of the vapour, and p the existing atmospheric pressure, we 



shall have 



P 



P—f 

 from which we deduce 



It was not my original intention to make any experi- 

 ments upon the force of aqueous vapour, believing the table 

 which I have hitherto employed, and which was calculated 

 by the author of the article " Hygrometry," in Brewster's 

 Encyclopaedia, from the experiments of Dalton, to have been 

 sufficiently exact. But the correctness of this table having 

 been indirectly called in question by so high an authority as 

 Mr. Kupffer, who has come to the conclusion, that the table 

 of the force of aqueous vapour, given by a German meteoro- 

 logist, of the name of Kamtz, is alone to be relied upon, I 

 resolved to commence with the vapour of water, in the hope 

 that I might be able, by the results of direct experiment, to 

 corroborate a conclusion previously drawn by Professor 

 Lloyd, from a discussion of some hygrometrical observations 

 of mine, viz., that for temperatures within the atmospheric 

 range, the table of Kamtz is less accurate than that of 

 Dalton, the values given in the former being all too low. 



The apparatus I have employed in my experiments is 

 composed of a glass ball prolonged on the one side into a 

 short tube, furnished with a cap and stopcock, and, on the 

 other, into a long tube of somewhat smaller diameter, di- 

 vided into 100 equal parts, each being .042 of a cubic inch, 

 or the .001 of the total capacity of ball and tubes down as 

 far as the division marked 1000. 



The first step consisted in filling this vessel with dry air, 

 which was done in the following manner : into the extremity 



