HORARY AND AIR TEMPERATURE CORRECTIONS. 91 
mean of several at the summit, or elsewhere, with the nearer camps. The coincidence of results, 
by single and by successive steps, has been so satisfactory as to warrant the conclusion that 
the grades and altitudes of these passes are quite accurately determined. The correct use of 
such data, in the joining of intermediate with main lines, and in the correspondence of single 
observations with the means of observation, is the severest test of barometric survey; and dis- 
crepancies cannot be wholly removed. 
'The principal cause of these discrepancies is in the non-periodic variation of atmospheric pres- 
sure, for which no constant or correction can be given except by reference to continued readings 
for a month or more at some station near. Mean results best eliminate this error, and they are 
therefore preferred in the order of their number, or of the number of days they cover. In the 
ascent of the Arkansas, the uniform grade of the stream permits the use of four or five successive 
camps as a mean result, and two or three non-periodic variations are thus eliminated. Subse- 
quently, to Great Salt lake, a less error from this cause is likely to occur, as this variation is 
least in August and September. 
The observations generally sustain the checking and criticism which rank the results as a 
determinate survey, with a near approximation to absolute accuracy, and, as among themselves, 
sufficiently conclusive of grades and points of comparison. 
_ Horary corrections of barometer. 
Hour. Inches. Hour. Inches. 
6 a.m -+. 007 3 p.m —. 030 
7 a.m +.020 | 4p.m —. 045 
8 a. m +.030 || 5p.m —. 050 
m +.040 | 6p.m —. 030 
10 a. m +. 050 7 p.m —. 020 
lla.m +.055 | 8p.m | | —.005 
12 +.025 | 9p.m |: 24000 
1p.m +.005 | 10 p.m | 4. 010 
2 p.m —, 015 1l p.m | +. 
Scale of corrections for extreme air temperatures. 
Low temperatures. High temperatures. 
At 35° add 25° At 95° subtract 15° 
At 40° add 23° i At 93° subtract 13° 
At 459 add 219 At 909 subtract 119 
A At 
At 509 add 1 At 85° subtract 8° 
At 53° add 15° At 83° subtract 7° 
At 55° add 13 At 
At 57° add 10° At 78° subtract. 3° 
add 5° At 75° subtract 2° (See note.) 
Nore.—The measures of this correction are variable, to some extent, with the season, and apply nearly to departures from 
the mean of the month in which observations are made. It is here given as required for summer months, in which most ef 
the observations in field surveys have been taken. 
