352 On Ilypsometricai Measurements. [No. 4. 



Log ( H' — H) = 0.708274 4- log (T — T') -, 



VD' (18) 



— log (167.319 + T + T' + ) -flog A + log G. ) 



600 



9. To facilitate computations of this kind, Tables V and YI have 

 been formed. Table VI gives the height in feet above the level 

 where water boils at 212° Faht. for every fifth part of a degree be- 

 tween 176° and 215° E. This Table and the column containing the 

 Multiplier for the mean temperature of the air in Table I will enable 

 us to obtain the heights, uncorrected for latitude, without the use of 

 logarithms. Table V containing the logarithmic pressures will be 

 of use when one of the observations is taken with a barometer. 



10. "When the observations are taken at the upper station only, it 

 becomes necessary to estimate t, the mean temperature of the stra- 

 tum of air between the sea level and that station approximately. 

 Laplace estimated the diminution of temperature with the elevation 

 at 16° or 17° cent, for 3000 metres of ascent,* but taking the mean 

 of observations made on mountain sides by Saussure, Kaemtz, 

 Bravais, Martins, Schouw, Humboldt, Boussingault, and the recent 

 [French Commission to the North, the diminution is 1° Faht. for 

 every 303 feet of ascent.f Hence we may reckon that for every 

 degree which the boiling-point falls, the temperature of the air 

 decreases 1°.8 F., so that the mean temperature may be esti- 

 mated at, — 



} (t + t') = t' + 0.9 (212° — T'), 



or when the observation is made with the barometer. 



190 B' 

 i (t + t') = 91* + t' 



30 +B' 



or, roughly — 



9 



a (t + t') s= 60 + t' B'. 



4 



* Laplace, Systeme du Monde, 'torn. i. p, 172 (Ed. 1836.) 



f On this subject, see a paper by Professor Challis, in the Transactions of the 

 Cambridge Philosophical Society, vol. vi. ; and Daniell'a Meteorology, vol. i. pp. 40, 

 41. 



