by Ken Alder
Sometime soon after June 28, 1792, Méchain finally left the capital, accompanied by his aides and bearing the two repeating circles he had waited so long to receive. His primary adjutant was a military cartographic engineer named Jean-Joseph Tranchot. The mountainous terrain of Catalonia had never been surveyed, and Méchain needed a skilled and hardy assistant. Tranchot was thirty-seven and a native of northeastern France, but he had spent half his life triangulating the Mediterranean island of Corsica, France’s most recent territorial acquisition—a terrain as rough and barren as any in Europe. The two men had already worked together to determine the position of Corsica relative to the map of the Mediterranean coast, and Méchain had personally instructed Tranchot in the finer points of astronomical observation and calculation. Méchain’s other assistant was the instrument-maker Esteveny, trained in Lenoir’s workshop. He was also accompanied by a manservant named Lebrun.
The team met only one obstacle on their voyage south. On the first day’s ride out of Paris, they were halted at a barricade near the town of Essonne, the sort of roadblock Delambre encountered at every turn. The local militia mistook their astronomical instruments for high-tech weaponry, and detained them while they conferred with local officials. In those days, however, before the fall of the monarchy, their royal proclamation saw them through. Past this barrier, they made good speed through a quiet countryside.
All was still calm when Méchain arrived a week later in Perpignan, the southernmost major town in France. The purple-walled Moorish city lay on a coastal plain of scorched vineyards and salt lagoons, locked in the long crooked arm of the Mediterranean as it reached out from Spain to embrace Italy. At the town’s back a range of hulking blue mountains, dominated by the Massif de Canigou, rose out of the parched lowlands like a dark muscular shoulder. These were the Pyrénées, where Méchain would begin his operations. The border with Spain lay along the crests of the mountains.
After presenting himself to the municipal assembly of Perpignan, Méchain and his team took the Grande Route toward Barcelona, then the major highway between the two kingdoms and still the route of a modern six-lane expressway today. Slanting away from the coast, the road traversed rich farmland before it climbed up hills broken by intermittent seasons of sun, rain, and frost toward a low mountain saddle, where it passed under the guns of the massive French fortress of Bellegarde and entered Spain. Thereafter, the French king’s magnificent highway became a “natural and miserable road,” descending through a desolate terrain of loose sandy soil, with cork trees growing on the upper slopes and olive trees cultivated sparsely below. Only as it once again approached the coast did the signs of human industry multiply. The air became fragrant with flowering shrubs and aromatic herbs. The road was bordered by hedges of aloe, Christ-thorn, and wild pomegranate. Chain pumps irrigated fields of maize and orange groves. The number of towns increased. Soon they had passed through the gates of Barcelona, a metropolis seized by the expectation of change.
Eighteenth-century Barcelona had prospered under the watchful gaze of its Castilian overlords. The Catalan town boasted silk manufactures, an Italian opera, and a half-mile-long quay that docked a hundred ships simultaneously. Gold streamed in from the Americas, and textiles and manufactured goods went out to the colonies. With the commercial boom, Barcelona also became an intellectual capital, in part because of its relative openness to its neighbor to the north.
This influx of French ideas was not always appreciated. With prosperity, the town’s population had tripled to 120,000. Many of the new arrivals were French, comprising nearly one-eighth of the residents by the end of the eighteenth century. These immigrants irritated the town’s residents and its Castilian rulers. Artisans viewed the newcomers as competitors, and the Castilians worried about radical ideas. The Revolution only confirmed their suspicions. The French were blamed for the rising price of bread and the downward spiral of wages. For several decades the writings of Enlightenment authors such as Voltaire and Rousseau had been smuggled into Spain, along with political pamphlets, antireligious tracts, and pornography, sometimes all folded together in one scintillating read. Madrid tried to staunch this flow of subversive works, even banning the scientific Journal de physique in 1791 for its purported atheism. Now, on top of these disturbing tracts came aristocrats and priests fleeing the godless Revolution. Not that these émigrés found a warm welcome themselves. The governor-general feared that Revolutionaries disguised as priests were fomenting trouble. In July he ordered the army to stop fugitives at the border. Refugees had to swear that they would remain in Spain and observe the Catholic religion.
Yet the Spanish Crown also wished to profit from the latest innovations in geodesy. For the past few years, the two nations had begun to cooperate on a venture to define their common border. The Spaniards were especially eager to have a look at the Borda repeating circle, so ideally suited to this task.
Immediately upon his arrival in Barcelona on July 10, Méchain met with Spanish officials and their team of scientific collaborators. Their leader was Lieutenant José Gonzales, commander of the frigate Corzo (Roebuck) and an expert in celestial navigation. Méchain was familiar with his work. He was seconded by Ensign Alvarez, as well as ship’s lieutenant Francisco Planez. Like most scientific men of the day, the Spaniards spoke French. They agreed to spend the rest of the month equipping their expedition. All told, they needed supplies for sixty men for several months.
During his stay, Méchain met the elite of the Catalan Enlightenment, savants in close touch with French ideas and thinkers. He was a man with a remarkable gift for friendship. At times melancholy, even petulant, he also inspired admiration and affection: a man of honor in a calling of integrity. His self-deprecating manner had its own charm. He had astronomical correspondents around the world—from Pisa to London and from Copenhagen to Madrid—men with whom he traded celestial data and discoveries. So it was perfectly natural that he befriended such Catalan intellectuals as the polymath general Antoni Martí i Franquès, an astronomer, mathematician, and chemist who was the first to calculate the correct mix of gases in air (revising Lavoisier’s estimates), and that he struck up a friendship with the medical innovator Doctor Francesc Salvà i Campillo.
Méchain had Barcelona’s artisans construct conical tents that would shade the repeating circle while marking the exact position of the station so that it could be located from afar. The tents could also double as shelters for the expedition at night. Méchain had designed the tents in the shape of tepees. A vertical spine composed of a heavy wooden rod having the dimensions of a carriage axle would be driven several feet into the earth, supported by three or four strong pieces of wood, then draped in canvas. Where the rod rose above the tent, it was capped with a double-backed cone like a giant child’s top, painted white to serve as a target for sighting. The bizarre design inspired rumors in talkative Barcelona, already abuzz with news of the tension between Bourbon Spain and Revolutionary France. As usual, the gossips got the story half right. A local grandee heard rumors that the tent-signals were to be planted on mountaintops and fortresses from Barcelona to the frontier to relay nightly news of war preparations against the French.
Once the tents were ready in early August, the team could set out north toward the mountains. In this first pass up the meridian, Méchain’s goal was to reconnoiter a workable chain of stations through the uncharted region between Barcelona and the high-mountain border, so that he might then double back south and measure the stations accurately with his repeating circle. The distance was not far as the crow flies: not much more than eighty miles. The terrain, however, was tortuous, the roads medieval. No carriage could negotiate the tracks, and Méchain had abandoned his custom-built carriage in Perpignan. Not even horses could negotiate the high mountain trails. To complicate matters, a two-week search in the Spanish archives had failed to turn up a single accurate map of Catalonia. The team hired mules to carry their supplies and local guides to lead them through the up-valley pastures and
the staggered crests of the pine forests.
MÉCHAIN’S SIGNALS IN CATALONIA
This drawing, in Méchain’s hand, shows the signals he designed for his triangulation through Catalonia. The lower conical section could be draped in canvas to form a tepee. The upper double cone was painted white to serve as a target for sighting from afar. The overall height was about twenty feet. (From the Archives Départementales des Pyrénées-Orientales, Perpignan, Méchain to Llucía, October 6, 1793)
In those days, the mountainous region straddling France and Spain was a zone of ambiguity and danger. The high country of the Pyrénées was uncharted, the border porous and ill defined. The Pyrénées march from the Atlantic to the Mediterranean in a series of interlaced chains that encompass fertile valleys and a population accustomed to moving freely between temperate farms, up-country pastures, and neighboring villages. Spain had ruled both sides of the range until the French conquered Roussillon (the part of Catalonia which lies north of the Pyrénées), and a 1659 treaty formally designated the border between the two kingdoms as lying along “the crests of the mountains.” But the French only considered the mountains a natural border as long as it suited their interests. In the 1710s, the Sun King’s armies had marched south toward Barcelona in an attempt to bring all Catalonia under Versailles’ rule. The conclusion of this war had seen a Bourbon placed on the Spanish throne and reaffirmed the border as the crests of the Pyrénées, without specifying which crests. At the end of the eighteenth century, the inhabitants on both slopes still spoke Catalan, and they asserted a fierce independence from both Paris and Madrid, now closely allied under the Bourbons. Smugglers and bandits—miquelets, in the local dialect—plied a dangerous trade in tobacco, firearms, and illegal books, harassing travelers, traders, and border patrols. This was another reason the French and Spanish governments were eager to cooperate on a meridian survey to Barcelona: the triangulations would help surveyors define the border between the two nations in scientific terms, so that trade between them might be policed, regulated, and taxed.
To expedite their search for workable stations, Méchain divided the men into two parties: Méchain and Gonzales took half the men, and Tranchot and Planez the rest. Each party advanced in parallel, planting signals the other party might sight. They began at the top of the Valvidrera ridge, which defines the western rim of the modern metropolis of Barcelona. From there, they worked their way north through the dry pine forests to the isolated monastery of Montserrat.
This medieval pilgrimage site was slotted into a narrow aerie halfway up a tremendous rack of cylindrical rock formations which resembled organ pipes and gave the monastery its name: “the serrated mountain.” Or as a Catalan poet put it: “With a saw of gold, the angels hewed twisting hills to make a place for you.” It took Méchain and his party three hours by mule to mount the thousand-year-old switchback trail, and even then the view was not grand enough. Méchain eventually planted his signal on the portico of the solitary Notre Dame chapel on top of the greatest of these stone organ-pipes, which rose another twelve hundred feet above the monastery and four thousand feet above the valley floor. From this vertiginous peak, he saw a full 360-degree panorama: from the cool northern curtain wall of the Pyrénées to the shimmering southern island of Mallorca. Directly below, in the terraced valley, he could trace out a tumult of broken forms: walnut trees and olive groves segregated by sinuous stone, red-roofed villages clustered along the river bank, dark ridges receding into barren mountains and, of course, the monastery itself directly below. “The peaks so closely surround the monastery that they seem about to crash down and destroy it,” he wrote.
At Montserrat, Méchain and his party were housed in a clean room and served good food and wine. Elsewhere in the region, the inns were wretched—three boards laid on trestles sufficed for a bed, and the windows were without glass. The team spent the next month traversing the desolate northern ranges of Catalonia, zigzagging their way further inland toward the high Pyrénées crests. The fields here were fallow or given over to hemp. The mountains were a savage country. Bears from the high country frequently attacked cattle and sheep, mounting their backs and smashing their heads. In winter, wolves attacked the bears. The shepherds carried firearms, and everyone smuggled.
By the time they approached the border in September the season was far advanced. Méchain had originally hoped to establish stations on the frontier mountains of Costa Bona (elevation 7,500 feet) and Massanet (elevation 6,000 feet). But snow had already put their summits out of reach. In the valleys it was raining heavily. More damaging still, political tension was rising along the border. News of the overthrow of the French monarchy had unleashed a violent reaction across southern France, and its repercussions had crossed over into Spain. The French feared a Spanish invasion; the Spaniards feared that the French Revolution would contaminate their virtuous kingdom. On the frontier near Costa Bona, Revolutionary enthusiasts had planted a Liberty tree, a symbol of Revolutionary regeneration. Bands of miquelets operated with impunity, and neither side could count on their loyalty. Under the circumstances, were Méchain’s joint party of French and Spanish officers to have begun taking telescopic sightings along the frontier, it might well have been considered a provocation to war. It might also have gotten them killed.
Indeed, on the same day that Delambre was delivering his impromptu geodesy lecture in Saint-Denis, Méchain was hunkered down in the Spanish hillside town of Camperdon, on the back slope of the Pyrénées. The governor-general of Catalonia had just ordered the expedition’s Spanish officers to move away from the border. And because Méchain’s passport obliged him to travel with his Spanish hosts, he would have to retreat too. This precaution was sensible. Just that week, he and his men had narrowly avoided an ambush. Twelve French partisans from the cross-border town of Prats de Mollò had been lying in wait for them on a little slip of French territory which happened to reach across the road between two stations. Luckily the expedition had taken another route. The price had been a three-day detour through “the roads of hell.” It had probably saved their lives.
Méchain was thoroughly frustrated. Didn’t these people understand that he was engaged in a peaceful scientific expedition? He wrote to the administrators of Perpignan to ask that a copy of his commission be posted in all the mountain villages. His mission was a scientific study, sponsored by both nations and dedicated to mankind’s highest aspirations for universal knowledge and peaceable commerce. Méchain acknowledged that his official commission—the same one Delambre was at that very moment reading out loud to the volunteers of Saint-Denis—had been signed by a king who no longer ruled. But it was all he had.
In the meantime, Méchain had no choice but to turn his back on the frontier and work his way south, station by station, back toward Barcelona. This time, he would conduct the definitive angle measurements using the repeating circle. He had brought two of them for the purpose: one ruled in the traditional 360-degree scale and the other ruled in the new decimal 400-degree scale, an expression of the new spirit of rationalization. The repeating circle was the brainchild of the Chevalier Jean-Charles de Borda, one of Méchain’s senior colleagues in the Academy. Borda was France’s leading experimental physicist, as well as a seasoned naval commander who had helped coordinate the French fleet’s campaign to liberate the American colonies—France’s first and last victory against the British at sea—with Borda himself commanding the sixty-four-gun Solitaire, captured in an action against overwhelming odds. In the mid-1780s, back in France, he had transformed one of his navigation instruments into a new device for the measurement of the earth. Elderly now, but as rigorous as ever, the stern aristocratic commander had worked with the dwarfish Etienne Lenoir, France’s finest maker of scientific apparatus, to create an instrument precise “beyond any ever conceived.”
The ingenious principle behind the repeating circle allowed the geodeser to take multiple readings of the same angle without resetting the instrument. This repetition promised virtu
ally to annihilate any errors due to the uncertain sense perceptions of the observer or deficiencies in the manufacture of the angular scale. The Borda repeating circle was composed of two scopes set one above the other on brass rings which rotated independently against a precision-ruled circular scale. To measure the angular distance between two points on the earth’s surface, the geodeser set the plane of the circle in the plane defined by the two points. He then zeroed the top scope by sighting the right-hand station, tightening the screws that held that ring in place. He then switched to the lower scope and used it to sight the left-hand station, clamping down that ring as well. At this point, the geodeser could have simply read off the angle between the two stations on the circular scale and called it quits. Instead, he did something counterintuitive. He returned directly to the lower scope and rotated it in the opposite direction, clockwise this time, moving both rings and both scopes together until he had sighted the right-hand station. In doing so, he had necessarily rotated the top scope that much further clockwise as well. Now he loosened the ring for the top scope and rotated it alone counterclockwise until it sighted the left-hand station. This meant that in total the top scope had traversed the double of the angle he wished to measure. Indeed, by repeating this procedure again, he could add another double angle, and so on. Ten such additional doublings could take as little as fifteen minutes if the stations were easy to observe, or all day if they were difficult to make out. Finally, he noted the final position against the graduated scale and divided it by the number of doublings. The repeating circle’s great advantage was that by multiplying readings it sliced error ever more finely. Uncertainties of ten seconds in the observer’s sightings or in the instrument’s manufacture, if spread over enough readings, would be diminished, in the words of its inventor, to the point where “an observer of sufficient patience should be able to eliminate nearly all error.”
