fuhjijling in the Atmofphere. 35 jr 



pervening decreafed temperature. The inferior ftrata of the 

 atmofphere are fcarce ever fiiperfaturated by vapours arifing 

 from water or moiihire warmer than the air into which they 

 afcend ; for, before the point of faturation can be attained, 

 their affinity to the portion of air to which they are united is 

 weakened, and thence exceeded by the unincumbered affinity 

 of the fuperior llrata : and this happens fucceffively on to the 

 higher regions ; but with diminifhed adtivity, by reafon of 

 the diminiffied denfity of the higher ftrata, until their ulte- 

 rior progrefs is checked by faturation ; but as they are ftill 

 continually recruited from below, their quantity is at laft fo 

 far increafed that they coalefce into clouds. Here the pro- 

 ccfs recommences ; for from the furface of thefe clouds a 

 frefli evaporation often takes place, which, after fome pro- 

 grefs, is again checked in its turn, and clouds are formed at 

 a fuperior height : thefe again give room to a further evapo- 

 ration, and a new ftage of clouds is formed, until the procels 

 is at lad arrefted by the intenfe cold of the fuperior regions. 

 But the mere cold of congelation is not fufficient to arreft it; 

 for Bouguer informs us that clouds are formed 2500 feet 

 above the lower line of congelation, and that ice itfelf eva- 

 porates, though cooled, feveral degrees below the freezing 

 point, is well known. The diftance of the particles, both 

 of air and vapour, from each other, when fo far rarefied as 

 they muft be in the fuperior regions of the atmofphere, pre- 

 vents their coalefcence in any but the extreme degrees of 

 cold. 



Hence we fee that in the warmer latitudes and feafons va- 

 rious llrata of clouds may be formed one above the other : 

 Mufchenbrooek attefts that even in Holland, in Auguft 1748, 

 hediftindlly dilccrned three. Thefe diflinft ftrata, varioufly 

 eletSlrificd and olherwife circumftanced, give occafion to va- 

 rious phtenomena, the detail of which would here be mis- 

 placed. 



The clouds which commonly crown the fummits even of 

 low mountains, and often announce rain, are caufed by the 

 near approach to faturation at tliofe elevations, and its a6lual 

 attainment through the evaporation from thofe fummits: 

 But the fummits of the lofliell mountains ever crowned with 

 fnow, are generally flirowded in clouds, from the cold they 

 impart to the air in contaft with them, and the lofs of elec- 

 tricity condu«Scd away from the vapours contained in that 

 air by the mountain. 



The heights at which the loweft clouds are formed, are 



various in various latitudes and feafons ; greater in the 



warmer, and fmallcr in the colder. In latitude 54' in Cum- 



V'oL. XI\^ JN(). 5_5, h bcrlaud. 



