question: What are the full names of the expedition's two ships after which the twin volcanoes they sighted were named?
context: James Clark Ross's 1839–43 Antarctic expedition in HMS Erebus and HMS Terror was a full-scale Royal Naval enterprise, the principal function of which was to test current theories on magnetism, and to try to locate the South Magnetic Pole. The expedition had first been proposed by leading astronomer Sir John Herschel, and was supported by the Royal Society and the British Association for the Advancement of Science. Ross had considerable past experience in magnetic observation and Arctic exploration; in May 1831 he had been a member of a party that had reached the location of the North Magnetic Pole, and he was an obvious choice as commander.
  The expedition left England on 30 September 1839, and after a voyage that was slowed by the many stops required to carry out work on magnetism, it reached Tasmania in August 1840. Following a three-month break imposed by the southern winter, they sailed south-east on 12 November 1840, and crossed the Antarctic Circle on 1 January 1841. On 11 January a long mountainous coastline that stretched to the south was sighted. Ross named the land Victoria Land, and the mountains the Admiralty Range. He followed the coast southwards and passed Weddell's Farthest South point of 74°15'S on 23 January. A few days later, as they moved further eastward to avoid shore ice, they were met by the sight of twin volcanoes (one of them active), which were named Mount Erebus and Mount Terror, in honour of the expedition's ships.The Great Ice Barrier (later to be called the "Ross Ice Shelf") stretched away east of these mountains, forming an impassable obstacle to further southward progress. In his search for a strait or inlet, Ross explored 300 nautical miles (560 km; 350 mi) along the edge of the barrier, and reached an approximate latitude of 78°S on or about 8 February 1841. He failed to find a suitable anchorage that would have allowed the ships to over-winter, so he returned to Tasmania, arriving there in April 1841.
  The following season Ross returned and located an inlet in the Barrier face that enabled him, on 23 January 1842, to extend his Farthest South to 78°09'30"S, a record which would remain unchallenged for 58 years. Although Ross had not been able to land on the Antarctic continent, nor approach the location of the South Magnetic Pole, on his return to England in 1843 he was knighted for his achievements in geographical and scientific exploration.
answers.text: HMS Erebus HMS Terror