THOMPSON, David, Canadian explorer, born in the parish of St. John. Westminster, England, 30 April, 1770; died in Longueil, near Montreal, 16 February, 1857. He was educated at Christ's hospital school, London, and at Oxford, and when he was nineteen years old entered the service of the Hudson bay company. He was afterward employed in exploration, and on 27 April, 1798, discovered Turtle lake, which he claimed to be the source of the Mississippi, as it is from this spot that the river takes its most direct course to the sea. His course in reaching the head-waters of this river is well delineated on his " Map of the Northwest Territory of the Province of Canada, made for the Northwest Com-puny in 1813-'14." He also surveyed the south shore of Lake Superior in 1798, in June, 1807, crossed the Rocky mountains by what is now known as the Howse pass, and in the spring of 1811 ascended Columbia river from the junction of Canoe river to its source, and then descended it to its mouth, where he arrived on 16 July the same year. On 27 May, 1812, he reached Red Deer lake, or Lac la Biche, which Schoolcraft, who visited it in 1832, claimed to be the true source of the Mississippi, and the same year resurveyed the north shore of Lake Superior to Sault Ste. Marie. He left the service of the Hudson bay company, 23 May, 1797, and entered that of the Northwest company, in which he was for many subsequent years a partner. From 1816 till 1826 he was engaged in surveying and defining the boundary-line on the part of Great Britain between Canada and the United States, being employed in 1817 on the St. Lawrence. Proceeding westward around the shores of the great lakes, he reached the Lake of the Woods in 1825. In 1834 he surveyed Lake Francis, in 1837 he made a survey of the canoe route from Lake Huron to Ottawa river, and a few years later a survey of Lake St. Peter. His last years were spent either in Glengarry county, Ont., or in Longueil. Of the early explorers, few rendered more valuable services or estimated their achievements more modestly.