28 INSTRUCTIONS FOR MAKING FOREST SURVEYS, ETC. 

 ANEROID BAROMETER. 



The pocket aneroid barometer is not a very accurate 

 instrument, but satisfactory results may generally be 

 obtained by using the following method : Two aneroids 

 are necessary. Both should be compared and set at 

 some established elevation, such as a bench mark of the 

 Geological Survey or at a railway station. Any neces- 

 sary correction may be made by sliding the rim or by 

 means of the small screw on the back of the barometer, 

 which will move the hand to the proper reading. After 

 arriving at the camp from which the survey is to be 

 made both aneroids should be read and the readings 

 entered in the notes. One aneroid should be kept in 

 camp while the other is used in the field, and they 

 should be compared twice a day, say, at 7 a. m. and 7 

 p. m. The camp barometer will then show the change 

 in atmospheric pressure from time to time during the 

 survey, and the difference between the two, when the 

 field barometer is being used at a distance, will give 

 the difference in elevation between the camp and the 

 point where the field barometer was read. If the two 

 barometers agree in the morning and do not agree at 

 evening the difference, if material, may be proportioned 

 during the day's notes, assuming the camp barometer 

 to be correct. The scale of "mercury inches," gen- 

 erally graduated on aneroids, is not to be used. If a 

 barometer gets out of order or does not give satisfaction, 

 it should be returned to the property clerk. Do not 

 attempt to repair it nor oil any of its parts. 



