234 TENTH ANNUAL REPORT OF 



wliere, an intense dry mist, wliicli is, probably, a different pbenome- 

 non, sometimes follows great earthquakes or volcanic eruptions.^ The 

 observer will carefully enter phenomena of this kind, and the circum- 

 stances under which they appear or disappear. If he has an oppor- 

 tunity, as ip 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 frequently 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 dis- 

 sipates, as falling 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, can be 

 easily estimated, in accordance with the following scale. Thus, we 

 understand by — 



0. A clear sky, entirely free from clouds ; 



10. The whole sky covered with clouds, or a dense fog, or rain; and 

 by 1, 2, 3, 4, 5, 6, 7, 8, 9, the different degrees of cloudiness 

 which lie between these : 



1. Denotes, for instance, nine times as much blue sky as clouds ; 



5. An equal amount of clouds and blue sky ; 



9. Nine times more clouds than blue sky. 



If, on account of the locality, it is impossible for the observer to 

 estimate the quantity of clouds in this way, he can make use of the 

 following expressions, which will mark at the same time the medium 

 character of tlie aspect of the sky during each day : 



Wcl. Wholly clear; a sky entirely free from clouds. 



CI. Clear ; when at least two-thirds of the sky is unclouded. 



