A number of events conspired to make it necessary for the North-West Company to be well acquainted with the location of its forts within the limits of the territory of the United States, in some parts of which it carried on operations of trade, and to understand its relation to the Hudson's Bay Company's territory. The treaty of amity and commerce, which is usually connected with the name of John Jay, 1794, seemed to say that all British forts in United States territory were to be evacuated in two years. This threw the partners at Grand Portage into a state of excitement, inasmuch as they knew that the very place of their gathering was on the American side of the boundary line. DAVID THOMPSON, ASTRONOMER AND SURVEYOR. At this juncture the fitting instrument appeared at Grand Portage. This was David Thompson. This gentleman was a Londoner, educated at the Blue Coat School, in London. Trained thoroughly in mathematics and the use of astronomical instruments, he had obtained a position in the Hudson's Bay Company. In the summer of 1795, with three companions, two of them Indians, he had found his way from Hudson Bay to Lake Athabasca, and thus showed his capability as an explorer. Returning from his Western expedition, he reported to Mr. Joseph Colon, the officer in charge at York Fort, by whose orders he had gone to Athabasca, and expressed himself as willing to undertake further explorations for the Company. The answer was curt--to the effect that no more surveys could then be undertaken by the Company, however desirable. Thompson immediately decided to seek employment elsewhere in the work for which he was so well qualified. Leaving the Bay and the Company behind, attended only by two Indians, he journeyed inland and presented himself at the summer meeting of the North-West fur-traders at Grand Portage. Without hesitation they appointed him astronomer and surveyor of the North-West Company. Astronomer Thompson's work was well mapped out for him. (1) He was instructed to survey the forty-ninth parallel of latitude. This involved a question which had greatly perplexed the diplomatists, viz. the position of the source of the Mississippi. Many years after this date it was a question to decide which tributary is the source of the Mississippi, and to this day there is a difference of opinion on the subject, i.e. which of the lakes from which different branches spring is the true source of the river. The fact that the sources were a factor in the settling of the boundary line of this time made it necessary to have expert testimony on the question such as could be furnished by a survey by Thompson. (2) The surveyor was to go to the Missouri and visit the ancient villages of the natives who dwelt there and who practised agriculture. (3) In the interests of science and history, to inquire for the fossils of large animals, and to search for any monuments that might throw a light on the ancient state of the regions traversed.