COMPUTATION OF ALTITUDES. 357 
The nineteenth station on the Coeur d'Aléne river, eleven miles above the Mission, was 
computed from two observations, taken at sunset and sunrise. Five other observations were 
also made at this station, but as they appeared to be much affected by abnormal oscillations, 
they were rejected, and the altitude computed from the first two observations. 
Station No. 26, the last crossing of the Coeur d'Aléne river before crossing the mountain, 
was made on the eastward trip, and a single observation taken at 2} p. m., which gives for the 
altitude 3,617 feet. This has been assumed as the best result which could be obtained; for 
though the party continued at the same place on the return trip and made both an evening and 
morning observation, the result was considered as only useful for determining the differences of 
altitude between this point and five other points higher up the stream, observed on the day 
following the evening we reached the crossing, and which are, respectively, I, 3, 4, 1, 1 mile 
from each other. 
The heights of the crossing and the several points of observation on the return trip are 
3,573.7, 3,518, 3,643.7, 3,709.6, 3,810.7, and 3,834.4; giving these differences: 4.3, 70, 135.9, 
231, and 260.7, between the points 1, 2, 3, 4, and 5, and the crossing; which, added to the 
adopted altitude of the crossing, 3,617, give for the heights of these points 3,621.3, 3,687, 
3,752.9, 3,854, and 3,877.7. 
At the camp of the 5th and 6th of July, on the Bitter Root river, there was a great rise of 
the barometer from evening to morning, the height of the column at sundown, when reduced 
to the freezing point, being 26.874, and at sunrise 27.090. The two observations have been 
combined, and the result thus deduced has been assumed as the altitude of the camp above the 
sea. Ап observation was taken near the mouth of the St. Regis de Borgia river half an hour after 
the morning observation at camp, which was computed, and the result compared with the alti- 
tude founded upon the morning observation at the camp. Though there was a great change 
from evening to morning, the change for half an hour would be inconsiderable; the difference 
between the two latter computations has, therefore, been taken as the absolute difference 
between the altitudes of the two points, and it has been subtracted from the result obtained by 
combining both observations at the camp, and considered as the absolute altitude of the mouth 
of the St. Regis de Borgia. 
The altitudes of the stations between 103 and 114, inclusive, were determined by a comparison 
with simultaneous observations made with instrument No. 769 stationed on Teton river, two 
miles west from Fort Benton; station No. 102 was compared with an observation made 14 miles 
m., where the camp was temporarily located. After taking the observa- 
up the Teton at 13 p. ontinued during 
tion the camp was moved to about two miles west of Fort Benton, where it c i | 
the remainder of this portion of the survey. The method of making the calculation with 
simultaneous observations is given and explained in the prefatory remarks to Guyot's tables. 
The only difference between the computation of this and the Biber portion of the trip, being 
that the reading of the barometer at the Teton, during the period of honê observations, was 
taken at the lower station, instead of the mean reading of the barometer at Suisun bay, (30.057,) 
which was taken at the lower station in computing the previous observations, as before 
explained. 
^: observations made by Mr. Doty between the 11th and 22d of Мау, 1854, from Fort 
Benton up the Missouri as far as the great falls and mouth of Sun river, thence up Sun river to 
the Rocky mountains, and along their eastern base to Badger river, were reduced to the неа. 
point, and corrected for horary oscillations by means of table B and table E, applied for 
