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Tentamen de motuum coelestium causis. A translation is given in Meli (1993). Leibniz claimed that when he wrote the Tentamen he had not seen Newton's Principia but only the review of it in the Acta eruditorum. According to Meli (1991), however, Leibniz almost certainly had seen Newton's work, but by claiming his ideas were formulated before Newton's, he hoped to elevate their status. This theme is expanded upon in Meli (1993).

about the nature of the solar vortex guaranteed that the planet would move in accordance with Kepler's second law.

For Leibniz, the centrifugal endeavour was a real force caused by the circulation of the body, the magnitude of which, for a body orbiting in a non-circular orbit, was v2/r, where v was the component ofthe velocity perpendicular to the radius vector. In his harmonic vortex - where this component varied inversely with r - Leibniz was led to the conclusion that the centrifugal force varied inversely with the cube of the distance from the Sun, and, utilizing Kepler's area law, he deduced that the magnitude of this force was h2/r3, where h = r2 6 is a constant. Hence, in his theory, the force on the planet in the radial direction was h2/r3 minus the effect of gravitation. Leibniz then considered elliptical motion subject to the area law and showed, using his differential calculus, that

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