xlii INTRODUCTION. 



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. 

 <70 (Table 90) = 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 l)arometer readings to standard gravity, 



980.735 — 980.665 „ • , r^, , • • ,1 



- — ~^^~ -Bn— +0.002 mch. Old correction, +0.007 ; correction to old 



980.665 ' 



records = 0.002 in. — 0.007 in. = —0.005 ^"• 



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 



, . 980.599 — 980.665 „ , „ _, . , ,, 



the ratio / /^ B— —0.000067 B. 1 he corrections are as follows : 



980.665 



Barometer reading. Correction. 



From 8 to 22 inches — o.ooi 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. 



REDUCTION OF BAROMETER READINGS TO SEA LEVEL. 



Tables 51 to 63 inclusive, " Determinations of Heights by the Barometer," 

 may be used for reducing barometric readings to sea level, provided the mean 

 temperature and vapor pressure of the atmosphere between the observing 

 station and sea level are known. 



See "Example: (English Measures)," p. xlix. 



Barometer at upper station corrected for temperature =23.61 in. 



Mean temperature of air column, 6, =35?o F. 



Latitude of station, </>, =44° 16' 



Altitude of station above mean sea level, Z, =6320 ft. 



The equation for computing the altitude Z is given on p. xlvii. This 

 equation is simplified after justifiable approximations to the form (in English 

 units) o r/i 29.9 , 29.9\ 



62583.6 (^ log ■;p -log ^ j = 



Z-Z [^0.0020390^-50°) +0.378 -|- +(y + ^)+ ^^^], 



where the terms are as defined on pp. xliv to xlvi, inclusive. Calling the 

 terms in the bracket (a), (&), (c) and (rf), respectively, to compute B^ we 

 have : 



from Table 52 with ^ = 6320 feet and ^=35?o F., Z(a) = — 194 



from Table 54 with Z = 6320 feet and average humidity, Z(&) = + 16 

 from Table 53 with 2 = 6320 feet and <^ = 44° 16', Z(c) = + 16^ 



from Table 55 with 2 = 6320 feet and ho = o, Z(d) = + 2 



Z[(a) + (b) + (c) + (d)]= =-160. 



