256 INSTRUMENTAL ERRORS AND HORARY CORRECTIONS. 
Cistern barometer, No. 790 十 .019 
Syphon barometer, No. 781 一 .031 
Do do 
783 —.024 
Do do 784 —.024 
Do do 785 —.012 
Comparison was made with the barometer of Dr. Ray, at Cincinnati, giving the following 
result : 
Dr. Ray's cistern, 0.000 
Cistern No. 790 十 .026 
Syphon No. 781 — .024 
Do 783 一 7ء‎ 
Do 784 一 .017 
This comparison confirms the corrections determined by mutual comparison of the instruments 
of the survey. 
Next to this verification of instruments, it became necessary to determine the constants proper 
to be applied in prosecution of the detail of the work. For this purpose there are no precedents 
in barometric determinations of heights in lines of such extent of continuous and connected 
survey, and for distances so great from the initial point. Such lines differ widely from isolated 
and scattered determinations ; and observations so made, require the correction of all the varia- 
tions of barometric reading belonging to constant or regular changes of pressure, at least, and as 
. much of correction for irregular or non-periodic alternations as may be applied. Of the known 
constant corrections, also, the elements are stil! imperfect. There is strong reason to suspect 
the accuracy of that for temperature of the air, particularly when applied to a line in which 
great changes of surface temperature occur. It is certain, indeed, that these corrections will 
not apply in case of considerable changes of surface temperature at places having nearly the 
same elevation ; and in the present case, the errors arising in this way were avoided by the use, 
as much as possible, of mean results from many observations in fixing the elevation of every 
camp or important point in the line from Fort Smith to Albuquerque. 
The horary variations of pressure are another constant too important to be neglected. For 
these a scale was first applied, deduced from the determinations at Washington and Philadelphia, 
modified in the extremes of its range to conform to the less considerable changes shown by 
recent observations to exist in the daily curve of atmospheric pressure at the coast of the Gulf of 
Mexico. This low scale of horary correction was applied to all the observations from Napoleon, 
Arkansas, to Camp No. 32, near the Washita river—thus bringing each observation to the mean 
pressure for the day. The signs, as given in the table, represent the relation of the several 
hours to the true daily mean of pressure, and are corrections with the opposite sign.* 
This degree of correction is not thought sufficient for the comparatively arid climates of the 
open plains, and is therefore carried only to Camp No. 32. 
Horary corrections used from e to Camp No. 32. 
4 a. m +.008 3 p.m +.006 
5 a. m +.006 4 p.m +.015 
6 a. m —.002 5 p.m 十 .020 
Ta. m 一 .006 6 p.m 十 .018 
8 a. m 一 .014 7p.m 十 .012 
9 a. m 一 .018 8 p. m. 十 .008 
10 a. m —.020 E یی سم مس ہپ کا‎ s 十 .005 
lla. m 一 .018 10 p. m 一 .005 
12m . 一 .010 11 p. m 一 .005 
1p.m —.006 HEB یئٰٰٰٰٰٰ‎ 87٤ 一 .000 
2p.m 十 .003 
The signs have since been changed to represent the corrections as they should be applied to the observations. 
