THE KATURAI/ HISTORY OF CLOUDS. 53 



Its particles must be niu,t,ually repulsive, and cannot come 

 into contact without a change of state : the same may be said 

 of the respective clouds in this modification, when they do 

 not differ too much in surface. 



Of the Nature of the Cirro-stnitns, 



When a portion of the atmosphere, charged with vapour, Nature of the 

 is brought over a tract of land of lower temperature than it- cirro-stiatus, 

 self, its caloric is abstracted in sufficient quantity, usually 

 to occasion a decomposition of some of the vapour, and a 

 'consequent general turbidness. 



The sweating, as it is improperly called, of walls and Dampness on 

 pavements in a thaw, and when rain is about to come on, is ^^''* '^^• 

 from this cause; the vapour being decomposed on their sur- 

 faces. The mist which ensues at ihese times obscures dis- Mist. 

 tant objects, and occasions the trees, agaiiist which it is borne 

 by the wind, to drip plentifully. It is in fact a cirro-stratus 

 in contact with the Earth, and no phenomenon is more fami- 

 liar to the inhabitants of hilly tracts. The same general de- 

 pression of temperature may happen in another way, and 

 higher in th& atmosphere. When a cold and moist air flows 

 over a warmer vaporous one, it is obvious, that the former 

 may be warmed, and become more transparent, at the ex- 

 pense of the latter; which, from the same cause, must be- 

 come turbid. The haze thus produced will not subside with 

 the uniform motion of dew, but rather in sheets, becoming 

 more dense as they descend, both from the approximation of 

 theii* particles, and addition from the vapour they meet with. 

 But the cirro-stratus is far from assuming always the simple 

 form, to which the mere effects of gravity might be supposed 

 to give rise. It exhibits changes, which can only be attri- 

 buted to the acquisition, or passage through it, of such small 

 portions of electricity, as in a humid medium we may con- 

 ceive a cloud to be susceptible of. On these occasions it 

 tends either to the state of cirrus, or that of cirro-cumulus, 

 of which we shall treat presently. 



The reason of the prognostic afforded by the cirro-stratus Indications, 

 win now be evident. It gives us notice of a change in the 

 »tate of the superior atmosphere, which we could not other- 

 wise 



