CHAPTER VI. 



COMPUTATION OF ALTITUDES FROM BAROMETRICAL OBSERVATOKS. 



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 J 4. FOB ABNORMAL OSCILLATION. METHOD OF COMPUTATION, WITH REMARKS : 1. ON THE READING OF THE BAROMETER 



AND THERMOMETER AT THE LOWER STATION; 2. ON THE READING OF THE THERMOMETER AT THE UPPER STATION. EXAMPLE. TEST 



OF THE COMPARATIVE ACCURACY OF THB DIFFERENT METHODS OF COMPUTATION, WITH TABLES SHOWING THK RESULTS OBTAINED. 



HEIGHT OF FORT READING. EXPLANATION OF TABLES OF BAROMETRIC OBSERVATIONS IN APPENDIX D, ETC. 



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 be 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 conditiou. 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 

 several 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 four cistern barometers, Nos. 1060, 10G1, 1068, and 108D, 

 made to order, by James Green, of New York, on the same pattern as those used by the Medic tl 

 Department of the army, but with scales graduated for greater altitudes, and with verniers 

 reading to thousandths of an inch. We also had an aneroid barometer, but it proved to be 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 rtading. 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, 



