  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 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 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 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 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 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 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 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, 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 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 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 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; 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 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. |
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