356 COMPUTATION OF ALTITUDES. 
Having thus explained the various corrections which were applied to the observations, we 
will now consider the methods which were used in computing the altitudes as derived from the 
observations. The observations taken on the trip to and from the Blackfoot council were / 
calculated by using Guyot’s tables, before referred to. The mean reading of the barometer at 
the level of mean tide at Suisun bay, near Benicia, was uniformly assumed for the reading at 
the lower station, which, from an extensive series of observations, was found to be thirty thou- 
sand and fifty-seven inches, the temperature of the mercury being 32° Fahrenheit, and that of 
the air 64° Fahrenheit; for the upper station, the mean air temperature for the time during the 
survey has been used, instead of the observed air temperature. The following is an example 
of the method of computation: 
Data for determining the altitude of Station No. 19, on the trip to the Blackfoot council on the Coeur 
d" Aléne river, eleven miles above the Mission, from two observations. 
Date. Hour. No. of ba- | Reading of Attached Detached 
rometer. barometer. | thermometer. | thermometer. 
June 26, 1855........| Sunset ..... 1006 27. 464 53 53 
June 27, 1856........| Sunrise..... 1006 21.434 53 53 
Corrections applied before computation. 
Sunset, June 26 | Sunrise, June 27. 
Barometric reading 27. 464 21. 434 
Correction for reduction to 329 Fahrenheit — .060 — .060 
Correction for index error + .025 + .025 
Correction for horary oscillations, (Table А) ......... + .003 — .022 
Corrected readings 27. 432 27. 377 
Computation by Guyot’ s tables. 
Н == 380.067 H = 32405 Tas 64. Т!->53 
Table I gives for Н--290.057......... css Fee ае Ves eves os 28725.4 
Table I gives for Н”--27,405........................... 26339.1 
Approximate dilterence of level D. ...... wv 2386.3 
D x FR ETB 3306.8 x 58 2 = 
900” E чымы Pe ийе 
Second approximate difference, )”................. 9596.7 
Table ІП gives for D’ = 2526.7, latitude 47° 30.......... — .6 
Table IV gives for D’= 2526.7 ...... ....... 522242. €. 4-66 
Altitude of station, in Гееі........................ 2532.7 
