In the judgment of D'Alembert the Cartesan theory was the best that the observations of the age admitted; and " its explanation of gravity was one of the most ingenious hypotheses which philosophy ever imagined."
He first visited Paris, where he saw a good deal of d'Alembert, Diderot, Barthelemy, Raynal, Helvetius, Baron d'Holbach and others of that circle, and was often a welcome guest in the saloons of Madame Geoffrin and Madame du Deffand.
He made his first appearance in public as the critic of Newton, and the arbiter between d'Alembert and Euler.
His treatise was remarkable, not only as offering a satisfactory explanation of the coincidence between the lunar periods of rotation and revolution, but as containing the first employment of his radical formula of mechanics, obtained by combining with the principle of d'Alembert that of virtual velocities.
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