Mr. J. H. Alexander on the Tensioji of Vapour of Water. 13 



to call attention to, I have taken only those epochs of tempe- 

 rature which were already in my table. 



Tlie experiments of the French Academy have been already 

 signalized. It is enough to establish their claim to distinction, 

 to say that they were executed by Dulongand Arago; names 

 that have been long since inscribed in the very highest rank 

 of physical philosophers. The numbers found in the appro- 

 priate column, are, agreeably to what I have already men- 

 tioned as governing me throughout, quantities actually ob- 

 served. The temperatures and pressures generally quoted in 

 the text-books on steam, as of the French Academy, are not, 

 in fact, what they observed, but what they deduced (in part, 

 by a formula of their own, and in part by Tredgold's) from 

 the present experimental series. The pressure 29*92 inches 

 corresponding to the temperature 212°, is marked with an 

 asterisk, because it is not expressly declared to have been 

 observed. It is the height which is constantly taken in France 

 for the barometric standard, as thirty inches are in England: 

 in the latter assumption, the temperature is rated at 60° F., — 

 in the former, at 32° F. ; and the difference of heights is 

 nearly identical with the difference of expansion at the re- 

 spective temperatures. 



The pressure in this series corresponding to the tempera- 

 ture 368°'51, is also noted with a dagger ; it may be presumed 

 to be erroneous, not only because it differs so much from the 

 result by my formula, but because it varies so much and so 

 suddenly from the rate accused by the pressure on either side 

 of it. Nor does it correspond at all with their own formula; 

 calculated by that, the pressure will be 335'87 inches. The 

 error is not, in this instance, of the press ; since it makes its 

 appearance in both ways of reckoning, by atmospheres and by 

 metres. 



I do not know how to account for another discrepant pres- 

 sure, corresponding to the temperature 405°*04<; which has 

 been indicated by a note of interrogation. On both sides, 

 above and below, the observed pressures are higher than the 

 calculated one ; in this instance it is suddenly lower. It 

 agrees, to be sure, with an independent calculation by the 

 formula of Dulong and Arago, at the temperature ; but very 

 manifestly breaks the uniformity or any regular progression 

 of the series. What adds to the difficulty, is that the same 

 observation is given again in another part of the Memoir of 

 the Academicians ; but the ciphers do not agree. I have 

 neither altered nor omitted either of these instances ; it is 

 obvious that they are not to be used in comparison with the 

 present formula. 



