Reports of explorations and surveys, to ascertain the most practicable and economical route for a railroad from the Mississippi River to the Pacific Ocean: Vol. 2, Pt. 4

REPORT.
CHIIAPTER I.
Methods pursued in determining tprethe Data upon whh are ase the aps and Reports of
the Survey.
To enable the department to judge advisedly of the amount of confidence due to the results
of an exploration of this character, it would seem proper to set forth the organization of the
exploring party, and to describe in full the instruments used and the methods pursued in
determining the data upon which these results are based.
As the object of this exploration was eminently practical in its character, and had in view
the investigation of a specific question, I have thought it advisable, in determining upon a plan
for the report, to give the-prominent place to the practical results which have a direct bearing
upon the construction of the railroad, and to set them forth under proper heads, as briefly as
is consistent with clearness. For this purpose I have carefully avoided embarrassing the
subject with a narrative of the daily incidents of the expedition, which must be more or less
irrelevant, or involving it in obscurit by the introduction of detailed descriptions, under scientific and technical names, of the specimens collected in the various departments of science. I
have only introduced into the reports such extracts from these subjects as are necessary to
illustrate some point having an immediate and important bearing upon the question of the
railroad, and have collected into an appendix the diary of the expedition and the reports upon
the geology, botany, and natural history of the route.
Met[hod of determni ng the geographical position of the route.-Of this duty I took charge
myself, and the following plan was adopted: Seven principal points were selected along the
line, as nearly at equal intervals as it was possible to place them, and at each of those points
the latitude and longitude were carefully and absolutely determined, by a complete series of astronomical observations. The sextant was the only instrument I had for this purpose, and it was
therefore necessary to determine the longitude by the method of "lunar distances." At least
seventy lunar distances of the sun, and of stars east and west of the moon, were observed at
each principal point; and the latitude was determined in all cases by at least one hundred and
fifty altitudes of stars north and south of the zenith. At each of these points the chronometer
was carefully rated by observing equal altitudes of the sun, and altitudes of east and west stars,
for several successive days. Twenty intermediate points were determined along the line by at
least sixty altitudes of north and south stars for latitude, and twenty altitudes of east and west
stars for the error of the chronometer. As the rate of the chronometer was carefully determined at each principal point, and the chronometric longitudes of intermediate places were
referred directly and at short intervals to the points thus absolutely determined, the time observations may be considered as furnishing at least very close approximations. Twenty-seven
points along the route, at intervals not to exceed twenty-five miles, were thus astronomically
determined; and as the observations exhibit no error on their face, and the results determined
by the computations of several able and experienced computers are in all respects satisfactory,
the position of the line of survey may be considered fixed with some considerable degree of
le