246 STATE OF VAPOUR SUBSISTING IN THE ATMOSPHERE. 



the procefs is at laft arretted 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 

 evaporates, 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 rarified as they 

 muft be in the fuperior regions of the atmofphere prevents their 

 coalefcence in any but the extreme degrees of cold. 

 Cloud sin ftrata Hence we fee that in the warmer latitudes and feafons, va- 

 atove each r j ous ft rata Q f c i ouc j s mav De f orm ed one above the other ; 

 MufchenbRook atteft*, that. even in Holland, in Auguft 1748, 

 he diftinclly difcerned three. Thefe diftind ltrata, varioufly 

 * v electrified and otherwife circumftanced, give occalion to vari- 

 ous phacnomena, the detail of which would here be mifplaced. 

 Clouds near The clouds which commonly crown the fummits even of low 



mountain tops, mountains, and often announce rain, are caufed by the near 

 approach to faturation, at thofe elevations, and its actual at- 

 tainment through the evaporation from thofe fummits. But 

 the fummits of the loftieft mountains ever crowned with fnow, 

 are generally (hrouded in clouds from the cold they impart to 

 the air in contact with them, and the lofs of electricity con- 

 duced away from the vapours contained in that air, by the 

 mountain. 

 Heights at which The heights at which the lowed clouds are formed are va- 

 clouds are r ; ous j n various latitudes and feafons ; greater in the warmer 



and fmaller in the colder. In latitude 54* in Cumberland, 

 Mr. Crofthwaite obferved none lower than 2700 feet, and 

 none higher than 3150 in the eourfe of feveral years*. But 

 this country being mountainous they are probably lower than 

 in others under the fame parallel. Lambert, in Berlin, lati- 

 tude 52° 32', in the month of July 1773, found their height 

 7792 (eet, thermometer 65°, and the barometer fomewhat be- 

 low its mean height f. Schuckburg alio remarks, that clouds 

 frequently reft below the fummit of Sateve, whofe height is 

 2831 feet. Phil Tranf. 1777, p. 538, and Gentil at Pondi- 

 cherry, latitude 12°, obferved fome at the height of 1024-0 

 feet. 2 Voy. p. 79. 



* D' Alton's Meteorological Obfervations, p. 41. 

 f Mem. Berlin, 1773, p. 44. 



The 



