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_. [Verse: "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_. [Illustration: 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. 349. _Why does dew form most abundantly on cloudless nights?_ Because the heat which is radiated by the earth does not return to it. The temperature of the earth, and the air immediately upon its surface, is therefore lowered, and dew is formed. It has been observed that sheep that have lain on the grass during the formation of dew have their backs completely saturated with it, but that underneath the line where their bodies turn to the earth, their coats will be dry. In the same manner glass globes suspended in the air, on dew forming nights, will be found loaded with globules of dew upon the top, but there will be no appearance of moisture underneath. [Verse: "Dost thou know the balancings of the clouds, the wondrous works of him which is perfect in knowledge."--JOB XXXVII.] 350. _Why are star-lit nights usually colder than cloudy nights?_ Because heat is _radiated_ from the earth, and passes away into the utmost regions of the atmosphere. 351. _Why is there little dew under branches of thick foliage?_ Because the foliage _acts as a screen_, which prevents the radiated heat of the earth from passing away. 352. _Why is there no dew formed on windy nights?_ Because, as winds generally consist of dry air, they _absorb and bear away_ the atmospheric moisture. 353. _Why are valleys and low places chiefly subject to dew?_ Because the elevated lands around them _prevent the disturbance of the air_ in which the moisture is held. 354. _What bodies are most likely to be covered with dew?_ All bodies that are _good radiators of heat_, such as wool, swansdown, grass, leaves of plants, wood, &c. 355. _What bodies are likely to receive little dew?_