CHAPTER XIX.

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_.

[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?_

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_.

[Verse: "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_!