As Long as the King’s Arm?

London Review of Books | Vol. 42, No. 5 | March 5, 2020
"As Long as the King’s Arm?" by James Vincent
A review of The Making of Measure and the Promise of Sameness by Emanuele Lugli
The​ length of Christ was a spiritual matter in the Middle Ages. Sometimes referred to as the mensura Christi or longitudo Christi, it appeared in manuscripts as a hand-drawn bar or linea, as broad as the width of the page would allow. These lines could be measured by the faithful with a string or ribbon and multiplied so that they might draw the Son of God up to his full height. One manuscript, made in Genoa around 1293, informs the reader that ‘prolonged 12 times this segment shows the height [mensura] of the body of our Lord.’ The resulting measurement had theological significance: it was a means of understanding the enigma of Christ’s Ascension by capturing a trace of his physical presence. It was also practical. The mensura could be kept about the person as a good luck charm – perhaps rolled up in a piece of jewellery – or nailed up inside the house, where it would ward off misfortune. As one manuscript promises, ‘those who wear this measure or keep it in their house or see it every day cannot die of sudden death on that day ... And they cannot be harmed by fire or water, nor by the devil, nor by a storm.’
The tradition of the mensura Christi dates back at least to the sixth century ad, when souvenir merchants in Jerusalem sold tapes and ribbons cut to the length of different parts of Christ’s body. Modern concepts of measurement are less personal. Modern metrology officially began in 1793 when the metre was defined as one ten-millionth of the distance from the equator to the north pole. The transition to metric units was designed, in part, to divorce measurement from human affairs. While earlier linear measures such as the cubit and the foot were based on parts of the body (before being standardised, often to the dimensions of the king’s forearm or foot), the savants who planned the metric system under Napoleon wanted to create standards that were ‘à tous les temps, à tous les peuples’. By defining the metre using the dimensions of the Earth, they intended not only to give the unit a basis in something permanent and unchanging, but also to make it accessible to all, re-measurable by any nation on Earth. This was undermined by the fact that the planet’s meridian differs depending on where you measure it – the only ‘correct’ meridian runs through Paris, which annoyed British and American politicians – but the idea of the metre as an arbitrary measure persisted and, in the long run, helped ensure its adoption worldwide.
Since the French Revolution, the metre, along with the six other units of the Système International d’Unités (SI), has been further abstracted and is now defined as ‘the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second’. In October 2018 the International Bureau of Weights and Measures voted to remove the last physical standard from the metric system, the International Prototype Kilogram, an egg-sized, platinum-iridium mass known as Le Grand K, kept in a sealed vault on the outskirts of Paris. It remains in the vault but is no longer the standard unit of mass. Like other standards derived from artefacts it generated problems. In recent decades, scientists have compared Le Grand K to its supposedly identical témoins, or witness weights, and found that it was losing mass. (Though, technically, as it was the definition of the kilogram, it couldn’t lose weight: the universe could only get heavier.) As of last May, the definition of a kilogram is based on Planck’s constant. Metrologists say that by excising the last physical vestige of the SI, they’ve fulfilled the dream of the savants and finally created an immaterial, universal basis for all metrology. But they have also removed many of the human and mystical aspects of measurement, those dimensions that allowed medieval peasants to believe that a length of ribbon could capture a faint impression of Christ’s salvation.
This journey from a medieval to a modern understanding of measurement is long, complex and muddled. It’s not helped by the fact that a great number of books on the history of measurement take the form of a glossary, listing as many archaic units as possible – ells, bins, stadion, firkins, jiggers, rods, barleycorns, and so on – before referring to Pythagoras’ usefully ambiguous ‘man is the measure of all things.’ Emanuele Lugli’s The Making of Measure and the Promise of Sameness avoids this by focusing on premodern understandings of measurement up to the early years of the metric system, and in particular on the period between the 12th and 14th centuries in Northern and Central Italy, where merchants, princes, medieval comunes and the Catholic Church were engaged in a series of power struggles in which metrological definition was an important weapon. ‘Metric discipline emerged as a way to reach consensus in an age of factions and profound cultural differences,’ Lugli writes. ‘It served as the plane of communication between antagonistic and often wildly different parties who spoke foreign languages and thought in codes.’ In this context, units existed as both fallible objects and unimpeachable ideals. Chipped measuring rods and battered capacity containers sparked riots in feudal villages. But they were also supposedly perfect, even holy standards.