Furlong

The 5 furlong (1006 m) post on Epsom Downs

A furlong is a measure of distance within Imperial units and U.S. customary units, and is equal to 660 feet or 201.168 metres, given that an inch is defined to be equal to 2.54 centimeters or 0.0254 meters. There are eight furlongs in a mile.

Distances for thoroughbred horse races in the United Kingdom, Ireland and the United States are still given in miles and furlongs (example), but the unit is otherwise no longer in common use. It is heavily used within the rail industry (especially maintenance and repair services) in the UK, due to the legacy measurements and distance markers from when the tracks were first laid down.

Coincidentally, 5 furlongs is approximately 1 kilometer (1.00584 to be exact). Since the original definition of the meter was a length that was one-quarter of one ten-millionth of the circumference of the Earth (along the great circle coincident with the meridian of Longitude passing through Revolutionary Paris, naturally), the circumference of the Earth is about 40,000 km or about 200,000 furlongs. Also, the speed of light, c, which is defined to be equal to 299.792458 million meters per second, is roughly 2 trillion furlongs per fortnight (see Trivia section below). A fortnight is two weeks or 14 nights or days, with 24 hours every day. Together with the firkin as a unit of mass, Furlongs, Firkins, and Fortnights make up the (less conventional) FFF system of units, with temperatures measured in Fahrenheit degrees, naturally.

History

The name "furlong" derives from the Old English words furh (furrow) and lang (long). Dating back at least to the ninth century, it originally referred to the length of the furrow in one acre of a ploughed open field (a medieval communal field which was divided into strips). The system of long furrows arose because turning a team of oxen pulling a heavy plough was difficult. This offset the drainage advantages of short furrows and meant furrows were made as long as possible. An acre is an area that is one furlong long and one chain (22 yards) wide. For this reason, the furlong was once also called an acre's length, though in modern usage an acre can of course be any shape.

The furlong was historically viewed as equivalent to the Roman stade (stadium), which in turn derived from the Greek system. For example the King James bible will use the term "furlong" in place of the Greek "stadion", whereas modern translations will translate into miles in the main text and relate the (often very symbolic) original numbers in footnotes.

In the Roman system, there were 625 feet to the stade, eight stade to the mile, and three miles to the league. A league was considered to be the distance a man could walk in one hour, and the mile consisted of 1000 passus (5 feet, or double-step).

After the fall of Rome, Medieval Europe continued with the Roman system, which proceeded to "diversify" leading to serious complications in trade, taxation, etc. Around the turn of the century of 1300, England by decree standardized a long list of measures. Among the important units of distance and length at the time were foot, yard, rod, furlong and mile. The rod was 5½ yards (16½ feet), and the mile was eight furlongs, so the definition of the furlong became 40 rods and that of the mile became 5280 feet. (In other words, the reason the prime number 11 slipped into the English and subsequently the British and US systems was to accommodate the rod.)

The official use of furlong was abolished in the United Kingdom under the Weights and Measures Act 1985, which also abolished from official use many other traditional units of measurement.

Trivia

An absurd unit of speed is the furlong per fortnight, which converts to:

• 0.0001663095 metre per second or roughly one centimetre per minute (in SI units)
• 0.001995714 inches per second or roughly a little less than one-eighth of an inch per minute (in Imperial units)

Thus:

• a car travelling at 60 km/h (37 mph) is travelling at a speed of 100,214.7 furlongs per fortnight;
• a Boeing 737 cruising at 420 knots or 216.2 m/s (i.e. typical 0.8 Mach cruise) is travelling at 1,300,013.7 furlongs per fortnight;
• the speed of light in vacuum is approximately 1.803×10¹² furlongs per fortnight;
• one furlong per fortnight is 0.166 millimetres per second, which would be barely noticeable to the naked eye (the tip of an hour hand on a clock, measuring 3.75 feet in length, travels at about 1 furlong per fortnight).

The city of Chicago's street numbering system allots a measure of 800 address units to each mile. Logically, streets were subsequently laid out 8 to the mile. This means that every block in a typical Chicago neighborhood (in either North/South or East/West direction but rarely both) is approximately one furlong in length. Salt Lake City's blocks are also each a square furlong in the downtown area. However, the blocks are irregular in shape as one moves further out from the center, but the numbering system remains the same (800 units to each mile) everywhere in Salt Lake County. Blocks in central Logan, Utah are similarly a square furlong in extent. City blocks in Melbourne's Hoddle Grid are also one furlong in length.