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The geologist now takes the level sheet from the instrument man 

 and by use of the aneroid barometer carries the elevation along the 

 outcrop of rock strata. For the results obtained with the barometer 

 to be of any value care should be taken that the barometer is checked 

 frequently. 



The method ordinarily used is to set the aneroid barometer at same 

 elevation as bench mark from which start is made also noting time 

 barometer was set, which is essential. Whenever an elevation reading 

 is made on the ovitcrop the time of reading should be noted. The 

 barometer must be checked at a known elevation every forty or forty- 

 five minutes and should not be more than an hour between checks for 

 accurate results. The barometer must not be changed after being set 

 at first station in the morning. 



At night, plot a curve showing amount of variation of barometer 

 from normal during day. By means of the curve correct all readings 

 for elevations made during the day by adding or subtracting the differ- 

 ence from normal, to the reading to be corrected. 



Example: Suppose correction curve shows aneroid barometer was 

 reading 22 feet high at 10:15 and elevation reading on outcrop was 

 953 feet at same time. To get correct elevation of that point subtract 

 22 feet from 953 feet which gives 931 feet, the correct elevation. If 

 aneroid barometer was reading low at 10:15 the 22 feet should be added 

 to give correct elevation which would be 975 feet, etc. 



While geologist is walking the outcrop, he should sketch the drain- 

 age, roads, trend of outcrop of rock strata and other features necessary 

 to make a complete geological map. 



After making correction of barometer readings the day's work 

 should be inked as finished, so that the work will not be lost by erasure 

 during work the next day. The inking should be up to date at all 

 times. 



The aneroid barometer most commonly used is 2^^ inches in diam- 

 eter graduated to record elevation of 3,000 feet with 10 feet divisions. 

 Frequently larger instruments are used, some as much as 6 inches in 

 diameter. The larger aneroids are the more accurate. 



