341. _Since all bodies expand by heat and contract by cold, why does
water, when it reaches the freezing point, expand?_
Because, in freezing, water undergoes crystallization, in which its
particles assume a new arrangement occupying _greater space_.
342. _Why does water never freeze to a great depth?_
Because the covering of ice which is formed upon the surface of the
water prevents the cold air from continuing to draw off the _caloric_
of the water.
[Verse: "For he saith to the snow, Be thou on the earth; likewise to
the small rain, and to the great rain of his strength."--JOB XXXVII.]
343. _Why has this exceptional law of the expansion of water, when
freezing, been ordained?_
Because, but for this, deep waters might be frozen through their whole
depth. This would destroy the myriads of fish and other living things
that inhabit the water. Parts of the earth, now clad in verdure, would
be lost in eternal winter; and even in the most temperate zones it
would take months to effect a thaw; and thawing would be attended with
such floods and subterranean commotion as are terrible to contemplate.
344. _Why are bed-room windows sometimes covered with crystalline forms
on winter mornings?_
Because the vapour of the breaths of the inmates has condensed upon the
window-panes, and formed water. The water has frozen with the cold, and
exhibits the beautiful crystalline forms into which its particles are
arranged.
Here we have another domestic illustration of the great laws of
nature. It is the same law which locks the arctic regions in ice
and decorates our window-panes. This beautiful phenomenon is
usually witnessed by us on frosty mornings when we rise from our
beds. It has a story which the observer of nature may read in its
sparkling eyes. It tells that, although without the air is biting
cold, God has wrapped a mantle around the face of nature to keep
it from injury; and that the earth and the waters, though looking
chilled and dead, have still the warmth of life preserved in their
bosoms.
345. _What is dew?_
Dew is _watery vapour_ diffused in the air, _condensed_ by coming in
contact with bodies _colder than the atmosphere_.
346. _Why does the air become charged with watery vapour?_
Because, during the day, under the influence of the sun's rays, vapours
are _exhaled_ from all the moist and watery surfaces of the earth.
These vapours are _held in suspension_ in the atmosphere until, by a
change in the temperature of the earth, and of bodies on the surface of
the earth, they are _condensed_, and deposited in translucid drops.
347. _What causes the decline of temperature that favours the
deposition of dew?_
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?_
All _bad radiators of heat_, such as polished metal surfaces, smooth
stones, and polished surfaces generally. Dew will be found to lie more
abundantly upon rough and woolly leaves than upon smooth ones.
356. _At what period of the night is the largest amount of dew usually
formed?_
It is generally supposed that dew is formed _most copiously_ in the
_mornings_ and _evenings_. But _such is not the case_. It is deposited
at all hours of the night, but _most plentifully after midnight_.
357. _Why is dew formed most plentifully after midnight?_
Because, as _radiation_ has been going on for some time, the
temperature of the earth, and of various bodies upon it, has been
_considerably reduced_.
[Verse: "Out of whose womb came the ice? and the hoary frost of
heaven, who hath gendered it?"--JOB XXXVIII.]
358. _In what parts of the world is the maximum of dew formed?_
In warm lands near the sea, or in the vicinity of rivers or lakes,
as the localities of the Red Sea, the Persian Gulf the coast of
Coromandel, in Alexandria, and Chili.
359. _In what parts of the world is the minimum of dew formed?_
It is quite absent in arid regions, in the interior of continents, such
as Central Brazil, the Sahara, and Nubia.
360. _Why is dew seldom formed at sea?_
Because of the defective _radiating_ quality of the surface of _water_.
361. _Why is a heavy dew regarded as the precursor of rain?_
Because a heavy formation of dew indicates that the air is _saturated
with moisture_.
362. _What is hoar-frost?_
Hoar-frost is frozen dew.
363. _Why is hoar-frost said to foretell rain?_
Because it shows that the air is saturated with moisture, and the
temperature of the air being low, the vapours are _likely to condense_,
and produce _showers_.
364. _What is honey-dew?_
Honey-dew is the name applied to a _sweet and sticky moisture_
occasionally deposited upon the leaves of plants. It is, however, an
error to call it _dew_, as it is procured by a class of _insects_
termed _aphides_.
365. _What are fogs?_
Fogs are _clouds_ formed near the earth's surface_;_ but London fogs
are distinguished from clouds by the fact that they embrace in their
vaporous folds the _smoke_ and _volatile matters_ imparted to the air
by the operations of man. This is also the case with fogs generally
that arise near large towns.
[Verse: "Hath the rain a father? or who hath begotten the drops of
dew?"--JOB XXXVIII.]
366. _Why are certain coasts liable to almost perpetual fogs?_
Because of local or geographical agencies which contribute to their
production. The coasts of California are almost constantly wrapped
in fog; and, almost as constantly, the western coast of the American
continent, as far south as Peru. Newfoundland, Nova Scotia, and
Hudson's Bay, are all subject to dense and frequent fogs arising
from the condensation of vapour from the water flowing from the hot
Gulf-stream, coming in contact with the colder air.
367. _What are dry fogs?_
Dry fogs are characterised by a dull opaque appearance of the
atmosphere. They are most common in certain parts of North America,
though they sometimes occur in Germany and in England. They are
generally referred to the _electrical state of the atmosphere_, but the
theory of them is still a matter of doubt.
368. _What is a mist?_
The term _mist_ is generally applied to vapours that rise over _marshy
places_, or the surfaces of _water_, and roll or move over the land.
369. _What is the difference between a mist and a fog?_
Fogs, as they are known to us, generally arise over the _land_, and are
usually mingled with the smoke of large towns. Mists generally arise
over water, or wet surfaces.
370. _Why do mists and fogs disappear at sunrise?_
Because the condensed vapours are again _expanded_ and _dispersed_ by
the heat of the sun's rays.
371. _Why do fogs frequently rise in the morning and fall again in the
evening?_
Because, warmed by the sun's rays, they become more rarefied, and fly
away at an altitude where they _appear_ to be altogether dispelled; but
at night, when the earth _cools by radiation_, the vapours near the
earth _again condense_, and settle in the _form of fog_.
372. _Why do fogs sometimes rest upon a given locality for several days
together, and then disappear?_
They are probably kept near to the surface of the earth by a
superstratum of cold air. A cold air lying _above_, or a cold air lying
_below_, might equally contribute to keep a fog near the surface of a
particular part of the earth, until a _flow of wind_, or a _fall of
rain_, altered the atmospheric condition.
[Verse: "He bindeth up the waters in his thick clouds; and the cloud
is not rent under them."--JOB XXVI.]
There are many interesting facts connected with the history of
dew. It has attracted the attention of natural philosophers in all
ages. But its true theory was never understood until recently.
The ancients imagined that dews were shed from the stars; and the
alchemists and physicians of the middle ages believed that the
dew distilled by night possessed penetrating and wonder-working
powers. The ladies of those times sought to preserve their beauty
by washing in dew, which they regarded as a "celestial wash." They
collected it by placing upon the grass heaps of wool, upon the
threads of which the magic drops clustered.