Xliv INTRODUCTION. 



midity is observed, the correction may be computed if desired, by the ex- 

 pression 



(0.378 1) 



where e is the mean pressure of vapor in the air column, and b the mean 

 barometric pressure. 



The side argument is the mean temperature of the air column, varying 

 by intervals of 2 from — 20 F. to 96 F., except near the extremities of the 

 table where the interval is 4 . The top argument is the approximate differ- 

 ence of altitude Z. 



Table 55 gives the correction for the variation of gravity with altitude 

 in its effect on the weight of the air. The side argument is the approximate 

 difference of altitude Z, and the top argument is the elevation of the lower 

 station h . 



The corrections given by Tables 53, 54, and 55 are all additive. 



Example: 



Let the barometric pressure observed, and corrected for temperature, 

 at the upper and lower stations be, respectively, B =23.61 and 

 B Q = 29.97. Let the mean temperature of the air column be 35 

 F., and the latitude 44 16'. To determine the difference of height. 



Table 51, argument 23.61, gives 

 Table 51, " 29.97, " 



Approximate difference of height (Z) 

 Table 52, with Z = 6484 and 6 = 35 F., gives 

 Table 53, with Z = 6300 and $ = 44 , gives 

 Table 54, with Z = 6300 and 6 = 35 F., gives 

 Table 55, with Z = 6300 and h Q = o, gives 



Final difference of height (Z) = 6320 



If in this example the barometric readings be observed with aneroid 

 barometers, the correction to be obtained from Table 53 will be simply the 

 portion due to the latitude factor, and this will be obtained by subtracting 

 the tabular value for 45 29' from that for 44 , the top argument being 

 Z = 6300. This gives 16 — 15 = 1. 



Tables 56, 57, 58, 59, 60, 61 , 62, 63. 



Metric and Dynamic Measures. 



The barometric formula developed on page xli is, in metric and dyna- 

 mic units, 



