BAROMETRICAL TABLES. xH 



Example 2. 



The barometer reading reduced to 0° C. is 637.42 mm., and the local 

 value of gravity is 981.51. The difference, ^z — (70= +0.845. From 

 the table, 



the correction for a barometer reading of 600 mm. = + 0-5i7 "ini. 



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 i^"'!^''- — + <^-55 n\n'i- 



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 variations 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 li Bn 

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

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

 reference plane. The 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 cos- 2 (f>)B 



where (f> is the latitude of the station, and B is the barometer reading. The 



1 r ..1 ^- ffi5'—go 980.621—980.665 _, . 



value of the ratio ^ = = — ^ ^ =—0.000045. Therefore, 



go 980.665 ^^ 



the expression for the gravity correction becomes 



( —0.00264 cos 2 ^ + 0.000007 COS" 2 (^ — 0.000045)5 



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

 an inch for every degree of latitude and for each inch of barometric pres- 

 sure from 19 to 30 inches, to reduce barometer readings to standard gravity, 

 computed from the equation 



C=(— 0.00264 cos 2<f> + 0.000007 cos^ 2 — 0.000045 ) B 



Table 50 (metric measures) gives the same corrections in hundredths 

 of a millimeter for each 20 millimeters barometric pressure from 520 to 780 

 millimeters. 



Example: 



Barometric reading (corrected for temperature) at latitude 



63° 55', =27434 inches 



Correction to standard gravity, Table 49, =0.043 inches 



Barometer reduced to standard gravity, =27.477 inches 



The adoption of this new value for standard gravity may require a slight 

 correction to old barometric records in order to make the entire series of read- 

 ings homogeneous. The amount of this correction will be the difference be- 

 tween the gravity correction computed by these new tables and by the old tables. 



