490 Tables 167—168 (concluded) 



ACCELERATION OF GRAVITY 



Computation of theoretical gravity for surface stations. 7 — Three types of reduction, 

 corresponding to the three types of interpolated gravity anomalies mentioned above, may 

 be considered for the computation of theoretical values of gravity. 



A combination of using the free-air and the Bouguer reductions turns out to give the 

 best results. This combination method consists of making a free-air reduction for the 

 elevation of the station, and a Bouguer reduction for the difference of elevation of the 

 station and the elevation of the general terrain. The equation is 



gx — g^ — 0.0003086/1 -f 0.0001118(/z — h') (3) 



where h' is the elevation of the general terrain for a radius of 100 miles, and the other 

 symbols have the same meaning as in equation (2). A similar formula for sea stations is 



g, = g t - 0.0003086/t - 0.0000688 (D — D') (4) 



where D is the depth of water in meters below the station and D' is the depth of water 

 in meters of the general level of the sea bottom for a radius of 100 miles. 



Acceleration of gravity in the free air. — Lambert 8 gives the following equation for 

 computing the acceleration of gravity g (cm. sec.~ s ) at height Z meters above sea level 

 in the free air : 



g = g*- (3.085462 X 10" 4 -f 2.27 X 10' T cos 2<p)Z 

 + (7.254 X 10" u + 1.0 X 10- u cos 2<t>)Z> 

 — (1.517xl0-" + 6Xl0- a> cos2<^)Z 3 (5) 



7 Computation of theoretical gravity in the free air is also discussed in Table 49. 



8 Lambert, W. D., Formula for the geopotential including the effects of elevation and of the 

 flattening of the earth, unpublished manuscript, Oct. 15, 1946. 



SMITHSONIAN METEOROLOGICAL TABLES 



