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¬ 
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 
