calculating Heights by the Barometer, 195 



c, the generally very small number which, in table C, faces 

 T— T'; the approximate height will be a—b—c. (If T— T' 

 is negative, it should be written a—b + c.) In order to apply 

 the correction necessary for the strata of air, it will suffice to 

 multiply the thousandth part of the approximate height by the 

 double sum 2 (t + t 1 ) of the detached thermometers ; the cor- 

 rection will be either positive or negative, according as t + t? is 

 itself either positive or negative. 



The second and last correction, that for the latitude and 

 the diminution of weight, is obtained by taking, in table D, 

 the number which corresponds vertically to the latitude, and 

 horizontally to the approximate height : this correction, which 

 can never exceed 28 metres, is always added. 



In those very rare cases where the lower station is itself con- 

 siderably elevated above the sea, it will be necessary to apply 

 a small correction to be found in table E. 



In order to understand the calculation of a height by means 

 of these tables, and those prefixed and appended, let us sup- 

 pose that in latitude =44° we had at the level of the sea, the 

 barometer = 30*04 English inches, temperature of the instru- 

 ment = 22°*5 centigrade, and of the air =22°. At the top of 

 a mountain, the barometer =26*57 English inches, tempera- 

 ture of the instrument =17*5, and of the air =17°. 



In order to obtain the equivalents of the English inches in 

 millimetres, search in table A ; where the number of millime- 

 tres corresponding to 30*04 inches observed at the sea will 

 be 763, and that of 26*57 observed on the mountain will be 

 675. Having obtained these equivalents, the calculation pro^ 

 ceeds : Mill. Metres. 



Barometer at sea level = 763 = 6182*01 rp , i r> 



Barometer on the mountain = 675 = 5206*1 / 



975*9 

 DifF. of attached thermometers = 5° = 7*4 Table C. 



Apparent height 968*5 



Double the sum of the detached ^ 



thermometers multiplied by the > ... 75*5 



thousandth part of 968*5 j 



1044* 



Correction for latitude 3*1 Table D. 



Height of the mountain 1047*1 



Height in English feet 3435 Table F. 



2C2 



