COMPUTATION OF ALTITUDES. 353 
The field-books оҒ the explorations of the main party in 1853, in which were kept the records 
of many important observations, were unfortunately lost on their way from Washington Terri- 
tory to New York, and we are thus deprived of all means of determining, by data actually 
obtained on the survey, the corrections for hourly and abnormal oscillations in the barometric 
column. 
In the absence of hourly corrections the following table, given by Lieutenant H. L. Abbot, 
of the Corps of Topographical Engineers, in volume VI, Pacific Railroad Explorations, p. 715, 
has been used for the observations taken on the trip to and from the Blackfoot council. 
TABLE A. 
Corrections for horary oscillations. 
TW RE ae EN teyr ccs d SA Mis ees xi dani xo 2. + .014 
ж mii oo jet - da sceyecuo wills vh —.032| Әр.ш.......................... + .028 
бөлшеікенлек ла Jill. a daa ЛАП a a ME]. Spinal) pace Rand» diteni) da + .034 
T dio Eno азы» татары GANS —.042 | 4р. т.е... o 4,088 
ғалыс TRAY ЛИ HGR HOST Єр. ауса ROT. O NIE + .029 
9 We EEN TU А. ee 008 6p. т.......................... + .020 
LEY ee С oe err WIS И MIR ESAE TU OT + .013 
HE е E i EME AP е е ТТЕРІ + .001 
DE Ee s er ЕО ОКИ ИОАНН ners dies Re rr + .004 
This table was deduced from a series of hourly observations taken during five days in the 
latter part of August, 1855, near the head of Des Chutes valley, Oregon Territory, at an eleva- 
tion of about four thousand feet above the level of the sea, and a mean temperature of about 
50° Fahrenheit. The corrections for 4 a. m. and 5 a. m. are interpolated. 
The above table was deduced in this manner: The hourly barometric readings being reduced 
to the freezing point,as before explained, a mean was taken of the whole number of observations 
made during the five days, six readings, which were omitted, being interpolated carefully from 
a constructed daily curve; this mean was called the grand mean, and from it were subtracted 
the several means of the observations taken at the same hour during the five days, giving the 
variations of each hour which were minus, or subtractive, when the hourly mean was greater 
than the grand mean, and plus, or addative, when the hourly mean was the less. 
In correcting the observations made between the 11th and 22d of May, 1854, by Mr. Doty, 
with Green’s syphon barometer Number 767, commencing at Fort Benton, thence up the 
Missouri as far as the mouth of the Sun river, to the Rocky mountains and along their eastern 
base as far as Badger river, where the instrument was unfortunately broken, the corrections 
for hourly variations were deduced from a comparison of simultaneous observations made at 
Fort Benton with the table of horary oscillations furnished by Lieutenant Abbot. The hours of 
observation at Fort Benton being at 7 a. m., 2 p. m., and 9 p. m., a mean of the observations 
at these hours, during this period, was taken, and the hourly variations for the hours of 7, 2, 
and 9 determined in the manner before stated, which were compared with the variations for 
the same hours in the table of Lieutenant Abbot, and the intermediate variations interpolated. 
The following is the table: 
45 8 
