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Parc Montjuic Barcelona – Birthplace of the Meter’s Hidden Error

parc montjuic barcelona
Borda’s Circle

Méchain took his initial measurements from the fortress in what is now Parc Montjuic Barcelona and then the Spanish arrested him. France and Spain were at war again.

From the inn room where they confined him, Méchain took a second measurement on the latitude of Barcelona and found that his initial calculation was off. He was distraught.

“Méchain possessed no conceptual tool to evaluate scientific errors. He did know that measurements were never perfect but at the time people could not even make a distinction between precision and accuracy. He thought that he’d made a mistake that was almost a moral failure.”

So wrote Northwestern University professor Ken Alder in his 2004 book The Measure of All Things: The Seven-Year Odyssey and Hidden Error That Transformed the World

It was a mistake that would kill the French astronomer.

“For All People For All Time”

parc montjuic barcelona
The Fortress at Montjuic

In 1792, at the height of the French Revolution the Academy of Sciences tasked two well-known astronomers Jean-Baptiste Delambre and Pierre Méchain to make a precise measurement of the meridian arc that ran from Barcelona, through Paris and to the port city of Dunkirk on the English Channel.

Its well known that until about 1800 pretty much every city and nearly every little town, village, valley had its own unit of measure. In many cases ways of measuring depended on the object you were measuring.  A pound of fabric could be very different in weight than a pound of metal.

France in particular suffered from this dissection of measurements given the diversity of its regions, its language divisions (langue d’oil in the north and “langue d’oc” in the south – not to mention various forms of patois) and its extremely decentralized nature where power was generally in the hands of lords and dukes and guilds.  A contemporary of the two astronomers labeled it “a preposterous and undefined chaos”.

Given the rebirth of science over the previous several centuries, the growth of the business class and the need for some administrative clarity, a forward looking, well-defined, well-regulated and universal system of measurements was seen as part and parcel with the Revolution.  It was seen as a way to break away from a pretty oppressive past.

Delambre and Méchain were sent from Paris to measure the width of the Earth and determine the precise measurement of the meridian arc that ran from Dunkirk to Paris to Barcelona. The meter was to be 1⁄10,000,000 part of the quarter of a meridian.

Delambre went north. Méchain went south.

To make the measurement possible, that quadrant of the Earth  was divided into two sections.  The two astronomers set out using a process known as triangulation. Simply put, triangulation is a method of location a place in space by measuring angles to that place from a point at the end of a known baseline.  This is much more accurate that measuring to the point directly. The two astronomers used a new and very accurate instrument called the “repeating circle” to measure the angles.

Thales used triangulation to measure the height of the pyramids sometime around 500BCE. The Rhind Papyrus suggests that the Egyptians used the process one thousand years before that! The Chinese were using triangulation around 200BCE. The Arab scientists held onto this knowledge but it was largely forgotten in the West during the Middle Ages. In 1533, Gemma Frisius “rediscovered” the method, proposing the use of triangulation to precisely pinpoint far-off places for the purposes of map making.

Parc Montjuic Barcelona

parc montjuic barcelona
Pierre Mechain

Parc Montjuic Barcelona is a rather expansive flat hill hanging over the Barcelona harbor. While not quite the Cova de Montserrat, it does dominate the view to the southwest. The hill has been fortified in one form or another for thousands of years. The Castle of Montjuïc, built in the 1600 and 1700s has stood until today. The castle has been used as a prison, a military garrison and an execution spot over the last 200 years.

After nailing a hotel with Hostelbookers you can take Bus 150 from Plaza Espana to get to the hill or you a combination of the metro and funicular to get there. This nice little map will help you plan the trip.

A lot of the hill is forested.  The trees have been nicely trimmed  for decades and it is great parkland for walking. There was also an Olympic stadium built there in the late 1920s. It was meant to be an alternative to the 1936 “Fascist Games” in Berlin. I hiked the hill and went straight for the swimming at the cool waters of the pools on Montjuic. This is always for me far better than at the crowded beaches below. It is one of my must visit places when I’m in Barcelona.

Méchain and his assistant Tranchot used the fortress as the base point for measuring his section of the meridian.

From his Barcelona incarceration Méchain not only determined that he had made a mistake on his original measurements from Montjuïc but he discovered a comet, his seventh celestial finding, in January 1793.  During The Terror in Paris he also lost all his property and much of his family.  His life however was spared by his absence.  The Spanish finally let him go and he returned to Paris in 1795.

When he returned to Paris he handed in his initial set of measurements, keeping secret the second measurement that, by that time, had nearly driven him to madness. The initial meter therefore was built on a bad measurement.

After several more years work on the calculations the meter was officially adopted as the national unit of measure in France on December 10, 1799.  In 1801 it became the sole legal system.  All others were outlawed.  In 1816 the Netherlands became the second nation to adopt the meter.

Méchain was so distraught by his mistake that, after years of obsessing on it, he requested that Napoleon allow him to re-measure his portion of the survey. Permission was granted and Méchain departed Paris in 1803.

A year later, in September 1804, he was dead in Castillion de la Plana. Yellow fever.

It was Delambre, who discovered Méchain’s error after his death. Historian Alder writes that Delambre had gotten hold of Méchain’s notebooks.  Delambre scribbled in the notebook that he wouldn’t reveal the error to the public.  The public, he thought, didn’t need to know what they didn’t know.

“Because after all, does it matter if it is wrong?” he wrote. “And can the meter be wrong?”




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