INTRODUCTION. 



Z (meters) = 18400 (log 5o- log -B) (1 + 0.00367 dC.) 



(1 + 0.378S 



(l + 0.002640 cos 2 (f) —0.000007 COS^ 2 (f> 



+ 0.000045) (l + 0.00239) 



/ Z + 2ho \ 



_V 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 Bo - log B) = 18400 hog ^ - log ^ j 

 and 18400 (log Bo - log B) = 18400 ( log 



Metric measures. 

 Dynamic 



measures. 



760 



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 Bq from the tabular value corresponding to B {B and B^ being 

 the barometric readings observed and reduced to 0° 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 w'ith the argument B^ gives an ap- 

 proximate value of the elevation of the lower station. 



1013.3 

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



B 



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

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

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

 metric readings observed and reduced to 0° 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 w'ith the argument B^ gives an approximate value of 

 the elevation of the lower station. 



Table 58 gives the temperature correction factor, a =0.00367^, for 

 each tenth of a degree centigrade, from 0° C. to 50.9° C. To find the cor- 

 rection corresponding to any mean temperature of the air column, 6, 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 6 is above 0° C; 

 subtract, if below 0° C. 



