110 A MANUAL FOR NORTHERN WOODSMEN 
barograph sheet a curve through these two new points 
and parallel to the one made by the barograph pen. 
From this curve he may take off th,e reading for any hour 
in the day to compare with a field reading taken at the 
same time. Such a supplementing curve is shown on the 
sheet illustrated. 
Example. — At 11 a. m. on the day in question at a 
point two miles away from camp the field barometer 
read 29.270. What was the elevation relative to the base 
station ? 
The field reading can not be compared with the morning 
reading at camp because the barometric pressure is known 
to have been changing. Neither can it be compared with 
the night reading, for the same reason. The short curve 
on the sheet, however, does tell what the field instrument 
would presumably have read at camp at any hour in the 
day. The curve at 11 a. m. is at 29.270, and the two points, 
therefore, are of equal elevation. 
In view of the low accuracy of aneroid work, different 
users of the instrument have devised schemes for shorten¬ 
ing or obviating the labor of computation. One that is 
serviceable where temperature at different seasons shows 
wide variation is as follows: 
On the foot scale of most instruments 1000 feet at the 
higher elevations will be found to occupy a smaller sector 
on the scale than 1000 feet at low elevations — as 5000- 
0000 as against 0-1000. This can be tested by comparing 
against identical marks on the inner scale. 
Now, being at a known or assumed elevation, set the 
corresponding graduation against the movable hand and 
observe where the thousand-foot marks above and below 
cut the inner or inch scale; next, take the values so ob¬ 
tained and compute difference of elevation accurately, 
correcting for temperature. If the result obtained varies 
seriously from 1000 feet, shift the foot scale by even 
thousands until a portion is found so graduated that it 
does correspond. With a constant correction of even 
thousands, elevations may now be had directly. Correc¬ 
tion is not thus made for weather changes, however. 
