182 Dr. Apjohn*s Formula for inferring the Dew-point 



against the necessity of metallic contact, — for and against the 

 origin of voltaic electricity in chemical action, — a duty which 

 I may not undertake in the present paper*. 

 [To be continued.] 



XXX. Formula for inferring the Dew-point from the Indica- 

 tions of the Wet-bulb Hygrometer, By James Apjoh n, M.D., 

 Professor of Chemistry in the Royal College of Surgeons, 

 Ireland. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 



WILL you allow me to give publicity, through your va- 

 luable Journal, to the following formula for inferring 

 the dew-point from the indications of the wet-bulb hygrome- 

 ter? It formed the subject of a paper read by me in Novem- 

 ber last before the Royal Irish Academy, and which will ap- 

 pear in their Transactions. As, however, the next number 

 of these will not be published for some months, I am per- 

 mitted by the Council of the Academy to present the sub- 

 stance of my communication to the world through the medium 

 of one of the scientific periodicals. I shall not enter here 

 into any historical or critical remarks upon the attempts made 

 by Leslie, Anderson, and others, to elucidate the same sub- 

 ject, for the methods of all are known to be imperfect ; and of 

 this no better proof can be given than that the theory of the 

 moist-bulb hygrometer is found among the questions sub- 

 mitted by the first meeting of the British Association held at 

 York to the renewed consideration of philosophers. For the 

 present I shall confine myself to an explanation of the me- 

 thod of investigation which I myself pursued, and of the re- 

 sults to which it led. 



When in the moist-bulb hygrometer the stationary tem- 

 perature is attained, the caloric which vaporizes the water is 

 necessarily exactly equal to that which the air imparts in de- 

 scending from the temperature of the atmosphere to that of 

 the moistened bulb. The air, also, which has undergone this 



♦ I at one time intended to introduce here, in the form of a note, a 

 table of reference to the papers of the different philosophers who have 

 referred the origin of the electricity in the voltaic pile to contact, or to 

 chemical action, or to both; but on the publication of the first volume of 

 M. Becquerel's highly important and valuable Traite de CElectricitc et du 

 Magnetism, I thought it far better to refer to that work for these refer- 

 ences, and the views held by the authors quoted. See pages 86,91, 104, 

 110, 112, 117, 118, 120, 151, 152,224, 227, 228, 232, 233, 252, 255, 257, 

 258, 290, &c.— July 3rd, 1834. 



