Tucked into the reference tables of a 1905 dyer's almanac — a book otherwise concerned with aniline colors, mordants, and the bleaching of cotton — is a definition of the metric system that almost no living person learned in school:

The entire metric system of weights and measures is based upon a fundamental unit called a meter, which is the ten-millionth part of the distance from the equator to the pole... The GRAM, or unit of weight, is the weight of a cube of pure water at its greatest density, the edge of which is the hundredth part of a meter.

Read that gram definition twice. The system was elegantly self-referential: a meter was a fraction of Earth itself. A liter was a cube one-tenth of a meter on each side. And a gram was the weight of one cubic centimeter of water at its densest point (4°C). That meant — by construction, not coincidence — that one liter of pure water weighed exactly one kilogram.

The Year-Book for Colorists and Dyers was an annual industrial reference compiled by Herman A. Metz, a New York dyestuffs importer (and later a U.S. Congressman), summarizing the year's advances in textile chemistry for working colorists. Practical men needed exact conversions, so volume after volume opened with this same crisp definition of the metric units — a definition that, by 1905, was already quietly obsolete.

What modern readers don't know:

• The "ten-millionth of the equator-to-pole distance" was measured by a heroic 1790s French survey — and was wrong. The original platinum meter bar was about 0.2 mm too short.
• The water-cube definition of the gram was abandoned in 1889 in favor of a platinum-iridium cylinder kept in a vault in Sèvres, France. Metz's text was already 16 years out of date when he printed it.
• In 2019, the kilogram was redefined again — this time based on Planck's constant, divorced from any physical object at all.

The "a liter of water weighs a kilogram" fact that everyone half-remembers from school is the fossil of this original design. Today it's a useful approximation; in 1799, it was the literal definition. The metric system was conceived as a single, interlocking idea — distance, volume, and mass all derived from the planet and from water. It was beautiful. It was also slightly wrong, which is why we kept redefining the units until they pointed at unchanging constants of physics instead.

Next time you fill a one-liter water bottle and notice it weighs about a kilogram, you're holding the ghost of the French Revolution's most ambitious act of standardization.