CHAPTER YI. 
COMPUTATION OP ALTITUDES PROM BAROMETRICAL OBSERVATONS. 
Preliminary remarks.—Instruments.—Instrumental errors.—Interpolation, and approximate test of accuracy in observer.— 
Corrections preparatory to computation : 1. For temperature of mercury; 2. For instrumental errors; 3. For horary 
oscillation ; 4. For abnormal oscillation.—Method op computation, with remarks : 1. On the reading op the barometer 
AND THERMOMETER AT THE LOWER STATION; 2. On THE READING OP THE THERMOMETER AT THE UPPER STATION.—EXAMPLE.—TEST 
OF THE COMPARATIVE ACCURACY OP THE DIFFERENT METHODS OP COMPUTATION, WITH TABLES SHOWING THE RESULTS OBTAINED.- 
Height of Fort Reading.—Explanation of tables op barometric observations in Appendix D, eto. 
PRELIMINARY REMARKS. 
To insure accuracy in the old method of determining altitudes by the barometer, it is theo¬ 
retically necessary that the observations at the upper and lower stations should he simultaneous. 
In obtaining the data for constructing the extended barometric profiles of the recent Pacific 
railroad surveys, many causes have rendered it impossible to comply, even approximately, with 
this condition. A new method of computation, based upon different principles, has therefore 
been required. Successive improvements have been introduced in computing the altitudes de¬ 
termined on the different surveys, until this object has been, in part at least, attained. Although 
seveial references to the subject have been made in the reports, the new system has never, to 
my knowledge, been published in a form sufficiently detailed for practical use. Partly to supply 
this deficiency, and partly to explain my reasons for believing that certain other slight changes 
in the old system are advisable, I have decided to describe in full the method used in reducing 
the field notes of our survey. 
INSTRUMENTS. 
On starting from Benicia we had tour cistern barometers, Nos. 1060, 1061, 1068, and 1089, 
made to order, by James Green, of New York, on the same pattern as those used by the Medic il 
Department of the army, hut with scales graduated for greater altitudes, and with verniers 
reading to thousandths of an inch. We also had an aneroid barometer, hut it proved to he so 
inferior an instrument that the few observations taken with it were rejected. We were likewise 
provided with four extra unfilled glass tubes. 
The barometers proved to be admirably adapted to mountain work ; but they had three defects, 
which gave us no little trouble. There were no portable tripods connected with them, which 
made it very inconvenient to take observations when there were no trees near the trail. Their 
verniers did not read higher than five hundredths of an inch, which rendered it necessary to 
look at both the scale and the vernier, and often to perform additions to determine the hundredths 
of the reading. This is very objectionable, as it renders mistakes almost inevitable, when the 
observations are taken during the hurry of the march. Lastly, the small pieces of wood to 
which the ivory points and the glass tubes were attached, were a little too large, and, in two 
cases, expanding from moisture while the glass cistern contracted from cold, actually cracked it, 
