XXXVI INTRODUCTION. 



where h is the altitude of the station above sea level. 



(3) Correct g$ for gravity anomaly. 1 



(4) Finally, g$ is to be corrected for topography and isostatrc com- 

 pensation. 2 



Example: 



To determine the value of local gravity g h at the Weather Bureau 

 Office, Atlanta, Ga., latitude 33 45' N., longitude 84 23' W., 

 height of barometer above sea level, 121 8 feet. 



From Table 83, mean sea level gravity for lat- 

 itude 33 45' 



Correction for height of barometer 



(—0.000094X1218) = — 



Correction for gravity anomaly, = — 



Correction for topography and compensation = + 



Local gravity at Weather Bureau Office, Atlanta, 



Ga. = 979.508 dynes. 



Having determined g h the reduction of barometer readings to stan- 

 dard gravity is easily and accurately accomplished by multiplying by the 

 ratio gi/g, or by applying a correction to the barometer reading, other- 



• \gi — i?) 

 wise corrected, derived from the expression — - — B. With gi < g the 



g 



correction is to be subtracted; with g t > g the correction is to be added. 



In general, sufficient accuracy will be attained by computing the gravity 



correction for a station once for all from the equation C = B n — — , 



g 

 in which B n is the normal station barometer pressure, and C is expressed 



in the same units as B n . 



Table 48 gives corrections to reduce barometer readings to standard 

 gravity. The top argument is the barometer reading. The side argument 

 is the difference, gi — g, for each tenth of a dyne up to 4.0 dynes. The 

 relation is a linear function of both gi — g and B, and for barometer read- 

 ings 10 or 100 times greater than those given in the argument the correc- 

 tion may be obtained by removing the decimal point in the tabulated val- 

 ues one or two places, respectively, to the right. The correction obtained 

 will be expressed in the same units as the barometer reading to be cor- 

 rected. 



1 In most cases the gravity anomaly may be obtained from Bowie's paper, op. cit., figure 

 II. 



2 In some cases this correction may be obtained from Bowie's paper, op. cit., pp. 50-52, 

 but in many cases, and especially in mountainous districts, it must be separately computed 

 for each station. 



