A Guide to the Scientific Knowledge of Things Familiar
CHAPTER XVI.
4.--RADIATION.
Q. _What is meant by RADIATION?_
A. Radiation means _the emission of rays_: thus the sun radiates both light and heat; that is, it emits _rays of light and heat_ in all directions.
Q. _When is heat RADIATED from one body to another?_
A. When the two bodies are _separated by a non-conducting medium_: thus the sun _radiates_ heat towards the earth, because the _air comes between_ (which is a very bad conductor).
Q. _On WHAT does RADIATION DEPEND?_
A. On the _roughness_ of the radiating surface: thus if metal be _scratched_, its radiating power is increased, because the _heat has more points to escape from_.
Q. _Does a FIRE RADIATE heat?_
A. Yes; and because _burning fuel emits rays of heat_, therefore we _feel warm_ when we stand before a fire.
Q. _Why does our FACE FEEL uncomfortably HOT, when we approach a FIRE?_
A. Because the fire radiates heat upon the face; which (not being _covered_) feels the effect immediately.
Q. _Why does the fire catch the FACE more than the REST of the body?_
A. The _rest_ of the body is _covered with clothing_, which (being a _bad conductor_ of heat) prevents the same sudden and rapid transmission of heat to the skin.
Q. _Do those substances which RADIATE heat, ABSORB heat also?_
A. Yes. Those substances which _radiate most_, also _absorb most heat_: and those which _radiate least_, also _absorb the least_ heat.
Q. _Does any thing ELSE radiate heat, BESIDES the SUN and FIRE?_
A. Yes; _all_ things radiate heat in _some_ measure, but _not equally well_.
Q. _What things RADIATE heat the NEXT BEST to the sun and fire?_
A. All _dull_ and _dark substances_ are _good radiators_ of heat; but all _light_ and _polished substances_ are _bad radiators_ of heat.
Q. _Why does a POLISHED METAL TEA-POT make BETTER TEA than a black earthen one?_
A. As polished metal is a very _bad radiator_ of heat, it _keeps the water hot much longer_; and the hotter the water is, the better it "draws" the tea.
Q. _Why will not a DULL BLACK TEA-POT make good tea?_
A. Because the heat of the water _flies off so quickly_ through the dull black surface of the tea-pot, that the _water is rapidly cooled_, and will not "draw" the tea.
Q. _Do not pensioners, and most aged cottagers, prefer the little BLACK EARTHEN TEA-POT to the bright METAL one?_
A. Yes; because they _set it on the hob "to draw;"_ in which case, the little _black tea-pot_ will make the _best tea_.
Q. _Why will a BLACK TEA-POT make better tea than a bright metal one, if it be set upon the HOB to DRAW?_
A. Because the black tea-pot will _absorb heat plentifully_ from the fire, and keep the water _boiling hot_: whereas, a bright _metal_ tea-pot (set upon the hob) would _throw off_ the heat by _reflection_.
Q. _Then sometimes a BLACK EARTHEN tea-pot is the best, and sometimes a bright METAL one?_
A. Yes; when the tea-pot is _set on the hob "to draw,"_ the black _earth_ is the _best_, because it _absorbs heat_: but when the tea-pot is _not_ set on the hob, the bright _metal_ is the _best_, because it _radiates heat very slowly_, and therefore _keeps the water hot_.
Q. _Why does a SAUCEPAN which has been USED, boil QUICKER than a NEW ONE?_
A. Because the bottom and back are _covered with soot_; and the _black soot_ rapidly _absorbs the heat_ of the glowing coals.
Q. _Why should the FRONT and LID of a SAUCEPAN be clean and BRIGHT?_
A. As they do not come in contact with the fire, they cannot _absorb heat_; and (being bright) they will not suffer _the heat to escape_ by radiation.
Q. _In what state should a SAUCEPAN be, in order that it may BOIL QUICKLY?_
A. All those parts which _come in contact with the fire_ should be covered with _soot_, to absorb heat; but all the _rest_ of the saucepan should be as _bright as possible_, to prevent the _escape of heat_ by radiation.
Q. _Why is it said that "SATURDAY'S KETTLE BOILS the FASTEST?"_
A. Because on Saturday the _front_ and _top_ of the kettle are generally _cleaned_ and _polished;_ but the _bottom_ and _back_ of the kettle are _never_ cleaned.
Q. _Why should NOT the BOTTOM and BACK of a kettle be CLEANED and polished?_
A. Because they _come in contact with the fire_, and (while they are covered with black soot) _absorb heat freely_ from the burning coals.
Q. _Why should the FRONT and TOP of a kettle be CLEAN and well polished?_
A. Because polished metal _will not radiate heat_; and, therefore, (while the front and top of the kettle are well polished) _the heat is kept in_, and not suffered to escape by radiation.
Q. _Why is the INSIDE of a KETTLE and SAUCEPAN WHITE?_
A. _White will not radiate heat_: if, therefore, the inside of a boiler be _white_, the liquor in it is _kept hot much longer_.
Q. _Why is the BOTTOM of a KETTLE nearly COLD, when the WATER is BOILING HOT?_
A. Black soot is a very _bad conductor of heat_; and, therefore, the heat of the boiling water is some considerable time, before it gets _through the soot_ which adheres to the bottom of the kettle.
Q. _Why is the LID of a KETTLE so intensely HOT, when the water boils?_
A. The bright metal lid of the kettle _is an admirable conductor_ of heat; and, therefore, _the heat from the boiling water pours into our hand_ the moment we touch it.
Q. _Show the benefit of SMOKE in COOKING._
A. The carbon of the fuel (which flies off in smoke) naturally _blackens_ all culinary vessels set upon the fire to boil, and thus renders them fit for use.
("Culinary vessels" are vessels used in kitchens for cooking, as saucepans, boilers, kettles, &c.)
Q. _How does SMOKE make culinary vessels FIT for USE?_
A. If it were not for the _smoke_, (which gathers round a kettle or saucepan) _heat would not be absorbed_, and the process of boiling would be greatly retarded.
Q. _Why is boiling water KEPT HOT best in a BRIGHT METAL pot?_
A. Because bright metal being a _bad radiator_ will not _throw off the heat_ of the boiling water _from its surface_.
Q. _Why is WATER KEPT COLD in summer-time in a BRIGHT METAL pot, better than in an EARTHEN vessel?_
A. Because bright metal _will not absorb heat_ from the hot air, like an _earthen vessel_; in consequence of which, the water is kept cooler.
Q. _Why are DINNER-COVERS made of BRIGHT TIN or SILVER?_
A. Light-coloured and highly-polished metal _is a very bad radiator of heat_; and, therefore, bright tin or silver will not allow the heat of the cooked food _to escape through the cover by radiation_.
Q. _Why should a MEAT-COVER be very brightly POLISHED?_
A. If the cover be _dull or scratched_ it will _absorb heat from the hot food beneath it_; and (instead of _keeping it hot_) will _make it cold_.
Q. _Why should a SILVER MEAT-COVER be PLAIN, and not CHASED?_
A. If the cover be _chased_, it will _absorb the heat of the food_ covered by it; and instead of _keeping it hot_, will _make it cold by absorption_.
Q. _What is DEW?_
A. Dew is the _vapour of the air condensed_, by coming in contact with bodies _colder than itself_.
Q. _Why is the GROUND sometimes COVERED with DEW?_
A. The _earth is more heated_ by solar rays _than the air_, during the _day_; but at _night_, the earth _parts with more heat_ than the _air_, and becomes (in consequence) 5 or 10 degrees _colder_.
Q. _How does the EARTH being COLDER than the AIR account for the deposition of DEW?_
A. As soon as the air _touches the cold earth_, its warm vapour is _chilled_, and _condensed into dew_.
Q. _Why is the surface of the GROUND COLDER in a FINE clear NIGHT, than in a CLOUDY one?_
A. On a fine clear star-light night, _heat radiates from the earth freely_, and is lost in open space: but on a _cloudy_ night, the clouds _arrest the process of radiation_.
Q. _Why is DEW deposited only on a FINE clear NIGHT?_
A. Because, when the night is _clear_ and _fine_, the _surface of the ground radiates heat most freely_; and (being cooled down by this loss of heat) _chills the vapour of the air into dew_.
Q. _Why is there NO DEW on a dull CLOUDY NIGHT?_
A. The clouds _arrest the radiation of heat from the earth_; and (as the heat cannot freely escape) the surface is not sufficiently cooled down _to chill the vapour of the air into dew_.
Q. _Why is a CLOUDY NIGHT WARMER than a FINE one?_
A. Because the clouds _prevent the radiation of heat from the earth_; and, therefore, the surface of the earth remains _warmer_ on a dull cloudy night.
Q. _Why is DEW most ABUNDANT in situations most EXPOSED?_
A. Because the radiation of heat _is not arrested_ by houses, trees, hedges, or any other thing.
Q. _Why is there scarcely any DEW under a shady TREE?_
A. The shady head of the tree both _arrests the radiation of heat from the earth_, and also radiates some of its own heat _towards the earth_; and, therefore, the ground (underneath a tree) _is not sufficiently cooled_ down to chill the vapour of the air into dew.
Q. _Why is there never much DEW at the foot of WALLS and HEDGES?_
A. 1st--Because the wall or hedge acts as a screen, _to arrest the radiation of heat from the earth_: and
2ndly--The wall or hedge also _radiates some portion of heat_ towards the earth.
Q. _How do these things prevent the deposition of dew?_
A. As the ground (beneath a wall, tree, or hedge) is _not cooled by the radiation of heat_, it remains of the _same temperature as the air_ above it; in consequence of which, the vapours of the air are _not chilled by it into dew_.
Q. _Why is there little or NO DEW beneath a FLOWER-AWNING, although that awning be open on all four sides?_
A. 1st--Because the awning _arrests the radiation of heat from the ground beneath_: and
2ndly--It _radiates some of its own heat downwards_; in consequence of which, the ground beneath an awning is _not sufficiently cooled down_ to chill the vapour of air into dew.
Q. _How can a thin covering of BASS or even MUSLIN protect trees from FROST?_
A. Because _any covering_ prevents the _radiation of heat from the tree_; and if the tree be _not cooled down by radiation_, the vapour of the air will _not be frozen_ as it comes in contact with it.
Q. _Why is the BASS or CANVASS itself (which covers the tree) always DRENCHED with DEW?_
A. The bass or canvass covering _radiates heat_ both _upwards and downwards_; and is, therefore, _so cooled down_, that it readily _chills all the vapour of the air_ (which passes over it) _into dew_.
Q. _Why does SNOW at the foot of a HEDGE or WALL melt sooner, than in an open field?_
A. Because the hedge or wall _radiates heat into the snow beneath_, which melts it.
Q. _Why is there NO DEW after a WINDY NIGHT?_
A. 1st--Because the wind _evaporates the moisture_, as fast as it is deposited; and
2ndly--It _disturbs the radiation of heat_, and diminishes the deposition of dew thereby.
Q. _Why are VALLEYS & HOLLOWS often thickly covered with DEW, although they are sheltered?_
A. The surrounding hills prevent the _repose of air_ (in the valleys) _from being disturbed_; but do not _overhang_ and _screen_ them, so as to _arrest their radiation_.
Q. _Why does DEW FALL more ABUNDANTLY on SOME THINGS than upon OTHERS?_
A. Because some things _radiate heat more freely_ than others, and therefore become _much cooler_ in the night.
Q. _Why are things which RADIATE HEAT MOST FREELY, always the most THICKLY COVERED with DEW?_
A. Because the vapour of the air is _chilled into dew_, the moment it comes in contact with them.
Q. _What kind of things RADIATE HEAT most FREELY?_
A. Grass, wood, and the leaves of plants, radiate heat _very freely_: but polished metal, smooth stones, and woollen cloth, part with their heat _very tardily_.
Q. _Do the leaves of ALL plants radiate heat EQUALLY WELL?_
A. No. Rough _woolly leaves_ (like those of a holly-hock) radiate heat much _more freely_, than the _hard smooth polished leaves_ of a common laurel.
Q. _Shew the WISDOM of GOD in making grass, the leaves of trees, and ALL VEGETABLES, EXCELLENT RADIATORS of heat._
A. As vegetables _require much moisture_, and would often perish without a plentiful deposit of dew, God wisely made them to _radiate heat freely_, so as to _chill the vapour_ (which touches them) _into dew_.
Q. _Will polished METAL, smooth STONES, and woollen CLOTH, readily collect DEW?_
A. No. While grass and the leaves of plants _are completely drenched with dew_, a piece of _polished metal_, or of _woollen cloth_ (lying on the same spot) will be _almost dry_.
Q. _Why would POLISHED METAL and WOOLLEN CLOTH be DRY, while grass and leaves are drenched with DEW?_
A. Because the polished metal and woollen cloth _part with their heat so slowly_, that the vapour of the air is _not chilled into dew_ as it passes over them.
Q. _Why is a GRAVEL WALK almost DRY, when a grass plat is covered thick with DEW?_
A. _Grass_, (_being a good radiator_) throws off its heat very _freely_; but _gravel (being a very bad radiator)_ parts with its heat very _reluctantly_.
Q. _Is that the reason why GRASS is SATURATED with DEW, and the GRAVEL is NOT?_
A. Yes. When the vapour of warm air comes in contact with the _cold grass_, it is instantly chilled into dew; but (as the gravel is _not so cold as the grass_) the vapour of air is _not so freely condensed_ as it passes over the gravel.
Q. _Why does DEW rarely fall upon hard ROCKS and BARREN lands?_
A. Rocks and barren lands are so _compact_ and _hard_, that they can neither _absorb nor radiate much heat_; and (as their _temperature varies but very little_) very little _dew_ distils upon them.
Q. _Why does DEW fall more abundantly on CULTIVATED soils, than on BARREN lands?_
A. Because cultivated soils (being _loose and porous_) _absorb_ heat freely during the day, and _radiate it_ by night; and (being _much cooled by the rapid radiation of heat_) as the vapour of the air passes over them, it is plentifully _condensed into dew_.
Q. _Shew the WISDOM of GOD in this arrangement._
A. Every plant and inch of land which _needs the moisture of dew_, is adapted to _collect it_; but _not a single drop even of dew is wasted_, where its refreshing moisture is _not required_.
Q. _Shew the WISDOM of GOD in making polished METAL and woollen CLOTH BAD RADIATORS of heat._
A. If polished metal collected dew as easily as grass, it could _never be kept dry_, and _free from rust_. Again, if woollen garments collected dew as readily as the leaves of trees, we should be _often soaking wet_, and subject to _constant colds_.
Q. _Shew how this affords a beautiful illustration of GIDEON'S MIRACLE, recorded in the book of Judges, VI. 37, 38._
A. The _fleece of wool_ (which is a _very bad radiator_ of heat) was _soaking wet_ with dew: when the _grass_ (which is a most _excellent radiator_) was _quite dry_.
Q. _Was not this CONTRARY to the laws of NATURE?_
A. Yes; and was, therefore, a plain _demonstration of the power of God_, who could change the very _nature of things_ at his will.
Q. _Why do our CLOTHES FEEL DAMP, after walking in a fine evening in SPRING or AUTUMN?_
A. Because the vapour (_condensed by the cold earth_) lights upon them, like dew.
Q. _Why are WINDOWS often covered with thick MIST, and the frames wet with standing WATER?_
A. The temperature of the _external air_ always _falls at sun-set_, and _chills the window-glass_, with which it comes in contact.
Q. _How does this account for the MIST and WATER on a WINDOW?_
A. As the warm vapour of the room _touches the cold glass_, it is _chilled_ and _condensed into mist_; and the mist (collecting into drops) _rolls down the window-frame_ in little streams of water.
Q. _Does the GLASS of a window COOL down more RAPIDLY than the AIR of the room itself?_
A. Yes; because the air is _kept warm by fires_, and the _animal heat_ of the people in the room; in consequence of which, the _air of a room suffers very little diminution of heat_ from the setting of the sun.
Q. _Whence arises the VAPOUR of a ROOM?_
A. 1st--The very _air_ of the room _contains vapour_:
2ndly--The _breath_ and _insensible perspiration_ of the inmates _increase_ this vapour: and
3rdly--_Hot dinners_, the _steam of tea_, &c. contribute to _increase it still more_.
Q. _What is meant by "the INSENSIBLE PERSPIRATION?"_
A. From every part of the human body an _insensible and invisible perspiration issues_ all night and day; not only in the hot weather of _summer_, but also in the coldest day of _winter_.
Q. _If the perspiration be both INSENSIBLE and INVISIBLE, how is it KNOWN that there IS any such perspiration?_
A. If you put your naked arm _into a clean dry glass cylinder_, the _perspiration_ of your arm will soon _condense_ on the glass, like mist.
Q. _Why are CARRIAGE WINDOWS very SOON covered with thick MIST?_
A. The warm vapour of the carriage _is condensed the moment it touches the cold glass_, and covers it over with a thick mist.
Q. _Why is the glass window COLD enough to condense the vapour of the carriage?_
A. Because the _inside_ of the carriage is much _warmer_ than the _outside_, and the glass window is made cold by contact with the _external air_.
Q. _Where does the WARM vapour of the carriage come from?_
A. The warm _breath_ and _insensible perspiration_ of the persons riding in the carriage, load the air of it with warm vapour.
Q. _What is the cause of the pretty FROST-WORK seen on bed-room WINDOWS in winter-time?_
A. The _breath_ and _insensible perspiration_ of the sleeper (coming in contact with the ice-cold window) is _frozen_ by the cold glass, and forms those beautiful appearances seen in our bed-rooms in a winter morning.
Q. _Why is the GLASS of a window colder than the WALLS of a room?_
A. Glass is a very _excellent radiator_; and, therefore, most _rapidly parts with its heat_.
Q. _Why is a TUMBLER of cold WATER made quite DULL with mist, when brought into a room FULL of PEOPLE?_
A. Because the _hot vapour of the room_ (coming in contact with the cold tumbler) _is condensed upon it_; and changes its invisible and gaseous form for that of a _thick mist_.
Q. _Why is a GLASS made quite DULL, by laying a HOT HAND upon it?_
A. The _insensible perspiration_ of the hot hand is _condensed_ upon the cold glass, and thus made perceptible.
Q. _Why are WINE-GLASSES made quite DULL when they are brought into a room FULL of COMPANY?_
A. The _hot vapour of the room_ (coming in contact with the cold wine-glasses) _is condensed_ upon them, and covers them with vapour like dew.
Q. _Why does this misty appearance GO OFF after a little time?_
A. Because the glass becomes of the _same temperature_ as the _air of the room_, and will no longer _chill the vapour_ which touches it, and _condense it into mist_.
Q. _Why is a WINE-GLASS (brought out of a CELLAR into the AIR) covered with a thick MIST in summer-time?_
A. The vapour of the hot air is _condensed_ by the cold glass, and covers it as a thick mist.
Q. _Why does BREATHING on a GLASS make it quite DULL?_
A. Because the hot breath is _condensed_ by the cold glass; and, therefore, covers it with a thick mist.
Q. _Why do WALLS stand thick with WET in a sudden THAW?_
A. The walls (being thick) cannot _change their temperature so fast_ as the thin air can; and, therefore, they _retain their cold_ after the thaw has set in.
Q. _How does RETAINING their COLD account for their being so WET?_
A. As the vapour of the warm air _touches the cold wall_, it is _chilled_ and _condensed into water_, which _sticks to the wall_, and sometimes trickles down in little streams.
Q. _Why does a thick WELL-BUILT HOUSE contract more DAMP of this kind, than an ORDINARY one?_
A. Because the walls are much _thicker_; and (if the frost has penetrated _far into the bricks_) it takes a long time to reduce them to the _same temperature as the air_.
Q. _Why are BANISTERS, &c. DAMP after a THAW?_
A. The wooden banister (being made of some very close-grained, varnished wood) cannot _change its temperature so fast_ as the air; and, therefore, _remains cold_ some time after the thaw has set in.
Q. _How does THIS account for the BANISTERS being DAMP?_
A. The vapour of the warm air (_coming in contact with the cold banister_) is _chilled_, and condensed into _water upon it_.
Q. _Why is our BREATH VISIBLE in WINTER and NOT in SUMMER?_
A. In _winter_ the coldness of the air condenses our breath into _visible vapour_; but in _summer_ the air is _not cold enough_ to condense it into visible vapour.
Q. _Why are our HAIR and the BRIM of our HAT often covered with little drops of pearly DEW in winter-time?_
A. The breath (issuing from our mouth and nose) _is condensed into drops_, as it comes in contact with our cold hair or hat; and (being condensed) hangs there in little dew-drops.
Q. _Why does the STEAM of a RAILWAY BOILER often pour down, like fine rain, when the steam is "let off?"_
A. The steam from the steam-pipe (when the air is cold) _is condensed by contact with the chill air_, and falls like fine rain.
Q. _Why is there LESS DEW when the WIND is EASTERLY, than when the wind is WESTERLY?_
A. _Easterly_ winds cross the _continent of Europe_, and, (as they pass over _land_) are _dry_ and _arid_; but _westerly_ winds cross the _Atlantic Ocean_; and (as they pass over _water_) are _moist_ and _full of vapour_.
Q. _How does the DRYNESS of an eastern wind PREVENT DEW-FALLS?_
A. As the easterly winds are _dry_, they _imbibe_ the moisture of the air; and, therefore, there _is very little_ left to be condensed into _dew_.
Q. _How does the MOISTNESS of a western wind PROMOTE dew-falls?_
A. As the westerly winds are _saturated with vapour_, they require a _very little reduction of heat_ to cause a _copious deposition of dew_.
Q. _When is DEW most COPIOUSLY distilled?_
A. After a hot day in summer or autumn, with the _wind in the west_.
Q. _Why is DEW distilled most COPIOUSLY after a HOT day?_
A. Because the surface of the earth _radiates_ heat very freely at sunset; and (becoming thus _much colder than the air_) _chills its vapour_, and condenses it into dew.
Q. _Does not AIR radiate heat, as well as the EARTH and its various plants?_
A. No. The air _never radiates heat_, nor is the air itself _made hot_ by the _rays of the sun_.
Q. _How is the AIR made HOT or COLD?_
A. By _convection of hot or cold currents_.
Q. _What is meant by "CONVECTION of hot and cold currents?"_
A. The air (which is heated by the surface of the earth) _ascends, warming the air_ through which it passes. _Other_ air (being warmed in a similar way) _also ascends, carrying heat_; till _all the air_ is made hot.
Q. _Is the AIR made COLD in a similar way?_
A. Yes. The air resting on the earth is _made cold by contact_: this cold air makes the _air above it cold_; and cold currents or winds _shake the whole together_, till all becomes of one temperature.
Q. _Why is MEAT very subject to TAINT on a MOON-LIGHT night?_
A. In a bright moon-light night, _meat radiates heat very freely_; and is, therefore, soon _covered with dew_, which produces _rapid decomposition_.
Q. _Why do PLANTS GROW RAPIDLY in MOON-LIGHT nights?_
A. In bright moon-light nights _rapid radiation is carried on_, and _dew is plentifully deposited_ on young plants, which conduces much to their growth and vigour.
Q. _Why is evening DEW INJURIOUS to HEALTH?_
A. Because the condensed vapours are always laden with _noxious exhalations from the earth_: this is especially the case in _marshy_ countries.
Q. _Is HONEY-DEW a similar thing to DEW?_
A. No. Honey-dew is a sweet liquid _shed by a very small insect_ (called the aphis), and deposited in autumn _on the under surface_ of favourite leaves.
Q. _Does HONEY-DEW INJURE leaves, or do them good?_
A. It injures them very much, because it _fills the pores_ of the leaf with a _thick clammy liquid_; and, therefore, prevents the leaf from _transpiring and absorbing_.
Q. _What EFFECT has honey-dew upon the APPEARANCE of a leaf?_
A. After a little time, the leaf (being _smothered_ and _starved_) begins to turn a _dingy yellow_.
Q. _Are not ANTS very FOND of HONEY-DEW?_
A. Yes; and they crawl up the loftiest trees, in order to obtain it.
Q. _What is the cause of MIST (or earth-fog)?_
A. If the _night has been very calm_, a _rapid_ radiation of heat has taken place in the earth; in consequence of which, the _air_ (resting on the earth) _is made so cold_, that its vapour is _chilled_, and condensed into a thick mist.
Q. _Why does not the MIST become DEW?_
A. Because the chill of the air _is so rapid_, that vapour is condensed _faster than it can be deposited_; and (covering the earth in a mist) _prevents any further radiation of heat_ from the earth.
Q. _When the earth can no longer RADIATE heat upwards, does it continue to CONDENSE the vapour of the air?_
A. No; the air (in contact with the earth) becomes about _equal in temperature_ with the surface of the earth itself; for which reason, the mist is _not condensed into dew_, but remains _floating above the earth_ as a thick cloud.
Q. _Why does this MIST seem to RISE HIGHER and HIGHER, and yet remain quite as dense below as before?_
A. The air _resting on the earth_ is first chilled, and _chills the air_ resting on _it_; the air which touches _this new layer of mist_ being also _condensed_, layer is added to layer; and the mist seems to be _rising_, when (in fact) it is only _deepening_.
Q. _Why does MIST and DEW VANISH as the SUN rises?_
A. Because the condensed vapour is _again rarefied by the heat of the sun_, and separated into invisible particles.
Q. _Why is a DEW-DROP ROUND?_
A. Because every part of the drop _is equally balanced_; and, therefore, there is no cause why _one part_ of the drop _should be further from the centre_ than _another_.
Q. _Why is the DEW-DROP on a broad leaf sometimes FLATTENED?_
A. Whenever two or more drops of dew _roll together_, they make one large _spheroid_ (or flattened drop).
Q. _Why will DEW-DROPS ROLL ABOUT CABBAGE-PLANTS, POPPIES, &c. without wetting the surface?_
A. The leaves of cabbages and poppies are _covered with a very fine powder_; and the dew-drop rolls over this fine powder, as a drop of rain _over dust_, without wetting the surface.
Q. _Why does not the drop of RAIN WET the DUST over which it rolls?_
A. Because it is driven from grain to grain by _capillary repulsion_.
Q. _Why does not the DEW-DROP WET the POWDER of the CABBAGE-plant?_
A. Because it is driven from grain to grain by _capillary repulsion_.
Q. _Why will DEW-DROPS ROLL over ROSES, &c. without wetting their petals?_
A. The leaves of a rose _contain an essential oil_, which prevents them from absorbing the dew immediately.
Q. _Why can a SWAN or DUCK dive under water WITHOUT being WETTED?_
A. Because their feathers are covered _with an oily secretion_, which repels the water.
Q. _What is the cause of MIST?_
A. When currents of air _from land_ mix with currents of air _from water_, the currents _from the water are condensed into mist_ by the colder currents _blowing from the land_.
Q. _Why are the currents of air from the LAND COLDER than those blowing over WATER?_
A. Because the earth _radiates heat very freely_, and (being greatly cooled down) _cools the air also_ which comes in contact with it.
Q. _Why is not the AIR, which passes over WATER, so COOL as that which passes over LAND?_
A. Because _water does not cool down at sun-set_, so fast as the _land_ does; and, therefore, the air in contact with it is _warmer_.
Q. _Why does not WATER cool down so fast as LAND?_
A. 1st--Because the _surface_ of water is _perpetually changing_, and as fast as _one_ surface is made cold, _another_ is presented: and
2ndly--The moment water is made cold _it sinks_, and _warmer portions of water rise to occupy its place_: therefore, before the _surface of water is cooled_, the _whole volume_ must be made cold; which is not the case with land.
Q. _What is the cause of a "pea-soup" LONDON FOG?_
A. These fogs (which occur generally in the winter time) are occasioned thus:--Some current of air (being suddenly _cooled_) _descends into the warm streets_, preventing the rise of the smoke, and _forcing it back in a mass_ towards the earth.
Q. _Why are there not ALWAYS FOGS every night?_
A. Because the air will always hold in solution a certain quantity of vapour, (which varies according to its temperature): and when the air is _not saturated with vapour_, it may be condensed without parting with it.
Q. _Why are there EVER FOGS at night?_
A. If the air be _pretty well saturated with vapour_ during the day, as soon as its capacity for holding vapour _is lessened by the cold night_, it deposits some of the superabundant vapour in the form of dew or fog.
Q. _Why is there very OFTEN a fog over MARSHES and RIVERS at night-time?_
A. The air of marshes is almost _always near saturation_; and, therefore, the _least depression of temperature_, will compel it to relinquish some part of its moisture in dew or fog.
Q. _What is the DIFFERENCE between DEW and RAIN?_
A. In _dew_, the condensation is made _near the earth's surface_:
In _rain_, the drops fall _from a considerable height_; but the cause of both is the same, viz.--COLD _condensing the vapour of the air_, when it is near the point of _saturation_.
Q. _Why does MIST and FOG VANISH at sunrise?_
A. Because the condensed particles are again _changed into invisible vapour_, by the heat of the sun.
Q. _What is the difference between a MIST and FOG?_
A. MIST is generally applied to _vapours condensed on marshes, rivers_, and _lakes_.
FOG is generally applied to _vapours condensed on land_, especially if those vapours are _laden with smoke_.
Q. _What is the reason why condensed vapour sometimes forms into CLOUDS, and sometimes into FOG?_
A. If the surface of the EARTH be _hotter than the air_, then the vapour of the earth (_being chilled by the cold air_) becomes FOG: but if the AIR be _hotter than the earth_, the vapour _rises through the air_, and becomes CLOUD.
Q. _If cold air produces FOG, why is it not foggy on a FROSTY MORNING?_
A. 1st--Because _less vapour is formed on a frosty day_; and
2ndly--The vapour _is frozen upon the ground_ before it can rise from the earth, and becomes HOAR-FROST.
Q. _Why are FOGS more general in AUTUMN than in spring?_
A. In spring _the earth is not so hot_ as it is in autumn. In AUTUMN the _earth_ is generally _warmer than the air_; and, therefore, the vapour (issuing from the earth) _is condensed into fog_ by the chill air.
Q. _Why are FOGS more common in VALLEYS than on HILLS?_
A. 1st--Because valleys _contain more moisture than hills_: and
2ndly--They are _not exposed to so much wind_, (which dissipates the vapour).
Q. _How does WIND dissipate FOGS?_
A. Either by _blowing them away_; or else by _dissolving them into vapour again_.
Q. _What is HOAR-FROST?_
A. There are two sorts of hoar-frost: 1.--FROZEN DEW: and 2.--FROZEN FOG.
Q. _What is the cause of the GROUND hoar-FROST, or frozen DEW?_
A. Very _rapid radiation of heat from the earth_; in consequence of which, the _surface is so cooled down_, that it _freezes the dew_ condensed upon it.
Q. _Why is HOAR-FROST seen only after a very CLEAR NIGHT?_
A. Unless the night has been very clear indeed, the earth will not have thrown off heat enough by radiation, to _freeze_ the vapour condensed upon its surface.
Q. _Why does HOAR-FROST very often COVER the GROUND and TREES, when the water of rivers is not frozen?_
A. Hoar-frost is not the effect of cold in the _air_, but the cold of the _earth_ (produced by excessive radiation); in consequence of which, _the dew_ (condensed upon it) _is frozen_.
Q. _Why is the HOAR-FROST upon GRASS and VEGETABLES much thicker than that upon lofty TREES?_
A. Because the air (resting on the _surface of the ground_) is much colder after sun-set, than the _air higher up_; in consequence of which, more vapour is condensed and frozen there.
Q. _Why is the AIR (resting on the surface of the EARTH) colder than that in the HIGHER regions?_
A. Because the _earth radiates more heat_ than the _leaves of lofty trees_; and, therefore, _condenses and freezes_ the vapour of the air _more rapidly_.
Q. _Why are EVERGREENS often FROST-BITTEN, when lofty trees are NOT?_
A. Evergreens do not _rise far above the surface of the earth_; and (as the air _contiguous to the earth_ is much _colder than that in the higher regions_) therefore, the _low evergreen is often frost-bitten_, when the lofty tree is uninjured.
Q. _Why are TOMB-STONES covered with HOAR-FROST, long after it has melted from every object around?_
A. _White is a very bad absorbent of solar heat_; and, therefore, the _white tomb-stone_ remains _too cold_ to thaw the frost congealed upon its surface.
Q. _Why is there little or NO HOAR-FROST under SHRUBS and shadowy TREES?_
A. 1st--Because the leafy shrubs and trees _arrest the process of radiation_ from the earth: and
2ndly--Shrubs and trees _radiate a little heat_ towards the earth; and, therefore, the _ground beneath_ is never _cold enough to congeal the little dew_ which rests upon it.
Q. _What is the cause of that HOAR-FROST which arises from FROZEN FOG?_
A. The thick fog (which invested the earth during the night) is condensed _by the cold frost_ of early morning, and _congealed upon every object_ with which it comes in contact.