The Castle of Mont-Jouy

There is almost nothing right or wrong which does not alter with a change in clime. A shift of three degrees of latitude is enough to overthrow all jurisprudence. One’s location on the meridian decides the truth, that or a change in territorial possession. Fundamental laws alter. What is right changes with the times. Strange justice that is bounded by a river or a mountain! The truth on this side of the Pyrénées, error on the other.


Cut off by war on the far side of the Pyrénées, Méchain knew little of these developments. For nine months he heard no news from France. The most recent letter was dated March 1793, from before his accident at the water-pumping station. For two months after that injury he convalesced in bed, until the summer sun lured him out of his dark room onto the terrace of the Fontana de Oro. The summer solstice was approaching, and Méchain insisted that he be carried out, not in search of a solar cure, but in search of solar knowledge. He was borne out into the dazzling Mediterranean summer and propped up on pillows under the repeating circle. The salt breeze breathed across the paving stones. The noonday heat silenced the town. Just out of sight, the sea sloshed against the quays. The fashionable crowd on the Rambla had retreated indoors. Only mad dogs, Englishmen, and solar astronomers go out in Barcelona’s noonday sun in summer.

Tranchot had prepared the instrument for him. For four thousand years stargazers had sought to define one of astronomy’s most fundamental constants: the obliquity of the ecliptic, or in other words, the angle of the earth’s tilt relative to the plane of its orbit around the sun. With the Borda circle to hand, and the summer solstice upon them, Méchain had an ideal opportunity to make the definitive measurement of this constant.

This was painful work under the best of circumstances, but Méchain insisted that he alone take the readings. While Tranchot held the smoked lens to the astronomer’s eye, Méchain tracked the sun until it reached its maximum altitude. Then Tranchot rotated the Borda circle for him, while Méchain fine-tuned the position of the scope. Working together, they managed to take a few preliminary readings before the heat of the sun began to distort the circle’s brass gauge. Méchain was having difficulty fine-tuning the scope with his left hand. For a man recovering from a shattered chest—for a right-handed man with a dangling right arm—the effort was too much. They were forced to break off. A relapse followed. For twenty years he had labored in the dark bureaus of naval cartography to map a Mediterranean coast he had never seen. Now its light suffused his mind, even when his eyes were shut.

Salvà was worried and proposed a medicinal cure at the thermal springs of Caldas. Chastened, Méchain took his advice. The hot baths and showers were comforting. But six months after his accident, his right arm still hung limp at his side. The doctors told him he might never recover its use. “Time has done more than art,” he would conclude a few years later when his arm had regained its strength.

By the time he returned from the spa—still impaired, if somewhat more capable—Spain was on the verge of a military victory that would make her mistress of both slopes of the Pyrénées, unifying Catalonia for the first time in 150 years. France may have declared war first, but Spain struck first. In May, General Ricardos, the supreme Spanish commander, ordered his main body of 40,000 troops through the saddle west of the Bellegarde fortress where Hannibal had attacked two thousand years before, while three columns of 3,500 of Ricardos’ soldiers spilled through the high inland passes that Méchain and Tranchot had triangulated the previous summer. Overpowering the French garrison at La Garde, they marched down the Tech valley to join the main body of troops occupying the plains of Roussillon. Had they pressed their advantage then, the Spaniards might well have conquered Perpignan. Instead they paused to fortify the heights, set siege to Bellegarde, and surround the city. All April, May, and June, within sight of the panicked citizens of Perpignan, they bombarded Bellegarde from nearby Puig Camellas, reducing the mighty fortress to rubble. When the brave French garrison finally surrendered, one thousand prisoners were marched to Barcelona, to be incarcerated in the Mont-Jouy castle where Méchain had conducted his celestial observations the year before. The captives were lodged in a cellar and guarded by cannon charged with shrapnel “in order to avoid insolence from so evil a people.”

Eighteenth-century warfare was suffused with its own contradictions. Courtesy coexisted with brutality, even between the defenders of the Catholic monarchy and the apostles of Revolutionary liberation. The Spanish generals allowed the captured French officers to spend a night in Perpignan before marching them off to prison. The Revolutionaries tried to persuade the Catalonians to adopt their cause. Like eighteenth-century scientific rivals, enemy officers often had more in common with one another than with the leaders who sent them into battle. With the advent of this new kind of mass war, however, the tug of nationalism began to pull even science apart.

Two months after the fact, Lavoisier at last wrote to Méchain to inform him that the Academy had been abolished, but that as a member of the Commission on Weights and Measures, he was entitled to a salary of 10 francs a day, which his wife might collect on his behalf. At least his family was now getting paid for his labor. Yet Méchain never received this letter. A few months later, at a time when Delambre was being purged from the meridian project and his former colleagues were being imprisoned and threatened with execution, Méchain was still writing to Paris to say that he would defer to the Academy’s wishes. He did hear rumors of the demise of the Academy, however, and believed (with some reason) that the Paris administrators were seeking to replace him. Indeed, he seems to have been spared from the general purge only because the Committee of Public Safety worried that any such threat would cause him to seek permanent asylum in Spain, along with his precious repeating circles and his detailed geodetic data.

So when he was offered a prestigious scientific appointment by the Spanish Crown, Méchain was understandably tempted. Tens of thousands of French men and women had fled their country. Thousands had taken up arms against their own nation. The dead king’s younger brothers were leading armies against their own people. Compared to this, what harm was there in measuring a few geodetic triangles for Spain? How could scientific work be traitorous? Lavoisier had said it best: “The sciences are not at war.” Moreover, Méchain was penniless. The Barcelona bankers had frozen his account, his French paper money was worthless, and a French law prevented his colleagues from sending him hard currency.

Most of all, Méchain despised the radical turn his country had taken since 1792. Even the comparatively mild upheavals of 1789 had distressed him painfully, while recent events had horrified him. But if the Revolution had frayed the knots of his patriotism, it had not loosened his sense of duty to his colleagues and to his mission. Méchain’s virtue was exactitude—a prosaic virtue perhaps, and one rarely associated with genius—but with it came a fierce determination to complete what he had begun. The frontier stations he had once hoped to approach from the French side of the border were now deep in Spanish-occupied territory. Delambre had offered to hurry down from the north of France to help measure them if that were necessary. If, on the other hand, Méchain wanted to measure them himself, this might be his only chance.

Fortunately, his arm had begun to heal, and he had the capable Tranchot to help him. So early in that autumn of 1793 he secured permission from General Ricardos to complete his triangles along the peaks of the Pyrénées. The general would now allow him—the emissary of an enemy power—to conduct sensitive geodetic measurements in a war zone. Already the Frenchmen had calculated the pinpoint locations of Catalonia’s major fortresses. In return, Méchain gave his solemn word that no member of his team would leave the country without official blessing, nor provide their data to the French until the war was over.

That September the two Frenchmen, accompanied by Captain Bueno, ventured back into the Pyrénées in a bold attempt to complete the high mountain stations. At the massive fort of Figuères—which Méchain wove into his triangles—they split into two parties, each taking one of the repeating circles. Méchain and Bueno angled toward Puig Camellas, the hilltop from which the Spanish cannon had pounded the Bellegarde fortress into rubble, while Tranchot struck out on his own for the high inland mountains.

Tranchot’s goal was Puig de l’Estella, a 5,800-foot peak on top of an old iron mine. The summit lay in the shadow of the Massif de Canigou, the glaciated blue behemoth which dominated the eastern Pyrénées. All this had once been French territory. Indeed, just as Tranchot arrived at Puig de l’Estella the French army, beefed up with reinforcements, broke free of their encirclement at Perpignan. Striking inland from the fort at Salses, just north of Perpignan, they began to drive up the Tech valley, forcing the Spaniards to regroup at Boulou, the strategic town where the Grande Route crossed the Tech River. Outnumbered, 9,000 to 29,000, the French were nonetheless supported by several thousand miquelets, who traveled along the flanks of the army, assailing the Spanish and terrorizing peasants. In response, the peasants organized into protective bands. The countryside was in turmoil.

Above the cool pine forests, the high dry air offered superb views of the neighboring geodetic stations, as well as the two armies jostling for position in the valley below. Each side was trying to maneuver their cannon onto the dominant hill. Over the course of twenty-four days the French probed the Spanish positions with eleven skirmishes and three general attacks. On September 22 alone they lost 3,000 men. On October 1, French reinforcements arrived in the face of a Spanish barrage and took a hilltop outpost. On October 5, the French let loose a cannonade of their own to protect a cavalry charge. On October 6, they established a new battery on the heights, which opened fire the next day on the main Spanish camp. Standing on the bare mountain peak beside his bizarre conical signal, sighting with his double-scoped instrument, Tranchot made a visible target.

He had been taking intermittent measurements for a week—the weather was changeable and strong winds threatened to topple his circle—when on the morning of October 7, just as the French battery at Banyuls opened fire on the Spanish positions below, a band of six villagers from the nearby hamlet of Vallmagne ambushed him in the name of the Revolution. Tranchot protested that he too was a French loyalist, an ardent Revolutionary, and on a mission from the National Assembly. He showed them his papers, his passports, and a copy of his commission, along with the newly certified documents sent by Delambre. But the villagers would not allow an unknown engineer to “conduct surveillance” along the front. Puig de l’Estella looked straight down on the valley where the French army was advancing. They bound Tranchot, gagged him, fastened a rope around his neck, and led him by garrote to their town, where the local mayor advised them to conduct him to the district capital, whence he was escorted to Perpignan.

This near disaster turned out to be a stroke of good fortune. Francesc-Xavier Llucía, the chief administrator of Perpignan, was familiar with the meridian mission; he secured Tranchot’s immediate release and more. A few weeks earlier, Méchain had begged Llucía to plant signals on the peaks of Mount Bugarach and Mount Forceral, well behind French lines, so that he might sight them from the frontier. With Tranchot suddenly on the French side of the front, Llucía now authorized him to carry out this task. Two weeks after his arrest the signals had been planted, and Tranchot was climbing back up the Puig de l’Estella to continue where he had left off, just as the battle of the “Battery of Blood” threatened to engulf his mountain aerie. For several days, 6,000 French troops assaulted Puig Singli, where the Spanish cannon commanded the heights over Boulou. The first seven attacks were repulsed, the next three gained the position temporarily, and the eleventh effort, with the Spanish out of ammunition, won the day—until a Spanish counterattack massacred the lot of them the next morning. Calm above the chaos, Tranchot performed his exacting labor.

This would be the only station Méchain ever allowed Tranchot to observe with the circle on his own, and one may well wonder why. It was not due to any lack of experience. Tranchot had toiled for two decades on the triangulation of Corsica, a country as rough as Catalonia. Born in Koeur-le-Petit, a hamlet of Lorraine where French mingles with German, he had survived a difficult birth to grow into a vigorous man. He may have lacked a formal education or an academic title, but he was among the nation’s most capable cartographers, a man of proven integrity. Toward the end of the Corsican project, accusations of scientific fraud had been bandied about. Tranchot’s final measures had resolved the controversy. Méchain had himself signed the Academy report which singled out Tranchot’s contribution as “infinitely precious for the precision of geography.” From Méchain, there could be no higher praise. By this date, moreover, Tranchot had mastered the repeating circle. Méchain himself admitted, “I could rely on him as I relied on myself.” Yet he always supplied Tranchot with prepared data sheets, and never let him perform his own calculations or look into the expedition notebooks. By way of contrast, Delambre allowed Bellet, a mere instrument-maker, to take observations, record data, and doublecheck calculations.

It is hard to imagine that Méchain’s scruples were justified on technical grounds. In 1790, the two men had collaborated on the navy’s charts of the Mediterranean coast. He knew Tranchot was a geodetic surveyor of consummate skill. Yet collaboration never came easily to Méchain. Despite his time in Lalande’s astronomical workshop, despite the assistance he accepted from his wife and the aides he employed on his journal, Méchain remained an astronomer who worked best alone. His virtue lay in his ability to know the earth, the planets, and the stars. He had less knowledge of his fellow man.

Or perhaps he knew his fellow man all too well. Perhaps Méchain learned what Tranchot had really been up to during his excursion across the border—and that at some time during his foray into France Tranchot had met with his military superiors and supplied them with the plans and geodetic locations of all the fortresses the team had triangulated in Catalonia: Figuères, Girona, Roses, Barcelona, and Mont-Jouy—all the major military installations of northeastern Spain. As a captain of military cartography, Tranchot was obliged to supply his commanders with this information. Failure to do so would have been treason, at a time when treason meant immediate execution. Besides, Tranchot was a patriot and committed to the Revolution, and these plans would help the Republican cause.

For Méchain, though, this would have been a betrayal. Scientific knowledge gathered for the benefit of the world’s people ought never to be used for harmful purposes. Méchain valued allegiance to science over allegiance to nation. At a minimum, Tranchot’s foray into France broke the promise Méchain had made to the Spanish general. His honor demanded that he keep his word. A savant’s reputation was the outward sign that he remained true to his science. To betray one’s own honor was worse than treason.

And this suggests the real reason Méchain did not trust Tranchot. Having betrayed the mission, what was to prevent Tranchot from betraying Méchain and usurping his command of the southern expedition? Tranchot deserved credit for many of the mission’s successes to date. And he was the most likely replacement for Méchain should the astronomer be declared incapacitated. It was this haunting thought which drove Méchain, despite his wounded right arm, back into the Pyrénées.

Méchain and Commander Bueno had their own view of the battle of the Tech from their station at the summit of Puig Camellas. From there, they could see across the battle lines into the besieged town of Perpignan, where the French generals were directing their breakout. They could see south into Spanish Catalonia and the district outside Figuères, where a crumbling sea-blackened turret, the Tour de Mala-Vehina (the “Tower of the Bad Neighbor”) stood on a crest of land belonging to Captain Bueno himself. They could see north toward Bugarach and Forceral, deep in French territory, where Tranchot had placed his signals. And on the bright, clear morning of October 25, as they turned their scopes across the Tech valley, they could see a figure on the summit of Puig de l’Estella, standing beside the double-cone signal—a dark figure against the blue sky hunched over a brilliant brass circle: Tranchot adjusting and readjusting the scopes of his circle while the battle boomed in the valley between them.

When Méchain completed his measurements ten days later, Tranchot was still triangulating. And when another week passed, and another, and Tranchot still had not returned, Méchain grew anxious. He was less concerned about Tranchot’s safety than about the possibility that his aide would remain in France. With these angle measurements done, their mission in Catalonia had ended. Lalande, for one, expected Tranchot to remain in France, and even urged Méchain to slip across the border to join him.

This was to underestimate Méchain’s scrupulousness. He sent a message to Tranchot demanding that he return to Spain immediately. “It is for the sake of my duty and my honor to enjoin you not to leave for France without my permission, neither by any route nor by any means. It is for this reason, and not for the continuation of our mission, that I insist so strongly. It would disgrace us greatly, and deservedly, for you to conduct yourself in any other fashion.” At stake was something greater than the success of a scientific project—“the most important any man has ever been charged with.” At stake was Méchain’s reputation as a man of his word.

The Spanish army chose just that moment to counterattack. They took advantage of their superior numbers and once again drove the French back down the Tech valley, conquering the coastal towns of Collioure and Bagnols and renewing their siege of Perpignan. In the process, they sealed off Tranchot. He was trapped in France, he said, by “force of arms.” Perpignan now became the scene of an all-out political struggle between moderates and radicals. Another failed French general was guillotined. Individuals suspected of counterrevolutionary activities—especially those with ties to the aristocracy, the Church, or the party out of favor—faced summary execution. Among these was Llucía, the French Catalan Revolutionary who had once declared that “It is time to electrify all souls.” He had saved Tranchot, but he could not save himself. Some fifty heads fell that month in Perpignan alone. Only the weather stopped the Spanish advance this time. The November rains ended both the military and the geodetic campaigns. In the Tech valley the soldiers slept in mud. Méchain returned to Barcelona. And some time that winter, Tranchot slipped back across the frontier to rejoin him.

This proof of the Frenchmen’s integrity did not persuade General Ricardos. Despite petitions and personal pleas, he insisted that Méchain and Tranchot remain in Barcelona until a peace was concluded. Nor would Méchain be allowed to send home any more communiqués with numerical data; these would be confiscated at the border as encoded letters. No military leader could knowingly allow this information to fall into enemy hands, although Ricardos was raising the drawbridge after the moat had been breached.

That spring, the fortunes of war shifted once again. In March, while visiting Madrid, the victorious General Ricardos died. His replacement, General La Unión, was the youngest general in the Spanish army, a devout Catholic of high moral sentiment, repelled by the populist atheism of the French Revolution. Soon after, a new French general, Jacques-Coquille Dugommier, fresh from his victory at Toulon (where he had commanded the young Napoleon Bonaparte), took charge of the Revolutionary army in the region. Dugommier quickly set in motion the republic’s plan to liberate—or rather, subjugate—Catalonia. Catalonia, he proclaimed, was ripe for revolution. The province was rich in mines and industry. The people loved liberty and hated their Castilian overlords. If they embraced equality and became an autonomous republic, the province might serve as France’s boulevard to the rest of the Iberian peninsula.

Dugommier attacked as soon as the season allowed. By mid-June, the French had recaptured the high mountain passes and had begun to push their way down the southern slopes of the Pyrénées. The Spanish retreated to their massive fortress at Figuères on the Grande Route, positioning 9,000 soldiers and thirty-two artillery pieces to hold their right flank at the Tour de Mala-Vehina on the lands of Méchain’s cartographic collaborator, the good Captain Bueno. Should Figuères fall, as seemed likely, the road was open to Gerona, and beyond that, Barcelona.

For the past nine months Méchain had heard no news from his colleagues in France, only rumors. He wrote them a long letter anyway. He had failed to secure passage on a vessel out of Barcelona, and the Spanish were holding him in “unjust detention.” With the Academy abolished, as reported in the public papers, the meridian project had no doubt been canceled as well, and the meter would be determined with a pendulum, as originally planned. If his mission had indeed been canceled he begged to be informed of this at once. If not, he could see a way to complete the meridian survey by the end of the year. He had the scenario all worked out. As soon as the Spanish general released him, he would return to France and triangulate his way north toward Delambre. Because the French terrain had already been surveyed and mapped by Cassini, he would simply revisit those old angle measurements with the repeating circle. If he began next month, he could triangulate as far as Evaux by July. Evaux was the halfway point of the meridian arc, well north of Rodez, where he had been scheduled to meet Delambre. With luck, he might even triangulate as far as Bourges, which would mean that he would have surveyed two thirds of the total arc, instead of the one third he had been assigned. Then the two astronomers would together measure the two baselines in August, one in Delambre’s northern sector and one in his own southern sector, and have the mission wrapped up by the end of the summer.

There was only one snag. The Spanish general refused to let him leave. “But alas, where am I? In irons! And yet I speak like a man free to indulge his passionate zeal for the success of this superb mission. No matter; at least I have tried to make my slavery useful, if not to the mission itself, then at least to astronomy.”

Méchain was too obsessive an astronomer to remain idle. They had barred him from Mont-Jouy, but they had not forbidden astronomical work at his Barcelona hotel. So in December he reorganized his observatory on the terrace of the Fontana de Oro. This time he would take advantage of the winter solstice to measure the angle of the earth’s rotation relative to its orbit around the sun. For this, he would also need an exact determination of the latitude of his hotel terrace; last winter’s data, gathered at Mont-Jouy, would not serve his purpose. Mont-Jouy, although readily visible to the south of the city, was over a mile distant. Equipped with the world’s most exact astronomical instrument, he intended to make this measurement with greater precision than any investigator in the past four thousand years. As an added bonus, his observations would also offer a double-check on his latitude results for Mont-Jouy.

Méchain’s motives for undertaking these observations—the results of which were to haunt him for the rest of his life—appear to have been mixed, as motives often are. Certainly he wished to prove to his Paris colleagues and his Spanish hosts that he remained the same meticulous astronomer as before, and that the accident of the previous April had not diminished his abilities. This would silence any talk of replacing him on the meridian expedition. It would also demonstrate his diligence at a time when individuals who refused to serve the public good risked execution.

There was also something about his earlier results that nagged at Méchain. To calculate the latitude at Mont-Jouy, he had measured the heights of six different stars: Polaris, Kochab, Thuban, Mizar, Elnath, and Pollux. More was always better. Thoroughness was always rewarded. For his final analysis, he had used the results of the first four of these stars, those for which he had gathered the most data. Of these, the results for the first three converged to a remarkable degree, giving average latitudes of 41°21‘44.91” (Polaris), 41°21'45.19” (Thuban), and 41°21'45.19” (Kochab). The total spread in these values came to an infinitesimal 0.3 seconds of a degree of arc. This suggested that Méchain had determined the global location of the castle tower of Mont-Jouy to within thirty feet. It was a stunning display of astronomical virtuosity, the kind of precise result that had won him the leadership of the southern expedition.

Results based on the fourth star, Mizar, however, diverged from this pattern, and indicated a latitude of 41°21'41.00”, which differed from the others by four seconds of arc, or some four hundred feet. This anomaly irritated Méchain. Why did the readings for this one star differ tenfold from the rest? It was a natural question for a natural philosopher to ask. Yet even then, he might have let sleeping data lie. Only a decade before, a discrepancy of four seconds would have been a remarkable achievement. It came to little more than 0.01 percent of the six-hundred-mile arc from Dunkerque to Mont-Jouy. Besides, he had already summarized these astronomical results for his Spanish hosts and sent a précis to Borda in Paris.

On the other hand, Méchain had a hypothesis that might explain this discrepancy. Ah, that other hand! Why is it always with that “other hand” that scientists open Pandora’s box? They do not open it to make their lives more difficult. More often they are simply seeking to reassure themselves, to confirm what they think they already know to a finer degree of certainty. But, for better or worse, they do not always know what they think they know. Sometimes they even have the good fortune to be mistaken. And then, as Enrico Fermi once said, they may make a discovery.

The problem with the Mizar data, Méchain hypothesized, was refraction. The corrections for the bending of light had been worked out by astronomers in London and Paris. Perhaps their corrections did not apply to southerly towns like Barcelona, where those stars crossed the meridian closer to the horizon and the higher temperatures distorted their sighting through the atmosphere. Of all the stars he had measured, Mizar crossed the meridian closest to the horizon. The corrections were small in the first place, of course, and any adjustment would necessarily be smaller still. But the meridian expedition was operating with an unheard-of degree of precision. The repeating circle promised precision limited only by the patience of the observer. And Méchain refused to believe the fault lay with the stars.

So he spent the winter of 1793–94 on his back on the terrace of his Barcelona hotel, taking nighttime observations. While Tranchot held the lantern and verified the spirit level, Méchain hunkered down as before, whirling the circle, then the scope; listening for the clock to beat the moment of the star’s meridian transit; then whirling the circle, then the scope, and repeating his eye-and-ear measurement. He conducted observations on Christmas Eve, on Christmas night, on New Year’s Eve, and on the first night of the new year, plus every clear night in December, January, February, and March. He took 910 stellar readings, each with ten or more repetitions, for a Herculean total of some ten thousand observations. Then, inside his hotel, during the daylight hours, he calculated his way through this mass of data, his refraction tables and logarithmic tables continually by his side. By early March he had determined the north latitude of his hotel to be 41°22'47.43” (based on Polaris), 41°22'48.38” (based on Kochab), and 41°22'44.10” (based on Mizar). Once again, the results for the first two stars (those in which he had the most confidence) agreed to within an impressive one second of arc (or one hundred feet), making the Fontana de Oro the most accurately located hotel on the face of the planet. Yet once again the Mizar data gave discordant results, differing some four hundred feet from the others.

One final step would clarify the mystery. Méchain would now need to compare his new latitude results at the Fontana de Oro with his old results for Mont-Jouy by subtracting the distance between them. Calculating this distance, of course, was just the sort of task his expedition had been equipped to perform. He laid out a triangulation that included his hotel, Mont-Jouy, and the cathedral of Barcelona, and to make doubly sure, a second triangulation which included his hotel, Mont-Jouy, and the Lanterna that served as the port lighthouse. There was only one snag: to carry out the triangulation accurately, he would need to take angle measurements at each station, and the Mont-Jouy castle was closed to him as a Frenchman. It was so near, yet just out of reach.

By mid-March, with Tranchot’s help, he had taken measurements from his hotel, the cathedral, and the Lanterna. In the meantime, he apparently persuaded the Mont-Jouy commander to grant him a single day at his old observatory tower at the castle. On Sunday, March 16, 1794, a slightly overcast spring day, Méchain climbed the hill of Mont-Jouy to perform a final triangulation—while several hundred of his fellow citizens languished in the prison below. Then he returned to his hotel to calculate.

The numbers were quickly tallied. According to the triangulations, Mont-Jouy was located 59.6 seconds of arc from his hotel, or a distance of 1.1 miles south. Comparing this distance with the two latitude measurements was a matter of simple subtraction. After subtracting 59.60 seconds from the average latitude of the Fontana de Oro (41°22'47.91”), the result should equal the average of his most reliable latitude data from Mont-Jouy (41°21'45.10”). It was a moment’s work.



This is Méchain’s own map of the triangulations he made within the city limits of Barcelona in 1794 to verify the distance between the Fontana de Oro and the fortress of Mont-Jouy. The Fontana de Oro is located at the center of the diamond. Méchain constructed two triangles that included Mont-Jouy and his hotel: one using the north tower of the cathedral and the other using the Lanterna (lighthouse). (From the Archives de l’Observatoire de Paris)

Imagine then his horror when the results fell short by 3.2 seconds of arc. Not 3.2 seconds to be folded into the six-hundred-mile arc from Dunkerque to Mont-Jouy, which would have been an insignificant difference of 0.01 percent, but 3.2 seconds over the course of a 1.1-mile arc, for a stunning discrepancy of 5.4 percent. Instead of explaining away the anomaly within his Mont-Jouy results, Méchain now confronted an anomaly of horrific proportions. Having pinned down his latitude to within forty to one hundred feet on two different occasions, he had now discovered that his two average results diverged by a horrifying three hundred to four hundred feet. He must have erred in his observations or his calculations. But which? Which set of data could he believe? Most horrible of all: he had already mailed a summary of one set of these results, those for Mont-Jouy, to his colleagues in Paris. From this they would want to calculate the length of the meter, the supreme standard for all people, for all time.

It was as if he had set out to fine-tune a Stradivarius and snapped the instrument’s neck. His integrity had plunged him into a crisis. His effort to rehabilitate his reputation had only caused him to doubt his own abilities. What had gone wrong?

Under normal circumstances, Méchain would simply have climbed back up Mont-Jouy and taken more stellar observations at the castle. But these were not normal circumstances. His one-day pass to the fortress had been a begrudging exception, not to be repeated for an enemy of the Spanish Crown. And day by day, as the Revolutionary armies advanced deeper into Catalonia, the political tensions worsened. The Republic promised the people of Catalonia a “sister” republic of their own; the Spanish Crown declared a religious war against atheism. Some residents of Barcelona supported the Revolution; others seethed against French godlessness. It was no time to be a Frenchman in Barcelona.

Moreover, nothing now seemed to prevent the team’s departure. Ricardos, who had opposed it, was dead. Tranchot and Esteveny were eager to return to France where their duty lay, as well as their colleagues, friends, and families. Méchain, however, faced a terrible dilemma. He had told no one of his error, not even Tranchot. He was free to go, but did he dare to leave his mistake behind? Once he left Spain, how would he ever return to Mont-Jouy? Yet how could he justify staying on in a foreign country—an enemy nation—now that his work there was done? He dared not risk giving the impression that he had decided to emigrate. Even a rumor to that effect might lead the Paris authorities to cut off his salary, imprison his family, and bar his return to France forever.

So, on the advice of his Spanish friends, he secured a passport for neutral Italy—ostensibly because it would not arouse suspicion—bypassing any need to inform General La Unión of his departure. In late May, after two years in Catalonia, Méchain booked passage on a Venetian vessel bound for Genoa, the Italian city nearest the French border. For someone who anticipated the worst, however, Méchain certainly attracted his share of calamities. His pessimism offered him no protection. On May 25, three days after he had loaded his precious repeating circles onto the vessel in Barcelona harbor, a bolt of lightning struck its mast, bursting the wooden boxes that carried the repeating circles and charring one instrument’s stand. The circles themselves appeared undamaged, but it was a fitting final salute from Catalonia. They sailed on June 4.


Nothing of these events was known in Paris. There, everyone assumed Méchain had been placed in detention by the Spanish generals (perhaps for having smuggled out the fortress plans). He himself had written home of his “unjust detention” in Barcelona. He had complained that he was being held “in irons.” The words were metaphorical, even melodramatic—Méchain had been comfortably lodged all this time at the Fontana de Oro—but his colleagues sent word to General Dugommier that the French astronomer was being held against his will. In mid-June, two weeks after Méchain sailed for Italy, Dugommier wrote an indignant letter to his opposite commander, the devout young General La Unión, demanding that the Frenchman be freed. “In the name of the French Republic, which protects the savants of all nations, and which knows how to avenge any outrages committed against its own, I seize this occasion to demand that in the name of the arts whose free exercise should be respected at all times and by all nations, you release the citizen Méchain and his two colleagues, charged with the measure of the arc of the meridian, and detained in Barcelona by the orders of your predecessor or by you.” This was not the only lesson the Republican general thought he would teach the barbarous monarchist. “Savants must not be considered soldiers,” Dugommier wrote, “nor treated as such. The peaceable arts have nothing to do with war. And unless you wish to perpetuate an extraordinary violation of those conventions which govern even the most uncivilized people, you cannot refuse to return him and his two collaborators to liberty and their homeland.” Méchain’s mission, he insisted, “must be respected around the globe.”

General La Unión knew no more of Méchain’s whereabouts than did the French. But he knew when his honor had been slurred. Never would he have impeded the advancement of human learning, nor dishonored his good name by holding an innocent civilian against his will. “If Méchain were to declare that he had been imprisoned by orders of either the Spanish government or myself, I would pass for an impostor in the eyes of the universe,” he wrote back. And then he added his own veiled accusation against the godless French. Like the rest of his countrymen, he announced, he appreciated “not only Méchain’s knowledge, but his moral virtues as well.” Yet just in case those virtues had gone unrecognized, he privately reminded Barcelona’s governors to treat Méchain honorably and provide him with financial assistance. Of course by then Méchain had been far from Spain for months before.

That fall, the siege of Figuères reached its climax. General Dugommier died in battle there on November 17, killed by an exploding shell as he surveyed his impending victory. “Dugommier is dead on the field of honor,” the proclamation read. “He demands vengeance and not tears.” Three days later, General La Unión followed him into the grave, killed by two musket shots during a bloody French assault. The French drove the Spaniards from the crest where Captain Bueno had his tower, forced the surrender of Figuères, and pushed east toward the coast. Their successes, however, quickly got the better of them. Their supply lines unraveled. Desertions mounted. The two nations entered into formal negotiations to end the war, and in July 1795 they signed the Treaty of Bâle, returning the border to the same ambiguous position it had occupied before the war. But by then, Méchain had no prospect of returning to Mont-Jouy.

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