35$ On the Modifications of Clouds, and 



the evening returns, if the vapour has not been carried oflT 

 in part by currents, it will often happen that more remains 

 difi'used in the general atmosphere than the temperature of 

 the night will permit to subsi'^t under the full pressure of 

 the aqueous atmosphere. A decomposition of the latter 

 then commences, and is continued until the general tempe- 

 rature and acjucous pressure arrive at an equilibrium, or until 

 the returning sun puts an end to tlic process. The caloric of 

 the decomposed vapour goes to maintain the general tempe- 

 rature; while the water iS separated in drops, which, minute 

 as they are, arrive successively at the earth in the space of 

 a few hours. That the ordinary production of dew is by a 

 fcal descent of water from the atmosphere, and not by de- 

 composition of vapour on surfaces previously cooled (as in 

 the experiment already mentioned), any one may readily 

 be convinced bv observing in what abundance it is collected 

 by substances w hich are wholly unfit to carry ofl" the requisite 

 quantity of caloric for the latter effect. 



Of the Forination of the Stratus. 



The case which has been just stated, of the decomposition 

 of vapour hij the almo'ipkere in u'hich it is already diffused, 

 goes but a little way in explanation of the production of a 

 cloud consisting of visible drops, and confined to a certain 

 space in the atmosphere : nuich less does it enable us to ac- 

 count for the diversity of its situations and appearances. In 

 attempting this we will begin with the stratus, as the most 

 simple in structure, and the next step, as it were, in the pro- 

 gress oi nuhificalion. 



When dew falls upon a surface the temperature of which 

 is superior to that of the atmosphere, it is plain that it will 

 not continue there, but will be evaporated again : and a body 

 so circumstanced will continue to refund into the atmo- 

 sphere the whole of the water thus gradually deposited on it, 

 so long as its substance can supply the requisite tempera- 

 ture to the surface. Moreover, water either in ma.ss, or 

 diftlised among sand, clay, vegetable earth, &c. will con- 

 tinue to be evaporated therefrom with a force proportioned 

 to its temperature, so long as the latter continues above that 

 point which coiiiilerbalances the pressure of the aqueous at- 

 mosphere. 



From the^^e causes it happens, that after the earth has 

 been superficially dried by a continuance of sunshine, and 

 heated, together witli the lakes and rivers, to a considerable 

 depth, there is an almost continual emission of vapour into 

 Uxt aUiiosphcr? by night. 



Thi* 



