THE ANEROID BAROMETER 



105 



return to the lake and take a second reading, using the 

 average of the two to compare with that observed at the 

 summit. A second, often available in cruising timber, is 

 to read on the same point two or more times during the 

 day and so ascertain the course of the barometer. The 

 third method of correction is by means of another instru- 

 ment which is left at the base station or some other 

 convenient point, and read by another person every hour 

 or half hour while the observer is in the field. Since in 

 ordinary weather the air changes are the same over large 

 areas, this arrangement tells what the field barometer 

 would have read on the base station at any hour during 

 the day. Better than this, however, is a self-recording 

 barometer, or barograph, which makes a continuous record 

 of pressure. The explorer compares his pocket instru- 



AROGRAPH 



ment with this as he starts out on his work, and again 

 when he comes in. If these comparisons are satisfactory, 

 he has the means of telling what his field instrument would 

 have read on the base station at any time while he was 

 gone, and so obtains the correct figure for comparison 

 with any given field observation. This arrangement en- 

 ables him to stay away from known elevations half a day 



