Pierre-Simon Laplace (1749-1827) publishes his Exposition du Systeme du Monde, his work on astronomy (mainly celestial mechanics) following his predecessors as Newton and Lagrange. He developed an analytical theory of tides, deduced the mass of the moon, improved the calculation of cosmic orbits, and predicted that Saturn's rings would be found to rotate. Most notably, he propounded the modern Nebular Hypothesis, independently outlined by Kant.
On 7 April, the gram was decreed in France to be equal to “the absolute weight of a volume of water equal to the cube of the hundredth part of the meter, at the temperature of melting ice.”1