April 24, the team laid down the first of their high-precision rulers to measure the baseline. Each of their four rulers was two toises (twelve feet) long and a marvel of artifice. Lenoir’s workshop had fashioned them out of pure platinum, the newest and most expensive metal on earth. Borda had then calibrated each one against a one-second pendulum and set it in a wooden sleeve alongside a strip of copper so that the relative expansion of the two metals could be read with microscopic precision. The routine was this: Bellet laid the rulers, Tranchot checked their alignment and level, Delambre read the temperature gauge, and each man recorded the results in his own separate logbook. An additional logbook was kept by a tall gray-eyed young man of seventeen named Achille-César-Charles de Pommard, the son of Delambre’s widow-companion. After the fourth ruler was placed, the first was removed and attached to the end of the fourth, and the process resumed. It took them all of the first day to advance 528 feet. At night they marked their point of progress by driving a lead-topped stake into a hole in the road and marking the extremity of the final ruler with a plumb line. They then covered the hole with heavy planks to shield the marker from passing carriages. It took them forty-one days, working from dawn till dusk, to traverse the six miles.
Eminent visitors came to watch their tedious crawl across the earth. Lalande rode down from Paris for the afternoon. A party of savants arrived on June 3 to celebrate the final measurements: among them Louis-Antoine de Bougainville, a seventy-year-old circumnavigator who had been the first European to discover Tahiti, and the youthful German geographer Alexander von Humboldt, on the threshold of a world tour that would make him the most famous explorer of the age. Both were impressed with Delambre’s approach. “Delambre’s personal character inspires as much confidence as the excellence of his instruments,” Humboldt wrote. “To complete such a task in the face of so many physical, moral, and political obstacles, it is essential that the expedition leader have this calm temperament, this tranquil joy, this perseverance.”
THE BASELINE OF MELUN
Masonry pyramids were placed at the terminal points of the Melun baseline, a six-mile stretch of the Paris-Melun highway now known as the N6. The pyramids were completed in 1798, in time for Delambre’s measurements. They were replaced in the 1880s when the baseline was remeasured. Later they were damaged in a road accident and subsequently destroyed. (From J.-B.-J. Delambre, Base du système métrique décimal, 3, plate VI; photograph by Roman Stansberry)
High-precision instruments mean nothing unless you trust those who wield them. Humboldt had secured one of Lenoir’s precious repeating circles for his own world tour. His first stop would be Barcelona, where he hoped to take latitude measurements and send Delambre his data. Trust, but verify.
The next order of business was the southern baseline at Perpignan. This was originally to have been Méchain’s baseline, linked to his chain of triangles, and so Delambre invited his colleague to join the measurement party. Méchain refused. Three years ago the Commission had not trusted him to measure the baseline on his own, and had insisted that he finish his triangles first. Well, he had yet to finish them. And there was another reason. He refused to have anything to do with Tranchot. The fact that the Commission had allowed Tranchot to assist Delambre proved that his colleagues trusted Tranchot more than Méchain. Let Tranchot have all the glory, then. Méchain should have surrendered command of the southern expedition to him long ago. Instead, he had botched his mission, failed his colleagues, and besmirched his reputation. Shame was all he felt. Painful, acute, and deserved shame. “After all that has happened,” he wailed, “I can no longer show myself anywhere and my only wish is to be annihilated.”
Madame Méchain agreed to help bring her husband to his senses. As he had refused to come and visit her in Paris, she would track him down in southern France. And once she found him, she assured Delambre, she would convince him to join his colleague at Perpignan for the measurement of the baseline. She would stay with him until his mission was accomplished, assisting him with his observations, as she had done in the halcyon days before the Revolution. In return, she had only one request. Scientific Paris was full of gossips; no one must know that Pierre-François-André Méchain, member of the Academy and joint commissioner for the measurement of the world, had been unable to complete his mission without the help of a woman, not even if that woman were his wife. She wrote to Delambre in the strictest confidence.
Paris, May 30, 1798
Monsieur,
You ask me to induce my husband to put the final touches on the important work with which you are conjointly charged. No one takes a greater interest in this than I, and I have long considered joining him myself, so that I might bring him words of consolation and peace. Until recently, diverse circumstances have prevented me from carrying out this plan. I now leave immediately for Rodez. I have notified my husband of my trip without awaiting his reply, so as to give him no opportunity to dissuade me. As I suspect he is no longer in Rodez, I have asked him to indicate a place where we might meet. Do not think that I will waste his time. On the contrary, my goal is to accelerate the measure of the triangles.
I have told him emphatically not to accommodate me by proposing a rendezvous in a town appropriate to a lady. I will not waste even a quarter-hour of his time, because he does not have the time to waste. I have told him that I will gladly meet him on the mountaintops, sleep in a tent or a stable, and live on cheese and milk; that with him, I will be content anywhere. I have told him that we will work together by day, and let the nights suffice for conversation. I am hopeful that the esteem and absolute trust he places in me will allow me to dissipate the unwholesome thoughts which devour his spirit, and which, against his will, distract him from his purpose. When I am done with him, he will be ready to be delivered into your hands. Perhaps I will wait until you have joined us before handing him over to you, so that we may together regenerate his spirits. You may judge for yourself, Monsieur, if the signs of your friendship for me have won for you my esteem and my gratitude.
This, regrettably, is all that it is in my power to do, my final effort for the good of the service, for the interests of my husband, and for glory. Needless to say, all of this must remain between you, me, and Monsieur Borda, who entirely approves of this plan. For all the world, I beg you to keep this all a secret. I have announced that I am going on a visit to the country and no one knows the purpose of my voyage, so as to give no one grounds to say, “She has gone to fetch her husband.”
I’ve not heard from him since the letter of 16 floréal [May 5, 1798], in which he says he is leaving for Rodez. I’ve waited until the last minute to find out whether he has resumed his triangles. As soon as I have joined him, I will inform you of the exact state of affairs. I will also ask him why he has not sent the document releasing Tranchot from his employ. All this will soon come to an end.
I have the honor to be, with feelings of the highest e
steem, your very humble servant,
Madame Méchain
One month later, on July 7, 1798, in the red-cathedral town of Rodez, Monsieur and Madame Méchain met for the first time in six years.
What did they talk about, Monsieur and Madame Méchain, upon seeing each other for the first time in six years? We do not know. There is no record of their conversation, just as there is no trace of the scores of letters they sent one another during his travels. What we want to know, of course, is whether he told her what was torturing him. Again, we do not know. All we know is that he was a man given to confession, and he had already confessed his error to at least one person (Giuseppe Slop). We also know that his wife had sufficient astronomical knowledge to grasp the import of his error. And we may presume that having traveled so far to lure him out of exile, heal his affliction, and talk some sense into him, she would have felt entitled to some kind of explanation. So it is quite possible that he told her. Indeed, Madame Méchain may have had sufficient astronomical knowledge to put Méchain’s error in perspective. Such knowledge, after all, did not depend on subtle mathematics, but on a grasp of the perils of observational science and the practical goals of the meridian mission. And Madame Méchain was a practical woman. . . .
Why had he not come home once in six years? It was not that far to travel. Paris was only a week away by carriage. Delambre had been back a dozen times, and although his northern sector lay closer to Paris, he had also traveled as far as Rodez, returned home, and then set out again for Perpignan—whereas in six long years Méchain had not found two spare weeks to visit his wife or his children (who hardly knew him) or to confer with his colleagues, not even during the dark winters, when all geodesy stopped. Think how much easier it would have been for him to visit her than for her to visit him.
And now that she had come all that distance, all he wanted to talk about was how they were measuring his baseline without him, how Tranchot was plotting a revolution to usurp his leadership of the southern expedition. What harm had Tranchot ever done him, anyway? The engineer was an honorable man, an able geodeser who had labored hard on this expedition. After a year of working together, Delambre had only words of praise for him. According to Delambre, Tranchot was hard-working, exact in his measurements, intelligent, never presumptuous; in short, an ideal collaborator. Moreover, Tranchot had never spoken a single word against Méchain in Delambre’s presence. To be sure, he lacked Méchain’s education and his skills in calculation. All the more reason to be generous with him. Had he insulted Méchain? Raised a hand against him? (Méchain had called Tranchot a violent man, and claimed that he had once threatened him. And for his part, Tranchot had admitted that once or twice, in Genoa and Marseille, he had expressed, perhaps “intemperately,” his frustration with Méchain’s delays and Méchain’s tight-fisted control over the mission’s purse-strings.) Well, if Tranchot had insisted that Méchain resume his mission, he was right to do so. Any further sulking would forever keep Méchain from the recognition he deserved.
As for this error Méchain kept worrying about: Who was to say whose fault that was? It could be a problem with the instrument (no matter what Borda said). Or it could be a flaw in the formulas or the correction tables. Or it might even have some natural cause. In a sense, it was presumptuous of Méchain to take all the blame. The burden of the mission—like its success—was great enough to be shared by all. No one’s work is perfect. And a relatively minor and inadvertent error was nothing to be ashamed of. His colleagues would not hold it against him, not even Laplace. They weren’t as judgmental as they sometimes seemed. They had no intention of replacing Méchain, nor of assigning someone else to finish his sector for him—provided he finished it. They were simply desperate to see the job done before the foreign savants arrived in Paris, and desperate to please the politicians who had committed such huge sums to the mission.
Or perhaps Méchain’s refusal to return to Paris and his self-reproach were just a roundabout way of casting the blame on others. Yes. By accusing himself of so small (and inadvertent) a sin, he underscored the guilt of all those who had accepted the Revolution, as if those who lived in Paris were somehow complicit in the crimes of those dark years. Well, this was not acceptable. The Méchain family had survived the awful year of 1789, when both of Thérèse’s parents had died and the mob had invaded their home in the Observatory. Méchain himself had missed the horrors of 1794, when so many honorable men and women went to prison or death—and thank God he missed it, because who knew whether he would have been among them. But in the end, things had not turned out so badly for the Méchain clan. Instead of their little house, they lived in the Observatory apartment. (And Cassini was perfectly content on his estate in Thury; he said himself that he wanted nothing more to do with science.) Now that inflation had settled down, Méchain’s salary was worth more than Thérèse’s inheritance ever was. And their new rights and liberties were nothing to belittle. So why did Méchain continue to hide in these remote provincial towns as if Paris were tainted, or as if he could somehow turn back the clock?
She could see he was suffering, that he was exhausted by his travels, by the tedium and terrible burden of his guilt. But he had to think of his children’s future, even if he could not contemplate his own. He always analyzed everything from every angle, turning things over in his mind, working himself into a state. It was all so counterproductive. As he himself wrote to Delambre, their sufferings were nothing alongside the horrible fate of “the millions of people who would have given their whole fortune to be in our shoes, not having suffered for an instant the dire needs which have befallen so many of our fellow citizens.” Consider their sufferings then, complete the mission, and come home. . . .
Of course we do not know what he told her, or whether he told her his secret—all his secrets, for what man spends six years away from his wife and does not amass a multitude of secrets? (And what of her secrets—a wife who was conspiring with her husband’s colleague to “hand him over”?)
All we know we know by triangulation. All our knowledge of nature, of history, of one another, or even (some might say) of ourselves, comes to us by mediation. Delambre and Méchain investigated the shape of the earth by measuring angles along a portion of its surface. We investigate their relationship by taking angular measurements of their letters to one another, their letters to their maître, their letters to third parties. They learned about one another through a similar angular measurement: a colleague triangulates the relationship between a husband and a wife; a wife triangulates the relationship between her husband and his colleague; and a husband triangulates the relationship between his colleague and his wife. We learn about ourselves by comparing ourselves with others, sometimes with others from the eighteenth century.
All we know is that Madame Méchain spent the next five weeks by her husband’s side, and that when they parted in the gloomy village of Rieupeyroux, he still refused to join Delambre at Perpignan. Her husband had rebuked her, she said, even lied to her. Or so she informed Delambre when she wrote to him, as promised, on her way home via Carcassonne.
Carcassonne, September 1, 1798
Citizen,
Having completely failed in my mission, and with a heart pierced by a thousand griefs, I return to Paris. As some business obliged me to pass through Carcassonne, Citizen Fabre has shown me your recent letter, which has disturbed me yet again. I will make every effort to relieve your worry. I informed you in May that I would soon be leaving Paris to take my place beside my husband. I promised you then to send you news as soon as I had joined him. A thousand fatal accidents conspired to prevent us from meeting in Rodez until July 7. Since that time, I have begged him in vain to write to you and to agree to measure the baseline of Perpignan with you. Not wishing to upset me, he always answered vaguely. For the first time, my husband has dissimulated with me. Not knowing his intentions, what could I tell you? I realized I was wronging you, but the situation dictated my actions. I only know that Citizen Tranchot, who
knows perfectly well what he is doing, informed all my husband’s acquaintances here that he alone would measure the baseline, not Méchain.
I told my husband that only a barroom savant would think a T—could replace a M—, that everyone knew who was the best man for the job, and that the matter would have to be resolved by the Academy. Above all, I urged him not to throw away the fruits of all his years of suffering and sacrifice for something so idiotic. I decided to force the issue. I insisted that I would not leave his side until he had finished all his triangles and had been reunited with you. He thereupon renounced the baseline irrevocably, and vowed that he would concede all the glory to those whom fortune had favored. He said that he would never show up unless Tranchot had been set aside, that in any case his presence in Perpignan would hardly be agreeable, and that he would rather be sent to his death than go to Perpignan. I lacked the will to oppose him further. But forgive me if even now I lack the courage to address any longer a subject that is killing me.
In the end, I managed to make him promise to inform you himself of his resolve. I wrote to Borda and explained my situation, no longer able to withstand the horrific blows which assail my heart. I know my husband to be a man of talent and virtue. I affirm and attest that his abilities and faculties have not been alienated in any way, that they remain unchanged, only his heart has been ulcerated by a man who has sworn to bring him down, to bring an entire family down. In my mind’s eye, I have seen my husband covered with glory and public acclaim. You put it well, Monsieur, when you say that a moment which should be the happiest of his life may yet consign us to oblivion. I do not complain. I accuse no one, least of all my husband. It is the extreme sensitivity of his soul which has ruined him. He is more unhappy than blamable.