BAROMETRICAL TABLES. XXIX 



Example : 



L,et the barometric pressure observed, and corrected for temperature, 

 at the upper and lower stations be, respectively, B = 23.61 and 

 B^ = 29.97. Let the mean temperature of the air column be 35° F., 

 and the latitude 44° 16'. To determine the difference of height. 



Feet. 



Table 20, argument 23.61, gives 6420 



Table 20, " 29.97, " — 64 



Approximate difference of height (Z) 

 Table 21, with Z^ 6484 and 9 ^ 35° F., gives 

 Table 22, with Z= 6300 and <^ — 44°, gives 

 Table 23, with Z= 6300 and 6 = 35° F., gives 

 Table 24, with Z= 6300 and /^^ = o, gives 



Final difference of height (Z) = 6321 



If in this example the barometric readings be observed with aneroid 

 barometers, the correction to be obtained from Table 22 will be simply the 

 portion due to the latitude factor, and this will be obtained by subtracting 

 the tabular value for 45° from that for 44°, the top argument being Z=6300.' 

 This gives 16—15 = 1. 



TABLES 25, 26, 27, 28, 29. Metric Measures. 



The barometric formula developed on page xxi is, in metric units, 



(i + 0.00367 6 C.) 



(1+0.378-) 



(i +o.do266cos2^) (i +0.00239) 



(Z + 2 /zj 



Z (metres) = 18400 (log B, ^ log B) 



I + 



6367323 



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 expression 

 is computed bj^ 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 

 transformation is made, which gives the following identity: 



18400 (log B, — log B) = 18400 (log ^ — log ^ j • 



TABLE 25. 



Table 25 gives values of the expression 18400 log -^-5- for values of B 



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

 mate value of Zis 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 reduced to 0° C. at the upper and lower 



