Neptune's satellite was discovered by Mr Lassel of Liverpool in 1846. This satellite differs from the other planets and satellites, except those of Uranus, in the direction of its motion, which is from east to west. After the satellite had been observed for a few years, it was pointed out by Mr Marth that the node and the inclination of its orbit were slowly changing. These changes were explained by Tisserand and Newcomb as arising from the spheroidal shape of Neptune. The observations prior to 1892 have been discussed by Dr Struve in the Mémoires de l'Académie Impérials des Sciences de St Pétersbourg, viie série, tome xlii. No. 4. Since then a large number of valuable observations have been made at American observatories, especially by Prof. Barnard, and at Greenwich Observatory.