104 A MANUAL FOR NORTHERN WOODSMEN 
face is graduated to correspond with the height of column of 
a mercurial barometer, 30, 29, 28, etc., inches, these even 
inches being divided into fractional parts. 
This change in pressure corresponds with definite change 
in altitude. One inch on the scale means roughly 900 feet 
in altitude; a half inch means 450 feet, and so on. As 
a matter of fact, there is a foot scale on most aneroids 
outside the inch scale, movable and graduated from zero 
up to the capacity of the instrument. Thus, if one knows 
how high he is above sea level, he may turn the foot scale 
of his instrument until the registering hand points to that 
height, and, going either up or down hill, read directly the 
elevation of any station which he may occupy. 
Just this process answers many purposes, but when best 
results are sought for, the operation is not quite so simple. 
First, there is the Correction for the Temperature of the 
Air. An inch difference in pressure at a temperature of 
32°, for instance, converted into height, means one thing; 
at 70° it means a good deal more. In order to get accu¬ 
rate results, therefore, on considerable elevations, it is 
necessary to read the inner or inch scale of the instrument, 
take the temperature of the air at the two points, and 
obtain the elevation from tables. Such tables will be 
found on pages 111 and 112 and full directions for their 
use accompany them. 
Correction for Weather Change. The other liability to 
error arises from the fact that the air pressure is frequently 
changing with the weather. This does not hamper work 
seriously in the western country where the weather and 
pressure remain steady for long periods at a time, but diffi¬ 
culty does arise from this source throughout the East. 
With an approaching storm the air grows lighter, and the 
reverse in clearing weather. This effect is best seen on a 
stationary barometer, but it has a like effect on one that 
is in motion. Thus, if an explorer starts at a lake of known 
elevation and takes two hours in going to the top of a hill, 
the air pressure meanwhile may have changed so as to 
throw his height readings off materially. 
There are three ways of obviating this, outside the evi¬ 
dent one of working only in steady weather. One is to 
