Washington D. C.

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Four Numbers That Explain a Capital

1. The library adds 10,000 items every working day.

The Library of Congress holds more than 178 million items across roughly 838 miles of shelving — further than the distance from Washington to Chicago. It receives around 15,000 items a day and keeps more than 10,000 of them, most arriving through copyright registration. That is roughly two million a year, permanently.

The collection includes 25.7 million catalogued books, 75 million manuscripts, 15 million photographs, 5.6 million maps and material in more than 470 languages. It began in 1800 with a small congressional collection, which British troops burned in 1814. Congress replaced it in 1815 by buying Thomas Jefferson's personal library — 6,487 books for $23,950.

2. No building may exceed the width of the street it faces, plus twenty feet.

The Height of Buildings Act of 1910 is still in force. It is why Washington has no skyscrapers: the limit is set by street width, not by a fixed ceiling. Individual exceptions have required acts of Congress — St Matthew's Church got 95 feet in 1910, the Harrington Hotel 130 feet in 1924.

3. There are more than 3,000 cherry trees, and they were a gift.

The mayor of Tokyo sent them in 1912. They are still there, around the Tidal Basin, and the city still schedules a festival around when they open.

4. About 80 atomic clocks sit in a compound on Embassy Row, and they define what time it is in the United States.

The US Naval Observatory has kept time for the Navy since 1830 and moved to its present site in 1893. Its Master Clock is the source of UTC(USNO), and it has stayed within 5 nanoseconds of international UTC since 1999.

The ensemble behind it runs to roughly 57 caesium atomic clocks and 24 hydrogen masers, held in 19 environmental chambers where the temperature is kept constant to within 0.1°C and the humidity to within 1%. A US Defense Department paper describes it as the largest assembly of atomic clocks for any single timing operation in the world.

Its output feeds the GPS satellite constellation. And in the international average that produces official world time, the Washington site alone carries about 35% of the weight; with its Colorado backup, more than half.

The Time Service Department that runs all this employs fewer than thirty people.

So: a library that keeps 10,000 things a day, a law that has stopped the skyline growing for 115 years, three thousand trees from another country, and thirty people whose clocks carry a third of the world's official time.

Washington is a city organised almost entirely around keeping things exactly as they are.

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