Miles, yards, feet and inches, pints, pounds and stones. Yes, fifty years after the UK embarked on the metric transition, we still need to be familiar with some of those old units. In this article, Ronnie Cohen looks at some of the less well known and largely forgotten features of the imperial system.
Readers may ask why Metric Views is discussing the measurement ‘system’ that the UK almost, but not quite, succeeded in abandoning a generation ago. Is this an attempt to kill it by kindness?
It is clear that many who profess enthusiasm for imperial measures know little about them. Would their enthusiasm be less, if they knew more? Alas, they are unlikely to come clean. But our readers may have an opinion. So please, take a look at some of the questions that follow and let us know if you think there is a place for such information in our campaign.
Do you know, for example, that there are two systems of pounds and ounces: troy and avoirdupois? The troy pound ceased to be legal for trade in 1879, and the troy ounce is now restricted to trade in precious metals. There are 12 troy ounces (31.3 g) in a troy pound , but 16 avoirdupois ounces (28.3 g) in an avoirdupois pound. When you see quotes of prices for precious metal per troy ounce, this is not the ounce on your kitchen scales.
Do you know that the British Empire and the US used different definitions of the yard until after WW2. As a result of problems with incompatible equipment, the US and Commonwealth countries agreed in 1959 on common definitions: one yard is equal to 0.9144 metres exactly and one avoirdupois pound is equal to 0.453 592 37 kg. In the UK, these definitions were incorporated into law in 1963.
Do you know that the imperial gallon was introduced in 1824, replacing several different gallons then in use? It was defined then as the space occupied by 10 pounds of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury and at a temperature of 62 degrees Fahrenheit. In 1985, it was redefined as 4.546 09 litres exactly.
Do you know that the Americans have two versions of each of the fluid ounce, pint and gallon, namely ‘dry’ and ‘liquid’, and that these differ from the imperial units? As well as the US customary fluid ounce (29.6 mL), the Americans also use a nutrition fluid ounce (exactly 30 mL). The imperial fluid ounce is approximately 28.4 mL.
Do you know that there are several types of ton? As well as the short or US ton (2000 lb) and long or imperial ton (2240 lb or about 1016 kg), there is also the water ton (a unit of volume of exactly 224 imperial gallons), the freight ton (40 cubic feet) and the register ton (100 cubic feet). None of these should be confused with the tun (spelled with a “u”, not an “o”), an old English volume measurement for measuring alcoholic drinks and other commodities. The tun varied in volume between 800 and 1000 litres, depending on the commodity being measured. By contrast, the metric system has one tonne (known in the US as the “metric ton”) equal to 1000 kg.
Do you know that an acre is based on an Anglo-Saxon strip field? Acre is a similar word to the modern German Acker or Dutch akker, both words for cultivated fields. The acre is the area that a yoke of oxen could plough in a day and is often visualised as a rectangular area, one furlong by one chain (220 yards by 22 yards), equivalent to 4840 square yards or about 4047 square metres. The acre and the closely related rood (a quarter of an acre) do not fit well with the use of square feet and square yards. A square that is the size of an acre would be 69 yards 1 foot 8½ inches on each side.
Do you know that stones of different sizes were once used for weighing different commodities? Historically, the number of pounds in a stone depended on the commodity being weighed and, in some cases, the area. The 1862 Report on weights and measures reported the use of “about 10 different stones; a stone of wool at Darlington is 18 lbs.; a stone of flax at Downpatrick is 24 lbs.; a stone of flax at Belfast is only 16 3/4 lbs.; but it is also at Belfast 24 1/2 lbs., having in one place two values.” The imperial stone is 14 pounds, about 6.35 kg.
Do you know that miles, feet and inches, pounds and ounces were once used throughout continental Europe? These old, pre-metric measurements differed from country to country, sometimes widely, and in some parts of Europe from region to region. They all differed from the English versions.
Do you know that the units names “inch” and “ounce” are derived from the same Latin word uncia, meaning a twelfth part? The names reflect the use of the Roman duodecimal systems of the 12-inch foot and 12-ounce pound. The 12-ounce pound is used in the troy system of weights.
Do you know that there were two types of chain used for surveying: the Ramsden chain with 100 links each of one foot, and the Gunter chain, 22 yards long, also with 100 links.
Do you know that the mile was originally divided into 1000 units? The mile was originally defined by the Romans and was the length of 1000 paces of a Roman legionary. Each pace was a double step. Clearly, the Romans saw the benefits of using powers of 10 for units of distance long before the invention of the metric system. This unit name is derived from the Latin word for a thousand.
Do you know that different miles are used on land and for sea and air transport? The statute mile is so-called because it was fixed by Parliament at 5280 feet, to settle disputes and confusion about the length of the mile and to replace the various miles that were in use at that time. The mile for sea and air transport is called the nautical mile and for many years was defined as 6080 feet. In 1970, the UK adopted the international nautical mile of 1852 metres. Speed at sea is measured in knots, that is nautical miles per hour.
Do you know that Britain had a different set of unit lengths for measurements at sea, namely fathoms, cables and nautical miles. A fathom is 6 feet, there are 100 fathoms to a cable and 10 cables are equal to about one nautical mile. Clearly, mariners saw the benefits of using units based on powers of 10.
Do you know that the imperial system has units for weight and volume for medical recipes? These form the apothecaries’ system, replaced in pharmacies in 1969 by the metric system. Although the apothecary pound weighs the same as the troy pound (373.2 g), the subdivisions of the pound differ from the subdivisions of the troy pound, and include drachms and scruples. The apothecaries’ system also contains subdivisions of the fluid ounce including fluid drachms and minims.
Do you know that the oil barrel is neither an imperial unit nor a physical container but a unit of volume equal to 42 US gallons (about 159 litres) or just under 35 imperial gallons. It is doubtful if oil was ever stored in 42 gallon barrels.
Do you know that there are different imperial units for thermal and mechanical energy and power. For thermal energy there is the British thermal unit, and for mechanical power there is the horsepower. Metric uses the same units for thermal, mechanical and electrical energy and power, namely the joule and the watt (yes, both named after Britons!)
And finally, do you know that by 2010, all imperial measures had ceased to be legally authorised in the UK, other than:
* those required by international treaties relating to transport,
* the mile, yard, foot and inch for road traffic signs, distance and speed,
* the pint for draught beer and cider, and doorstep milk, and
* the troy ounce for trading in precious metals.
So what does this tell us about the UK measurement muddle?
Imperial measures may be an interesting area of study for historians, but they are not a coherent system – they are a collection of separate, stand-alone systems that developed independently for unrelated purposes. This is not surprising. Many of these sets of measures are application-specific (e.g. drachms for dispensing medicines, troy weights for precious metals) while some were adopted for general use (e.g. avoirdupois system of weight). For length, there are the general units such as the inch, foot, yard and mile, but there were also maritime units and survey units, as noted above. And so on for area, volume, energy and power.
That said, is any of this likely to influence the imperial enthusiasts?
Happily, no country now has to tolerate a measurement muddle, which, as we know only too well, increases costs, confuses shoppers, leads to serious misunderstandings, causes accidents, wastes children’s education, impairs numeracy and handicaps efforts to compete in global markets. All countries have the option of using just one unit for each physical quantity with one definition, accepted internationally, and all wrapped up in a single, simple, logical, coherent and universal measurement system. Most have taken this option. How long before the UK does so is anyone’s guess.