MEASUREMENT OF HEIGHTS. 53.3 



ing on Avhich it is founded : it is far exceeded by the constantly- 

 existing disturbances of the equihbrium of the atmosphere, as 

 well as by the uncertainty of the law (applied in the 2nd Sect.) 

 of the variation of temperature between two heights at which 

 the thermometer has been observed. Even the geometrical 

 measurement of the difference of elevation could scarcely be made 

 with sufficient certainty to determine a quantity so small as that 

 upon which a decision between the two assumptions would de- 

 pend. 



6. 



The necessary following out of Dalton's supposition, in its 

 relation to barometrical measurements of altitude, gives me an 

 occasion of expressing my own view of this much-discussed sub- 

 ject. The supposition rests principally on the comportment of 

 aqueous vapour when mixed wdth air, and when by itself. 



A fluid brought into an empty space gives off vapour until 

 the vapour has attained a density dependent on the temperature 

 of the space. Dalton has determined this density, in the case 

 of the vapour of water, for all degrees of temperature between 

 freezing and boiling water ; and has shown by indubitable ex- 

 periments, that the vapour attains precisely the same density 

 when the space is occupied by dry air, of any density whatso- 

 ever, as when it is originally a vacuum. Every attempt to in- 

 crease the density, when the temperature remains the same, fails. 

 If the space, when filled with the densest vapour consistent with 

 the temperature, be contracted in the ratio of 1 : 1 — w, a part 

 of the vapour, proportioned to the whole as ?z : ] , is converted 

 into fluid : precisely the same change takes place if a space filled 

 with the densest vapour consistent with the temperature, and 

 containing air of any density whatsoever, be contracted in the 

 same proportion : in such case the air undergoes no correspond- 

 ing change, but merely an increase of density in the ratio of 

 \—n:\. This is the pure result of Dalton's experiments. 

 They show a difference between vapour and air, assigning to 

 vapour a maximum of density dependent on temperature, which 

 does not exist in the case of air. They show further, that the 

 forces at the surface of the fluid, which cause it to rise in va- 

 pour in a vacuum, are not counteracted by the pressure of the 

 air in contact with it. In respect to the latter point, I may re- 

 mark that Poisson derived from phaenomena of another class, 

 i. e. the capillary, that the density at the surfaces of fluids is in- 



VOL. II. PART VIII. 2 X 



