108 A MANUAL FOR NORTHERN WOODSMEN 
multiply a above by this percentage. Let us call this b. 
If b has a plus sign, add it to a; if a minus sign, subtract 
from a. The result will be the desired elevation. 
Example. — The barometric reading on a lake of known 
elevation is 29.500 inches, and the temperature there 72° F. 
Shortly after, the reading on a hill not far away is found to 
be 28.760 and the temperature 63°. How high is the 
hilltop above the lake ? 
From Table I we have 
Barometric elevation of hill 1150 feet 
Barometric elevation of lake 458 feet 
Difference (a above) 692 feet 
From Table II we have for t+ t' = 135°, C = + .042. 
b therefore = 692 X .042, is = 29 feet. This must be 
added to a, since the sign of the factor is +, and the 
result (692 + 29= 721) gives 721 feet as the required 
answer. 
A short cut to the same result, which is accurate enough 
and which will save much labor in reducing a number of 
readings referred to the same base station, is as follows: 
Between 29.500 and 28.760 inches the difference of eleva¬ 
tion corresponding to .1 inch pressure is 94 feet. This 
is obtained instantly by inspection of column 3 of Table 
I. Stated another way, the difference of elevation in feet 
is 6 per cent less than the difference between barometric 
readings expressed in thousandths of an inch. But the 
temperature correction for the conditions is + 4 per cent, 
leaving a net loss of 2 per cent on the difference in the 
barometric readings. 
Now 29.500- 28.760= .740, and 740- 2 per cent = 
725. Answer, 725 feet. 
Problem II. — To correct for changes of pressure due 
to the weather , as shown by regular readings on a station 
barometer or the record of a barograph. 
The barograph sheet reproduced herewith show T s for 
the working hours of that Friday a steady fall of pressure. 
At 6.30 in the morning when the party left camp the 
indicated pressure was 29.250 inches. When they got in 
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