Because the clouds _radiate back to the earth_ the heat which the earth evolves? Because, also, the clouds radiate to the earth the heat they have _derived from the solar_ rays during a cloudy day. 398. _Why is the earth warmer than the air during sunshine?_ Because the earth freely _absorbs the heat of the solar rays_; but the air derives _comparatively little heat_ from the same source. 399. _Why does the earth become colder than the air after sunset?_ Because the earth _parts with its heat freely by radiation_; but the air does not. 400. _Why do glasses, mats, or screens, prevent the frost from hitting plants?_ Because they prevent the _radiation of heat from the plants_, and also from the earth beneath them. 401. _Why are the screens frequently covered with dew on their exposed sides?_ Because they radiate heat from _both their surfaces_. A piece of glass, laid horizontally over the earth, would radiate heat both _upwards_ and _downwards_. But on its lower surface it would _receive_ the radiated heat of the earth, while from its upper surface it would _throw off its own heat_ and become cool. Therefore dew would be deposited upon the _upper_, but not on the _under_ surface. 402. _Why does dew rest upon the upper surfaces of leaves?_ Because the under surfaces receive the _radiated warmth of the earth_. [Verse: "And it was so: for he rose up early on the morrow, and thrust the fleece together, and wringed the dew out of the fleece, a bowl full of water."] 403. _Why are cultivated lands subject to heavier dews than those that are uncultivated?_ Because cultivation breaks up the hard surface of the earth, and thus _its radiating power is increased_. 404. _Why is the gravel walk through a lawn comparatively dry while the grass of the lawn is wet with dew?_ Because gravel is a _bad radiator_, but grass is a _good radiator_. 405. _What benefit results from this arrangement?_ In cultivated lands, where moisture is required, it is _induced_ by the very necessity which demands it; while in rocky and barren places, where it would be of no good, dew _does not form_. 406. _Why does little dew form at the base of hedges and walls, and around the trunks of trees?_ Because those bodies in some degree _counteract the radiation_ of heat from the earth; and they also _radiate heat_ from their own substances. 407. _Why do heavy morning dews and mists usually come together?_