Mr. J. H. Alexander on the Tension of Vapour of Water. 1 09 



mulse in terms of the pressure instead of the temperature. 

 This method enables me, by an easy and safe interpolation, 

 to extract the proper quantities from M. Biot's table, and 

 thus to avoid the portentous labour of working out the nume- 

 rical transformation of his theorem. 



Not to embarrass this table with so many columns, I omit 

 the individual deviations of the two formulae, and present the 

 general result as under. 



Biot. Alexander. 



Mean deviation from experiment'! o-02'ififi O-ISIQI 

 without regard to signs . • J M 



Mean deviation from experiment I _o*01704< —0-09114 

 with signs j 



It is hardly necessary to repeat that the first of these for- 

 mulae is founded in part upon the very experiments with which 

 it accords so well, and that the other was not. 



The table of M. Biot goes up as far as 220*^ C. ; but he 

 supposes that his formula is applicable much further; and in 

 fact he has given results, in a small supplementary table, as 

 high as 300^ C. or 572° F., at which temperature it makes the 

 pressure equal to almost exactly eighty- five atmospheres. 

 The present formula would make, corresponding to this pres- 

 sure, a temperature of 559°-92 F. or 293°-3 C, differing from 

 the other within the correction between a mercurial and an 

 air-thermometer. 



It is at the other extremity, where we still have opportu- 

 nity of referring to experiment, that the difference between 

 the two formulae becomes more marked; and where that of 

 M. Biot, neither in its terms nor its progression, can be con- 

 sidered applicable. This may be seen as under: — 



