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_!