Because they both have their origin in the _humidity of the atmosphere_. The temperature of the earth having fallen, dew has been deposited; but, at the same time, the condensation of the vapour in the air _has formed a screen over the surface of the earth_, which has checked _the further radiation of heat_, and, consequently, _the further formation of dew_. The sun rises, therefore, upon an atmosphere charged with visible vapour at the earth's surface, and his first sloping rays, _having little power to warm the atmosphere_, the mist _continues visible for some time_. 408. _What effect have winds upon the formation of dew?_ Winds, generally, and especially when rapid, prevent the formation of dew. But those winds that are moist, and _contribute to the formation of clouds_, indirectly aid the formation of dew _through the formation of clouds_, and also by the _moisture they impart to the air_. [Verse: "And Gideon said unto God, * * * Let it now be dry only upon the fleece, and upon all the ground let there be dew."] 409. _Why does the humidity of the atmosphere sometimes form clouds, and at others form fogs, mists, dews, &c.?_ The result depends upon the varying _temperature_, _motion_, and _direction_ of the atmosphere. A _warm light atmosphere_, of a few day's duration, will elevate the vapours to the region where they are formed into _clouds_. A _chill air_, lying upon the surface of the warmer earth, will occasion _mists_ or _fogs_. A _cold earth_, acting upon the vapours contained in a _warmer atmosphere_, will condense them and occasion _dews_. 410. _Why are frosty mornings usually clear?_ Because, in the cold atmosphere which preceded the frost, _there was but little evaporation_; and now that the frost has set in, the vapours that existed have become _frozen_ in the form of _hoar-frost_. 411. _Why are clear nights usually cold?_ Because the "screen" afforded by the clouds does not exist; therefore the heat of the earth escapes, while the vapours of the air are abstracted from it by condensation into dew, thereby imparting great _clearness to the nights_. 412. _Why are hoar-frosts, or, as they are termed, "white frosts," so frequent, and "black frosts" so unusual?_ Because white, or _hoar frosts_, result from the _coldness of the earth_, which, from its great radiating power, is always varying. But _black-frosts_ result from the _coldness of the air_, which is liable to less variation of temperature than the earth. 413. _What is a black-frost?_ A _black-frost_ results from the _coldness of the atmosphere_, which is at the time overshadowed by a dull cloud, giving a darkness to everything, and a leaden appearance to the _frozen surface of water_. 414. _Why are black-frosts said to last?_ Because as they result from the temperature of the air, which is less likely to vary than that of the earth, there is a probability that the coldness thereof _will last for some time_.