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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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