The Translator of Newton

Gabrielle Émilie Le Tonnelier de Breteuil, Marquise du Châtelet, translated Newton's Principia into French. It was published in 1759, ten years after her death, and it remains the standard French translation.

The translation alone would secure the entry. What makes it more interesting is the accompanying commentary, which is not an explanatory gloss. It recasts Newton's geometrical arguments in the analytical calculus that had developed since, incorporates work by the Bernoullis and by Clairaut, and in doing so makes the Principia usable to a generation of French mathematicians who had been working in a different formalism.

Her independent work is more consequential than the translation and is less known. In her Institutions de Physique of 1740, she took up the dispute over how to measure the force of a moving body. The Newtonian position measured it as proportional to velocity. Leibniz had argued for a quantity proportional to velocity squared, which he called vis viva.

Du Châtelet argued for the Leibnizian quantity, and she supported it with reference to experiments by Willem 's Gravesande in which brass balls were dropped into soft clay. A ball at twice the speed made an impression indicating four times the effect, not twice. The observation fits the square.

That quantity is kinetic energy. The debate she intervened in was resolved substantially in the direction she argued, and the modern formulation retains the square.

She was elected to the Bologna Academy of Sciences. She was not eligible for the Académie des Sciences in Paris, which did not admit women, and the story of her being refused entry to a café where the mathematicians met, and returning in men's clothing, is repeated often enough to have become the memorable fact about her.

That is the Hypatia mechanism again in a milder form. The anecdote about the café travels. The argument about vis viva does not, and the argument is the reason she matters.

Her collaboration with Voltaire is the other thing that crowds out the work. They lived and worked together at Cirey for over a decade, and the standard framing makes her his companion who also did physics. In the specific matter of Newtonian mechanics, the direction of instruction ran the other way.

She died in 1749 following childbirth, at forty-two, having completed the Principia translation and commentary in the preceding months while explicitly working against the clock.

The translation is still in print. It has been the French Principia for two hundred and sixty years, which is a longer working life than most scientific books achieve.