Q. Why does the moon appear larger at her rising and setting, than when above our heads?
Q. Why do cats rub their ears when it is likely to rain?
A. Either because the air is full of vapour, and its humidity (piercing between the hair of the cat) produces an itching sensation; or more probably, because the air is overcharged with electricity.
Q. How can the electricity of air produce a sensation of itching?
A. If the air is overcharged with electricity, the hair of the cat is overcharged also; and this makes her feel as if she were covered with cobwebs.
Q. Why does the cat keep rubbing herself?
A. Her hair will not lie smooth, but has a perpetual tendency to become turgid and ruffled; so the cat keeps rubbing her coat and ears, to smooth the hair down, and brush away the feeling of cobwebs.
A. Probably because the air is overcharged with electricity; and, therefore, a sensation (like that of cobwebs) irritates the skin, and produces an itching.
Q. Why do we hear distant CLOCKS more distinctly when rain is near at hand?
A. Because the air is filled with vapour, and water is a better conductor of sound than dry air.
Q. Why do we hear church-bells further, just previous to rain?
A. Because the air is filled with vapour, and vapour is a better conductor of sound than dry air.
Q. Why do doors swell, when rain is at hand?
A. Because the air is filled with vapour, which (penetrating into the pores of the wood) forces the parts further apart, and swells the door.
Q. Why do doors shrink in dry weather?
Q. Why is the air filled with offensive smells previous to a coming rain?
A. Because the volatile parts, (which rise from dunghills, sewers, &c.), being laden with vapour, are unable to rise so readily, as when they are rarefied by a bright sun.
Q. Why do flowers smell sweeter and stronger just previous to rain?
A. Because the volatile parts (which constitute the perfume of flowers) are laden with vapour; and (being unable to rise) are confined to the lower regions of the air.
Q. Why do horses and other animals stretch out their necks, and snuff up the air, just previous to a fall of rain?
A. Because they smell the odour of plants and hay, and delight to snuff in their fragrance.
Q. Why does smoke fall when rain is at hand?
A. The air being less dense in wet weather, cannot buoy up smoke so readily, as when more dry and heavy.
A. Because the insects (of which they are in pursuit) have fled from the cold upper regions of the air, to the warm air near the earth: and as their food is low, the swallows fly low.
Q. Why do these insects seek the lower regions of the air in wet weather, more than in fine weather?
A. Because they are forced downward, by some current of cold air which drives them down.
Q. Why does a downward current of cold air bring rain?
A. Because it condenses the warm vapour; which then descends in rain.
Q. The proverb says, “a single magpie in spring, foul weather will bring:” why is this the case?
A. In cold stormy weather, one magpie alone will leave its warm snug nest in search of food, while the other stays with the eggs or young ones; but in fine mild weather (when their brood will not be injured by cold) both the magpies will fly out together.
A. Because when magpies fly abroad singly, the weather is cold and stormy; but when both birds fly out together, the weather is warm and mild, which is favourable for fishing.
Q. Why do sea gulls fly about the sea in fine weather?
A. Because they live upon fish, which are found near the surface of the sea in fine weather.
Q. Why may we expect stormy rains, when sea gulls assemble on the land?
A. Because the fish (on which they live) leave the surface of the sea in stormy weather, and go down too deep for the gulls to get at them; they are obliged, therefore, to feed on the worms and larvæ which are driven out of the ground at such times.
Q. Why does the petrel always fly to the sea during a storm?
A. Because the petrel lives upon sea insects, which are always to be found in abundance about the spray of swelling waves.
(The Petrel is a bird of the duck-kind, which lives in the open sea. They run on the top of the sea, and are called Petrels, or rather Peter-els, from “St. Peter,” in allusion to his walking on the sea, to go to Jesus.)
Q. Why do candles and lamps spirt when rain is at hand?
A. Because the air is filled with vapour, and the humidity penetrates the wick; where (being formed into steam) it expands suddenly, and produces a little explosion.
Q. Why does a drop of water sometimes roll along a piece of hot iron without leaving the least trace?
A. If the iron be very hot indeed, the bottom of the drop is turned into vapour, before the drop can evaporate; and the vapour thus formed buoys the drop up, without allowing it to touch the iron at all.
Q. Why does it roll?
A. The current of air (which is always passing over the heated surface) drives it along.
Q. Why does a laundress put a little saliva on an ironing-box to know if it be hot enough?
Q. Why is the box hotter if the saliva runs along the box, than if it adheres to it till it is evaporated?
A. If the saliva runs along the box, the iron is hot enough to convert the bottom of the drop of spittle into vapour; but if the saliva will not roll, the box is not hot enough to convert the bottom of the drop of spittle into vapour.
Q. What is meant by evaporation?
A. The dissipation of liquid by its being converted into vapour.
Q. What effects are produced by evaporation?
Q. If you wet your finger in your mouth, and hold it up in the air, why does it feel cold?
A. The saliva quickly evaporates; and (as it evaporates) absorbs heat from the finger, which makes it feel cold.
Q. If you bathe your temples with ether, why does it allay inflammation and feverish heat?
A. Ether very rapidly evaporates; and (as it evaporates) absorbs heat from the burning head, producing a sensation of cold.
Q. Why is ether better for this purpose than water?
A. Because it requires less heat to convert it into vapour; and therefore it evaporates much more quickly.
(Ether is converted into steam with 104 degs. of heat, but water requires 212 degs. of heat to convert it into steam.)
Q. Why does ether very greatly relieve a scald or burn?
A. Because it evaporates very rapidly; and (while it is converted into vapour) carries off the heat of the burn.
Q. Why do we feel so cold when we have wet feet or clothes?
Q. Why do wet feet or clothes give us “cold?”
A. Because the evaporation absorbs heat from the body so abundantly, that it is lowered below its natural standard; and therefore health is injured.
Q. Why is it dangerous to sleep in a damp bed?
A. Because the heat of the body is continually absorbed in converting the damp of the sheets into vapour; and as heat is abstracted from the body, its temperature is reduced below the healthy standard.
Q. Why do we not feel the same sensation of cold, if we throw a macintosh over our wet clothes?
A. The macintosh prevents evaporation, because the steam cannot escape through the air-tight fabric; and (as the wet cannot evaporate from the clothes) no heat is absorbed from our bodies.
A. The salt of the sea retards evaporation; and (as the heat of the body is drawn off very gradually) the sensation of cold is prevented.
Q. Why does sprinkling a hot room with water cool it?
A. The heat of the room causes a rapid evaporation of the sprinkled water; and as the water evaporates, it absorbs heat from the room, and cools it.
Q. Why does watering the streets and roads cool them?
A. The hot streets and roads part with their heat to promote the evaporation of the water sprinkled on them.
Q. Why does a shower of rain seem to cool the air in summer-time?
A. The earth (being wet with the rain) parts with its heat to promote evaporation; and as the earth is cooled, it cools the air also.
Q. Why is linen dried by being exposed to the wind?
Q. Why is linen dried sooner in the open air, than in a confined room?
A. Because the particles of vapour are more rapidly removed from the surface of the linen by evaporation.
Q. Why are wet summers generally succeeded by cold winters?
A. Because the great evaporation (carried on through the wet summer) reduces the temperature of the earth lower than usual, and produces cold.
Q. Why is england warmer than it used to be, when agues were so common?
A. Because it is better drained and better cultivated.
Q. Why does draining land promote warmth?
A. Because it diminishes evaporation; in consequence of which less heat is abstracted from the earth.
Q. Why does cultivation increase the warmth of a country?
Q. Why do hedges and belts of trees promote warmth?
A. Because they retard evaporation, by keeping off the wind.
Q. If belts of trees promote warmth, why do forests produce cold?
A. 1st—Because they detain and condense the passing clouds:
2ndly—They prevent the access of both wind and sun:
3rdly—The soil of forests is always covered with long damp grass, rotting leaves, and thick brushwood: and
4thly—There are always many hollows in every forest full of stagnant water.
Q. Why do long grass and rotting leaves promote cold?
A. Because they are always damp; and the evaporation which they promote, is constantly absorbing heat from the earth beneath.
Q. Why do digging and ploughing help to make a country warm?
Q. Why are France and Germany warmer now, than when the vine would not ripen there?
A. Chiefly because their vast forests have been cut down; and the soil is better drained and cultivated.
Q. What becomes of the water of ponds and tubs in summer-time?
A. Ponds and tubs in summer-time are often left dry, because their water is evaporated by the air.
Q. How is this evaporation produced and carried on?
A. The air contains heat, and changes the surface of the water into vapour; this vapour (blending with the air) is soon wafted away; while fresh portions of air blow over the water, and produce a similar evaporation; till the pond or tub is left quite dry.
Q. Why are the wheels of some machines kept constantly wet with water?
Q. Why is mould hardened by the sun?
A. Because (when the moisture of the mould has been evaporated by the sun) the earthy particles come into closer contact, and the mass becomes more solid.
Q. Show the wisdom of god in this arrangement.
A. If the soil did not become crusty and hard in dry weather, the heat and drought would penetrate the soil, and kill both seeds and roots.
Q. Why is tea cooled faster in a saucer than in a cup?
A. Because evaporation is increased by increasing the surface; and as tea in a saucer presents a much larger surface to the air, its heat is more rapidly carried off by evaporation.
(The subject of “convection” will be treated of in a future chapter, and would scarcely be understood in this place.)
Q. Why is not the vapour of the sea salt?
A. Because the salt is always left behind, by the process of evaporation.
A. The white crust is the salt of the water left on the clothes by evaporation.
Q. Why does this white crust always disappear in wet weather?
A. In wet weather the moisture of the air dissolves the salt; and, therefore, it no longer remains visible.
Q. Why should not persons, who take violent exercise, wear very thick clothing?
A. When the heat of the body is increased by exercise, perspiration reduces the heat (by evaporation) to a healthy standard: as thick clothing prevents this evaporation, and confines the heat and perspiration to the body, it is injurious to health.
A. 1. By Conduction. 2. By Absorption. 3. By Reflection. 4. By Radiation: and 5. By Convection.
Q. What is meant by conduction of heat?
A. Heat communicated from one body to another, by actual contact.
Q. Why does a piece of wood (blazing at one end) not feel hot at the other end?
A. Wood is a bad conductor of heat; and, therefore, heat does not traverse freely through it: hence, though one end of a stick be blazing-hot, the other end may be quite cold.
Q. Why do some things feel so much colder than others?
A. Principally because they are better conductors; and, therefore, draw off the heat from our body (which touches them) so much faster.
Q. What are the best conductors of heat?
A. Dense solid bodies, such as metal and stone.
Q. Which metals are the most rapid conductors of heat?
Q. What are the worst conductors of heat?
A. All light and porous bodies, such as hair, fur, wool, charcoal, and so on.
Q. Why are cooking vessels so often furnished with wooden handles?
A. Wood is not a good conductor, like metal; and, therefore, many vessels (which are exposed to the heat of the fire) have wooden handles, lest they should burn our hands when we take hold of them.
Q. Why is the handle of a metal tea-pot made of wood?
A. As wood is a bad conductor, the heat of the boiling water is not so quickly conveyed to the wooden handle, nor so quickly poured into the hand by it, as when the handle is made of metal.
Q. Why would a metal handle burn the hand of the tea-maker?
A. As metal is an excellent conductor, the heat of the boiling water rushes quickly into the metal handle, and into the hand that touches it.
A. By touching the metal collar into which the wooden handle is fixed: though the wooden handle is quite cold, this metal collar is intensely hot.
Q. Why do persons use paper or woollen kettle-holders to take hold of a kettle with?
A. Paper and woollen are both very bad conductors of heat; and, therefore, the heat of the kettle does not readily pass through them to the hand.
Q. Does the heat of the boiling kettle never get through the woollen or paper kettle-holder?
A. Yes; but though the kettle-holder became as hot as the kettle itself, it would never feel so hot.
Q. Why would not the kettle-holder feel so hot as the kettle, when it really is of the same temperature?
A. Because (being a very bad conductor) it disposes of its heat so slowly, that it is scarcely perceptible; but metal (being an excellent conductor) disposes of its heat so quickly, that the sudden influx is painful.
A. Because it gives out a much greater quantity of heat in the same space of time; and the influx of heat is, therefore, more perceptible.
Q. Why does money in our pocket feel so hot, when we stand before a fire?
A. Metal is an excellent conductor; and, therefore, becomes rapidly heated. For the same reason it becomes rapidly cold, when it comes in contact with a body colder than itself.
Q. Why does a pump-handle feel intensely cold in winter?
A. As metal is an excellent conductor, when the hot hand touches the cold pump-handle, the heat passes rapidly from the hand into the iron; and this rapid loss of heat produces a sensation of intense coldness.
Q. Is the iron handle of the pump really colder than the wooden pump itself?
A. No; every inanimate substance (exposed to the same temperature) possesses the same degree of heat.
A. Merely because the iron is a better conductor; and, therefore, draws off the heat from our hand much more rapidly than wood does.
Q. Why does a stone or marble hearth feel to the feet so much colder than a carpet or hearth-rug?
A. Because stone and marble are good conductors, but woollen carpets and hearth-rugs are very bad conductors.
Q. Why does the stone hearth make our feet cold?
A. As soon as the hearth-stone has absorbed a portion of heat from our foot, it instantly disposes of it, and calls for a fresh supply; till the hearth-stone has become of the same temperature as the foot placed upon it.
Q. Do not the woollen carpet and hearth-rug, also, conduct heat from the human body?
A. Yes; (but being very bad conductors) they convey the heat away so slowly, that it is scarcely perceptible.
Q. Is the cold hearth-stone and warm carpet then of the same temperature?
Q. How long will the hearth-stone feel cold to the feet resting on it?
A. Till the feet and the hearth-stone are both of the same temperature; and then the sensation of cold in the hearth-stone will go off.
Q. Why would not the hearth-stone feel cold, when it is of the same temperature as our feet?
A. Because the heat would no longer rush out of our feet into the hearth-stone, in order to produce an equilibrium.
Q. Why does the hearth-stone (when the fire is lighted) feel so much hotter than the hearth-rug?
A. The hearth-stone is an excellent conductor; and, therefore, parts with its heat more readily than the woollen hearth-rug; which (being a very bad conductor) parts with its heat reluctantly.
Q. Why does parting with heat rapidly make the hearth-stone feel warm?
Q. Why does the non-conducting power of the hearth-rug prevent its feeling so hot as it really is?
A. Because it parts with its heat so slowly and gradually, that we scarcely perceive its transmission into our feet.
Q. When we plunge our hands into a basin of water, why does it produce a sensation of cold?
A. Though the water (in which we wash) is really warmer than the air of our bed-room; yet because it is a better conductor, it feels colder.
Q. Why does the conducting power of water make it feel colder than the air, though in reality it is warmer?
A. Because it abstracts heat from our hands so rapidly, that we feel its loss; but the air abstracts heat so very slowly, that its gradual loss is hardly perceptible.
Q. Is water a good conductor of heat?
Q. Why is water a better conductor of heat than air?
A. Because it is less subtile; and the conducting power of any substance depends upon its solidity, or the closeness of its particles.
Q. How do you know that water is not a good conductor of heat?
A. Because water may be made to boil at its surface, without imparting sufficient heat to melt ice a quarter of an inch below the boiling surface.
Q. Why are not liquids good conductors of heat?
A. Because the heat (which should be transmitted) produces evaporation, and flies off in the vapour.
Q. Why does a poker (resting on the fender) feel so much colder than the hearth-rug, which is further off the fire?
Q. Why are hot bricks (wrapped in cloth) employed in cold weather to keep the feet warm?
A. Bricks are bad conductors of heat, and cloth or flannel still worse: therefore a hot brick (wrapped in flannel) will retain its heat a very long time.
Q. Why is a tin pan (filled with hot water) employed as a foot warmer?
A. Because polished tin (being a bad radiator of heat) keeps hot a very long time; and warms the feet resting upon it.
Q. What is meant by being a “bad radiator of heat?”
A. To radiate heat is to throw off heat by rays, as the sun; a polished tin pan does not throw off the heat of boiling water from its surface, but keeps it in.
Q. Why is the tin foot-warmer covered with flannel?
Q. What harm would it be if the polish of the tin were injured by the perspiration of our feet?
A. Polished tin throws off its heat very slowly; but dull, scratched, painted, or dirty tin, throws off its heat very quickly: if, therefore, the tin foot-warmer were to lose its polish, it would get cold in a much shorter time.
Q. Why are furnaces and stoves (where much heat is required) built of porous brick?
A. As bricks are bad conductors, they prevent the escape of heat: and are, therefore, employed where great heat is required.
Q. Why are furnace doors, &c. frequently covered with a paste of clay and sand?
A. Because this paste is a very bad conductor of heat; and, therefore, prevents the escape of heat from the furnace.
Q. If a stove be placed in the middle of a room, should it be made of bricks or iron?
Q. Why does the Bible say, that God “giveth snow like wool?”
A. As snow is a very bad conductor of heat, it protects vegetables and seeds from the frost and cold.
Q. How does the non-conducting power of snow protect vegetables from the frost and cold?
A. As snow is a bad conductor, it prevents the heat of the earth from being drawn off by the cold air which rests upon it.