445 



with the wettened bulb in these experiments ; and that the 

 values which they give for the elastic force of vapour, for 

 these temperatures, are too low. 



{Additional Note by Dr. Apjohn.) 

 " M. Kupffer has recently been engaged in discussing the 

 value of m, the coefficient in the hygrometric formula. In a 

 note read by him at the Petersburgh Academy, on the 22nd 

 of last January, and published in the Bulletin Scientifique, 

 No. 132, after a detailed examination of the experiments 

 of August, Gay-Lussac, Erman, Bohnenberger, and 

 Kamtz, he comes to the conclusion, that the theoretic value 



48 a 

 of m, or , agrees sufficiently well with that deduced 



from the most trust-worthy comparative observations on 

 the dew-point. The formula which he definitively adopts, is 



/' -/' - -267 {t - t') ; 

 ./' and/" being expressed in tenths of an English inch, and 

 t and t' in degrees of Reaumur's thermometer. But this, ex- 

 pressing/ 7 and/" in inches, and t and t' in degrees of Fah- 

 renheit's scale, becomes 



/'=/■— -01142 (#_*>, 



an expression in which the coefficient is almost identical with 

 that which has been deduced above from the three series 

 of experiments to which I have so often referred. This for- 

 mula, however, M. Kupffer observes, gives results in ac- 

 cordance with direct observation, only when the table of the 

 elastic force of vapour drawn up by Kamtz is employed; 

 from which he infers, that it alone represents with accuracy 

 the relation between the tension and the temperature of 

 steam — an opinion from which, notwithstanding the high 

 authority of M. Kupffer, I am compelled to differ, on the 

 grounds already stated by Professor Lloyd. 



" There is another statement of less importance made by 



