Delisle

The Delisle scale was introduced in 1732 by the French astronomer Joseph‑Nicolas Delisle while working at the Imperial Academy of Sciences in Saint Petersburg. Delisle established a fixed point at the boiling point of water and measured downward as liquids cooled, producing an inverted temperature axis. The freezing point of water lies at 150°De, creating a convenient round‑number span between common reference points.

Delisle thermometers originally used alcohol, whose volume change with temperature is large and easy to observe. Readings were taken by the contraction of the liquid as it cooled, which naturally motivates a scale that increases when the thermometer sinks. Later 18th‑century instrument‑makers linearized the spacing and related it to the emerging Celsius scale, preserving the inversion while fixing the degree size.

Because the scale was tied to widely reproducible phase points of water, it provided a practical laboratory tool before international standardization. It coexisted with Réaumur, Fahrenheit and early Celsius conventions during a period of rapid refinement in thermometry.

The Delisle scale gained traction in Russia and appeared in observatories, medical practice and chemical laboratories. It is found in 18th‑ and early‑19th‑century meteorological journals, expedition notes and instrument catalogues. Cartographers and engineers sometimes preferred Delisle alcohol thermometers for cold‑weather work because of their wide sub‑zero range.

As standardized Celsius/Kelvin conventions became dominant in science and industry, Delisle fell out of practical use. Today it survives in museum collections, historical instruments and archival weather records where faithful conversion is required for comparative climate studies.

The Delisle scale is affine‑equivalent to Celsius: it differs only by orientation and a constant degree size. One Delisle degree represents two‑thirds of a Celsius degree, but the axis is inverted (larger Delisle values indicate colder temperatures). In practice, historical Delisle values are converted via Celsius/Kelvin for scientific comparison.

Early instruments sometimes exhibited slight non‑linearities due to fluid and glass behavior. Modern treatments therefore reference standardized mappings to ensure consistent interpretation of archival data.

Delisle is a historical, non‑SI scale. No current standards body maintains it for metrological use; it is retained in scholarship for interpreting historical data and for cataloguing instruments. Recommended symbol: °De. Primary references in modern practice are conversions through the Celsius/Kelvin definition.