The North-Going Astronomer

Fabrice showed them his passport indicating he was a barometer salesman travelling with his wares. “Are they fools!” cried the border-guard, “This goes too far!”

—STENDHAL, The Charterhouse of Parma

The countryside was strangely silent, the roads deserted. The local militia had been ordered to stop “any unknown person travelling by foot, horse, or carriage, and with the amiability which Equality and Liberty prescribe, check their identity against their passports, and if the passports prove false, conduct them to the town hall to be judged according to the law.” That afternoon, a gendarme told a man traveling by carriage with his wife and daughter to hurry home. The fortress of Verdun had fallen and eighty thousand Prussian soldiers were crossing the plains of Champagne, marching toward Paris to restore the French king to his throne. Everything was in readiness for the onslaught. A proclamation had gone out from the capital that the people of the surrounding hamlets should prepare themselves to “share with their fellow citizens the honor of saving their fatherland—or of dying in its defense.” Inside the Paris walls, the gendarme told the traveler, patriots had begun to massacre all the city’s prisoners lest they rise in aid of the aristocrats.

That same day—September 4, 1792—in the highest reaches of a château set on top of the region’s most elevated prominence, a man was bent over a strange apparatus, sighting across the horizon. The man—a savant by all appearance—had set up an observatory inside a lofty twenty-two-foot pyramid that normally served as a belvedere where diners might admire the delightful prospect. At intervals, he lifted his head from the instrument to manipulate the two telescopes on their interlaced brass rings, pivoting them first one way, then the other, as if solving a mechanical puzzle. Then he bent his eye to the eyepiece to take another sighting, while one assistant verified the gauge and another recorded the value. It was a delicate operation, sensitive to the least vibration. The men dared not shift their weight lest the floorboards transmit their motion to the instrument and perturb those values destined to serve as the unique and permanent measure of all things.

The Château de Belle-Assise was aptly named. It was indeed “beautifully situated,” famous for its view over the fertile valley of Brie. A château had stood on the hill since the thirteenth century. The current owner, the comte de Vissec, had permitted the expedition to labor in his pleasure pavilion. On the western horizon, the savant could pick out twin domes rising from the gray jumble of Paris: the leaden dome of the new Panthéon and the golden dome of the old Invalides. On the southern horizon he could make out the Gothic church at Brie-Comte-Robert. And on the northern horizon he could identify the church belfry of Dammartin, due to be demolished. Nearer to his position he could see the medieval dungeon of Montjai, from which he had originally hoped to conduct his measurements. His task was to measure the horizontal angle separating these sites with a precision never before achieved.

That evening, just as the savant completed his fourth and final day of observations at Belle-Assise—night had fallen and his assistants were packing their instruments into their carriages in preparation for the post-horses they had ordered from the town of Lagny—a party of militiamen arrived instead. They were well armed with muskets and well fortified with wine. They had secured permission from the local municipal council to search all the surrounding châteaux. Rumors of treason were circulating through the countryside. It was widely suspected that the four visitors to Belle-Assise were spying for the Prussians. Was it not true they had paid the local carpenter Petit-Jean to build a platform on the ruined tower at Montjai, which, as everyone knew, was haunted by the demons of a murderous priest? And why had they been peering across the valley in the direction of the Prussian advance? They would have to show their papers.

The savant presented his passport. It identified him as Jean-Baptiste-Joseph Delambre, “jointly charged by the National Assembly to carry out, in conjunction with Monsieur Méchain, the mathematical measurement of the meridian from Dunkerque to Barcelona.” Delambre was a solid, well-set man of forty-two, of average height for that time—he was five foot four—with a round face, a strong nose, blue eyes, and brown hair swept back from his forehead. It was a frank and open countenance, yet curiously observant, with a mouth inclined toward irony. His blue eyes were disarmingly naked, and on closer inspection it was clear why: Delambre had no eyelashes. He was an observer rather than a man readily observed.

His assistants presented their papers as well. The first was Michel Lefrançais, a twenty-six-year-old apprentice astronomer, nephew of the illustrious astronomer Jérôme Lalande. The second was Benjamin Bellet, a thirty-two-year-old instrument-maker, an apprentice to Etienne Lenoir, whose workshop had built the expedition’s newfangled “Borda repeating circle,” the instrument that was to bring unrivaled precision to their survey. And the third was a manservant named Michel.

The leader of the militia seemed satisfied by these documents. But his followers did not agree. They complained that the passports had expired—or more precisely, had been issued by a political authority which had itself expired. In the four months since they had been signed, an uprising had deposed Louis XVI and installed a republic.

Delambre tried to explain that he had been sent on a mission to measure the size of the world. He was a practitioner of geodesy, the science of measuring the size and shape of the earth. Improbable as it sounded at a time of national emergency, the government had assigned his mission its highest priority. His mission was to travel up and down the meridian of France. The Academy of Sciences—

“There is no more ’Cademy,” interrupted one of the militiamen, “the Cademy is no more. We’re all equal now. You’ll come with us.”

It was not true, not yet; the Academy still existed, as far as Delambre knew. Earlier that week, Antoine-Laurent Lavoisier, the great chemist and treasurer of the Academy, had admonished him not to quit his mission until he had “exhausted every reserve of strength within him.” Any halt or failure would have to be justified to the National Assembly itself. But at the moment, further resistance seemed pointless. As Delambre wrote to a friend, “They were armed and we had only reason; the parties were not equal.”

So Delambre and his team accepted the militia’s “invitation” to accompany them across the nighttime fields. The mud was thick, the sky black. A heavy rain had begun to fall. “Luckily I had time to place a frock coat over my clothes,” Delambre wrote. “And as we marched, we could talk to the men and make them see reason, so that they began to show us some courtesy, warning us of treacherous footing ahead, and giving us a hand when we needed to be pulled out of the muck.” For the next four hours they accompanied the militia on their rounds, searching houses for arms and requisitioning horses. After struggling for six miles through the dark, they finally arrived in Lagny shortly before midnight, just as a squall soaked them to the bone.



This portion of the great Cassini map of France (1740–95) shows the area around the Château de Belle-Assise. The château is here labeled Belleassise, southeast of Lagny on the way to Villeneuve. After the Revolution the château came into the possession of the Baron de Rothschild and it was demolished at the end of the nineteenth century. Its formal gardens (pictured on the map) are today a tangle of muddy forest. Only the windmill (likewise indicated) still stands. The town of Lagny is now a suburb of Paris, and the land to the east of the town is today the valley of Euro Disney. (From the Earth Sciences Library, University of California, Berkeley)

The municipal council was in candlelight session. The town was on a wartime footing. Mayor Aublan, a former financial agent for the local abbey (now abolished), had recently congratulated his constituents for overthrowing the “odious king” and unmasking the “perfidious proclamations of the corrupt ministers and other vampires of the realm.” Delambre presented his papers to the assembled officialdom. One alderman recognized the signature of the district official on the papers, and argued that Delambre should be released. But Mayor Aublan was more suspicious. He ordered all four members of the expedition escorted by armed guard to the Hôtellerie de l’Ours (the Inn of the Bear), where they were “not to consider themselves arrested, but merely detained.” In the meantime, Delambre should send a message to the district office so that they might vouch for the legitimacy of his mission.

“That night we had nothing to change into, no nightclothes, nothing; and to dry ourselves, only a few sticks of firewood and a couple of glasses of bad wine.” Their two guards had a worse night of it, however; they had to spend all night in a drafty corridor, detaining men who had no intention of escaping. As Delambre noted in his expedition logbook: “Consigned to the Hôtellerie de l’Ours, two sentinels on guard at the exits; September 4, 1792, the second year of liberty and the first of equality.”

In the morning, when confirmation came from the district office that the mission was indeed sanctioned by the highest authority in the land, Delambre thought it advisable before leaving town to thank the municipal council in person for their overnight hospitality. As he entered the town hall, the mayor rushed over from his office to apologize for the “little trouble” of the previous evening—while the impatient militiaman who despised academies stood by with a sullen expression, having apparently slept off his wine. According to the municipal records, Delambre then“thanked the municipality for so promptly allowing him to continue on his way.”

“And so ends the true and tragicomic history of the memorable arrest of the former Cademician,” Delambre wrote that evening to a friend—as if his troubles had not just begun.


Delambre’s wry equanimity seems to have been due in part to his late start in science. He did not take up astronomy until his mid-thirties—an age when many scientists are either at the height of their powers or already on the downhill slope. He was born in the cathedral town of Amiens on September 16, 1749, the eldest child of cloth-sellers of modest means. The family name Delambre probably derives from lambeau, meaning “rag.” When he was still an infant, fifteen months old, he was stricken with smallpox, which nearly cost him his eyesight and permanently denuded him of eyelashes. If the latter loss ultimately made it easier for him to take up the telescope (lashes tend to get in the way of beginners), his weak eyesight hardly presaged a promising career in observational astronomy. Until the age of twenty he was acutely sensitive to sunlight, and could hardly read his own handwriting. He grew up assuming he would one day go blind. For just that reason, he devoured every book he found. He learned English and German, and studied with the Jesuits until the order was expelled from France, at which point the town brought in three replacement teachers from Paris.

Delambre might have aspired at most to a position as a local curé had not one of these teachers put him up for a scholarship at du Plessis, a famous Paris school where adolescent boys absorbed Roman virtues through an endless diet of Latin classics. Graduates included devout theologians, atheist physicians, military republicans, and illustrious savants. The high hopes for young Delambre were not fulfilled at exam time, however. He failed his finals because he could not read his exam papers. Without a scholarship for a university, his parents urged him to return to Amiens and take up holy orders.

Instead Delambre stayed in the capital, living on bread and water, studying ancient Greek by day and carousing with demimonde literati by night. It was the high tide of the Enlightenment. While the elderly Voltaire issued epigrams from Ferney, and the moody Rousseau wrote diatribes from the country, their would-be usurpers plotted utopias in cafés and wrote subversive pamphlets in garrets. Delambre and his friends formed their own literary club. To support himself he took a temporary position tutoring a nobleman’s son in nearby Compiègne; to instruct his pupil he was obliged to learn mathematics himself. He read Milton’s Paradise Lost in the original, and composed his own English primer, which included such homilies as: “To love riches is the property of a base and groveling soul, as to live [poorly] in comparison of virtue is the property of a noble and generous mind.”

He was certainly poor enough. At the age of twenty-two he returned to Paris to tutor the son of Jean-Claude Geoffroy d’Assy, a member of the prosperous elite who managed the kingdom’s finances. For the next thirty years, Delambre remained a part of the d’Assy household. The grateful parents even offered him a sinecure in their financial offices, but Delambre accepted a more modest annuity that would enable him to devote the rest of his life to study. Thus did many a promising young man from the provinces set himself up as a lay cleric in the Ancien Régime, a bachelor scholar on a small pension. In those days, Delambre styled himself the “abbé de Lambre.” It was his dream fulfilled. He was a cosmopolitan humanist, rigorous in his learning, tolerant in his poverty, a connoisseur of human absurdity. He had narrow eyes, quizzical eyebrows, and a mouth framed by skeptical curves. Already in his mid-thirties, he still had no career.

For the past several years, he had been reading ancient Greek science. To supplement his studies he looked into modern astronomy; this led him to the standard textbook in the field, Astronomy by Jérôme Lalande. While he was at it, Delambre decided to audit Lalande’s lectures at the Collège Royal. One day he heard the teacher comment that the Milky Way had the width of the celestial sphere. After class, Delambre informed the professor that this observation had also been made by the Greeks. From then on, whenever Lalande wanted to check to see whether his students had understood his lecture, he called on Delambre, who always supplied the right answer—not surprising really, since Delambre had gotten all his information out of Lalande’s own textbook. Even two hundred years ago, this was a well-worn student ruse. “You’re wasting your time,” Lalande finally told him one day. “What are you doing here?” For no other reason, Delambre confessed, than to get to know Lalande.

Everyone knew Lalande. He was France’s foremost scientific publicist, enemy of every human prejudice. He was an outspoken atheist. He ate spiders to prove that arachnophobia was irrational. And he had recently calculated the likelihood that a comet would devastate the earth, causing all Paris to panic. He was a small, ugly man, and impossibly vain. He liked to boast he was as ugly as Socrates. If he was not the world’s greatest astronomer—as he sometimes seemed to think—he was certainly its most famous, having trained many of the world’s leading practitioners, the most recent of whom was Pierre-François-André Méchain. In 1783, looking for a new recruit among the dozens of auditors in his astronomy course, Lalande judged “the abbé de Lambre, already very able.”

Lalande lent Delambre a three-and-a-half-foot sextant and began to incorporate his student’s observations into the third edition of his Astronomy. Delambre’s eyesight had steadily improved with the years. Despite his late start in science, he had become a superb calculator. When he returned for yet another assignment from his maître, Lalande refused. “Don’t be a fool,” he told him. “Work for yourself, and to get into the Academy.” In short order, Delambre made himself into one of the nation’s leading astronomers. When in 1787 the d’Assy family moved to a new home in the Marais—at 1, rue de Paradis—they built him his own private observatory on the roof.

For the next twenty years, Delambre lived at the d’Assy residence. The elegant neoclassical structure still stands, renumbered more prosaically as 58 bis, rue des Francs Bourgeois, now the administrative offices of the French National Archives. For Delambre, paradise was on the roof. After climbing the ninety-three steps to his bedroom, he had only one short flight more to enter an observatory built to the most exacting specifications and outfitted with state-of-the-art equipment. In 1789, when the observatory was complete, he had every reason to think he had arrived at astronomer’s heaven.


The French Revolution, which convulsed Paris that year, overturned the comfortable hierarchies of the Ancien Régime, dragging its unexamined standards of conduct and deference into the harsh light of reason. Among these were the standards which governed the nation’s economic life. The wealth of the d’Assy family, for instance, derived from its marketplace monopoly over the up-and-coming Temple district of Paris. Any butcher or baker who wished to set up shop in the neighborhood near the Marais had had to apply to Geoffroy d’Assy for a license. That monopoly had now vanished, along with the rest of the nobility’s legal privileges. French citizens were henceforth free to trade, independent of the personal control of others. Among the unexamined Ancien Régime standards subject to this searching review were measurement standards—and here it was the savants who led the revolution. In 1790, the newly elected National Assembly authorized the Academy of Sciences to design a system of uniform measures. These savants had the courage to look beyond their immediate historical circumstances and build their standards on a permanent foundation. They vowed to choose a set of measures which would “encompass nothing that was arbitrary, nor to the particular advantage of any people on the planet.” They decided to base their new measures on the size of the earth itself.

In April 1791, the Academy of Sciences confided this meridian mission to three of its members: Pierre-François-André Méchain, Adrien-Marie Legendre, and Jean-Dominique Cassini. These three eminent savants were the logical choice for a body that prided itself on its logic. Méchain was an astronomical workhorse, the editor of the Connaissance des temps, the annual tables of celestial events that guided French navigators at sea. Legendre was a gifted mathematician who had perfected the calculations for the measurement of the globe. And Cassini—or Cassini IV, as he was known—had every reason to consider the meridian mission his birthright. He had been born in the Royal Observatory, which his father, grandfather, and great-grandfather had each directed in turn. The Cassinis were one of the most illustrious examples of sustained family achievement in the history of science. Each Cassini had surveyed the meridian of France with the most advanced equipment of his day. Since his youth, Cassini IV had worked with his father to create the great Cassini map of France, surveying the stations which would serve as the template for the new meridian mission. No one was more capable of following in the footsteps of Cassini III than Cassini IV.

If pedigree, seniority, and disciplinary turf mattered in the Academy, it was for such logical reasons as these. Yet Cassini was reluctant to begin the mission. For one thing, his wife’s recent death had left him with five young children to care for. Then there was the problem of his royalist sympathies. On June 19, 1791, Cassini had secured a royal audience for the members of the metric commission. At six in the evening, they presented themselves at the Palais des Tuileries: Cassini, Méchain, Legendre, and a fourth savant, Chevalier Jean-Charles de Borda, the inventor of the repeating circle, the new instrument which would push the expedition to a new level of precision. In the eyes of history, Louis XVI has earned a reputation as a political simpleton. But he had his talents. He was a skilled watchmaker and, like his grandfather Louis XV and his great-great-great-grandfather Louis XIV, a connoisseur of cartography. After all, if the Cassinis owned the map of France, the Bourbons owned the real thing.

The king also took a surprisingly close interest in the spending of the royal purse. “How’s that, Monsieur Cassini?” he asked the savant. “Will you again measure the meridian your father and grandfather measured before you? Do you think you can do better than they?”

Cassini managed to preserve filial respect while holding out the promise of progress. “Sire,” he answered, “I would not flatter myself to think I could surpass them had I not a distinct advantage. My father and grandfather’s instruments could but measure to within fifteen seconds; the instrument of Monsieur Borda here can measure to within one second. That is the sum of my merit.”

The king’s sangfroid was all the more remarkable because that evening the royal family had been secretly preparing to flee the country. The next morning, the king and his immediate circle set out on the infamous “flight to Varennes” which ended when a provincial innkeeper spotted the king, disguised as an English merchant, walking with the notorious Bourbon waddle. Louis was hauled back to an enraged capital and confined to his palace under the city’s watchful eyes. Cassini henceforth considered himself released from all obligations to an “illicit, usurping, seditious government of assassins.” If Louis XVI refused to serve France, how could Cassini IV serve her?

But while Cassini dithered, the rest of the government grew impatient. Jean-Marie Roland, chief minister of the nation, was an expert on the new economy transforming Britain. He wanted France to enjoy the advantages of a uniform system of measures. Such a reform would aid the free circulation of grain, and so help resolve the food crisis at the heart of the nation’s troubles. A modern nation needed a standard, any standard, and the surest course of action would be to declare the units used in Paris the national units. On April 3, 1792, Roland threatened to do just that.

Roland’s demand threw the Academy into consternation. Their dream of a universal measure seemed about to evaporate. At their next meeting, they divided the meridian expedition into manageable sectors—and urged Cassini to set out. One commissioner would take charge of the northern portion, from Dunkerque to Rodez, while the other would take the southern portion, from Rodez to Barcelona. If the northern sector was twice as long as the one in the south, that was because the north had been previously surveyed, most recently by Cassini’s father in 1740, whereas the south was more mountainous and included the uncharted Spanish section. This division of labor was only provisional, of course; the two teams were to work their way toward each other as rapidly as possible and meet up where they would.

Yet even as he refused to set out, Cassini asserted his right to command any meridian expedition. The Revolution may have upended the kingdom, but the Academy still stood upon certain formalities. He offered to remain in Paris while an adjutant carried out the actual surveying. In the end, the Academy rejected this proposal. A savant needed to have direct contact with nature, to travel and measure for himself, so that he might personally vouch for the reliability of his findings.

This was where Delambre came in. On February 15, 1792, he had been unanimously elected to the Academy—in part, as Lalande informed him, because the members thought they might need him for the meridian mission. When Cassini refused a final plea to set out, Delambre was elected on May 5 to lead the northern portion of the meridian survey, with Méchain to lead the southern sector. Decades later, Delambre would recall pleading with Cassini to change his mind. The two men had graduated from the same school a year apart. In revolutionary times, Delambre warned his colleague, a citizen must demonstrate his devotion to the national good, if only to shield himself from reproach. But Cassini would not serve a régime he considered illegitimate. For Delambre, it was the sort of career opportunity the Revolution made possible.


As soon as the king’s authorization arrived on June 24, Delambre began to scout out stations in the vicinity of Paris. His plan was to revisit the stations of the 1740 Cassini survey of the meridian, conduct improved measurements with his new instruments, and wrap up his mission by the end of the year, applying to his geodetic measurements of the earth the same thoroughgoing precision he had so recently brought to his astronomical measurements of the heavens.

Delambre was a quick learner—a humanist in his mid-thirties who had, in the course of one decade, become one of the nation’s leading astronomers—and the central method of geodesy was in principle quite simple, little more than Euclidean geometry on a curve. The method was known as triangulation and for two hundred years cartographers had been using it to map terrain, and would continue to do so right up to the advent of the satellite. Triangulation relied on an elementary theorem in geometry: if you know all three angles of a triangle, plus the length of any one side, you can calculate the length of the other two sides. Hence, if you know all the angles in a set of triangles connected side by equal side in a chain, plus the length of any single side, you can calculate the lengths of all their sides (since every two connected triangles share at least one side). The geodeser simply took advantage of this. First he identified a series of observation stations that might serve as the nodes (the vertices or “corners”) of his triangles—church steeples, fortress towers, open hilltops, purpose-built platforms—each node visible to at least three other stations, such that they formed a chain of triangles that straddled the meridian. He then traveled from station to station measuring the horizontal angles separating adjacent stations. Next he measured along the ground the actual length of one side of one of the triangles—a “baseline”—typically by placing rulers end to end over the course of several miles, and used this value to calculate the lengths of all the sides of all the interconnected triangles. From this, he could derive the distance along the meridian arc from his northernmost to his southernmost station. Finally, he determined the respective latitudes of the northernmost and southernmost stations using astronomical observations, so that he might extrapolate from the length of that arc to the full quarter meridian. And that gave him the size of the earth.

This was the principle, anyway. But as in any science where extreme precision is sought, the practical challenges were considerable. First, because the geodeser necessarily measured the angular distances from stations that were somewhat elevated, he had to adjust all his values to a common surface-level triangle. Second, because he could not always place his measuring instrument at the exact vertex of that triangle, another correction had to be included. Third, because atmospheric refraction distorted apparent sightings, all the angles had to be adjusted for the bending of light. And fourth, because the angles of a triangle on a curved surface do not quite add up to 180 degrees, this had to be corrected for as well. All these adjustments complicated the calculations. They did not change the basic principles involved.

By revisiting the stations used in Cassini’s 1740 survey of the French meridian, Delambre hoped to skip one of the most laborious steps in any triangulation: the identification of workable stations. But first he had to verify that Cassini’s sites could still serve his purpose. For their station in the capital city of Paris, the surveyors of 1740 had chosen the belfry of the church of Saint-Pierre near the summit of Montmartre, a Benedictine abbey which still stands today, a stone’s throw from the current site of the church of Sacré-Coeur. On June 24, Delambre set out with his two assistants to climb the hill of grapevines, quarries, and windmills. Even then, Montmartre offered a famous panorama over Paris. From the hilltop, they could look back on the jumble of low gray buildings which swarmed like angry insects around the city’s massive royal and religious edifices.

But when they climbed still higher, to set up the instrument on the platform of the bell tower of Saint-Pierre, Delambre met with a disappointment. The view was miserable. None of the surrounding sites used in 1740 was visible. Indeed, the church tower barely cleared the roof. In every direction he looked, the view was blocked by surrounding buildings.

Back in the capital an old etching cleared up the mystery. Fifty years before, the church had been capped by a tall wooden belfry, since destroyed. A half-century of urban construction and demolition had transformed the Paris cityscape, leveling steeples, raising palaces, filling empty lots. The abbey of Saint-Pierre would no longer serve, and Delambre would have to locate an alternative station elsewhere in the capital. This, he decided, could best be done from the outside looking in. He decided he would travel counterclockwise around the capital, visiting the peripheral stations which ringed the city, scanning for an appropriate landmark in the center.

The next few weeks of travel showed him just how much the past fifty years had transformed the French countryside as well. To the south of the city, the observers of 1740 had adopted the tower of Montlhéry, an abandoned medieval fortress which guarded the main route into Paris. The tower still stood, as it does today, pigeons fluttering in its broken interior. Delambre discovered that the first ten steps of the staircase had crumbled away, and though he sent a workman up to verify the view, he had no appetite for hoisting himself and his instruments up the ninety-six-foot turret. He settled for an observation point on the overgrown fortification wall below.

Next, Delambre visited the Malvoisine farmhouse, perched on a low ridge twenty miles to the southeast, which had been used by Cassini III in 1740. The site is still a working farm, its muddy courtyard piled with machinery and patrolled by dogs. But even from the farmhouse roof Delambre could barely make out the adjacent station of Montlhéry. Stands of tall trees had grown up around the property in the intervening fifty years. He secured permission from the owners to add six feet of height to the farmhouse chimney so as to create a workable observation signal there, and continued his travels counterclockwise around the capital.

The Gothic church tower at Brie-Comte-Robert still suited his requirements. But at Montjai, Delambre encountered new obstacles. Even in 1740, Cassini III had hesitated to climb the medieval tower—the eastern twin of the tower at Montlhéry—not for fear of the demons that supposedly haunted its ruins, but because he had been warned that the tower might fall down. Delambre decided the better part of valor was to hire a local carpenter named Petit-Jean to rig a freestanding observation platform alongside the structure. While that work got underway, he continued to Dammartin, a town on top of a steep ridge just outside today’s Charles de Gaulle airport. There he learned that the Collégiale chapel, which had served Cassini III in 1740, was about to be sold off as part of the Revolutionary sale of church lands, and the buyer intended to demolish the church for building materials. On the spot, Delambre decided to make this site his highest priority. But first he had to verify that he could see Dammartin from Saint-Martin-du-Tertre, the next station to the north; there too his preliminary observations did not match those of 1740, suggesting that the position of the church belfry had been shifted by several hundred feet at some point during the past fifty years.

Geodesy is a natural science. It is the science which measures the size and shape of the earth, a planet formed by the same gravitational forces that had spun the solar system out of a disk of luminous nebular dust (according to Pierre-Simon Laplace’s reigning theory). What is the earth’s shape? Even more: what is meant by shape? Our planet’s surface is not smooth. It is scarred with mountains and valleys, roiled by geological processes. The imaginary shape that our planet would possess if its surface were everywhere at sea level, which scientists today call the “geoid,” was known in the eighteenth century as the “figure of the earth.” A meridian, for these savants, was the surface of an imaginary canal that ran unswervingly from north to south; in this case, from the North Sea to the Mediterranean. Yet to measure the length of this canal, and hence the figure of this imaginary earth, geodesers relied on the very geological processes that have distended the surface of the planet, creating the mountains and hilltops from which they surveyed the terrain.

Geodesy is a natural science. It is also a science that depends on human history and human labor. Where the surface of the earth is too level, with neither mountains nor hilltops available for triangulation, the geodeser must commandeer manmade structures for his view: church steeples, fortress towers, observation platforms, any lofty site. As Delambre’s whirlwind circuit of Paris had taught him, however, the same human purposes that had raised churches, towers, and platforms could tear them down. The world would not hold still while he measured it. In such times, with commercial and political revolutions converging at violent speed, the past was a treacherous guide.

On August 10, 1792, Delambre was finally ready to take his first definitive measurements. He set up his delicate instrument in the belfry of the Collégiale at Dammartin and sent young Lefrançais back to Montmartre with instructions to light a parabolic reflector from a rooftop observatory there, so that he might pick out the station amid the hodgepodge of buildings on the hill. At ten o’clock that night Delambre had still not detected a signal from Montmartre. But he did see flames from an unexpected direction: the Palais des Tuileries was on fire. That day, unknown to Delambre, some ten thousand Parisians had stormed the royal palace where the king was a virtual prisoner, set it on fire, and with the help of the turncoat Paris militia, massacred six hundred of the king’s Swiss Guard, some by defenestration, others with cold steel. As the dominant hill to the north of the city, fortified with its own battery of cannon, Montmartre possessed strategic value to those jostling for control of the capital. Later that night, three militiamen were killed at a Montmartre barricade while arresting members of the Swiss Guard. To light signal flares from the hilltop that night, as Lefrançais had planned, would have been suicidal. The next night, with the help of his uncle Lalande, he did manage to set a flare, though it did not burn long enough for Delambre to get a good reading. The uprising of August 10 brought the monarchy to an end. Delambre would never again risk lighting signal flares at night.

This meant giving up on Montmartre, however, and selecting another site as his central Paris station. Delambre had just settled on the dome of the Invalides for that purpose, and had begun retaking his angle measurements using the golden dome as his sighting target, when he received news that the residents of Montjai, armed with muskets, had forced Petit-Jean the carpenter to tear down the observation platform he had been building alongside the crumbling tower. Delambre rushed to Montjai to insist that the local town council order their constituents to stop harassing the carpenter. In a Republic, the council responded, one might exhort citizens, but could not command them. If Delambre wanted the carpenter to build the platform, he would himself have to persuade the citizenry to allow it. Delambre did make the effort. But his explanations only succeeded in rousing the surrounding villages against his mission. Montjai would have to be abandoned. Searching for a substitute station, he noticed the Château de Belle-Assise on a nearby prominence and secured permission from its owner, the comte de Vissec, to use his charming belvedere as an observation station. Four days later, the local militia came and escorted him out of the château to the town hall of Lagny and thence to the Hôtellerie de l’Ours, where he was “not arrested, but merely detained.”


On the morning of September 6, 1792, when they were finally able to leave Lagny, Delambre and Bellet drove their carriages toward the hilltop town of Saint-Martin-du-Tertre, skirting Paris counterclockwise as they continued their circuit of the capital. On the way, as a precaution against further trouble with local militiamen, they stopped in the district office of Saint-Denis to obtain a certificate of safe passage. The district office had recently been relocated to the ancient abbey, the most sacred pilgrimage site in France.

For fifteen hundred years, the basilica of Saint-Denis had served as the ancestral tomb of royal France. Monarch to monarch, and dynasty to dynasty, the dead kings of France had been borne to the crypt at Saint-Denis so that a new king might live. King Dagobert, first of his line, lay beside Queen Nanthilde. Henry IV, assassinated in his prime, lay between his two wives, Marguerite de Valois and Marie de Médicis. The Sun King, Louis XIV, lay beside Marie-Thérèse. Above their tombs, sculpted effigies rested on marble beds, some robed in stony gowns, others naked in their final agony. Bronze angels and bishops knelt at their feet. Fifteen hundred years of royal succession had ended the previous month.

The town council of Saint-Denis had ordered the fleurs-de-lys (the symbol of French royalty) effaced from the abbey as a sign of despicable feudalism. They had erected furnaces in the chapels of the basilica so that the bronze statues of Charles VIII, Henry II, and Catherine de Médicis might be melted into cannon. And just that week, they had debated whether to dig deeper and extract lead from the royal coffins so that the casings of kings might be made into cannonballs to hurl against the enemies of the new Republic.

The chief administrator of the district, Denis-Nicolas Noël, signed Delambre’s certificate of safe passage, but warned the savant that this piece of paper would not provide much protection. Saint-Denis straddled the main road north from Paris, and every village from here to the frontier had raised barricades to stop aristocrats fleeing the capital. Peasants were digging fortifications. The Prussians were expected at any moment.

The savant, however, refused to delay. He directed his two carriages onto the Route de Poissy, which ran northwest along the loop of the Seine. Fifteen minutes out of town, at a barricade at the approach to the village of Epinay-sur-Seine, the local militia halted their carriages and demanded passports.

Regrettably, their passports made no mention of the strange instruments they were transporting. What was the purpose of this equipment, and why were they bearing it toward the frontier? The instruments seemed designed to spy out actions at a distance. Might they not also have a military purpose?

Delambre explained that these were astronomical instruments that would enable him to measure the size of the earth.

And why would he want to do that?

So by the side of the road, within sight of the reedy banks of the Seine, Delambre unpacked his instruments and explained his mission. It was a warm end-of-summer afternoon—perfect weather for an outdoor seminar—and the size of the crowd swelled as word spread that a scientific sideshow was underway at the edge of the town. The local militia had stopped some highfalutin carriages bearing mysterious instruments toward the frontier. As newcomers joined the throng, they insisted on being brought up to speed. Delambre was obliged to restart his seminar several times. Mayor Louis Beaudoin, a local wine grower, joined the crowd, as did two land surveyors. Delambre appealed to these men for help. He showed the mayor his official papers, including the guarantee of safe passage signed by the district official that morning. He begged the surveyors to vouch for him. Surveying and geodesy were brotherly trades. Both measured the earth; while surveyors measured fields, geodesers measured planets.

The surveyors, however, refused to confirm Delambre’s words. And Delambre could easily understand why. “They sensed the mood of the crowd, realized it would be useless to speak in our behalf, and dared not back me up.” As for the mayor, he too preferred to exercise caution. He ordered the guards to escort Delambre and his carriages back to Saint-Denis for questioning.

Where the main square of Saint-Denis had been empty that morning, a thousand exuberant men and women were now assembled under the mismatched spires of the basilica, the taller tower on the left draped with the tricolor flag, the shorter tower on the right capped with a gigantic, toque-like Liberty bonnet. Among the crowd, several hundred young men wore the insignia of the First Division of the Paris National Guards, volunteers marching north to help their comrades repel the Prussian invaders. They had paused in Saint-Denis only long enough to encourage the local lads to join them. The fatherland was in danger! The Prussian army was on its way to restore the king. To save the Republic, the newly formed government had pleaded for 300,000 volunteers. Volunteers! The very idea was revolutionary. For centuries soldiers had died for pay, glory, booty, and loyalty to comrades and commanders. That men were now willing to die for an abstraction called a nation meant that for the first time they conceived of themselves as citizens of a nation, rather than retainers of a feudal lord or subjects of a king. Some eight hundred young men from Saint-Denis and the surrounding hamlets had answered the call. They had left their bakeries, workshops, farms, and families to defend a Revolution which promised them liberty and equality. They had assembled in the main square of Saint-Denis to demand that in return for their sacrifice, the municipal council supply them with firearms, a thousand pounds of bread, and carriages to transport their provisions to the front.

Then, like a miracle, two carriages appeared, escorted by the militia of neighboring Epinay. As the militia cleared a path through the crowd, they boasted to their comrades of their prize: two suspects caught on their way to the frontier with spying instruments. The square rang with their joyous cries: “Long live the nation! Behold the aristocrats!” Shouts of abuse were showered on Delambre as he was hustled into the town hall beside the cathedral, and then into the district offices in the half-moon courtyard of the abbey. Inside their offices, the huddled administrators chastised the mayor of Epinay for creating this volatile situation.

Outside, in the square, the people of Saint-Denis thought they had good grounds for suspicion. France was full of traitors. Two weeks earlier, General Lafayette, the hero of the American Revolution, had tried to subdue the capital by force and, when his troops refused obedience, fled to Belgium. Generals and aristocrats had gone over to France’s enemies. Now that Verdun had fallen, only the common people could halt the Prussian advance on the capital.



This early-nineteenth-century view by Giuseppe Canella down the main street of Saint-Denis shows the facade of the basilica as it appeared at the time of the Revolution. Street lighting had been introduced in the 1770s. The basilica has since been shorn of the tower on the left. (From the Musée d’art et d’histoire, Saint-Denis; photograph by Irène Andréani)

The crowd was growing impatient. While Delambre was conferring inside the abbey with the officials, a fiery band of volunteers stormed the carriages and pulled down the leather cases that contained his instruments. Inside, they also found a cache of fourteen letters, all sealed with the royal signet. This was a stupendous find! Might not these letters bear a message from the imprisoned king to the northern front? Only with great difficulty did the militia persuade the men to reload the cases onto the carriage. In return, the crowd was promised a full explanation of the royal letters. The call went out for Delambre. The crowd was summoning him.

To Noël and the other public officials, this summons sounded like Delambre’s death knell. In the early days of the Revolution, the assistant to Saint-Denis’ mayor had been stabbed fourteen times in the church tower for refusing to lower the price of bread to twosous. All that week in Paris, in the Revolution’s worst outbreak of popular violence, ordinary prisoners had been hauled out their cells, accused of participating in aristocratic conspiracies, and murdered by mobs of men, including members of the Paris National Guard, some of whom might have been in the crowd that day outside the basilica. Chief Administrator Noël told Delambre to hide in a closet before he stepped outside to see what the crowd wanted. Only when he was satisfied that they would not tear Delambre to pieces did he bring the astronomer out to explain his mission, his instruments, and above all the royal letters. Delambre took his stand on the steps of the town hall. He broke the royal seal.


concerning the observations and experiments to be performed by the commissioners of the Academy of Sciences in execution of the law of August 22, 1790, which ordered that weights and measures be rendered uniform, dated June 10, 1792. . . .

And so on for three dense pages of royal legalese, in which the king commanded local administrators along the length of the meridian to aid the appointed commissioners with horses, food, and lodging, and to permit them to erect signals, scaffolding, and reflectors “on the roofs and exterior of steeples, towers, and fortresses.”

Even at a rapid clip, the letter would have taken fifteen minutes to read. Yet the crowd insisted on hearing every word. Who knew but that some malevolent plot might be concealed among the verbiage? Apparently satisfied that this letter—albeit a royal proclamation—was innocent, the crowd turned its attention to the thirteen other letters, likewise sealed with the royal signet. Delambre was obliged to unseal a second letter so that it too might be read to the crowd. The second letter proved identical to the first. But what of the others? Perhaps a single traitorous letter had been concealed among the innocuous ones? So Delambre agreed to have a third letter read, and then a fourth, and then a fifth. And so an hour passed and more. The reader’s voice was tiring. To read all fourteen letters would have taken all night. The September evening was drawing down. Moreover, each broken seal rendered that letter null and void. So Delambre offered his audience a deal. He would agree to read one more letter, selected at random, and if that letter did not match the previous letters word for word, he agreed to answer for it with his life. The deal was struck. A letter was chosen, unsealed, and read. It matched.

Still the crowd was not satisfied. (The volunteers apparently still had their eye on those two carriages, ideal for transporting their provisions to the front.) They demanded to know the purpose served by his instruments. It was up to Delambre to explain.

He did his best. As free men and women they had the right to know why this work was being done in their name, and how it was being carried out. His mission may have sounded arcane, remote from their immediate concerns. But its successful completion would one day transform their lives more than any battlefield triumph.

Ought not a single nation have a uniform set of measures, just as the soldier fought for a single patrie? Had not the Revolution promised equality and fraternity, not just for France, but for all the people of the world? By the same token, should not all the world’s people use a single set of weights and measures to encourage peaceable commerce, mutual understanding, and the exchange of knowledge? That was the purpose of measuring the world.

As everyone in the crowd there that day knew, measures in France varied from province to province, town to town, and parish to parish. Despite the similarity of their names, they also varied from trade to trade, and for different goods. When a volunteer from Saint-Denis visited Paris to hoist a pinte of beer to salute his Paris comrades, he discovered that the pinte of Paris held two-thirds the beer of his hometown pinte. The bakers in the crowd used a livre (pound) that was lighter than the livre of the ironmongers. In many parts of France, a pound of bread really did weigh less than a pound of lead. For instance, the standard measures of Saint-Denis were enshrined in masonry just inside the basilica doors immediately behind him: two receptacles for two different types of grains, two for salt, plus an aune (an ell, about three feet long) mortised into the wall. The aunewas used exclusively for measuring cloth, and Paris had three different aunes for three different kinds of fabric, while in Delambre’s hometown of Amiens, his father’s shop used one aune to buy wholesale and a shorter one to sell retail, while thirteen different aunes were used in the surrounding villages. All across France such discrepancies caused endless confusion, disrupting trade, baffling administrators, inviting fraud.

For centuries royal administrators had been trying to wrest authority over weights and measures from the hands of local nobles, guild masters, and town aldermen. Uncertainty about the true value of local measures hindered the state’s efforts to extract revenue on sales, implement a fair property tax, assess imports, and regulate the supply of grain and the price of bread. The army likewise aspired to uniform measures in order to coordinate better the production of war matériel, fortifications, and maps. In the last few decades the mood among the educated population had swung decisively against the diversity of measures. Alexis Paucton, the age’s foremost compiler of weights and measures, both ancient and modern, urged reform: “They are the rule of justice which must not vary, and the guarantee of property which must be sacred.” The enlightened authors of Diderot and d’Alembert’s famous Encyclopédie bemoaned the “encumbrance” of France’s diverse measures. But like so many others in those days, they had found the fault “beyond remedy.” Even the king’s most capable minister, Jacques Necker, had decided that metrical uniformity was beyond the monarchy’s power.

Where faint-hearted kings had failed, however, the Revolution was determined to succeed. The nation, to be a nation, had to be coaxed into a uniformity of weights and measures. In the Cahiers de doléances, the famous litany of grievances solicited by the king in 1788, it was the people themselves who had called for the reform of weights and measures. Some 128 of the regional Cahiers demanded uniformity, as did 32 of the nobility’s, and 18 of the clergy’s. And at the local level, thousands of village Cahiers echoed the call for “one law, one king, one weight and one measure.” The townsfolk of Saint-Denis had themselves made this demand in their own Cahier, and the mayor of Epinay had himself signed his town’s Cahier asking that France be governed by a single set of weights and measures. It was up to the Revolution to make good on that demand.

We do not know what the crowd made of Delambre’s explanations. We only know that they were primed for battle, not for an impromptu lecture on measurement and geodesy. Delambre himself detected a certain impatience.

The instruments were spread out on the square, and I was obliged to recommence my lecture on geodesy, the first lessons of which I had given earlier that day in Epinay. I was not heard any more favorably this time. The day was coming to a close; it was increasingly difficult to see. My audience was quite large. The front rows heard without understanding; the others, further back, heard less and saw nothing. Impatient murmurs began to be heard; a few voices proposed one of those expeditious methods, so in use in those days, which cut through all difficulties and put an end to all doubts.

Before the hecklers made good on their threats, administrator Noël intervened. Feigning severity, he ordered the suspect carriages placed under seal and towed into the abbey’s courtyard for safekeeping. Then he forced Delambre back into the town hall on the pretense that he wanted a more convincing account of his mission. Once inside, he obliged Delambre to spend the rest of the night in the company of the aldermen—for his own safety—while they requested instructions from Paris. Delambre and Bellet slept that night in armchairs in the town hall of Saint-Denis. Only at dawn were they allowed to take lodgings in the nearby inn known as the Trois-Maillets.

The National Convention voted later that evening, September 7, 1792, to make Delambre and Méchain official emissaries of the Republic and ordered local authorities to assist them on their route. An expedition licensed by the King had become the people’s mission. Lefrançais brought Delambre the decree as soon as it was released, and together they took it to the Sunday morning meeting of the municipal council so that they might get the seals removed from their carriages and continue on their mission. That night the Benedictine monks performed their last holy Mass after a thousand years of continual prayer in the kingdom’s greatest abbey.



The corpse of Henry IV, the most popular of all French kings, was briefly exhibited in the Basilica of Saint-Denis after its exhumation in 1793. (Photograph by Roman Stansberry)


In the interim, the volunteers of Saint-Denis had been marched to a barracks outside Paris for accelerated training, and thence to meet the Prussian invaders at Châlons-sur-Marne. The volunteers of Saint-Denis helped save the Revolution. But the Revolution was not finished with Saint-Denis. In December, a crowd invaded the basilica, not to slaughter the living but to disinter the dead. The militia protected the royal graves, but popular calls for a mass exhumation multiplied after Louis XVI was guillotined in January 1793. The king’s body was dumped in an anonymous grave. His ancestors did not deserve a better resting place.

Again the National Convention scrambled to lead the populist charge. In honor of the first anniversary of the uprising of August 10, they ordered the tombs of Saint-Denis destroyed so that the royal dead might be reburied in a mass grave at Valois and the lead from their coffins—nine tons in all—refashioned into cannonballs and musket shot. Only the statues of François I and other high Renaissance works were saved because of the excellence of their art. Chief Administrator Noël turned the first spade of earth. The first royal corpse brought to the surface was that of Henry IV, the nation’s most popular king, perfectly preserved, his face black as pitch. A young soldier cut a lock from Henry’s beard before laughing onlookers, held it to his chin, and declared: “Well, I’m a soldier too! Now I’m sure to vanquish those English bastards.” When the Sun King was exhumed, a worker sliced open the corpse’s belly to the applause of the crowd. To mask the stink, officials burned a mixture of vinegar and gunpowder and closed the basilica.

Not long after, the municipality was debating whether to permit local patriots to use cannonballs to shoot down the basilica’s belfry, when the Commission of Weights and Measures intervened. The tower, they said, was crucial to the survey of the meridian that ran from Dunkerque to Barcelona. In consideration of its “great utility” for determining the new Republican measures and triangulating the territory of the Republic, as well as for other cartographic and scientific purposes, the council ought merely to efface those remaining crucifixes and fleurs-de-lys that offended the good patriots of Saint-Denis, and leave the tower standing. So science saved the basilica, even as science itself came under attack.

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