The earth, which during the day received heat from the solar rays, radiates the heat back into the air, and therefore becomes itself colder. All the various objects upon the face of the earth also radiate heat in a greater or lesser degree. And dew will be found to be deposited upon the surfaces of such bodies in proportion to the fall of their temperature through radiation.

"The Lord is my shepherd, I shall not want. He maketh me to lie down in green pastures."—Psalm xxiii.

348. Why is there little or no dew when the nights are cloudy?

Because clouds act as secondary radiators; and when the earth radiates its heat towards the clouds, the clouds again radiate it back to the earth.

Fig. 3.—ILLUSTRATING THE FORMATION OF DEW.

If plates of glass be laid over grass-beds, as in the engraving [Fig. 3], no dew will be deposited on the grass underneath the glass plates, although all around the grass will be completely wetted. The explanation is that the glasses, being radiators of heat, act in the same manner as the clouds, returning the heat to the bodies underneath them, and preventing the formation of dew thereon.