1825.] Baionietrkat Measuremeitt vf Heights. 437 



vessel a quantity of any fluid specitically lighter than the mer- 

 cury, the resulting additional pressure doivnwards will give rise 

 to a corresponding pressure upwards, and consequent ascent of 

 the mercury withni such parts of the vessel D and A, as well as 

 within the tube C, and the closed branch of the syphon B, to 

 which the rarer fluid has not access, and upon whose upper sur- 

 faces it cannot exert any part of the pressure incumbent on the 

 mercury in the body of the vessel. The equilibrium of pressure 

 will have taken place when a cylindrical column of the mercury, 

 equal in height to the (vertical) difference of level of that fluid 

 in any one of the closed tubes (their heights m, in', m" , and m'" , 

 being the same), and in the body of the vessel (or open branch 

 of the syphon) form an exact counterpoise to ■&. similar column of 

 the superincumbent fluid of the same base, and of its total height 

 above the (reduced) level I' , V (or surface of contact of the two 

 fluids). 



Remark. — The subsidence of the level of the mercury in the 

 vessel from / / to V , V , is occasioned by that part of its original 

 volume having ascended, on the introduction of the superior 

 fluid, into the closed tubes k, B, C, D. 



Conceiving the light fluid pressing upon the exposed surface 

 of the mercury to represent the atmosphere, then will the vessel 

 U with its various tubes form so many varieties of the barometer 

 having one common cistern. 



15. Whatever the construction of the barometer, the vertical 

 height of that surface of the mercury to which the atmosphere 

 has not access above the surface in contact with it, equals, and 

 is termed the pressure of atmosphere, or more generally the 

 height of the barometer. 



Of the Demit 1/ of Dry Air. 



Variation from Pressure. — 16. A volume of perfectly dry air' 

 of any temperature, contained in the cylindrical ves- 

 sel C, supporting only the pressure of the gravitat- 

 ing uniformly dense cylindrical weight W, will have 

 its volume and height diminished in proportion to 

 the augmentation of the weight (height) of the com- 

 pressing column. 



17. The weight (height) of the siiperincumbent 

 cylinder W being diminished in any ratio, the volume 

 (height of the column) of air will be increased in the 

 same proportion. 



18. Hence the elasticities of perfectly dry air are directli/, and 

 the volumes and heights reciprocalli/ as the pressures (or heights 

 of the compressing weights). 



I'J. The volume occupied by any fluid being increased or 

 diminished in any ratio, its density will be altered inversely in 

 the same proportion. The densities will consequently be 



