BAROMETRICAL TABLES. 



XXVH 



Z=K{\ogB^-\ogB) 



{1+0.6) 



(i4-)8) 



(i + k cos 2<^)(l +77) 



in which A' (log B„ — log B) is an approximate value of Z and the factors 

 in the brackets are correction factors depending respectively on the air 

 temperature, the humidity, the variation of gravity with latitude, the varia- 

 tion of gravity with altitude in its effect on the weight of mercury in the 

 barometer, and the variation of gravity with altitude in its effect on the 

 weight of the air. With the constants already given, the formula becomes 



in English measures : 



Z (feet) = 60368 (log B, - log B) 



[i + 0.002039 (9 — 32°)] 



(l + 0.002662 cos 2^) (l + 0.00239) 

 /Z+ 2/l,\ 



In order to make the temperature correction as small as possible for 

 average air temperatures, 50° F. will be taken as the temperature at which 

 the correction factor is zero. This is accomplished by the following trans- 

 formation : 



I + 0.002039 {6 — 32°) = [i + 0.002039 (6 — 50°)] [i + 0.0010195 X 36°]. 



The second factor of this expresssion combines with the constant, and 

 gives 60368 (i +0.0010195 X 36°) = 62583.6. 



The first approximate value of Z is therefore 



62583.6 (log ^. - log ^). 



In order further to increase the utility of the tables, we shall make a 

 further substitution for log B^ — log B, and write 



62583.6 (log B^ - log B) =-- 62583.6 log (-^ - log ^y 



Table 20 contains values of the expression 



2Q.Q 



62583.6 log -^ 



for values of B varying by intervals of o.oi inch from 12.00 inches to 30.90 

 inches. 



The first approximate value of Z is then obtained by subtracting the 

 tabular value corresponding to B^ from the tabular value corresponding to 

 B (B and B^ being the barometric readings observed and corrected for 

 temperature at the upper and lower stations respectively). 



