[237 3                            HO
AH these circumstances tended, in a singular manner, to contract the ex.
tensive region between the Mississippi and the Missouri ; so that there was
not (so to speak) room for the intermediate territories which 1 had explored.
But my astronomical observations have, throughout, restored order. The
Missouri being thrown over to the west, and the Mississippi to the east,
while, at the same time, the latitudes are generally increased, each interme-
diate country has found its natural place.
In fine, the Mississippi may be said to be now well known. I give, in
the Table of Geographical Positions, already referred to, the numerous po-
sitions by which it is determined ; not that the lower part enters into my
map, but because these positions form a complete system in regard to this
majestic river—one of the most magnificent in the world, and even also
qow one of those whose course is astronomically the best determined from
its embouchures to its very source.
I return now, in relation to the map, to the problem of the longitudes;
which here, as everywhere else on the earth, demands numerous observa-
tions, against which a war-cry will continually be heard among geogra-
phers, as long as in their works of exploration they are restricted to chro-
nometric methods, lunar distances, eclipses, and other phenomena of rare
occurrence, without being provided with the ample means which science
can indicate, but cannot furnish, except when national governments con-
tribute by an allowance of both time and funds. In this aspect, I look upon
it as my duly to expose here the elements which have served in the deter-
mination of the principal longitudes of my map ; so that observers who
may follow me over the ground it covers, may know where to direct their
efforts for improving these first results.
When the calculations of the astronomical observations in which we
were engaged were a little advanced, I detached Lieutenant Frémont to
commence the construction of the map—charging myself with the calcu-
lation of the remaining observations, the long and tedious discussion of the
longitudes, and the computation of the meteorological observations which
I had brought together for determining the altitudes of the stations above
the sea.
It was necessary for us, in the outset, to have some positive meridians
as well known as possible. In this regard, I had been fortunate enough to
have observed some occultations of fixed stars, and also the solar eclipse of
the 18th of September, 1838. The reverend Mr. Verot had the kindness
to make for me the calculation. But at this period I had not been able to
procure the particulars of corresponding observations, nor the errors of the
tables as to the epochs of these phenomena. 1 was obliged, therefore, to
employ the elements for the calculations as they were given in the Nautical
Almanac. However, as the results could not be so much in error as to affect
sensibly the exactness of the construction of the map, I did not hesitate to
adopt them for the concurrent determination of the longitudes, which I
especially wanted.
. At a later period I had the pleasure to make acquaintance with Mr. S-
C. Walker, of Philadelphia, a distinguished astronomer, whose zeal and
industry are efficaciously contributing to the progress of astronomy in
the United States. Mr. Walker had taken the pains to form an extensive
catalogue of all important astronomical phenomena which have been ob-
served at different times in North America. He has progressed far in
the discussion and the reduction of the observations upon occultations