■ 462 Defcription of a new Hygrometer and Photometer. 



the cafe, for the cold induced never pafles certain limits. It is manifeft, therefore, that 

 the evaporablc matter muft at laft receive heat from fome other fource exadlly in propor- 

 tion as it lofes its own. Nor is it difficult to perceive how that will be derived, for 

 each fucceeding portion of air which, in diflblving its due quantity of moifture, touches the 

 humid furface, muft be cooled down to the fame ftandard, and confequently muft depofit 

 the excefs of its heat. Hence, while the repeated ahftraEllotis of heat arc uniform, the cor- 

 refponding additions of heat will continually increafe, till they at laft counterbalance the for- 

 mer, and then the deprcfled temperature of the humid furface will remain conftant. But each 

 portion of air, parting with its furplus heat, muft diffolve as much water as will faturate it, 

 and confequently will carry off a quantity of heat proportional to that moifture, and necef- 

 fary to its gafeous conftitution and union with atmofpheric air. As thefe two caufes ulti- 

 mately equalize together, the one will ferve as a meafure of the other ; that is, the cold pro- 

 duced by evaporation ivill accurately denote the degree of drynefs of the air, or its dijlance from the 

 foint of faturation. Thus, the efFeft of that procefs depends entirely on the difpofition of 

 the air, and is not modified at all by agitation, or the frequent renewal of furfaces. Such 

 means can only accelerate the term of equilibrium, in the fame manner as wind brings a 

 thermometer more quickly to the ftandard than ftill air ; though this ftandard in both cafes 

 continues the fame. 



Let it be obferved that this reafoning is independent altogether of any hypothefis. By 

 whatever mode the procefs of evaporation is carried on, our general conclufion will hold 

 true, if it be only granted that the conveying of heat and the diflblving of moifture are 

 effeds which take place at the fame time. Suppofe it even poflible for the air to hang ftag- 

 nant around the humid mafs, and'the moifture to be transferred along the contiguous 

 ftrata ; ftill the heat being conduced by the fame agents the refult would be unchanged. 

 In reahty, however, air, acquiring elafticity by its a6tion on moifture, is quickly fucceeded 

 by frelh portions, which thus maintain a perpetual circulation. 



To difcover the drynefs or humidity of the air, therefore, we have only to find the 

 change of temperature induced on a body of water infulated or expofed on all fides to evapo« 

 ration. This principle I firft eftabliftied in the year 1790- My fituation at that time afforcied 

 me all the facilities I could defire of reducing it into praiftice. Living with the late Mr. 

 Wedgwood, I enjoyed the advantages of his celebrated manufaQory, while ray efforts were 

 ftimulatcd by the example of his eminent talents, and by his liberal zeal for the improve- 

 ment of the fciences. 



I procured a cup of unglazed bifcuit-ware which is quite bibulous, about the fize and 

 {hape of a pigeon's egg. This was filled with water, and fufpended freely by a filk thread ; 

 beCde it was placed a very nice thermometer, having the ordinary degrees fubdivided into 

 tenths, and of a peculiar conftrudion, to indicate the difference between its prior and fub- 

 fequent ftate. Being plunged into the cup, the mercury quickly defcended and marked on 

 the fcale the depreffion of temperature which the water had fuffered, that is, the meafure 

 of the drynefs of the ambient air. The performance of this double inftrument was very 



complete, 



