100 Mr. J. H. Alexander on the Tension of Vapour of Water, 



practice, and for whose benefit the whole of the present dis- 

 cussion is mainly intended. 



It is obvious, then, in the first place, that the idea of free- 

 dom from error is as.sociated with symmetry in the results. 

 Such symmetry will always be observable in quantities that 

 progress (as natural quantities may be assumed to do) accord- 

 ing to some constant law ; and as, in our ignorance of what the 

 true law is in this case, all that we can deal with is relative 

 symmetry, it is of no importance what law or formula we take 

 as the other term of comparison, provided there be no material 

 difference between the origin and termination of the two. 

 I shall therefore compare a few of M. Regnault's observations 

 at the lower temperatures with the results of the present for- 

 mula, as under: — 



It is apparent, then, that so far these observations do not 

 follow any uniform or symmetrical yirogression ; and without 

 pretending to criticise the experiments themselves, which 

 doubtless have as much accuracy as the nature of the research 

 admitted, it follows that, in spite of all the extraordinary tact and 

 skill of the observer, there is yet prima J'acie evidence against 

 the absolute accuracy of the results. It is to be remarked upon 

 the column of temperatures, both here and hereafter, that the 

 remote decimals result from the reduction of Centigrade de- 

 grees to those of Fahrenheit, and are preserved because they 

 added to the accuracy, while they did not increase the labour 

 of the calculation. Nevertheless the thermometer of M. Reg- 

 nault could be read directly to the j^th of a degree Centigrade, 

 corresponding very nearly to ^^\h of a degree Fahrenheit; 

 and by estimation, to the next decimal place. 



The temperatures of this table under 32° F. are lower than 

 pressures have ever been observed at before, and rest upon 

 single observations. They do not admit, therefore, of a com- 



