72 EXPLANATIONS OF BAROMETRIC COMPUTATIONS. 
construction is thus avoided, and the greater advantage gained of combining all the observations 
at a station in a correct mean reading, to be used in a single computation of the altitudes. 
The mean of the observed air temperature is used in these cases also, as avoiding to some 
extent a source of error in extremes of surface temperature, for which, in single observations, a 
table of corrections is appended. All the observations were also corrected for horary variations 
of atmospheric pressure through the day, thus bringing each to the true mean position for the 
day. The accompanying scale of horary corrections gives the value employed for each hour ; 
they are derived primarily from well-determined curves of daily variations of pressure for the 
eastern United States, but with material and important modifications and additions established 
by the observations of other surveys in the interior of the continent, principally by that of Lieut. 
Whipple. By the observations through the winter months at Great Salt Lake City, the 
measures of this horary scale are shown to be less for that season, and to conform these more 
nearly than in summer to those observed in the eastern United States and in Europe. For the 
months occupied in the field-work of this survey, however, and for the districts traversed, the 
measure of the correction here employed is fully confirmed. At the sea-level, or so near it as 
both extremities of the line are, the measures of horary variation again fall off to those belong- 
ing to well known districts; yet as no determinations of importance occur at these extremities, 
it is not necessary to give the scale belonging to them. A correction previously found to be 
required for extremes of air temperature has been so well determined by the comparison of survey 
by levels and with the barometer, at the passes of the Sierra Nevada, surveyed by Lieut. 
Williamson, that a scale of corrections sufficiently precise for practical use has been constructed. 
When the error from this cause could not be eliminated by the use of mean temperatures, this 
scale has been employed in the determinations here made. The measures given for this cor- 
rection belong to extremely arid climates, and to elevated districts, requiring modification in 
the position of the point where no correction is required; also in different seasons. As it affects 
great elevations in these arid districts by an extreme amount, of at least 150 feet, it is too im- 
portant to be neglected, notwithstanding a discretionary use of the value is usually necessary. 
The reduced observations at stations.on the Plains, from Pawnee fork to camp 33, the first 
after crossing the Arkansas river, were referred to the mean barometric*readings noted by Dr. 
Engelmann at St. Louis, for July, 1853, the month in which they were made. 
The altitude of this station above the Gulf of Mexico, as determined by him from a long series 
of observations, was added to make up the entire altitude. For these stations and dates, the 
results thus obtained are very nearly identical with those computed by direct reference to the 
barometric mean at the level of the sea for the latitude. 
For altitudes beyond this point, direct comparison of each camp is made to an assumed mean 
barometrie reading at the level of the sea, in this latitude, of 30.000 inches, the barometer cor- 
rected to 32°, and a mean air temperature taken of 57°. The constant belonging to the latitude 
and climate of the Gulf is 30.050 inches of the barometer, and 64? of air temperature, which 
would add unduly to the altitudes. 
The principle is assumed that the constants of pressure and temperature employed belong to 
the latitude, and that the resulting determinations of elevations belong, correctly and alike, to 
both the Gulf, the Atlantie and Pacific. "There are no well-determined mean readings of the 
barometer on the Pacific coast, yet the most of those recently made in California give the 
impression that a slightly greater mean atmospheric pressure exists there than in the same 
latitude of the Atlantic. The constant has not, however, been altered for any portion of 
this line. 
The discussion of observations at the principal passes has been, in part, upon simultaneous 
Observations at an hour's interval in time and distance. The slower movement of one baro- 
meter, miscner, rendered it necessary in many cases to take successive readings of the best one, 
corrected for horary variation, for determinations of successive differences. The termini of these 
lines of ascent an descent were also checked by comparison of preferred results, as of the mean 
