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



The curve for alcohol extends from 32° to 264° of Fahren- 

 heit. It is divided into two portions, having different verti- 

 cal scales, suitable to high and low pressures respectively. 



The curve for the less volatile ether extends from 105° to 

 210° ; that for the more volatile ether, from 34° to 104°. 



The results of Dr Ure's experiments ai'e marked by small 

 crosses. 



The irregular and sinuous manner in which those crosses 

 are distributed, indicates that the errors of observation, espe- 

 cially at high temperatures, must have been considerable. 

 This does not appear surprising, when we recollect how many 

 causes of uncertainty affect all the measurements required 

 in such experiments, especially the thermometric observa- 

 tions, and how little those causes have been imderstood until 

 very recently. The data from which the constants have been 

 calculated, are, of course, aff'ected by the general uncertainty 

 of the experiments. 



When those circumstances are taken into account, it is 

 obvious, from inspection of the diagram, that the curves re- 

 presenting the formulae agree with the points representing the 

 experiments, as nearly as the irregularity of the latter and the 

 uncertainty of the data permit ; and that there is good reason 

 to anticipate, that, when experiments shall have been made 

 on the vapours of alcohol and ether with a degree of preci- 

 sion equal to that attained by M. Regnault in the case of 

 the vapour of water, the equation will be found to give the 

 elasticities of those two vapours as accurately as it does that 

 of steam. 



Although the diagram affords the best means of judging 

 of the agreement between calculation and experiment, three 

 Tables (III-, IV., and V.) are annexed, in order to shew the 

 numerical amount of the discrepancies at certain tempera- 

 tures. The data, as before, are marked with asterisks. 



It is worthy of remark, in the case of alcohol, that although 

 the lowest of the experimental data is at the temperature of 

 lll°-02, the formula agrees extremely well with the experi- 

 ments throughout the entire range of 79 degrees below that 

 point. 



