BAROMETRICAL MEASUREMENT OF HEIGHTS. 



353 



Height of barometer — ; 761.35. 



The second column gives 81 ; the column headed 0.3 gives, on the same line, 64.2. 

 Tne corresponding number is then 8164.2. Adding the value of five hundredths of 

 miilim., being 0"-.5, as indicated in the last column, we have 8164.7 metres, cor- 

 responding to 761.35 miilim. 



The other four tables need no further explanation. 



To calculate, by means of the tables, a difference of level from two barometrical 

 observations, proceed in the following manner : — 



1. Take the height of the barometer at the lower station, or A, and seek in 

 Table I. the number corresponding to this height. Seek likewise the number cor- 

 responding to the height of the barometer at the upper station. Subtract the second 

 from the first. The remainder is the approximate difference of level between the two 

 stations. Then apply the following corrections. 



2. Correction to be applied for the temperature of the barometers. 



If T' be the temperature of the attached thermometer at the upper station, and T 

 that of the attached thermometer at the lower station, take the difference, or T' — T, 

 and seek in Table II. the number corresponding to this difference. 



When T' is smaller than T, that is, when the temperature of the attached ther- 

 mometer of the upper station is lower than that of the lower station, the correction 

 is to be subtracted from the approximate height ; when T' is greater than T, it is to 

 be added. 



3. Correction for the temperature of the air. 



The first correction having been applied, multiply the number obtained, or N, by 

 the double sum of the temperatures of the air at both stations, and divide the prod- 

 uct by 1000 ; the number thus found, or the quantity expressed by j^. 2 {t -{- t') 

 is the correction in metres which is to be added to the preceding number N. 



4. Tables III. and IV. give two corrections ; the first due to the decrease of 

 gravitation in latitude, which is to be added when the mean latitude of the places 

 of observation is between the 45th parallel and the equator ; and to be subtracted 

 when it is between the same parallel and the poles, as indicated at the head of the 

 columns. The second correction, due to the decrease of gravitation on the vertical 

 line, is always additive. 



5. Table V. gives another small correction to be added in the case of the lower 

 station being veiy elevated above the level of the ocean. 



Examples of Calculation. 

 Measurement of the Height of Guanaxuato. By M. de Humboldt. 



mill. - o 



Barometer at the upper station, h' = 600.95 T' = 21.3 t' = 21.3 



Barometer at the level of the sea, h = 763.15 T -= 25.3 t = 25.3 



D l"- 



