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 



