One of the earliest types of measurement concerned that
of length. These measurements were usually based on parts of the body. A
well documented example (the first) is the Egyptian cubit which was derived
from the length of the arm from the elbow to the outstretched finger tips.
By 2500 BC this had been standardised in a royal master cubit made of black
marble (about 52 cm). This cubit was divided into 28 digits (roughly a
finger width) which could be further divided into fractional parts, the
smallest of these being only just over a millimetre.
In England units of measurement were not properly
standardised until the 13th century, though variations (and abuses)
continued until long after that. For example, there were three different
gallons (ale, wine and corn) up until 1824 when the gallon was standardised.
In the U S A the system of weights and measured
first adopted was that of the English, though a few differences came in when
decisions were made at the time of standardisation in 1836. For instance,
the wine-gallon of 231 cubic inches was used instead of the English one (as
defined in 1824) of about 277 cubic inches. The U S A also took as their
standard of dry measure the old Winchester bushel of 2150.42 cubic
inches, which gave a dry gallon of nearly 269 cubic inches.
Even as late as the middle of the 20th century there
were some differences in UK and US measures which were nominally the same.
The UK inch measured 2.53998 cm while the US inch was 2.540005
cm. Both were standardised at 2.54 cm in July 1959, though the U S
continued to use 'their' value for several years in land surveying work -
this too is slowly being metricated.
In France the metric system officially started in
June 1799 with the declared intent of being 'For all people, for all time'.
The unit of length was the metre which was defined as being one
ten-millionth part of a quarter of the earth's circumference. The production
of this standard required a very careful survey to be done which took
several years. However, as more accurate instruments became available so the
'exactness' of the standard was called into question. Later efforts were
directed at finding some absolute standard based on an observable physical
phenomenon. Over two centuries this developed into the S I. So maybe their
original slogan was more correct than anyone could have foreseen then.