At that time, questions of serious import were agitating the minds of the partners. By the treaty which closed the American Revolutionary War, the boundary between the British dominions and the territories of the United States had been fixed as a line drawn from the northwest corner of the Lake of the Woods to the source of the Mississippi, which was then thought to lie considerably north of its true position. Nine years later, the forty-ninth parallel of latitude had been accepted as the boundary; and by the Treaty of Amity and Commerce subsequently passed, it had been agreed that all British trading posts south of this line should be withdrawn. This made it necessary for the Company to fix at once the location of their houses on the upper Red river, and incidentally if possible to discover the true source of the Mississippi. The partners were also anxious to have a route surveyed to the headwaters of the Missouri river, where there existed the villages of an Indian people who got their living from the cultivation of the soil. The scientific spirit of the merchants was shown by their further desire to search for the fossil bones of prehistoric animals and any monuments which might throw light on the earlier condition of the regions surveyed. Finally, they wished to ascertain the courses of the rivers and the situation of the lakes, in order that they might rearrange their network of trading houses so as to cover more completely the territory under their sway. Thompson's arrival came therefore at an opportune moment. He was promptly engaged as astronomer and surveyor to the company, and the partners and agents all agreed to send orders to their various trading posts that he should be given any assistance he required. In August, the great canoes arrived from Montreal with the goods for the Indian trade. When these were unloaded, the merchandise was assorted and made up in parcels each of ninety pounds' weight, so that the boatmen might be able to shoulder them over the portages. These were then taken in charge by the various agents and their shouting crews of voyageurs. Five days of hard labour were needed for the carry over Grand Portage to the upper waters of Pigeon river. When all was ready, Thompson climbed the long trail from the lake. Armed with his precious instruments, he joined the Swan river brigade of four canoes, and embarked on what was to be the longest, fastest and most brilliant piece of survey in his whole career.