Thompson now determined to survey what had been an object of much interest, the lake which was the source of the great River Mississippi. To do this had been laid upon him in his instructions from the North-West Company. Making a détour from Grand Forks, in order to avoid the ice on the Red Lake River, he struck the upper waters of that river, and followed the banks until he reached Red Lake in what is now North-Eastern Minnesota. Leaving this lake, he made a portage of six miles to Turtle Lake, and four days later reached the point considered by him to be the source of the Mississippi. Turtle Lake, at the time of the treaty of 1783, was supposed to be further north than the north-west angle of the Lake of the Woods. This arose, Thompson tells us, from the voyageurs counting a pipe to a league, at the end of which time it was the fur-traders' custom to take a rest. Each pipe, that is, the length of time taken to smoke a pipe, however, was nearer two miles than three, so that the head waters of the Mississippi had been counted 128 miles further north than Thompson found them to be. It is to be noted, however, that the Astronomer Thompson was wrong in making Turtle Lake the source of the Mississippi. The accredited source of the Mississippi was discovered, as we shall afterwards see, in July, 1832, to be Lake Itasca, which lies about half a degree south-west of Turtle Lake. Thompson next visited Red Cedar Lake, in the direction of Lake Superior. Here he found a North-West trading house, Upper Red Cedar House, under the command of a partner, John Sayer, whose half-blood son afterward figured in Red River history. He found that Sayer and his men passed the winter on wild rice and maple sugar as their only food. Crossing over to Sand Lake River, Mr. Thompson found a small post of the North-West Company, and, descending this stream, came to Sand Lake. By portage, reaching a small stream, a tributary of St. Louis River, he soon arrived at that river itself, with its rapids and dalles, and at length reached the North-West trading post near the mouth of the river, where it joined the Fond du Lac. Having come to Lake Superior, the party could only obtain a dilapidated northern canoe, but with care it brought them, after making an enormous circuit and accomplishing feats involving great daring and supreme hardship, along the north shore of the lake to Grand Portage. On hearing his report of two years' work, the partners, at the annual meeting at Grand Portage, found they had made no mistake in their appointment, and gave him the highest praise.