Iviii INTRODUCTION. 



Example : 



Mean pressure at Augusta, October, 1891, 29.94; temperature, 60.8 F. 

 Mean pressure at Atlanta, October, 1891, 28.97; temperature, 59?4 

 Mean pressure of air column B = 29.455; S = 6o°i 



Entering the table with 29.455 and 60.1 as arguments, we take out 

 94.95 as the difference of elevation corresponding to a tenth of an inch dif- 

 ference of pressure. Multiplying this value by the number of tenths of 

 inches difference in the observed pressures, viz. 97, we obtain the difference 

 of elevation 921 feet. 



TABLE 70. 



Table 70. Difference of height corresponding to a change of one millimeter 

 in the barometer — Metric measures. 



This table has been computed by converting Table 69 into metric units. 

 The temperature argument is given for every 2° from — 2° C. to + 36° C; 

 the pressure argument is given for lo-mm. intervals from 760 to 560 mm. 



TABLE 71. 



Table 71 . Babinet's formula for determining heights by the barometer. 



Babinet's formula for computing differences of altitude 1 represents 

 the formula of Laplace quite accurately for differences of altitude up to 1000 

 meters, and within one per cent for much greater altitudes. As it has been 

 quite widely disseminated among travelers and engineers, and is of con- 

 venient application, the formula is here given in English and metric meas- 

 ures. It might seem desirable to alter the figures given by Babinet so as to 

 conform to the newer values of the barometrical constants now adopted; 

 but this change would increase the resulting altitudes by less than one-half 

 of one per cent without enhancing their reliability to a corresponding degree, 

 on account of the outstanding uncertainty of the assumed mean temperature 

 of the air. 



The formula is, in English measures, 



Z (feet) = 52494 [i + ,00 J K+B' 



and in metric measures, 



2 (/o + /)1 Bo-B 



Z (meters) = 16000 i + 



1000 J Bo-\- B' 



in which Z is the difference of elevation between a lower and an upper 

 station at which the barometric pressures corrected for all sources of in- 

 strumental error are Bq and B, and the observed air temperatures are ^^ 

 and /, respectively. 



For ready computation the formula is written 



^ ^^Bo+B' 

 1 Comptes Rendus, Paris, 1850, vol. xxx., page 309, 



