DETERMINATION OF ALTITUDES BY BAROMETER. 
121 
TEST OF THE COMPARATIVE ACCURACY OF THE DIFFERENT METHODS OF COMPUTATION. 
On a former survey in Southern California, Lieutenant Williamson surveyed two passes, the 
Tejon and the Canada de las Uvas, with a spirit level and a barometer, for the purpose of testing 
the latter instrument. It was from these observations that Mr. Blodget deduced his empirical 
table referred to above. In accordance with Lieutenant Williamson’s request, I took the original 
notes of the Canada de las Uvas survey, which he considered rather more accurate than the 
other, and carefully tested by them these various methods of computing altitudes from barometric 
observations. Before referring to the results obtained, a slight description of the pass will be 
given. 
The Canada de las Uvas is a pass from the Tulare valley, in southern California, through the 
Sierra Nevada to the basin east of the range. A better place for experimenting with the 
barometer could not have been found, had this been toe sole object of the survey. The stationary 
barometer was observed in a brush hut, at a depot camp situated near the head of the wide and 
open Tulare valley, at an elevation of 1,447 feet above the level of mean tide at Suisun bay, 
near Benicia, and distant 12.8 miles from the entrance of the pass. For about six miles this 
.pass is a narrow gorge, bordered by ridges several hundred feet in height. It then becomes an 
open valley, from half to three-quarters of a mile in width. This character is preserved nearly 
to the first summit, a distance of about 5.5 miles. The road then crosses several branches of 
the Santa Clara, a river discharging into the Pacific, gains the summit of a second divide, 
and descends to the basin, which is elevated about 1,500 feet above the head of the Tulare valley. 
From the first summit to the basin, a distance of about 12 miles, the trail is bordered by low 
rolling hills ; but the high ridges of the Sierra Nevada intervene between it and the Tulare 
valley. 
By this description it will be seen that some of these test observations were taken in a narrow 
gorge, others in a wide valley, and others in an open undulating country, separated by a high 
range of mountains from the stationary barometer. The altitudes of the stations varied from 
192 feet to 2,809 feet above Depot camp ; and their distances, from 12.8 miles to 36.4 miles from 
the same place. A greater diversity in their positions could not have been desired. 
The observations in the pass were taken with one of Green’s cistern barometers, similar to 
those used on our survey. At the depot camp, another barometer of the same kind was used, 
together with two syphon barometers, which, although greatly inferior instruments, furnished 
a useful check upon errors of observation. 
In making the test computations, I prepared the table of abnormal corrections from the 
observations at the depot camp. The table of horary corrections used was the one already 
mentioned, deduced from Lieutenant Parke’s observations on his recent survey in the vicinity. 
The altitudes were first computed by the old method, with Lee’s tables, using the carefully 
interpolated simultaneous readings at the depot camp for the barometric and thermometric read¬ 
ings at the lower station. To prevent the slight error arising from taking the ratio between 
two equally erroneous barometric readings, the corrections for horary and abnormal error were 
applied to the observations at both stations. The altitudes were then computed by the new 
method, first using the observed, and then the mean daily air temperatures. As the results 
were the heights of the stations above mean tide at Suisun bay, near Benicia, the altitude of the 
depot camp above that level was subtracted from each in order to institute a comparison between 
them and those determined by the spirit level and by the old method of computation ; both of 
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