Later the anthropomorphic approach reached a first level of abstraction, characterized by ``the shift from concrete representations to abstract ones, from `my or your finger' to `finger in general' ''4 . Nevertheless, even when the stage was reached of conceiving measurement units as abstract concepts, differences in establishing the value of these units remained, depending on region or time [6,7,8,9].
Only in the eighteenth century, with the consolidation of the experimental method on one hand, and the drive towards international co-operation and trading on the other, strong emphasis was placed for the first time on the need for standardized units. In fact, the plethora of units in use in the various countries, and even in different towns and regions of the same country, made it difficult for researchers at different places, observing the same physical phenomenon, to quantitatively compare their results. In addition, the coveted improvement in international commerce was in this situation strongly hampered.
Changing a system of weights and measures (and currency) in a country encounters notoriously much resistance, due to reasons of several natures: practical, like having to renew all instruments of measurement and converting values between old and new systems in order to be back compatible with all the information (measurements, book keeping and contracts) acquired in the old system; sociological, due to human tendency to make profit of somebody else's ignorance or good faith (the recent switch from national European currencies to Euro is a good example of it, especially in the authors' country); psychological, that, apart from a general inertia to changes of a large fraction of people, is objectively due to the fact that adults have difficulty in modifying their mental representations (and, taking again the Euro introduction as an example, we authors still use to roughly convert Euro values into `thousand Lire' -- the practical unit of currency in Italy before the Euro -- multiplying the Euro value by two). National laws can help to impose the new system,5 but the process of changing can take decades and sometimes it can simply result into a fiasco.6
The problem becomes even more difficult to solve when one aims to reach an international standardization of units. To the above mentioned problems, we have to add national pride that refuses to accept foreign rules, no matter how reasonable they might be. Indeed, a similar regional pride existed in the eighteenth century also among towns and regions of the same country. Any attempt of standardization would have then been perceived as an imposition of the most powerful town or region over the others.7This is the reason that led several scientists and thinkers to seek for units that were not just as arbitrary as any materialization of the parts of the human body. Standards `taken from nature' (``pris dans la nature'' ) were then seen as a possible chance to achieve a reform of units of measurements acceptable to all citizens of the country and possibly to all nations. Besides these sociological aspects, well chosen units based on nature have advantage of being constant with time and reproducible in the case that the practical standards are corrupted or destroyed.8
The problem of having a universal system of units of measurement was first tackled seriously about at the same time in France, Great Britain and United States at the end of the 18th century, though the only program eventually accomplished was that launched by the French National Assembly at the time of the French Revolution.
The French program started at the beginning of 1790 when the Academy was entrusted for the reform by the National Assembly. As the historian Alder puts it , one of the aims of the French scientist was to facilitate communications among scientists, engineers, and administrators: their ``grander ambition was to transform France - and ultimately, the all world - into a free market for the open exchange of goods and information''. (For some `external' influences -- economical, philosophical, political and cultural -- on the works of the French Academy see e.g. Refs. [9,12,13].) However, for the French legislator the issue had primarily a sociological priority, as can be perceived in the project of the 1793 decree  in which citoyen Louis Arbogast requires the attention of the National Assembly on a ``subject of universal beneficence, [having been] the uniformity of weights and measures for a long time one of the philanthropist wishes; claimed at once by the sciences and the arts, by commerce and by the useful man who lives of the work of his hands and who, the most exposed to frauds, is the less capable of bearing their effects.''
The success of the French attempt to create a universal system of units of measurement was due to several reasons that, besides the driving illuministic spirit, certainly include the political drive to support the project with education and imposition, even when the revolution had ended. But perhaps the main reason that forced rulers to be so determined was the special chaotic situation of units of measurement there at that time , where about 800 different names for measures have been estimated, whose units varied in different towns, for a total of about 250000 differently sized units . ``It is quite evident that the diversity of weights and measures of different countries, and frequently in the same province, are a source of embarrassment in commerce, in the study of physics, in history, and even in politics itself; the unknown names of foreign measures, the laziness or difficulty in relating them to our own give rise to confusion in our ideas and leave us in ignorance of facts which could be useful to us'', complains Charles de La Condamine .
Sometimes order outsprings from chaos. That was just the case with metrology in France.
Remarkably, the American, British and French attempts of reform were originally based on the length of a pendulum that takes one second per swing. However, as it is well known, the French metric system was finally based on the size of Earth. But the decision to switch from the pendulum to the meridian was so sudden and hurried, especially when analyzed after two centuries, that it looks like a coup de main. Therefore, before describing the steps that led to the definition of the meter based on the Earth meridian, let us shortly review the several proposals of relating units of length to the period of the pendulum.