396. Why are cloudy days colder than sunny days?

Because the clouds intercept the solar rays in their course towards the earth.397. Why are cloudy nights warmer than cloudless nights?

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.

"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?

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.

"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.

"And God did so that night: for it was dry upon the fleece only, and there was dew on all the ground,"—Judges vi.

415. What benefits result from the radiation of heat, &c.?

But for the radiation of heat, we should be subjected to the most unequal temperatures. The setting of the sun would be like the going out of a mighty fire. The earth would become suddenly cold, and its inhabitants would have to bury themselves in warm covering, to wait the return of day. By the radiation of heat, an equilibrium of temperature is provided for, without which we should require a new order of existence.

The amount of heat which our earth receives from the sun, and the economy of that heat by the laws of radiation, reflection, absorption, and convection, are exactly proportionate to the necessities of our planet, and the living things that inhabit it. It is held by philosophers that any change in the orbit of our earth, which would either increase or decrease the amount of heat falling upon it, would, of necessity, be followed by the annihilation of all the existing races. The planets Mercury and Venus, which are distant respectively 37 millions of miles, and 63 millions of miles, from the great source of solar heat, possess a temperature which would melt our solid rocks; while Uranus (1,800 millions of miles), and Neptune (whose distance from the sun has not been determined), must receive so small an amount of heat, that water, such as ours, would become as solid as the hardest rock, and our atmosphere would be resolved into a liquid! Yet, poised in the mysterious balance of opposing forces, our orb flies unerringly on its course, at the rate of 63,000 miles an hour; preserving, in its wonderful flight, that precise relation to the sun, which takes from his life-inspiring rays the exact degree of heat, which, being shared by every atom of matter, and every form of organic existence, is just the amount needed to constitute the heat-life of the world!