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 the 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- 

 6000 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. 



