Letter, from Henry Oldenburg to John Borelli, dated at London Glad that books for Borelli had arrived in Rome; congratulates him for completing his book on the movement of animals which had gone to press; concerning Mariotte's treatise on the impact of bodies; that Monsieur St Hilaire at Paris can determine the point of section by a magnet; that Cassini had discovered two planets around Saturn; concerning Boyle's work which had gone to press; wishes to know about the eruption of Mount Etna 7 June 1673