De/cription of a new Hygromettr and Photometer. 461 



IX. 



De/cription of an Hygrometer and Photometer. By Mr. Jour; Leslie. Communicated 



by the Isvestor. 



X HESE two inftruments, in their form analogous, however dlfllmilar in their appli- 

 cation, were fuggefted by the fame train of rcfledions. Experience has amply confirmed 

 the juftnefs of the principles on which they are founded. But the developement of thofe 

 principles, befides the profefled obje6ls of the inftruments, exhibits feveral clafles of nice 

 and important phyfical inquiries for which they are moft happily calculated. Regarding them 

 as valuable acquifitions to our philofophical apparatus, I was induced to beftow much atten- 

 tion on their improvement, and have through perfeverance fucceeded to my utmoft 

 wiflies. By fucceflive fteps, they have at length attained that fimplicity of conftru6lion in 

 which the mind refts fatisfied. I confider it my duty, therefore, without farther delay to 

 communicate them to the public. Of the refearches, however, which, by their means, I 

 have made, embracing a confiderable extent of inveftigation, I muft defer the account, as 

 it will form part of a work fliortly to be committed to the prefs, and which it is prefumed 

 will be found to contain a variety of new views in the phyfical fciences. For the prefent, 

 I (hall content myfelf with briefly ftating the progrefs of ftiy ideas, with accurately de- 

 fcribing the mode of conftru£ting thofe inftruments, and with pointing out in general 

 terms the fubjefts of inquiry for which, by their delicacy and facility of application, they 

 are peculiarly fitted. 



My attention was firft direded to the fubjeft of hygrometry by the perufal of the late 

 Dr. Hutton's very ingenious Theory of Rain. The affedlion of air to humidity, as differ- 

 ently modified by heat, appeared to perform a moft important part in the oeconomy of 

 nature. But to afcertain the adual difpofition of the atmofphere was ftill a deftderatum. 

 Senfible of the imperfedion, if not the abfolute inutility, of the ordinary contrivances for 

 that purpofe, I was foon convinced that they depended not only on arbitrary aflumptions, 

 but on hypothefis altogether erroneous. Abandoning, therefore, the various expedients 

 to ferve as hygrofcopes, I fought to difcover other principles, and to introduce that geo- 

 metrical precifion which fo eminently diftinguiflies the more cultivated branches of fcience. 

 An examination of the circumftances attending the adlion of air on a humid furface 

 feemcd to offer the beft profpeft of fuccefs. 



It is well known that evaporation produces cold, but the nature of the procefs and the 

 true conditions which determine the eSe£^, require ftill to be inveftigated. Water, ex- 

 pofed to free air, fuiFers a continued wafte by exhalation -, it muft at the fame time fuftain 

 a correfponding expencc of heat. In this view, therefore, the temperature of the humid 

 mafs (hould undergo a progrefHve and unlimited diminution. Yet fuch is not aftually 



Vol. III. — January i8oo. 3 O the 



