[ 266 ] 



XXXII. Experimental Investigation of a Formula for in- 

 f erring the Dew-point from the Indications of the Wet-bulb 

 Hygrometer. By James Apjohn, M.D., Professor of Che- 

 mistry in the Royal College of Surgeons, Ireland.* 

 A T the meeting of the Academy held in November last, 

 •*** I was permitted to read a short memoir f on the subject 

 of a formula, at which I had a considerable time previously 

 arrived, for inferring the dew-point from the indications of 

 the moist-bulb hygrometer. This formula was deduced alto- 

 gether from general considerations, and, though satisfied from 

 some hasty observations of my own that it represented facts 

 with considerable accuracy, I was not, at the time, in posses- 

 sion of evidence which could be considered as establishing 



• • • o 



this important point in an unequivocal manner. The table 

 which is subjoined to my paper undoubtedly shows that, 

 within certain limits, my formula is in accordance with expe- 

 riment; but the observed depressions in the table are, generally 

 speaking, so small, that a formula in itself incorrect might, it 

 must be admitted, yield results which would deviate from the 

 observed dew-points, by quantities not exceeding the possible 

 errors of observation. Berzelius, for example, states (Traite 

 de Chimie, torn. viii. 6. 254?.) that from the experiments of Au- 

 gust, Bohnenberger, and others, it appeared that the tempera- 

 ture of a thermometer, with moistened bulb, was an arithmetic 

 mean between that of the air and the dew-point ; and this rule, 

 which would make /" = 2 t' — t, though utterly erroneous, 

 would apply to the table appended to my paper nearly as well 

 as the formula I have deduced. The validity, therefore, of 

 my method required to be more rigorously tested, and having 

 been for some time engaged in experimental researches insti- 

 tuted with this object, which have led to interesting, and to 

 me most satisfactory results, I am anxious to submit them with 

 as little delay as possible to the judgement of the Academy. 



The equation which, as I believe, comprehends the theory 

 of the wet-bulb hygrometer is as follows : 



f>=f'->ndx£, 



* Communicated by the Author ; having been read before the Royal 

 Irish Academy, on April 27, 1835. 



+ The substance of this memoir has since been published in the Lond. 

 and Edinb. Phil. Mag., vol. vi. p. 182. Those who would refer back to 

 it should make the following corrections. Page 183, line 25, for " water'' 



read air. Same page, line 33, for (27x448*' X — ) substitute 



^27x448"+? X— Y Page 184, line 2, for 37, in both places where it 

 occurs, write 87. Page 186, line 17, for " elimination" read dimension. 



