by Ken Alder
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 soldi
ers 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.
THE BASILICA OF SAINT-DENIS
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 two sous. 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.
PROCLAMATION OF THE KING
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 aune was 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 forem
ost 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.
