1858.] On Ilypsometrical Measurements. 341 



and Regnault 0.36706. Adopting 0.367 as the total expansion be- 

 tween 32° and 212° Faht. we have for 1° Faht.— 



a = 0.0020389. 



5. The value of d, or the density of aqueous vapour in terms of that 

 of the air, used by Bessel was 0.62 as found by Berzelius ; Eeg- 

 nault's experiments give 0.621 as a more accurate determination, 

 and hence l — d= 0.379. 



6. Again p, the pressure of aqueous vapour in terms of that of the 

 atmosphere at the temperature t° reckoned from freezing point, 

 may be determined by Eegnault's formula, or from any table 

 calculated by means of that formula. 



In the usual tables of tensions computed from Eegnault's formula, 

 the tensions are expressed in inches of mercury,* and if we put p' 

 for these values we have — 



p' = 29.9218 p. 



7. Lastly, the geometrical mean of the two semidiameters of the 

 earth, according to Airyf is 



?• = 20,888,733 feet. 



S. Now if B and B' be the heights of the barometer at the lower 

 and upper stations reduced to 32° Faht. — 



B s r x 2 B' 



30 



V + H 7 30 V-j-H'' 



and since h is nearly equal to H, — - 



P B 2 M (*' — *) 



Log — = log — -f . 



P' B' r 



and with sufficient accuracy for our purpose — 



. ^ITb 



V p p' = , . 



30 



* Sec a Tabic of this kind in Dixon's Treatise on Heat, j>. 257 — 2C>0. 

 t See Herschel's Outlines of Astronomy, sec. 206. 



2 y 



