DETERMINATION OF ALTITUDES BY BAROMETER. 
115 
of abnormal error is not balanced. Should one of the hourly observations be wanting on any 
day, a value, interpolated as correctly as possible by comparing the character of the curve 
between the hours preceding and following it on other days, should be substituted. A moment’s 
consideration will show the necessity of this interpolation when there is any abnormal change 
from day to day. Still the great mistake of omitting it has often been made. 
The observations having been prepared as explained above, a mean of all the observations at 
each hour is taken, and a curve plotted to represent these mean results. It should be a smooth 
curve, generally with two maximum and two minimum points in the twenty-four hours, the exact 
times of which vary somewhat. Should this curve not be smooth, some error of observation or 
calculation has been made. It now only remains to find the mean reading for this mean day, 
and to take the difference between it and each mean hourly reading, affecting the result with 
the positive sign when the hourly reading is the less, and with the negative when it is the 
greater. The correction from this table, applied with its sign to an observation taken at any 
hour, eliminates the error due to horary oscillation. 
It may be well to remark, that it is a very good test of the value of a table of horary correc¬ 
tions to apply it to the curve representing observations taken for a few days at a depot camp. 
If a more sweeping line is produced, without a daily recurrence of any peculiar form, the table 
may be considered good for observations taken in the vicinity, where the mean temperature is 
about the same. 
From the observations taken on our survey, the following tables of horary corrections were 
deduced. They proved to be well adapted to the peculiar characteristics of the different tracts 
of country through which we passed. The manner in which they were computed is fully shown 
in Appendix E. 
Corrections for Horary Oscillation. 
Hour. 
Table 1. 
Table 2. 
Table 3. 
Hour. 
Table 1. 
Tabic 2. 
Table 3. 
Inches. 
Inches. 
'■Inches. 
Inches. 
Inches. 
Inches. 
6 a. m._ 
— . 069 
— . 040 
2 p. m__ 
-f- .017 
+ .028 
4- .033 
7 a. m... 
— . 072 
— . 042 
— . 045 
3 p. m__ 
+ .034 
4- .034 
4- .041 
8 a. m._ 
— . 072 
— . 037 
— . 049 
4 p. hi. __-_ 
4- .048 
4- . 033 
043 
9 a. m. ... 
— . 060 
— . 028 
— . 051 
5 p. m__ 
-{- . 061 
-f .029 
4- .038 
10 a. m.. 
— . 050 
— . 016 
— . 043 
6 p. m__ 
4- .064 
4- .020 
4- .026 
11 a. m._ 
— . 032 
— . 007 
— . 022 
7 p. in.. 
4- . 058 
4- .013 
4- on 
12 m . 
— . 018 
-f- .008 
-f .001 
8 p. m.. 
4- .047 
4- .001 
— . 003 
1 p. m__ 
— . 002 
+ .014 
+ .020 
9 p. m__ 
4- .027 
— . 004 
The curves on Plate XIII illustrate these tables. They represent the oscillation of the barometer as twenty times greater 
than it actually is, in order to clearly show its character. 
Table No. 1 was deduced from six days’ observations, taken in the latter part of July, at 
Fort Reading, at an elevation between five and six hundred feet above the level of the sea, 
and a mean temperature of 83° Fahr. The condition above stated was very well satisfied, and 
the table was applied to the observations taken in the Sacramento valley, where the mean tem¬ 
perature was very high. 
Table No. 2 was deduced from five days’ observations, taken in the latter part of August, near 
