Geoscience Reference
In-Depth Information
accessible than the circumference of the Earth. It was presented to the French
Assembly in June 1799 without a mention of the uncertainties in the natural standard
from which it was derived, and copies were made and displayed publicly around Paris
so that anyone could check a length of cloth or chain that they had bought.
Places
Standard lengths
On the face of the Palais de Luxembourg opposite number 36 is mounted one of the 16
marble meters issued by the revolutionary government between February 1796 and
December 1797 as a public reference standard, propagating the meter to practical use.
Following the French example, there is a similar device dating from 1876 mounted on the
north wall of Trafalgar Square in London, labeled “Imperial Standards of Length, placed
on this site by the Standards Department of the Board of Trade MDCCCLXXVI - Standards
of Length at 62 degrees F.” It marks out lengths of one foot, two feet, a yard, and an inch.
Along the bottom of the wall is marked a standard chain (66 feet).
In practice the platinum rod itself was declared to be the standard meter,
weakening the founding principle of the metric system that it should be based on
natural reproducible measurements. To be sure, the speeches made to the Assembly
reminded the legislators that the meter was based on the size of the Earth and made
everyone a “co-owner of the world” - fine, democratic stuff. The very basis of the
metric system, however, had turned into a political sham. Not until 1960 when
the meter was redefined in terms of the wavelength of a particular spectral line
of the element krypton did the metric system return to the ideal scientific principles
with which it had started.
The 1960 definition was replaced in 1983 when the meter was defined as the distance
that light will travel in a specified fraction of a second and where a second is defined in
terms of a certain number of the oscillations of a caesium atom. Today, the meter has
become a secondary standard and the standard unit is effectively the light-second.
The Paris Observatory maintains its connections with these definitions through
the Bureau Internationale de l'Heure (International Office of Time), maintaining
at the Observatory the world's standards of time and position by intercomparison
of the world's atomic clocks and position-measuring telescopes even though time
and longitude is conventionally reckoned as starting in Greenwich (see Section
13). To keep its own clocks stable and isothermal, they are housed in the base-
ment of the building, its foundations extending as far below ground (27 m) as the
building is high.
What happened to Delambre and Méchain, you may ask? Honors and accolades
were showered upon both but Méchain was still severely depressed. He avoided
visitors and buried himself in his scientific work as Director of the Paris
Observatory; he had not changed and suffered from self-doubt and depression. In
1801 he called for more measurements that would extend the meridian past
Barcelona into the Balearic Islands. The southern part of the measured arc would
be distant from mountains and the irregularities in the Earth's surface would be
revealed by the earlier survey, but the survey would give him the chance to repeat
and supersede the worryingly discrepant measurements which he had made ten
years before. Eventually the Bureau of Longitudes approved the plan but appointed
