XXXll INTRODUCTION. 



The temperature argument is given for every 5° from 30° F. to 85° F., 

 and the pressure argument for every 0.2 inch from 22.0 to 30.8 inches. 



This table may be used in computing small differences of altitude, and, 

 up to a thousand feet or more, very approximate results may be obtained. 



Example : 



Mean pressure at Augusta, October, 189 1, 29.94 ; temperature, 6o.°8 F. 

 Mean pressure at Atlanta, October, 1891, 28.97 5 temperature, 59.°4 

 Mean pressure of air column, B = 29.455 ; d = Soli 



Entering the table with 29.455 and 60.° i as arguments, we take out 94.95 

 as the difference of elevation corresponding to a tenth of an inch difference 

 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 31. Difference of height of air correspondhig to a change of i 

 tnillinietre in the barometer — Metric measures. 



This table has been computed by converting Table 18 into metric units. 

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

 pressure argument is given for every millimetre from 760 to 560 mm. 



Table 32. Babinef s formula for determining heights by the barometer. 



Babinet's formula for computing differences of altitude* represents the 

 formula of lyaplace quite accurately for differences of altitude up to 1000 

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

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

 venient application, the formula is here given in English and metric measures. 

 It might seem desirable to alter the figures given by Babinet so as to con- 

 form 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 L^ + 900 J ^-T^ ' 

 and in metric measures, 



Z (metres) — 16000 i H ^^^ 75^— — 7;, 



L 1000 -\ B^ + B 



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

 at which the barometric pressures corrected for all sources of instrumental 

 error are B^ and B, and the observed air temperatures are 4 and /, respectively. 



* Comptes Rendus, Paris, 1850, vol. xxv., page 309. 



