A pint is a pound, the world around
Except everywhere
In elementary school, I was taught the rhyme “A pint is a pound, the world around.” It’s memorable but in fact useless without the gloss “for they both have sixteen ounces.” Even then, it’s important to remember that the pint is sixteen fluid ounces, while the pound is sixteen dry ounces.
“Ounce” derives from the Latin uncia, meaning twelfth, which explains why the inch, derived from uncia, is one twelfth of a foot, but doesn’t explain why an ounce is one sixteenth of a pound (or pint). Just as the Roman foot was divided into twelve unciae (i.e., inches), the Roman pound (or libra, whence the abbreviation “lbs.” for pounds), was divided into twelve uncia (i.e., ounces).
The Roman pound was about three-quarters the weight of the pound we know, the avoirdupois pound. For reasons lost to time, we decided, rather than divide the avoirdupois pound into twelve bigger-than-Roman ounces, to make the avoirdupois ounce similar in weight to the Roman ounce (although not exactly the same), so that the avoirdupois pound is made up of sixteen almost-Roman ounces.
In fact, we didn’t go right from the Roman pound to the avoirdupois pound, but had other medieval units in between, like the Troy pound and the tower pound. Each of these pounds was used to measure different things, in different places, at different times. The avoirdupois pound was originally called the “wool pound,” and we still use troy ounces to measure the weight of gold and silver. The tower pound was used by the Normans to weigh silver, whence the “pound sterling,” the currency of the United Kingdom, which originally referred to a tower pound of silver, which was fashioned into silver pennies called, for reasons lost to time, “sterlings.”
Now, the most pernicious thing about “a pint is a pound, the world around” is not that most of the world uses the metric system rather than pints and pounds, but that a pint and a pound both have sixteen ounces only in the United States, since in the United Kingdom, a pint has twenty ounces. (The Imperial fluid ounce is very similar, but not exactly the same, as the US fluid ounce, so a UK pint is about one-quarter larger than a US pint. This is why a pint of English ale feels, to an American, like a lot of liquid.)
The difference between the UK pint and the US pint is because they were derived from systems used to measure different things. Just as the wool pound and the pound sterling were for measuring different things, so in Shakespeare’s day there was a wine gallon and an ale gallon. The wine and ale gallons were intended to represent the same amount of liquid at rest, accounting for the fact that beer, in a barrel being transported and otherwise moved around, would froth to approximately one-quarter larger than its resting volume. (Or so, at least, the little museum in Winchester claims.) The US adopted the wine gallon, and the UK the ale gallon, which explains why the UK pint is one-quarter larger than the US pint and thus has twenty ounces, one quarter more than the sixteen ounces in the US pint.
As I walked along the South Downs Way, in southern England, I noticed ripples in the landscape, and learned that these were the intentional result of medieval farming. Plows were asymmetrical, and always pushed soil to one side. The plowing team would mostly walk in a long, straight line to avoid the inconvenience of turning. When it did turn, it wouldn’t make an immediate about-face, but would instead move over some distance, thus plowing a single strip while piling up the soil toward the middle of the strip, creating an elevated and well-drained ridge for wheat and then furrows for drainage. The strip plowed by a single farmer in a day was an important measurement, and it came to be one furlong long (that is, one furrow long) by one rod wide. The furlong (one-eighth of a mile) and the rod (sixteen and a half feet) are no longer common measurements, but together they define a strip that is one-quarter acre (with an acre being the land you could plow in a day with a larger, post-medieval team of oxen).
I can’t easily prove it, but I have the sense from going apple-picking that a bushel was about the amount that a single person could lift and carry.
As a scientist, I’m naturally very pleased that we live in a world of precisely defined units. Even if you don’t know what a peck or a bushel is, you can feel confident that it converts to a precise number of grams, and that a gram truly is a gram the world around. But as a regular person, I wish we made more daily use of units of measures that reflect our daily lives. For example, I would vote for the recognition of the zoomlong, measuring about forty minutes, which is about the length of time I can pay attention to a video call before I wish it were over.