# Sciences. 1791, the revolutionary adoption of the subway

Measuring lengths is difficult. It is difficult to express in the same unit the distance between cities, the length of a strip of cloth, the distance to the neighbor, the sizes of the land, an individual or something. For long distances we use periodic, covered in an hour. The prohibitions were just a faction away, an hour’s walk from the city, this is the origin of the suburbs! For short distances one foot length was very suitable … the reference foot still needed to be matched. It was often the foot length of the reclining statue of the deceased king. For shorter lengths, the inch: one twelfth of the foot, the line: one twelfth of an inch, and finally the stitch: one twelfth of the line. Some other units were used: the meter (its wingspan, six feet), the alder (four feet), and the elbow (the length of the forearm, from the elbow to the tip of the middle finger). All of this lacked precision and has remained local.

## The length of the pendulum exceeds the second

In 1583, 19-year-old Galileo discovered that the stroke length of a pendulum depends only on its length, regardless of the mass of the oscillating body. In the 1650s, Christian Huygens developed a pendulum that wins the second. So he thinks he’s found a pattern for overall height. It is suggested that the length of the second pendulum hit is three feet per hour! 86,400 seconds separating the Sun’s return to its meridian, we have a precise global pattern. Italian Tito Livio Poratini suggests naming this abacus pendulum the Greek metron. That would compare three feet an hour, half a head of fat, with a meter. Unfortunately, we soon discovered that the length of the pendulum hitting the second depends on where it is being measured. Required modifications.

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## Pickard worked on the meridian

In the 17th century, the accuracy of the measurements made by astronomers finally allowed great distances to be determined. To go from angular measurements (latitude and longitude) to distances (leagues), it is necessary to accurately measure the longitude of the arches of the earth. This is what Picard did by measuring the longitude of a portion of the meridian that passed through the Paris Observatory. Starting in 1672, starting with the measurement of the largest accessible base in the Kingdom of France, the Paris-Cayenne distance, the distance from the Earth to the Sun was finally determined: 33 million parsecs of 2,000 taes from Paris, or Toises du Châtelet, its ‘cosmic’ length Specified by the pendulum in seconds.

On March 26, 1791, the National Assembly approved a report from the Academy of Sciences (which is still proprietary) in order to establish a system of measures “It does not present anything arbitrary nor specific to the situation of any person in the world.”. The association had entrusted the preparation of this study to the Academy in March 1790, and the formalization of the study on May 8, which was approved by Louis XVI on August 22 of the same year. It is suggested that the unit longitude is the tenth part of a millionth of a quarter of the longitude of the Earth and that the unit be calculated from the measurements to be made between Dunkirk and Barcelona. Astronomers Méchain and Delambre are responsible for determining the longitude of the meridian. The work begun by Jean Picard continued in 1669, and followed for more than a century by Lair, Lacel, and Cassini on geometry, the first reliable measurements dating back to the third century BC. .

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## The path is traveled in space by light

Measurements of the Dunkirk-Barcelona line began in June 1792, sometime after the fall of the kings … Méchain and Delambre encountered many difficulties, taken by spies. The licenses approved by Louis XVI seem suspicious, and access to high points in the Pyrenees is extremely difficult. Work is very slow. The agreement decides to adopt a temporary meter, as a result of the measurements made in 1740. The academy was dissolved, the task was discontinued at the end of 1793 and resumed on April 7, 1795, and ended in the autumn of 1798. An extension of the period that the meter was adopted was slightly shorter than the temporary meter (1 / 3 by a millimeter). The patterns were built in June 1799, in platinum iridium, an alloy that is particularly hard and not deformable (90% platinum, 10% iridium). The metric system will be able to superimpose it on almost everyone.

Material standard lengths can only be measured with an accuracy of one hundredth of a millimeter. This is insufficient for measurements of wavelengths of light and for spatial measurements … Therefore, it was necessary to improve the accuracy of the standard. In 1960, a new definition of the meter was adopted: the length equals 1,650,763.73 times the wavelength corresponding to a given transmission of 87 krypton. Since October 20, 1983, the scale is the length of a path that is traveled in a vacuum by light in 1/299792458 seconds. So the “global” unit of length is related to the speed of light … which was first measured in 1676, at the Paris Observatory.

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