28 DEW FOG CLOUDS. 



enter phenomena of this kind, and the circumstances under which 

 they appear or disappear. If he has an opportunity, as in a 

 high station, he should endeavor to ascertain if there is an upper 

 limit, and what is the thickness of the layer of haze or dry mist. 

 Observations made in the Alps prove that the atmosphere is 

 often entirely free from it at a height of two thousand feet, when 

 it is very intense in the plain. Does a thunder-storm or rain 

 always cause it to disappear? Do the prairie fires have any 

 relation with kindred phenomena? Does it appear more fre- 

 quently in certain seasons than in others ? 



HYDRO-METEOROLOGICAL PHENOMENA. 



DEW. 



The dews, especially when they are abundant, and 

 The white frosts, or frozen dew, particularly the first and last 

 of the year, and their intensity, must be entered. 



FOG. 



Fog The moment must be noted when it forms and when it 

 dissipates, as fatting fog, rising fog ; its density, as dense fog, 

 slight fog. 



Mists hanging over forests, moors, meadows, rivers, or the 

 like. 



Notice must be carefully taken of the time of their appearance 

 or disappearance ; these are the most important facts in regard 

 to them. 



These fogs must not be confounded with the dry fog, which 

 belongs to another class of phenomena, which have been spoken 

 of above. 



CLOUDS. 



For noting these the observer must go out to a place entirely 

 free, in case his residence has too confined a horizon. 



The cloudiness or the quantity of clouds, after some practice, 



