Geoscience Reference
In-Depth Information
were roughly equivalent to the British inch and foot but they were not standardized.
They varied throughout France, with the Parisian
pied-de-roi
11 percent longer than
the one in Strasbourg and 10 percent shorter than the one in Bordeaux. This was
not the end of the proliferation of units, which were as abundant as in Britain,
sometimes even more so. For example, cloth was sold by the
aune
(ell) and land by
the
perche
(rod).
There were two additional principles of the metric system. One was that all units
other than the base units should be derived from these base units. The
pouce
and the
pied-du-roi
had been defined quite independently one from the other and it was only
later that the foot and the inch were related by the convention that 12 inches make a
foot. The second principle of the metric system was that multiples and sub-multiples
of the units should be decimal. For a time there was a debate as to whether the sub-
multiples should be twelfths. Peasants and trades people could halve or quarter
lengths of cloth or an amount of produce by folding or dividing it. Twelfths made
this easy but tenths were much more difficult; one can see this even today in a French
market where eggs are sold in boxes of ten (made as two rows of five). If you want
fewer eggs you can have four or six but not five because this means splitting the box
lengthwise or stepwise and the seller will shrug, refusing to try.
The principles laid out by the Academy still underpin the modern metric system
(now called the Système International d'Unités, abbreviated as the SI system).
France created world-wide interest with this development and it resulted in 15
countries subscribing to the Convention on the Meter in 1875. The USA and Britain
maintain their own everyday units of measurement, such as feet and inches, but
define them in metric terms, and the SI system is used for all scientific and most
but not all engineering applications. Where there is no universal common standard
there is the risk of confusion, and in 1999 this risk became evident with the loss of
the
Mars Climate Orbiter
spacecraft (
Fig. 29
). NASA lost the $125 million space-
craft because a Lockheed Martin engineering team used English units of measure-
ment to make it while the NASA team used the SI system for spacecraft operation.
The relationship between the two sets of documentation describing the manufacture
and the operation of the spacecraft went awry. After a 286-day journey from Earth
to Mars, the spacecraft fired its engine to push itself into orbit around Mars. The
engine fired but the confusion in units meant that the wrong thrust was applied.
Ultimately the spacecraft came much closer to Mars than planned and, after entering
the dense parts of the atmosphere, bounced off into space and was lost.
A hundred years before the metric system was discussed by the eighteenth century
scientists of the Academy, there had been debate as to the basis of the standard of length.
In 1660, the Royal Society in London suggested that the standard should be a “new
yard” or the length of a pendulum that swung uniformly back and forth each in one
second (period 2 seconds). This would have led to a standard of about 39.2 inches, a
few millimeters shorter than the meter used now. The proposal was supported by Isaac
Newton. In 1675 the Italian polymath, Tito Livio Burattini (1617-1681), promoted this
suggestion. He referred to this unit of length as a
mettro cattolico
, the first suggestion
of the name
meter
(meaning
measure
) with the adjective contrasting the proposed stand-
ard for use in Catholic countries against English units like the original yard.
