Daniell on a new Hygrometer. 299 



that time was shortly this : I made a mixture of two salts, cal- 

 culated to produce cold by their solution ; I then arranged half 

 a dozen drinking-glasses upon a board, each furnished with a 

 thermometer, and poured water into one of them. I added a 

 tea-spoonful of the freezing mixture, which invariably produced 

 a copious dew upon the exterior of the glass. I poured the 

 contents of the first glass into the second, and so into the third, 

 till the liquor, gradually acquiring heat by the process, arrived 

 at such a temperature as no longer to produce any condensa- 

 tion upon the vessel. This point, as marked by the thermo- 

 meter, was accurately noted, and was found to vary very con- 

 siderably, according to the different states of the atmosphere. 

 I kept a journal of the weather several months, registering 

 the variations of the barometer, thermometer, De Luc's hygro- 

 meter, and the highest degree of the thermometer, at which 

 moisture was condensed, and obtained some very interesting 

 results. I afterwards varied my apparatus in the following 

 manner : I procured five small brass hollow cylinders, three 

 inches in diameter and four inches in height, fitted with a small 

 cock in the bottom of each. These were very highly polished, 

 and placed in a frame, one immediately over another, so that by 

 turning the cock, the contents of the upper would flow into 

 that directly beneath it. I put the cold liquid into the top 

 bucket; and when steam was produced upon its surface, suffered 

 the solution to run into the next, and so into the third, till all 

 condensation ceased, when the temperature was marked as be- 

 fore. I found this apparatus very sensible; the bright surface 

 of the metal being visibly obscured by the slightest film of mois- 

 ture. These experiments were, however very troublesome, and 

 required much time to ensure accuracy. The results I forbear 

 from particularly detailing, as they are superseded by the 

 more accurate observations which I have been enabled to make 

 with the instrument which I am about to describe. 



It was not till many months after I had commenced this course 

 of inquiry, that I discovered that the idea which had occurred to 

 me, was not as new as I had conceived it to be. I found that Mr. 

 Dalton, in his " Essay upon the force of steam or vapour, from 



