The adventure of the metre...
One
can wonder what is the actual worth of the works that led to the
determination of our units of length and mass.
Here
is the opinion of the contemporary people of that time, according to Tarbé
(1813):
"They
took the value of each angle, without caring about neither the other angles
nor the sum of the three angles of a triangle […] Observations were taken as
they were, without correcting them, without putting right anything
afterwards. Yet, out of the 115 triangles which join the meridian ends
together, there are 36 for which the error made when calculating the three
angles altogether is nearly one second. There are triangles in which the
error - for which it is the most important - is nearly 5 seconds, that is to
say one 720th of degree for the three angles.
"Two
bases were measured, one between Melun and Lieusaint, the other between
Vernet and Salces, near Perpignan. […] Measuring them led to […] the two
following measures, at sea level and at the temperature of 16 degrees and
one quarter on the centigrade thermometre: base of Melun, 6,075.90 toises;
base of Perpignan, 6,006.25. One indisputable evidence of the operations
accuracy is that the base of Perpignan, deduced from the base of Melun
thanks to the chain of triangles that link the first to the latter, differs
from its actual measure from only 10 to 11 inches, although these bases are
700,000 metre apart."
And
here are the results from modern verifications:
From
1870 to 1896, step by step, the Armed Force Geography Service realized a new
French meridian measurement, with methods and instruments that had made
great progress in comparison to the methods and instruments that Delambre
and Méchain had. The results of these modern operations coincided
outstandingly with the 1799 results: the Mètre des Archives is nearly 0.2 mm
shorter than the ten-millionth part of the earth quadrant to which its
definition depended.
On
the other hand, in its original definitions, the kilogramme had the weight
of one cubic decimetre of water at 4 degrees centigrade. Several checkings
made between 1895 and 1905 by the IBWM have shown that the International
Kilogramme exceeds the mass that results from this definition by 28
milligrammes.
These
differences are so slight - 2 ten-thousandth and 28 millionth - that they
show the 1792-1799 operations were led with the utmost accuracy.
Moreover,
whether it is about the 1889 standards or the meter's definitions of 1960
and the following years, every new definition was chosen within the limits
of uncertainty of the previous definition.
Therefore,
the present metre and kilogramme are really close to their "natural"
definition of 1793.
History to be continued !