would be nothing at all. On the other hand, if l»he aqueous 



at^iofphere were leis than that, fuppofe half of it, correfpond- 



ing to 39° of heat, then the elfedive evaporating force would 



^^ TTS of that from boiling water; in fliorf, the evaporating 



force niijft be univerfally equal to that of the temperature of 



the water, diminiflied by that already exifting in the atmof- 



phere. In order to find the force of the aqueous atmofphere Experiment ta 



I ufiiaHy take a tall cylindrical glafs jar, dry on the oulfide, ^^^^^^"^^"^j^'J^^^ 



and fill it with cold fpring water frefti from the well; if dew mofpherk 



)be immediately formed on the outfide, I pour the water out, "^^P^"'''' 



let it ftand a while to increafe in heat, dry the outfide of the 



glafs well with a linen cloth, and then pour the water in again ; 



this operation is to be continued till dew ceafes to be formed, 



and then the temperature of the waterrauftbeobferved; andop- 



pofite to it in the table (p, 264, Vol. VI.) will be found the force It is the fame as 



of vapour in the atmofphere. This muft be done in the open ^^^' of water, 



' . ^ . , * which js at che 



ajir, or at a window ; becaufe the air within is generally more loweft temper- 

 humid than that without. Sprinoj water is generally about *^"'^' which 

 r. 1 -n ri r t /• i i i /-, coadenfes no 



^0°, and will moltly anlwer the purpole the three hotteit dew on its canr 



pionths in the year: in other feafons an artificial cold mixture taiaing veffeU 



is require^-. — The accuracy of the refult obtained this way I 



think fcarcely needs to be infifted upon, Glafs, and all other 



hard fmooth fubflances I have tried, when cooled to a degree 



below vyhat the furrounding aqueous vapour can fupport, caufe 



it to be condenfed on their furfaces into water. The degree 



of cold h ufually from 1 to 10^ below the mean heat oF the 24' 



hours ; in fummer I have often obferved the point as high as 



58° or 59°, correfponding to half an inch of mercury in force, 



and once or twice have feen it at 62° : in changeable and 



windy weather it is liable to confiderable fluduation ; but this 



is not the place to enlarge upon it. 



For the purpofeof obferving the evaporation in atmofpheric Narrative of 



temperatures I got two light tin veflfels, the one fix inches in ^^^ expcnrnents 



'^ ° -■ .... .or evaporation 



diameter, and half an inch deep, the other eight inches dia- in atmofpheric 



meter and ^ inch deep; and made to be fufpended from a *®™P"*'"'""» 



balance, like the former one. When any experiment defigned 



as a tefl of the theory was made, a quantity of water was put 



into one of thefe (generally the fix inch one, which I preferred) 



the whole was weighed to a grain ; then it was placed in an 



open window or other expofed fituation for ten or fifteen mi- / 



nutes^ and again weighed to afcertain the lofsby evaporation ; 



at 



