xl INTRODUCTION. 



where li is the akitude of the station above sea level. 



(3) Correct (/^ for gravity anomaly.^ 



(4) Finally, <70 is to be corrected for topography and isostatic com- 

 pensation.^ 



Example : 



To determine the value of local gravity, gj, at the Weather Bureau 

 Office, Atlanta, Ga., latitude t>2>° 45' N., longitude 84° 23' W., 

 height of barometer above sea level. 1218 feet. 



From Table 90, mean sea level gravity for lat- 

 itude 33° 45' = 979-631 dynes. 



Correction for height of barometer 



(—0.000094x1218) =— 0.1 14 " 



Correction for gravity anomaly, = — 0.023 " 



Correction for topography and compensation = -f- 0.014 " 



Local gravity at Weather Bureau Office, Atlanta, 



Ga. = 979-508 dynes. 



Having determined gu the reduction of barometer readings to stan- 

 dard gravity is easily and accurately accomplished by multiplying by the 

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

 wise corrected, derived from the expression --^^ — ^—-5. With giKQo the 



correction is to be subtracted; with gi'>go 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 = Bn ~ -i!^ 



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

 in the same units as En- 



Table 48 gives corrections to reduce barometer readings to standard grav- 

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

 difference, gi—go, for each tenth of a dyne up to 4.0 dynes. The relation 

 is a linear function of both gi—go and B, and for barometer readings 10 or 

 100 times greater than those given in the argument the correction may be 

 obtained by removing the decimal point in the tabulated values one or two 

 places, respectively, to the right. The correction obtained will be expressed 

 in the same units as the barometer reading to be corrected. 



Example i. 



The barometer reading corrected for temperature is 29.647 inches, and the 

 local value of gravity is 978.08. The difference, gi—go, =—2.585. 

 From the table, 

 the correction for a barometer reading of 20 inches = — 0.0527 in. 



the correction for a barometer reading of 9 inches = — 0.0237 in. 



the correction for a barometer reading of 0.65 inches = — 0.0017 ^^• 



Correction for a barometer reading of 29.65 inches = — 0.078 in. 



Corrected barometer reading =29.647 in. —0.078 in. = 29.569 in. 



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. 



