Your friend catches a fish that gets him bragging rights; it was over eighteen inches long, and he has the picture to prove it, his fish right alongside a ruler. But how do you know he didn't use one of those fisherman's gag rulers that are shrunk, making the fish look bigger? Or if he used a regular ruler, how do you know it was in line with other regular twelve-inch rulers? What are the odds that he took that ruler from one that had been carefully calibrated to a standard foot? What is a standard foot? The enigmas involved in measuring are among the subjects in _World in the Balance: The Historic Quest for an Absolute System of Measurement_ (W. W. Norton) by Robert P. Crease. Crease writes the "Critical Point" column for _Physics World_, and some of the chapters here are from those columns, which means that digressive chapters might not deal specifically with world measurement standards. It doesn't matter; this is a sweeping history of how humans measure things, and since Crease is also the chairman of a philosophy department, it is about the meaning of measurement and its place in human thinking. It is a fascinating story, and all the more so because it is full of optimism. The international community of metrologists (experts in measuring) have spent centuries working on the problem of universal measurement standards, and have cooperated pretty well, and further cooperation seems assured.
In the past, every country, and even particular regions within countries, had idiosyncratic systems of measurement. The search for absolutes forms the main part of Crease's book. It was in the 17th and 18th centuries that it became clear that it would be handy to have one standard measuring system. The best proposal was by French scientists at the end of the 18th century. They wanted it to be universal, but they hooked it to the meridian that came south from the North Pole, took in Paris, and went to the equator. One ten-millionth of this distance was to be a meter. There were problems with the system, beyond the considerable ones of other cultures being slow to convert to it. There had been errors in measuring that meridian through Paris, so that the meter rod reverentially stored in the Archives was short. As early as 1827, scientists were fretting over the inexactitude and ephemerality of such rods. If a comet struck the Earth, they said, the axis of rotation or the shape of the Earth might be changed, and so the meridian measurement could not go back to a universal standard. It made no practical difference, as long as everyone was using the same meter rod, but rods may not last forever. In 1834 in London, the House of Lords was set on fire, along with the rods that were the standards for imperial measure; there were then no official standard lengths to turn to. There was no reason this could not happen to the meter rod in the Archives. Much of Crease's book has to do with tying the meter to a natural standard (it is defined now as a particular number of wavelengths of a particular kind of light). The unit of mass, the kilogram, has yet to be given a natural standard, and this is troubling. Right now, there is the "real" kilogram weight stored with the utmost care in Sèvres, but for reasons no one really understands, it seems to be getting lighter compared to the weights that are its official copies. If metrologists can tie it to Planck's Constant, the kilogram, too, will no longer be vulnerable to the vicissitudes which can afflict any physical object.
The funniest chapter here is about American resistance to the metric system. That we still use feet and miles is no joking matter, but in the 1880s there was a wacky American anti-metric movement which was "born in Ohio and exhibited the classic signs of American antireform movements: xenophobia, rabid rhetoric, fabrication of `facts,' reimagining history, conspiracy theories, and appeals to preserve the purity of nature and nation." In this view, Noah was the architect who designed the pyramids for Egypt, imbuing them with the "sacred cubit", one twenty-fifth part of which was the "Pyramid Inch," exactly one five-hundred-millionth of the Earth's axis of rotation. Thus, inches came from the Bible and the Lord (and from Egypt). The proponents of this view cast themselves as downtrodden combatants against the atheists and their meters. Crease also has a funny chapter about the difficulties of measuring human bodies, especially measurements for brassieres. The whole book proves to be surprisingly entertaining. Cease in his role as philosopher knows that measuring is more than just applying precision, but is a human endeavor that must always be tied to human enthusiasms and activities.