XXXVlll INTRODUCTION. 



sure from 19 to 30 inches, to reduce barometer readings to standard grav- 

 ity, computed from the equation 



C — ( — 0.00264 cos 2 4> + 0.000007 cc, s 2 2 4> — 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', . = 27.434 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 readings homogeneous. The amount of this correction will be the differ- 

 ence between the gravity correction computed by these new tables and by 

 the old tables. 



Example: 



Seattle, Wash., Lat. 47 38' N. Long. 122 20' W., height of barometer 



above sea level 125 feet, normal station barometer 29.89 inches. 

 gt (Table 83) = 980-859 dynes. 



Correction for height ( — 0.000094 X 125) = — .012 



Correction for topography and compensation = — .019 



Correction for gravity anomaly = — .093 



Value of local gravity 980.735 dynes. 



Correction to reduce barometer readings to standard gravity, 



980.735 -980.665 „ . . , „. . 



- , B n =+0.002 inch. Old correction, +0.007; correc- 



980.665 



tion to old records = 0.002 in. — 0.007 in. = — 0.005 i n - 



For correcting back records of readings at sea, or at any place where 



the value of local gravity cannot be determined, the correction is equal to 



the ratio - — ^-^ — — - — ■ — - B = — 0.000067 B. The corrections are as 

 980.665 



follows: 



Barometer reading. Correction. 



From 7 to 22 inches — 0.001 in. 



From 23 to 32 inches — 0.002 in. 



From 380 to 520 mm. — 0.03 mm. 



From 530 to 670 mm. — 0.04 mm. 



From 680 to 820 mm. — 0.05 mm. 



