32 W. J. M. Rankine, Esq., on the Elasticity of Vapours. 



In comparing the I'esults of the formula with those of ex- 

 periment, as exhibited in Table I., the following circum- 

 stances are to be taken into consideration : — 



First, That the uncertainty of barometric observations 

 amounts in general to at least one-tenth of a millimetre. 



Sccondli/, That the uncertainty of thermometric observations 

 is from one- twentieth to one tenth of a degree, under ordi- 

 nary circumstances, and at high temperatures amounts to 

 more. 



Thirdly, That, in experiments of the Ivind refei'red to in 

 the Table, those two sorts of uncertainty are combined. 



The fifth column of the Table shews that, from 30° below 

 the freezing point to 20° above it, where the minuteness of 

 the pressures makes the barometric errors of most importance, 

 the greatest difference between experiment and calculation is 

 xVo of a millimeti'e, or a J^^ of an inch of mercury, a very small 

 quantity in itself, although, from the slowness with which the 

 pressure varies at low temperatures, the corresponding dif- 

 ference of temperature amounts to yVo of a degree. 



The sixth and tenth columns shew^ that, from 20" to 230° 

 above the fi-eezing point, the greatest of the discrepancies 

 between experiment and observation corresponds to a diflFer- 

 ence of temperature of only x%-5 of a degree, and that very 

 few of those discrepancies exceed the amount corresponding 

 to s'o of a degree. 



A comparison between the sixth and tenth columns shews 

 that, for four of the temperatures given, viz., 120°, 150°, 

 200°, and 210°, the pressures deduced from M. Regnault's 

 curve of actual elasticities, and from his logarithmic curve re- 

 spectively, diflPer from the pressures given by the formula in 

 opposite directions. 



If the curves represented by the formula were laid down 

 on M. Regnault's diagram, they would be almost undistin- 

 guishable fi'om those which he has himself drawn, except near 

 the freezing point, where the scale of pressures is very large, 

 the heights of the mercurial column being magnified eight- 

 fold on the plate. In the case of the curves of logarithms of 

 pressures above one atmosphere, the coincidence would be 

 almost perfect. 



