Metre
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The metre is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second, where the second is defined by a hyperfine transition frequency of caesium.
The historical standard metre in Paris
The Meridian room of the Paris Observatory (or Cassini room): the Paris meridian is drawn on the ground.
Gravimeter with variant of Repsold–Bessel pendulum
Triangulation near New York City, 1817
American and British English spelling differences
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Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography, the two most notable variations being British and American spelling. Many of the differences between American and British/English in the Commonwealth of Nations date back to a time before spelling standards were developed. For instance, some spellings seen as "American" today were once commonly used in Britain, and some spellings seen as "British" were once commonly used in the United States.
An 1814 American medical text showing British English spellings that were still in use ("tumours", "colour", "centres", etc.)
Image: Webster Orthography 1828 (4 14)
Image: Webster Orthography 1828 (15)