94 METHOD OF COMPILING THE MAP. 
1823, and subsequently verified by Mr. Nicollet, (see page 116 of his report,) is about longi- 
tude 93° 05/, and is undoubtedly the best determined point, by astronomical observations, on 
the Upper Mississippi. It would place the fourth principal meridian in longitude 90° 20’, 
which is the position I have assigned it in the State of Wisconsin. The longitude of St. Louis, 
quite well established by the observations of Mr. Nicollet, is about 90° 15’ 10". The fourth 
meridian is about twelve miles to the westward, making it here in longitude about 90° 30’. 
If both of these are correct, an error of about one degree deviation to the east was made in 
running this meridian—an error which I felt obliged to assume in locating it. 
The fifth principal meridian passes through the mouth of the Arkansas river, and in Missouri 
lies about forty miles west of St. Louis. Its longitude, referred to this last point, is therefore 
about 90° 58’. It was made to conform, in direction, to the sixth principal meridian. 
The siath principal meridian, in Missouri, lies about 114 miles west of St. Louis, making its 
longitude there 92° 13’. This meridian, in Louisiana, lies about 146 miles west of New 
Orleans, whose longitude (Nicollect’s Report, page 121) is 89° 59’. It is therefore in this 
latitude, in about longitude 92° 23’. But the longitude of Nut cape, at the mouth of the 
Sabine, as determined by Major J. D. Graham, Topographical Engineers, is 95° 50’ 15". 
This point is about ninety miles west of the sixth principal meridian, which would therefore 
place the latter in longitude 92° 20’. The difference of longitude of this meridian, as referred 
to New Orleans and Cape Nut, amounts to 3’, which is probably within the limit of error in 
these astronomical results. I have therefore given the sixth principal meridian, in Louisiana, 
the position obtained from a mean of these two references, that is, in longitude 92° 21’ 30", 
Here, therefore, with the sixth principal meridian, as with the fourth, we find a difference in 
the longitude of its northern and southern extremities of 8' 30", requiring a deviation from 
the true meridian of 1? to the east.* . 
The positions of the fourth and sixth principal meridians having boen fixed thus, the surveys 
made with reference to them established the longitudes approximately of all points of the land 
office surveys from Lake Superior to the Gulf of Mexico; of the whole western frontier; of the 
west boundaries of the States; and of the starting points of all the expeditions from that region. 
Previous astronomical determinations placed these points from 12’ to 15’ too far to the east. 
The adoption of the determinations of positions for the mouth of the Minnesota river, for St. 
Louis, for New Orleans, and for Cape Nut, to the exclusion of all others, was not made without 
careful investigation and comparisons. ۱ 
Mr. Warner Lewis, surveyor general of Iowa, furnished me with several measurements along 
the base lines in that State, and the gentlemen in the General Land Office in Washington 
afforded me every facility for investigation and comparison. 
The longitudes of places thus determined on the general map will, m not be found i in 
error by more than 5 of arc.t All astronomical determinations of latitude were used, when 
carefully made, and they generally agreed with the land surveys. The eastern portion of the 
© The commission appointed to run the west boundary of Arkansas south of Fort Smith have found the north end of the 
land survey meridian inclining to the east, as I had been obliged to represent it on the map. 
t A discussion of the difference of longitude, as determined by the land surveys and by the astronomical observations, was 
made by Mr. Charles Whittlesey and published with the report of Foster and Whitney on the geology of the Lake Superior 
district, —(Senate Executive Doc. No. 4, special session, March, 1851.) Mr. Whittlesey did not, in this discussion, use the 
astronomical determination of the mouth of the Minnesota river, which I consider the best one in the region of the Upper 
Mississippi. Probably the land surveys had not then progressed sufficiently to enable him to compare its results with the 
other at this point. 
