BAROMETRICAL TABLES. XXXVll 



Example 1. 



The barometer reading corrected for temperature is 29.647 inches, 

 and the local value of gravity is 978.08. The difference, gi — g, 

 = — 2.585. From the table, 



the correction for a barometer reading of 20 inches 

 the correction for a barometer reading of 9 inches 

 the correction for a barometer reading of 0.65 inches 

 Correction for a barometer reading of 29.65 inches 

 Corrected barometer reading = 29.647 in. — 0.078 in. 



Example 2. 



The barometer reading reduced to o° C. is 637.42 mm., and the local 

 value of gravity is 981.51. The difference, gi — g = + 0.845. From 

 the table, 



the correction for a barometer reading of 600 mm. = + 0.517 mm. 



the correction for a barometer reading of 30 mm. = + 0.026 mm. 



the correction for a barometer reading of 7 mm. = + 0.006 mm. 



Correction for a barometer reading of 637.4 mm. = + 0.55 mm. 



Corrected barometer reading = 637.42 + 0.55 = + 637.97 mm. 



In the case of barometer readings made at sea, and also at some land 

 stations, it is not practicable to determine local gravity with greater ac- 

 curacy than it can be computed from the equations for variation with lati- 

 tude and altitude given above. The reduction to standard gravity, accord- 

 ingly, consists of two parts — a correction for altitude, and a correction 

 from the computed sea-level gravity for the latitude of the station to stan- 

 dard gravity. The first part of the correction, or the correction for altitude, 

 may be computed once for all from the expression c = — 0.0003086 h B n 

 (metric measures), or c = —0.000094 h B n (English measures), and is usually 

 combined with the reduction of the barometer to sea level or to some other 

 reference plane. Th e second part has heretofore consisted of a correction for 

 the difference between the mean value of gravity for the latitude of the 

 station and for latitude 45 ; and, in accordance with the equation given 

 above, it may be derived from the expression 



( — 0.002640 cos 2 + 0.000007 cos2 2<f>) B 



where <$> is the latitude of the station, and B is the barometer reading. The 



1 r 1 • £i=; — £ 980.621 — 980.66s ^, 



value of the ratio 2^ = — — ~ - = - 0.000045. There- 



g 980.665 



fore, the expression for the gravity correction becomes 



( — 0.00264 cos 2 <f> + 0.000007 c °s 2 2 — 0.000045) -S 



Table 49 (English measures) gives the corrections in thousandths of 

 . an inch for every degree of latitude and for each inch of barometric pres« 



