Plus or Minus One Ear

Steven Shapin

  • World in the Balance: The Historic Quest for an Absolute System of Measurement by Robert Crease
    Norton, 317 pp, £18.99, October 2011, ISBN 978 0 393 07298 3

The geeks at the Massachusetts Institute of Technology are fond of merry japes, locally known as ‘hacks’. One of the more memorable happened one night in October 1958 when an MIT fraternity had the idea of initiating new members by making them measure a bridge over the Charles River connecting the Cambridge campus with Boston. Crossing the bridge was often a wet, windy and unpleasant business and it was thought that students returning at night from downtown would like to know, by visible marks and with some precision, how far they still had to go. The older fraternity brothers decided to use one of the new pledges as a rule, and selected Oliver R. Smoot, the shortest of the lot at 5ft 7in. The other pledges laid Smoot out at one end of the bridge, marked his extent with chalk and paint, then picked him up and laid him down again, spelling out the full measurement every ten lengths, and inscribing the mid-point of the bridge with the words ‘halfway to Hell’. In this way, it was determined that the span was 364.4 smoots long, ‘plus or minus one ear’ (to indicate measurement uncertainty).

The hack was too good to let fade away, so every now and then the fraternity makes its pledges repaint the markings. You might think this isn’t the sort of vandalism the police would tolerate, but they do. The smoot markings soon became convenient in recording the exact location of traffic accidents, so (as the story goes) when the bridge walkways needed to be repaved in 1987, the Massachusetts Department of Public Works directed the construction company to lay out the concrete slabs on the walkway not in the customary six-foot lengths but in shorter smoot units. Fifty years after the original hack, the smoot markers have become part of civic tradition: the City of Cambridge declared 4 October 2008 ‘Smoot Day’. MIT students ran up a commemorative plaque on a precision milling machine and created an aluminium Smoot Stick which they deposited in the university’s museum as a durable reference standard: the unit-smoot is now detached from the person-Smoot. Through the legions of MIT graduates driving global high-tech culture, the smoot has travelled the world. If you use Google Earth, you can elect the units of length in which you’d like distances measured: miles, kilometres, yards, feet – and smoots.

The history of the smoot recapitulates much of the deep history of measurement standards. Most stories about the emergence of length measures track back to the human body. The cubit ran from the elbow to the fingertip; the yard was the distance from the tip of an outstretched hand to the middle of the chest (or to the tip of your nose); the fathom was the distance between the extremes of a person’s outstretched arms, and the ell (an abbreviation of elbow) was traditionally an arm’s length, though English, Scottish and Flemish ells were reckoned differently. Human bodies and their parts vary in size and so do the measures derived from them.

Central European foot measures generally ranged from 101/2 to 121/2 modern inches, but the Sicilian foot was 8.75 inches and the Genevan foot 19.2, so we can’t be certain that all foot standards really did come from any human foot. Maybe the Genevan or even the 12-inch foot belonged to heroic specimens, or maybe the original foot measure included a generously proportioned shoe. Maybe both human feet and the length that the foot measure measured increased over time. Maybe too there were other ways of establishing the foot. Sixteenth-century writers claimed that French workmen calculated it by joining the extremities of their thumbs, clenching the fingers, and extending the thumbs as far as they could. Try it yourself and you’ll see that you can get pretty close to a 12-inch foot. The concept of the ‘average foot’ (understood as the mean of the population) probably wasn’t intelligible before the emergence in the 19th century of the notion of the ‘average man’, but a 16th-century German source reported an ingenious way of arriving at a reliable foot measure: lurk outside church on Sunday and, when the worshippers come out, ask 16 men to stop – both short and tall – and make them line up their left feet, one after the other. The length you get will constitute the local land measure called the rood, and a sixteenth part of that ‘shall be the right and lawful foot’, even if it corresponds to the foot length of no one of the 16. Similarly, one story about the inch says that it was taken as the width of a man’s thumb at the base of the nail, and another derives it from the Latin word for a twelfth (uncia), as in 1/12th of a foot is an inch and 1/12th of a Troy pound is an ounce.

Length units could be systematically related because bodily dimensions were understood as organically related. ‘Man is the measure of all things,’ and Leonardo’s Vitruvian Man represented confidence in the proportionality of human body parts: ‘The length of the outspread arms is equal to the height of a man; from the hairline to the bottom of the chin is one-tenth of the height of a man; from below the chin to the top of the head is one-eighth of the height of a man.’ Tailors as well as artists knew some of these systemic relations: the Lilliputian seamstresses in Gulliver’s Travels measured up their giant guest using a rule of thumb – ‘twice round the thumb is once round the wrist.’ The human body was a cosmologically and aesthetically resonant measuring-kit. It was metrically intelligible, useful and, above all, it was at hand. ‘Traditional measures were “human” in many respects,’ the Polish historian Witold Kula wrote in his great study of Measures and Men. ‘They were expressive of man and his work.’

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