I thought this was interesting. Not so much for the story (although it is 
good), but for the fact that it shows that an alert publisher can get a 
national newspaper to devote half a full page to a professional history of 
science piece. Well done Ken Alder, and well done Little, Brown. J.

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How the metre fell martyr to short measure

Revealed after 210 years - how French astronomers hid 0.2mm blunder

Jamie Wilson
Tuesday August 27, 2002
The Guardian

In a revelation to put a spring in the step of metric martyrs everywhere, it 
has emerged that the metre - which is supposed to be one 10 millionth of the 
distance from the pole to the equator - is in fact too short.
The award-winning science writer Ken Alder claims in a book that the two 
18th century French astronomers given the task of working out the length of 
the measure made a minute error in their calculations that means the metre 
is in fact short by 0.2mm - roughly the thickness of two pages of this 
newspaper.

Worse still, Dr Alder alleges, the scientists knew they had made a mistake, 
but not knowing how to correct it decided to conceal it, which means the 
error has been perpetuated to this day.

The tale of intrigue and cover-up took place in 18th century France amid the 
scientific fervour of the revolution.

At the time measurements differed not only from nation to nation, but from 
province to province, district to district and in some cases town to town. 
It is estimated that under the cover of some 800 names, France used a 
staggering 250,000 different units of weights and measures.

In June 1792, in an attempt to create a universal measure that would be one 
10 millionth of the distance from the north pole to the equator, two French 
astronomers set out from Paris in opposite directions. Their quest was to 
extract a single number from the curved surface of the planet by measuring 
the meridian arc that ran from Dunkerque through Paris to Barcelona.

The erudite and cosmopolitan Jean-Baptiste-Joseph Delambre made his way 
north, while the cautious and scrupulous Pierre-François-André Méchain made 
his way south.

Once they had reached their destinations they measured their way back 
towards each other. For seven years the pair travelled through revolutionary 
France. Delambre was lucky to escape the guillotine on the outskirts of 
Paris while Méchain was trapped behind enemy lines in the war against Spain 
and imprisoned.

But finally the two met up in the southern fortress town of Carcassonne, 
from where they returned to Paris to present their data to an international 
commission. The astronomers received a hero's welcome, and the result of 
their calculations - the metre - was enshrined in a bar of pure platinum.

"Conquests will come and go," declared France's new ruler Napoleon 
Bonaparte, "but this work will endure."

But what nobody knew, and Dr Alder in his book, The Measure of All Things, 
claims he has uncovered, is that Méchain had discovered a minute error in 
his calculations, an error he concealed, the guilt over which eventually 
drove him to an early grave.

According to Dr Alder, Méchain discovered that sets of readings he took from 
the same place over two different summers did not add up. According to 
modern satellite surveys, the length of the meridian from the pole to the 
equator equals 10,002,290m.

Although only minutely out - possibly, Dr Alder said, because of a slight 
wearing down of the surface of the measuring instrument - the discrepancy 
means the metre calculated by Delambre and Méchain falls roughly 0.2mm 
short.

Letters written by Méchain show he was driven to the brink of madness by the 
discovery of his error. In the end he died of malaria while on an expedition 
to try to correct himself.

Dr Alder, who discovered the cover-up from the log books of the meridian 
expedition, kept at the Observatory of Paris, also found that Delambre was 
party to the deception.

In an extended commentary in one of Méchain's log books, Delambre wrote: "I 
deposit these notes here to justify my choice of which version of Méchain's 
data to publish. Because I have not told the public what it does not need to 
know, I have suppressed all those details which might diminish its 
confidence in such an important mission."

According to Dr Alder, the error in the original metre has been perpetuated 
in every subsequent redefinition of its length, including the current 
definition of the metre that relates to the distance travelled by light in a 
fraction of a second.

But he also claims that Méchain was probably too hard on himself, and that 
the task he and Delambre had set themselves was probably virtually 
impossible to attain.

"Basically they made a false assumption that the Earth is uniform when it 
turns out it is lumpy and warped," he said yesterday.

"The distance between the pole and the equator will be different depending 
on where you measure it from. The mistake he made was believing in the 
perfection of nature."

But Neil Herron, one of the leaders of the metric martyr campaign, was less 
forgiving.

"As far I know, they have changed the length of the metre three times 
anyway," he said. "It is quite an irony that this system that is supposed to 
be based on science is in fact a fallacy ... We are not and never have been 
anti-metric, we just prefer to use a system that people understand and is 
based on human measurements rather than a system made up by scientists who 
do not live in the real world anyway."

Whatever the rights and wrongs, the error has certainly had some 
repercussions: 100m sprinters, for example, have actually been running only 
99m, 99cm and 98mm - roughly the thickness of a vest.

The Measure of All Things by Ken Alder, published September 16 by Little, 
Brown


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