If you’ve been following my blog for a while, you may remember that early last year I started a series of posts on the subject of units of length measurement based on the Distance Measurement Tool feature of Google Maps, using the span of the Menai Suspension Bridge as a test object for comparing the different measurements.

The last couple of units I looked at (beard-seconds and Olympic swimming pools) were decidedly esoteric, but both derived essentially from standard metric units (a beard-second is 5nm and a swimming pool is 50m so it’s not surprising that, as I’ve just noticed, an Olympic swimming pool is exactly 10,000,000,000 beard-seconds long).

This time I want to return (mostly) to somewhat more mainstream units and also diverge from the metric path as we consider inches and related measurements. These form the length part of the so-called Imperial system of measurements which, according to Wikipedia, was formally introduced to the UK by the Weights and Measures Act of 1824, although the actual units themselves are somewhat older. Since 1995, all Imperial measurements in use in the UK are defined in terms of metric units and measuring devices used in trade are legally required to display metric measurements. In practice, many people continue to use Imperial measurements for many purposes.

In principle, I’ve always been a firm supporter of the metric system as it makes a lot more logical sense to me. For instance, I find it much easier to remember that there are 1000 millimetres in a metre and 1000 metres in a kilometre than to remember that there are 12 inches in a foot and 5280 feet in a mile (I had to look that last one up on Google!). However, a couple of years ago I read an interesting book entitled *About the size of it* (by Warwick Cairns, published in 2008 by Pan Books), which advances the thesis that Imperial units are actually based on various measurements relating to the human body and are thus easier for people to visualise.

For instance, a foot (which is about the same as 30cm – apparently it’s actually 304.8mm) is the length of a standard British size 10 boot, which is the average shoe size for an adult male. As it happens, my feet are size 10. This is obviously fairly approximate as individual boots (for a size 10 foot) vary somewhat in length but it means that it’s quite easy to pace out a length of, say, roughly 6 feet and consequently somewhat easier (or so Mr Cairns says, and I’m inclined to agree with him) to visualise a length of 6 feet than one of 1.8m.

An inch (now officially 25.4mm) is, according to Cairns, essentially the thickness of a human thumb. Although different people have hands of different sizes there is, apparently, less variation in the thickness of thumbs than you might think and certainly they are close enough to a standard size to be quite useful for approximating distances.

I would have to do some experimenting to be sure, but I’m fairly certain that I’m more accurate when I try to estimate (suitable) distances in feet or inches than when I try to do it in metric units (despite my best efforts, in the past, to make myself work in metric). For accurate measurements or calculations, metric would still be my first choice in general, but for estimating or visualising distances (on a human scale, at least) I have always found imperial units to be somewhat more natural and now I have a better understanding why that is.

Another Imperial unit that Cairns mentions, which as far as I can tell was not included in the Weights and Measures Act, is the barleycorn. This, as the name suggests, is the length of an average grain of barley and happens to be exactly a third of an inch. Although no longer regularly used as a unit of measurement, the barleycorn is apparently the basis of the British system of measuring shoes. Here’s a condensed explanation (see Cairns’ book for more detail): A child’s size 0 shoe is based on the size of an average child’s foot when they first start needing shoes, which is 4 inches (aka. 1 hand – you can probably guess where that unit came from); thereafter, shoe sizes go up in barleycorns (e.g. a size 1 is 4 inches + 1 barleycorn, i.e. 4.33 inches, size 2 is 4 inches + 2 barleycorns, size 3 is 4 inches + 3 barleycorns, i.e. 5 inches, etc.); a child’s size 12 shoe is 8 inches (or 2 hands) long (i.e. 4 inches + 12 barleycorns (4 inches)). The next size up (8 inches + 1 barleycorn) is considered a child’s size 13 or an adult size 0 and thereafter the adult sizes continue by adding barleycorns. This means that an adult size 10 is 8 inches + 11 barleycorns or 11.66 inches (11 and two thirds, to be precise), slightly shy of a foot (i.e. 12 inches); the discrepancy is explained by the fact that shoe sizes measure the size of the insole, which is slightly smaller (by about a barleycorn, in fact) than the external size of your boot. In other words, if I want to measure distances in feet as accurately as possible, I should put on a fairly sturdy pair of boots.

The span of the Menai Suspension Bridge, as measured with the Google DMT for the purposes of this blog series, is 10444.7 inches, which is the same as 31334.1 barleycorns or 870.393 feet.