diamond geezer

 Wednesday, October 19, 2005

Trafalgar Square (7): Imperial Standards of Length

How long is a foot, or a yard, or an inch? It was once important to be sure because Britain's trade relied on everyone having the same definition for these key units of length. King Henry I in his wisdom decided that a yard should be the distance from the tip of his nose to the end of his outstretched thumb, while King Edward I decreed that "three grains of barley, dry and round make an inch". Not very standard. It was left to Elizabeth I to come up with a nationally-agreed 'standard yard', but it was not until 1824 that the first Imperial Standard Length was created. This was a metal bar of precise length which was kept locked away safely in Westmister... at least until the massive fire that burnt down Parliament ten years later. A brass copy of the replacement standard yard was affixed to the northeast wall of Trafalgar Square by the Board of Trade in 1876, in full public view. The horizontal distance between pairs of metal marks - here one foot, two feet and the full yard - allowed people to check the accuracy of their rulers [proper photo, bit of history]. All perfectly accurate at precisely 62 degrees Fahrenheit, apparently. Nowadays scientists define the metre as "the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second", which is nowhere near as romantic. Maybe that's why the girl reading a magazine sat on a bench directly in front of the brass plaque gave me a really funny look when I invaded her personal space to take the above close-up photo. From about a yard away, I think.

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