1 10 Mr. J. H. Alexander on the Tension of Vapour of Water. 



About the same time with M. Biot, other formulae claiming 

 (like that of M. Roche) a foundation on abstract theoretical 

 principles were proposed by Mr. Russell, who has also ap- 

 plied their somewhat extensive logarithmic apparatus to the 

 calculation of a table of pressures for each degree from 32° to 

 2.50° F., and then for intervals of one or more atmospheres up 

 to fifty. This does not properly come into this discussion, 

 because the author has found it necessary to employ different 

 terms above and below the point of boiling water, and in point 

 ol fact to liave two formulaj; an inconvenience, the same in 

 kind though not in degree, with what the object of the very 

 research is to avoid. Nor do they counterbalance this by a 

 proportionate accuracy which would warrant their results to 

 be substituted for those of experiment. On the contrary, 

 starting from their common zero, 212°, they both deviate in 

 their respective directions from the curve given by observation; 

 the pressures calculated by them are, at the two extremities, 

 very much above any experimental ones. Not to trouble 

 ourselves with the part of the scale below the boiling tempe- 

 rature, where the errors are not of so much practical import- 

 ance, I give a few instances in the higher degrees, contrasted 

 with the results of the French Academy. 



The formulae of M. Regnault, to whose experimental re- 

 searches such resort has been had, are in one respect in the 

 same category as those of Mr. Russell: they are three\ one 

 adapted to pressures below the melting of ice, the second 



* Mr. Russell, in his comparison, as well as the Franklin Institute in 

 theirs, give this temperature at 30>5°-8 ; an error which has arisen from 

 hastily reducing the actual Centigrade temperature of 153°-08, as if it were 

 153°-80. 



