But national tradition and habit was a powerful force in resisting the adoption of another country's standards.

The most obvious problem with such units is the fact that men and women come in different sizes. Who do you measure as your standard? The king or queen is the obvious candidate. Even so, this results in a recalibration of units every time the throne changes hands. One notable response to the problem of the variation in human dimensions was that devised by David I of Scotland in 1150 to define the Scottish inch: he ordained that it was to be the average drawn from measurements of the width of the base of the thumbnail of three men: a ‘mekill’ [big] man, a man of ‘messurabel’ [moderate] stature, and a ‘lytell’ [little] man.

The modern metric system of centimetres, kilograms and litres, and the traditional ‘Imperial’ system of inches, pounds and pints are equally good measures of lengths, weights and volumes so long as you can measure them accurately. That is not the same thing as saying they are equally convenient, though. The metric system mirrors our counting system by having each unit ten times bigger than the next smallest. Imagine having a counting system that had uneven jumps. So, instead of hundreds, tens and units we had a counting system like that used in England for non-technical weights (like human body weights or horse-racing handicaps) with 16 ounces in one pound and 14 pounds in one stone.

The cleaning up of standards of measurement began decisively at the time of the French Revolution at the end of the eighteenth century. Introducing new weights and measures brings with it a certain upheaval in society and is rarely received with unalloyed enthusiasm by the populace. The French Revolution therefore provided an occasion to make such an innovation without adding significantly to the general upheaval of everything else.8 The prevailing trend of political thinking at the time sided with the view that weights and measures should have an egalitarian standard that did not make them the property of any one nation, nor give any nation an advantage when it came to trading with others. The way to do this was believed to define measure against some agreed standard, from which all rulers and secondary measures would be calibrated. The French National Assembly enacted this into law on 26 March 1791, with the support of Louis XVI and the clear statement of principle submitted by Charles Maurice de Talleyrand:

‘In view of the fact that in order to be able to introduce uniformity of weights and measures it is necessary that a natural and unchanging unit of mass be laid down, and that the only means of extending this uniformity to other nations and urging them to agree upon a system of measures is to choose a unit that is not arbitrary and does not contain anything specific to any peoples on the globe.’9

Two years later, the ‘metre’10 was introduced as the standard of length, defined as the ten millionth part of a quarter of the Earth's meridian.11 Although this is a plausible way to identify a standard of length it is clearly not very practical as an everyday comparison. Consequently, in 1795, the units were directly related to specially made objects. At first the unit of mass was taken as the gram, defined to be the mass of one cubic centimetre of water at 0 degrees centigrade. Later it was superseded by the kilogram (1000 grams) defined as the mass of 1000 cubic centimetres of water at 4 degrees centigrade. Finally, in 1799, a prototype metre bar12 was made together with a standard kilogram mass and placed in the archives of the new French Republic. Even today, the reference kilogram mass is known as the ‘Kilogramme des Archives’.

Unfortunately, the new metric units were not at first successful and Napoleon reintroduced the old standards in the early years of the nineteenth century. The European political situation prevented an international harmonisation of standards.13 It was not until New Year's Day 1840 that Louis Phillipe made metric units legally obligatory in France. Meanwhile they had already been adopted more universally in the Netherlands, Belgium and Luxembourg twenty-four years earlier, and by Greece in 1832. Britain only allowed a rather restricted use of metric units after 1864 and the USA followed suit two years later. Real progress only occurred in 1870 when the International Metre Commission was established and met in Paris on 8 August for the first time, to coordinate standards and oversee the making of new standard masses and lengths.14 Copies of the standards were distributed to some of the member states chosen by the drawing of lots. The kilogram was the mass of a special cylinder, 39 mm in height and diameter, made of an alloy of platinum and iridium15 kept under three glass bell-jars and stored inside a vault at the International Bureau of Standards in Sèvres near Paris. Its definition is simple:16

‘The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.’

The British Imperial units, like the yard and the pound, were defined similarly and standard prototypes were kept by the National Physical Laboratory in England and the National Bureau of Standards in Washington DC.

This trend for standardisation saw the creation of scientific units of measurement. As a result we habitually measure lengths, masses and times in multiples of metres, kilograms and seconds. One unit of each gives a familiar quantity that is easily imagined: a metre of cloth, a kilogram of potatoes. This convenience of size witnesses at once to their anthropocentric pedigree. But its inconvenience also becomes obvious when we start to use these units to describe quantities that are super- or sub-human in scale. The smallest atoms are 10 billion times smaller than a metre. The Sun is more than 1030 kilograms in mass. In Figure 2.1 we show the span of sizes and masses of significant objects in the Universe with ourselves added for perspective.