BAROMETRICAL TABI.ES. XXXlll 



For ready computation the formula is written 



B.-B 



Z=C-x 



B^ + B 



and the factor C computed both in English and metric measures, has been 

 kindly furnished by Prof. Cleveland Abbe. The argument is i (/„ + 

 given for ever>' 5° Fahrenheit between 10° and 100° F., and for every 2° 

 Centigrade between 10'' and 40° Centigrade. 



In using the table, it should be borne in mind that on account of the 

 uncertainty in the assumed temperature, the last two figures in the value of 

 C are uncertain, and are here given only for the sake of convenience of 

 interpolation. Consequently one should not attach to the resulting altitudes 

 a greater degree of confidence than is warranted by the accuracy of the 

 temperatures and the formula. The table shows that the numerical factor 

 changes by about one per cent of its value for everj^ change of five degrees 

 Fahrenheit in the mean temperature of the stratum of air between the 

 upper and lower stations ; therefore the computed difference of altitude will 

 have an uncertainty of one per cent if the assumed temperature of the air 

 is in doubt by 5° F. With these precautions the observer may properly 

 estimate the reliability of his altitudes whether computed by Babinet's 

 formula or b}- more elaborate tables. 



Example : 



Let the barometric pressure observed and corrected for temperature at 

 the upper and lower stations be, respectively, B = 635 mm. and 

 ^_^ = 730 mm. Let the temperatures be, respectively, ^=15" C, 

 4 = 20° C. To find the approximate difference of height. 



With i(4 + /) =?^-^tI5_ = i7°5 C, the table in metric measures gives 



_ B^-B 95 



C = 1 7 1 20 metres. „ — r. = /- ' 



B^^B 1365 



The approximate difference of height = 17120 x ^^_ = 1191.5 metres. 



THERMOMETRICAL MEASUREMENT OF HEIGHTS BY OBSERVATION OF THE 

 TEMPERATURE OF THE BOILING POINT OF WATER. 



When water is heated in the open air, the elastic force of its vapor 

 gradually increases, until it becomes equal to the incumbent weight of the 

 atmosphere. Then, the pressure of the atmosphere being overcome, th« 

 steam escapes rapidly in large bubbles and the water boils. The tem- 

 perature at which water boils in the open air thus depends upon the 

 weight of the atmospheric column above it, and under a less barometric 

 pressure the water will boil at a lower temperature than under a greater 

 pressure. Now, as the weight of the atmosphere decreases with the 

 elevation, it is obvious that, in ascending a mountain, the higher the 



