FOG is generally applied to _vapours condensed on land_, especially if those vapours are _laden with smoke_. Q. _What is the reason why condensed vapour sometimes forms into CLOUDS, and sometimes into FOG?_ A. If the surface of the EARTH be _hotter than the air_, then the vapour of the earth (_being chilled by the cold air_) becomes FOG: but if the AIR be _hotter than the earth_, the vapour _rises through the air_, and becomes CLOUD. Q. _If cold air produces FOG, why is it not foggy on a FROSTY MORNING?_ A. 1st--Because _less vapour is formed on a frosty day_; and 2ndly--The vapour _is frozen upon the ground_ before it can rise from the earth, and becomes HOAR-FROST. Q. _Why are FOGS more general in AUTUMN than in spring?_ A. In spring _the earth is not so hot_ as it is in autumn. In AUTUMN the _earth_ is generally _warmer than the air_; and, therefore, the vapour (issuing from the earth) _is condensed into fog_ by the chill air. Q. _Why are FOGS more common in VALLEYS than on HILLS?_ A. 1st--Because valleys _contain more moisture than hills_: and 2ndly--They are _not exposed to so much wind_, (which dissipates the vapour). Q. _How does WIND dissipate FOGS?_ A. Either by _blowing them away_; or else by _dissolving them into vapour again_. Q. _What is HOAR-FROST?_ A. There are two sorts of hoar-frost: 1.--FROZEN DEW: and 2.--FROZEN FOG. Q. _What is the cause of the GROUND hoar-FROST, or frozen DEW?_ A. Very _rapid radiation of heat from the earth_; in consequence of which, the _surface is so cooled down_, that it _freezes the dew_ condensed upon it. Q. _Why is HOAR-FROST seen only after a very CLEAR NIGHT?_ A. Unless the night has been very clear indeed, the earth will not have thrown off heat enough by radiation, to _freeze_ the vapour condensed upon its surface. Q. _Why does HOAR-FROST very often COVER the GROUND and TREES, when the water of rivers is not frozen?_ A. Hoar-frost is not the effect of cold in the _air_, but the cold of the _earth_ (produced by excessive radiation); in consequence of which, _the dew_ (condensed upon it) _is frozen_. Q. _Why is the HOAR-FROST upon GRASS and VEGETABLES much thicker than that upon lofty TREES?_ A. Because the air (resting on the _surface of the ground_) is much colder after sun-set, than the _air higher up_; in consequence of which, more vapour is condensed and frozen there. Q. _Why is the AIR (resting on the surface of the EARTH) colder than that in the HIGHER regions?_ A. Because the _earth radiates more heat_ than the _leaves of lofty trees_; and, therefore, _condenses and freezes_ the vapour of the air _more rapidly_. Q. _Why are EVERGREENS often FROST-BITTEN, when lofty trees are NOT?_ A. Evergreens do not _rise far above the surface of the earth_; and (as the air _contiguous to the earth_ is much _colder than that in the higher regions_) therefore, the _low evergreen is often frost-bitten_, when the lofty tree is uninjured.