108 A MANUAL FOR NORTHERN WOODSMEN 



multiply a above by this percentage. Let us call this b. 

 If 6 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 stall on 

 barometer or the record of a barograph. 



The barograph sheet reproduced herewith shows for 

 the \\orking hours of that Friday a steady fall of pressure. 

 At (5.30 in the morning when the party left camp the 

 indicated pressure was 29.250 inches. When they got in 



