XXVI INTRODUCTION. 



Tables 17, 18, 19. Redudioyi of the barometer to sea level — Metric measures. 



For reducing to sea level readings of the metric barometer, the baro- 

 metric formula in metric measures derived on page xxii is treated in the 

 same manner as the formula in English measures just described in detail, 

 and the method of construction of the tables is the same. 



Table 17 gives values of 2000 wz. 



Z I 



VI = 



18444 .+ 67.53 + 0.003 Z ' I + ^ 



The temperature B varies by intervals of 2° from — 16° C to + 36° C. 

 except near the extremities of the table where the interval is 4° The alti- 

 tude Z varies by 10 metres from 10 to 3000 metres. The values of 2000 m 

 are given to one decimal. 



Table 18 gives the small correction to 2000 fn for latitude. The argu- 

 ments are 2000 t/z, which varies by tens from 10 to 350, and the latitude 

 which varies by 5° from 0° to 90° The correction is to be subtracted for 

 latitudes below 45° and added for latitudes above 45° The tabular values 

 are given to one decimal. The value of 2000 m thus corrected is then used 

 in entering Table 19. 



Table 19 gives the correction which must be applied to the barometric 

 reading B (corrected for temperature) to reduce it to sea level. The argu- 

 ments are B, which varies by lo^mm. from 790 mm. to 480 mm., and values 

 of 2000 m which vary by units from i to 345. The tabular values B^ — B 

 are given to o. i mm. 



Example : 



Let B= 648.7 mm. be the barometric reading observed and corrected 

 for temperature at a station whose altitude is 1353 metres and latitude 

 32.° Suppose the mean temperature of the air column 6 = i4°3 C. 



Table 17 gives (p. 83) for 6 — 14° and Z— 1353, 2000 wz = 138.6 

 The proportional j)art for o.°3 is .15 



Hence the approximate value of 2000 ?;? is 138.45 



Table 18, with 20ooni= 138 and latitude 32°, gives the subtractive 

 correction 0.15. Hence the corrected value of 2000^2 is 138.3. With this 

 value and B = 649 mm. as arguments, Table 19 gives B^ — B — 11 2.0 mm. 



Accordingly the barometric reading reduced to sea level is 



B^ = 648.7 + 112. o = 760.7 mm. 



THE DETERMINATION OF HEIGHTS BY THE BAROMETER. 



Tables 20, 21, 22, 23, 24. English Measures. 



The barometric formula developed in the preceding section (see p. xxi) 

 is arranged in the following form for determining heights by the barometer. 



