438 Lieutenant Rennt on a new Barometric 



may be found in Poisson's " Mecanique" (Paris edition, 1811, vol. ii. book iv. 

 chap. V. p. 429), with such additions and modifications as the more correct 

 consideration of the subject renders necessary. The method employed by 

 PoissoN is substantially the same as that employed by La Place, which may 

 be found in the " Mecanique Celeste." 



2. Let— 

 TT be the elastic force of a given mixture or union of dry air and vapour of wa- 

 ter, being measured by the pressure to which the said mixture is exposed. 

 T be the temperature of said mixtui'e, measured by Fahrenheit's thermometer. 

 F be elastic force or tension of vapour of water of said mixture. 

 p be specific gravity or density of said mixture, 

 A be ratio of elasticity to density (also called modulus of elasticity) of said 



mixture, and equal to -. 

 P 



a be modulus of elasticity of dry air at freezing point. 



z be heio-ht above the level of the sea of an extremely thin stratum of a mix- 

 ture of dry air and vapour of water, having a temperature t, and under a 

 pressure tt, the elastic force of the vapour of water being F. 



g' be force of gravity at distance z above the level of the sea. 



g be force of gravity at the level of the sea. 



•^ be latitude of the stations of barometric observations. 



f be half the increase of gravity from the equator to the pole of the earth = 

 0-002695. 



r be the radius of the earth = 20898240 feet. 



M be modulus of common logarithms. 



I be expansion of gas for each degree of Fahrenheit - 0-002084. 



m be expansion of quicksilver for each degree of Fahrenheit = 0-0001. 



Let, moreover, — 

 h, h' be heights above the level of the sea of the lower and upper stations of 



barometric observations. 

 p,p' be pressures of the atmosphere at lower and upper stations. 

 /,/' be tensions or forces of aqueous vapour at lower and upper stations. 

 /3, ji' be barometric observations at lower and upper stations. 



