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The term light-year looks like an adjective-noun combination naming a kind of year. Instead it denotes a distance, how far light travels in a year. Can the deceptive name be justified?

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Similarity is a relation, more or less formal, depending on context. We naturally associate similar things, as in rhyming; and especially as having similar uses, for need detects likeness: necessity is the mother of resemblance. Euclidean geometry, which is much concerned with shape, does away with “similarity” in the general sense, narrowing its meaning to mere “sameness of shape.” In certain contexts, as with words and living things, it is “relatedness” that can be reduced to a fairly precise meaning: “relation by descent,” which can (as a rule, in principle) be exactly diagrammed. Without losing the useful breadth of “similarity” in these contexts—admitting, therefore, such categories as appearance, sound, shape, function, behavior—one can try to limit it to refer to properties which can be measured. In the case of living things, the modes of classification by (familial) relatedness and (measurable) similarity have respectively been given the unlovely names of cladistics (referring to branching of a genealogical tree) and phenetics (referring to appearance).

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A reader of Ernest Rutherford’s famous paper “The Scattering of α and β Particles by Matter and the Structure of the Atom,” which reports the discovery of the atomic nucleus, may be perplexed by a couple of expressions at the beginning of §3. An explanation follows.

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Finally, in this last part, the lame Meter.

It was on the 4th of Messidor in the year VII that the standard meter, and with it the standard kilogram, were presented to the legislature. —What, Messidor? VII? Yes, the Republic had been inaugurated on September 22, 1792, and on that day (it was eventually decided) began Year I. Years of the Republic had twelve months of 30 days each, plus some festival days; and the months, each beginning around the 20th of an English month, were Vendémiaire (Vintage-y), Brumaire (Misty), Frimaire (Frosty), Nivôse (Snowy), Pluviôse (Rainy), Ventôse (Windy), Germinal (Bud-y), Floréal (Flowery), Prairial (Meadowy), Messidor (Harvest-giver), Thermidor (Heat-giver), Fructidor (Fruit-giver). These names, bestowed by Fabre d’Églantine, whose head was cut off for his pains, were probably thought more dignified and poetical than those contrived by Sir Gregory Gander for the English calendar:

Snowy, Flowy, Blowy,
Showery, Flowery, Bowery,
Hoppy, Croppy, Droppy,
Breezy, Sneezy, Freezy.

But they were hardly more exact. Besides, Sir Gregory kept his head. —Messidor began on June 20th, so the big day must have been June 23, 1799.

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It is by no means only the introduction of arbitrary subdivisions (mere arithmetical adjustment) that removes the “natural” unit of length from nature. No, the Earth itself refuses to sanction the aspirations of the founders of the metric system.

Since the 17th century it has been known that the length of a pendulum beating seconds varies with its location on the Earth; specifically, with the latitude. This is because the constant of proportionality between time and the square root of length depends on the acceleration of gravity g—the rate at which the speed of a freely falling body increases—and g varies with latitude. Hence if the pendulum is to deliver a unit of length, the arbitrary choice of the second of time must be supplemented by something still less justifiable: the selection of a place on the Earth to put the pendulum.

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In this part, “wretched matter” begins to assert itself.

If we want to measure length, we need a unit of length. Given such a unit, not only lengths, but areas and volumes as well, can be expressed in terms of it: foot, square foot, cubic foot. The founders of the new system desired a natural unit, one that would not depend upon an arbitrary choice, and that would be perpetually available in the natural world, so that it could readily be consulted in case of doubt. Neither criterion would be satisfied by (for example) the length of the king’s foot.

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Milton explained that rhyme was “the Invention of a barbarous Age, to set off wretched matter and lame Meter,” so of course he didn’t need it. No more do I need his explanation, only his phrase, to introduce a delightful book, Les Origines du Système Métrique (The Origins of the Metric System), by Adrien Favre, Professeur au Lycée du Toulouse—which means a high-school teacher in a provincial city, then of a couple of hundred thousand people, far from Paris—published in 1931.

“Before the establishment of our Metric System,” he begins, “there reigned in France, among the units of measure, an incredible confusion.” Naming twenty-two of them, he invites us to think of Rabelais’s lists, and then reveals that the twenty-two were merely the subdivisions of the agrarian measures in one area of the country, Haute-Garonne—a place where, while the measures themselves had only five different names, these designated forty-three different actual measures.

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