BAROMETRICAL TABLES. xlv 



Z (meters) = 18400 (log B Q -\og B) l~(i + 0.00367 d C.) 



(1 + 0.378;) 



(i + O.OO264O COS 2 0—O.OOOOO7 COS 2 2 <f> 

 + O.OOOO45) C 1 + O.OO239) 

 ( Z±2K\ 



\ 6 367 324/ __ 



The approximate value of Z (the difference of height of the upper and 

 lower station) is given by the factor 18400 (log B — log B). This expres- 

 sion is computed by means of two entries of a table whose argument is the 

 barometric pressure. In order that the two entries may result at once in an 

 approximate value of the elevation of the upper and lower stations, a trans- 

 formation is made, which gives the following identities: 



18400 (log B - log B) = 18400 flog '-=r- - log ^r- J — Metric measures, 



and 18400 (log B - log B) = 18400 (log ^^ - log ^^) —Dynamic 

 measures. 



Table 56 gives values of the expression 18400 log ~- for values of B 



varying by intervals of I mm. from 300 mm. to 779 mm. The first approxi- 

 mate value of Z is then obtained by subtracting the tabular value corre- 

 sponding to B Q from the tabular value corresponding to B (B and B Q being 

 the barometric readings observed and reduced to o° C. at the upper and 

 lower stations respectively). The first entry of Table 56 with the argu- 

 ment B gives an approximate value of the elevation of the upper station 

 above sea level, and the second entry with the argument B Q gives an ap- 

 proximate value of the elevation of the lower station. 



JOJ-7 "I 



Table 57 gives values of the expression 18400 log — -^ for values of 



B 



B varying by intervals of 1 mb. from o mb. to 1049 mb. The approximate 

 value of Z is then obtained by subtracting the tabular value corresponding 

 to B Q from the tabular value corresponding to B (B and B Q being the baro- 

 metric readings observed and reduced to o° C. at the upper and lower sta- 

 tions respectively). The first entry of Table 57 with the argument B gives 

 an approximate value of the elevation of the upper station above sea level, 

 and the second entry with the argument B Q gives an approximate value of 

 the elevation of the lower station. 



Table 58 gives the temperature correction factor, a = O.OO3670, for 

 each tenth of a degree centigrade, from o° C. to 50. 9 C. To find the cor- 

 rection corresponding to any mean temperature of the air column, 0, mul- 

 tiply the approximate altitude as determined from Table 56 or 57 by the 

 value of a obtained from this table, and add the result if is above o° C; 

 subtract, if below o° C. 



