Passage: Steamboat pilot Horace E. Bixby took Twain on as a cub pilot to teach him the river between New Orleans and St. Louis for $500 (equivalent to $14,000 in 2018), payable out of Twain's first wages after graduating. Twain studied the Mississippi, learning its landmarks, how to navigate its currents effectively, and how to read the river and its constantly shifting channels, reefs, submerged snags, and rocks that would "tear the life out of the strongest vessel that ever floated". It was more than two years before he received his pilot's license. Piloting also gave him his pen name from "mark twain", the leadsman's cry for a measured river depth of two fathoms (12 feet), which was safe water for a steamboat.As a young pilot, Clemens served on the steamer A. B. Chambers with Grant Marsh, who became famous for his exploits as a steamboat captain on the Missouri River.  The two liked each other, and admired one another, and maintained a correspondence for many years after Clemens left the river.While training, Samuel convinced his younger brother Henry to work with him, and even arranged a post of mud clerk for him on the steamboat Pennsylvania. On June 13, 1858, the steamboat's boiler exploded; Henry succumbed to his wounds on June 21. Twain claimed to have foreseen this death in a dream a month earlier, which inspired his interest in parapsychology; he was an early member of the Society for Psychical Research. Twain was guilt-stricken and held himself responsible for the rest of his life. He continued to work on the river and was a river pilot until the Civil War broke out in 1861, when traffic was curtailed along the Mississippi River. At the start of hostilities, he enlisted briefly in a local Confederate unit. He later wrote the sketch "The Private History of a Campaign That Failed", describing how he and his friends had been Confederate volunteers for two weeks before disbanding.He then left for Nevada to work for his brother Orion, who was Secretary of the Nevada Territory. Twain describes the episode in his...

Question: What profession did Twain hold until 1961?
river pilot