On a new Hygrometer or Dew-point Instrument. 143 



*. 0. *. F. 



1. 50-] 58-3 50-1 1-3538 



2. 303 58-5 50-35 1-3536 



When more hydrochloric acid was added to the solution of 

 perchloride of iron, the electromotive force of the element sunk to 

 F = 1-3250. 



Finally, with perchloride of iron and a little hydrochloric acid, 

 and solution of salt in the porous cell, we obtained, — 



F = 1-3908. 

 This constant galvanic apparatus stands thus almost exactly 

 in the middle between the constant charcoal element and the 

 constant copper element. Before the latter, it has the prefer- 

 ence of greater force and constancy ; while in common with it, 

 it has the advantage that it may be used in confined apartments 

 without evolving injurious vapours. 



XIX. On the new Hygrometer or Dew-point Instrument. By 

 A. Connell, F.R.S.E., Professor of Chemistry in the Univer- 

 sity of St. Andrews. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, St. Andrews, Nov. 29, 1854. 



I BEG to request that you will do me the favour of giving 

 insertion in your Journal to the following observations of 

 Dr. Buist of Bombay, contained in a letter which he was so good 

 as to address to me, after making use, on his voyage to India, of 

 the hygrometer described by me in your Number for last August. 

 Dr. Buist is well known as a distinguished meteorologist, both 

 theoretically and practically, and presided over the Meteorolo- 

 gical Observatory at Bombay for a considerable time, and in that 

 capacity published many highly valuable reports on this science. 

 I may therefore well consider any opinion of his in regard to a 

 meteorological instrument as possessing peculiar value. Un- 

 luckily his hand-writing is occasionally so indistinct and so full 

 of contractions, that the task of reading his letters, however 

 interesting they may be in their details, is frequently a very dif- 

 ficult one ; and on the present occasion I have found it to be 

 impossible to make out many portions of his letter, and an 

 individual here accustomed to his hand was equally unsuccessful. 

 What, however, I have quoted below has been distinctly made out ; 

 and it is quite sufficient to establish two points; first ,that Dr. Buist 

 found this hygrometer fully to answer its purpose as a dew-point 

 instrument ; and secondly, that he prefers it to the elegant hy- 



