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Units of measure
Pecks, bushels, and the story of Mohenjo-Daro
Every autumn, I get confused about pecks. I show up at the apple-picking farm, and I get two choices: I can pay to pick one peck of apples, or I can pay to pick a half bushel of apples. I furiously google —over the spotty rural wireless— and re-discover that 4 pecks make one bushel, so a half bushel is two pecks, or exactly 1,075.21 cubic inches, or approximately 24 pounds.
Autumn is also the start of the season for buying firewood. Firewood comes in cords. One cord, you will recall, is 128 cubic feet of wood, equal, for example, to a pile 8 feet long, 4 feet wide, and 4 feet high. My New Hampshire friend was proud that she had finally developed a sense for how many cords of wood she needed for winter to heat her house.
I could go on and on, reminding you how many rods are in a furlong and so on, but I think the overall historical picture is pretty clear. But there are plenty of other standard units that are truly weird, but also so familiar that we don’t think of them, like how a fish doesn’t think about water. For example, why are there 7 days in a week? There’s no reason a week has to be 7 days. The Romans had an 8-day week; every eighth day was a market day and rest day. The Egyptians had a 10-day week; perhaps with 2 “weekend” rest days.
The 7-day week arose out of the ancient Mesopotamian and Jewish calendars, which spread through the Near East. The 7-day week had religious significance: the Epic of Gilgamesh has multiple 7-day plot elements, and the Old Testament creation story covers 7 days. There was also an astronomical reason: the lunar month, which varies between 27 and 28 days, nearly divides into four 7-day weeks. I expect the 7 to 10 day period was also about the amount of time that people could go before they needed a weekend. Proximally speaking, we have a 7-day week today because the Romans converted to Christianity and adopted the Jewish 7-day week.
In the beginning, units of measure were mostly local phenomena. As trade became widespread, those units became more standardized and more widespread. Over time, centralized political powers more carefully regulated those measures. Science and engineering matured in ways that made the standards more reliable.
In prehistoric South Asia, there is a mysterious counterexample to this neat historical narrative. The Indus Valley Civilization, a Bronze Age culture generally contemporaneous with the ancient Mesopotamians, Chinese, and Egyptians, built the amazingly standardized city of Mohenjo-Daro without any signs of strong central authority. The city housed something like 40,000 people, making it one of the largest cities in the world at the time, but there are no obvious temples, palaces, monuments, or other signs of strong central authority. The houses are mostly of the same size, and grave goods are roughly similar between people.
Most amazingly, to me, the whole city is laid out on a common grid system, and all structures are made with bricks that come in only two sizes, big and small, both with the same relative proportions. There is also evidence of very well-defined standards of weight. How did such a large city come into being, with such careful standardization, without one king lording over everything?
No one really knows. Archaeologists might decipher the Indus valley writing system, or we might unearth a new archaeological site that gives new answers.
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