Q. Why does mortar crumble away in frost?
A. If the mortar was not dried in the warm weather, its moisture freezes, expands, and thrusts the particles of the mortar away from each other; but (as soon as the frost goes) the water condenses and leaves the mortar full of cracks and chinks.
Q. Why does stucco peel from a wall in frosty weather?
Q. Why cannot bricklayers and plasterers work in frosty weather?
A. Because the bricks and plaster would start from their position as soon as the frost came and expanded the mortar.
Q. Why do bricklayers cover their work with straw in spring and autumn?
A. Because straw is a non-conductor, and prevents the mortar of their new work from freezing during the cold nights of spring and autumn.
Q. Why are water-pipes often covered with stall-litter in winter time?
A. Because straw (being a non-conductor) prevents the water of the pipes from freezing, and the pipes from bursting.
Q. Why are delicate trees covered with straw in WINTER?
A. Because straw (being a non-conductor) prevents the sap of the tree from being frozen.
A. Yes; in very many ways. For example—a bottle of water wrapped in cotton, and frequently wetted with ether, will soon freeze.
Q. Why would water freeze if the bottle were kept constantly wetted with ether?
A. Because evaporation would carry off the heat of the water, and reduce it to freezing point.
Q. Why does ether freeze under the receiver of an air-pump, when the air is exhausted?
A. Because evaporation is very greatly increased by the diminution of atmospheric pressure; and the ether freezes by evaporation.
FREEZING MIXTURES.
1. If nitre be dissolved in water, the heat of the liquid will be reduced 16 degrees.
2. If 5 oz. of nitre, and 5 of sal-ammoniac (both finely powdered) be dissolved in 19 oz. of water, the heat of the liquid will be reduced 40 degrees.
3. If 3 lbs. of snow be added to 1 lb. of salt, the mixture will fall to 0° (or 32 degrees below freezing point).
The two following are the coldest mixtures yet known:—
1. Mix 3 lbs. of muriate of lime with 1 lb. of snow.
2. Mix 5 lbs. of diluted sulphuric acid with 4 lbs. of snow.
A. Because the specific gravity of salt water is greater than that of fresh, and therefore it buoys up the swimmer better.
Q. How do cooks ascertain if their brine be salt enough for pickling?
A. They put an egg into their brine. If the egg sinks the brine is not strong enough, if the egg floats it is.
Q. Why will the egg sink if the brine be not strong enough for pickling?
A. As an egg is heavier than water, it will sink if immersed therein; but if as much salt be added as the water can dissolve, the egg will float.
Q. Why will the egg float in strong brine?
A. Because the specific gravity of salt and water is greater than that of water only.
Q. Why do persons sink in water when they are unskilful swimmers?
A. 1st—Because (in their floundering about) they take in water at their nose and mouth, which makes them heavier:
2ndly—Fear contracts the body; and as the body is compressed by fear into a smaller compass, it becomes heavier: and
3rdly—The water and fear take away the breath; and when the breath is taken from the body, its bulk is reduced, and it becomes heavier.
Q. Why can quadrupeds swim more easily than man?
A. 1st—Because the trunk of a quadruped is lighter than water, and this is the greatest part of them:
2ndly—The position of a beast in water is a natural one.
Q. Why is it more difficult for a man to swim than for a beast?
A. Because the head and limbs of a man (like those of a beast) are heavier than water, and these compose more than half his body:
2ndly—The position of a man in water is unnatural to him.
Q. Why can fat men swim more easily than spare men?
Q. How are fishes able to ascend to the surface of water?
A. Fishes have an air-bladder near their abdomen: when this bladder is filled with air, the fish increases in size; and (being lighter) ascends through the water to its surface.
Q. How are fishes able to dive in a minute to the bottom of a stream?
A. They expel the air from their air-bladder; in consequence of which, their size is diminished, and they sink instantly.
Q. What is light?
A. Rapid undulations of a fluid called ether, striking on the optic nerve of the eye. (See p.46.)
The heat of fire or of the sun sets the atoms of matter in motion; and these atoms, striking against the fluid ether, cause it to undulate.
Q. How fast does light travel?
A. Light travels so fast, that it would go eight times round the earth, while a person counts “one.”
Q. Does all light travel equally fast?
A. Yes; the light of the sun, or the light of a candle, or the light from houses, trees, and fields.
Q. Where does the light of houses, trees, and fields come from?
A. The light of the sun (or of some lamp or candle) is reflected from their surfaces.
Q. Why are some surfaces brilliant like glass and steel, and others dull like lead?
A. Those surfaces which reflect the most light, are the most brilliant; and those which absorb light are dull.
Q. What is meant by reflecting light?
A. Throwing the rays of light back again, from the surface on which they light.
Q. What is meant by absorbing light?
Q. Why can a thousand persons see the same object at the same time?
A. Because it throws off from its surface an infinite number of rays in all directions; and one person sees one portion of these rays, and another person another.
Q. Why is the eye pained by a sudden light?
A. Because the pupil of the eye is burdened with rays, before it has had time to contract.
Q. Why does it give us pain, if a candle be brought suddenly towards our bed at night time?
A. In the dark the pupils of the eyes dilate very much, in order to admit more rays. When a candle is brought before them, the enlarged pupil is overladen with rays, and feels pained.
Q. Why can we bear the candle-light after a few moments?
A, Because the pupil contracts again almost instantly, and adjusts itself to the quantity of light which falls upon it.
A. Because the pupil (which contracted in the bright room) does not dilate instantaneously; and the contracted pupil is not able to collect rays enough (from the dark road or street) to enable us to see before us.
Q. Why do we see better, when we get used to the dark?
A. Because the pupil dilates again, and is able to gather together more rays; in consequence of which, we see more distinctly.
Q. If we look at the sun for a few moments, why do all other things appear dark?
A. Because the pupil of the eye (which was very much contracted by looking at the sun) is too small to collect sufficient rays from other objects, to enable us to distinguish their colours. (See “accidental colours.”)
Q. If we watch a bright fire for a few moments, why does the room seem dark?
Q. Why can we see the proper colour of every object again, after a few minutes?
A. Because the pupil dilates again, and accommodates itself to the light around.
Q. Why can tigers, cats, and owls see in the dark?
A. Because they have the power of enlarging the pupil of their eyes, so as to collect several scattered rays of light; in consequence of which, they can see distinctly when it is not light enough for us to see any thing at all.
Q. Why do cats and owls sleep almost all day?
A. As the pupil of their eyes is very broad, daylight fatigues them; so they close their eyes for relief.
Q. Why do cats keep winking, when they sit before a fire?
A. As the pupil of their eyes is very broad, the light of the fire pains them; and they keep shutting their eyes to relieve the sensation of too much light.
A. As these animals cannot see distinctly in strong daylight, they sleep during the day: and as they can see clearly in the dark, they prowl then for prey.
Q. Why do glow-worms glisten by night only?
A. Because the light of day is so much stronger, that it eclipses the feeble light of a glow-worm; in consequence of which, glow-worms are invisible by day.
Q. Why can we not see the stars in the day-time?
A. Because the light of day is so powerful, that it eclipses the feeble light of the stars: in consequence of which, they are invisible by day.
Q. Why can we see the stars even at mid-day, from the bottom of a deep well?
A. As the rays of the sun never come directly over a well, but the rays of the stars do; therefore the light from those stars (in such a situation) is more clear than the light of the sun.
A. The use of two eyes is to increase the light, or take in more rays of light from the object looked at, in order that it may appear more distinct.
Q. Why do we not see things double, with two eyes?
A. 1st—Because the axis of both eyes is turned to one object; and, therefore, the same impression is made on the ret´ina of each eye.
2ndly—The nerves (which receive the impression) have one point of union, before they reach the brain.
Q. Why do we see ourselves in a glass?
A. The rays of light from our face strike against the surface of the glass, and (instead of being absorbed) are reflected, or sent back again to our eye.
Q. Why are the rays of light reflected by a mirror?
A. Because they cannot pass through the impenetrable metal with which the back of the glass is covered; so they rebound back, just as a marble would do if it struck against a wall.
A. The line of the angle of incidence.
Q. When a marble rebounds back again, what is the path it then describes called?
A. The line of the angle of reflection.
Q. When the light of our face goes to the glass, what is the path through which it goes called?
A. The line of the angle of incidence.
Q. When the light of our face is reflected back again from the mirror, what is this returning path called?
A. The line of the angle of reflection.
Q. Why does our reflection in a mirror seem to approach us as we walk towards it, and to retire from us as we retire?
A. Because the line of the angle of incidence is always equal to the line and angle of reflection.
Here CA, EA and DB, FB are the lines of the angle of incidence; and GA, KA and HB, LB are the lines of the angle of reflection. When the arrow is at CD, its shadow will appear at GH, because the line CA=GA and the angle CAB=angle GAB, &c.; and the same may be said about the point D.
Q. Why can a man see his whole person reflected in a little mirror not 6 inches in length?
A. Because the line of the angle of incidence is always equal to the line and angle of reflection.
Take the last figure—CD is much larger than the mirror AB; but the head of the arrow C is reflected obliquely behind the mirror to G; and the barb D appears at H.—Why? Because the line CA=AG and the angle CAB=angle GAB, &c. The same may be said of the point D.
Q. Why does a shadow in water always appear topsy-turvy?
A. Because the line of the angle of incidence is always equal to the line and angle of reflection.
Here the arrow-head A strikes the water at F, and is
reflected to D; and the barb B strikes the water at E, and is reflected
to C.
If a spectator stands at G, he will see the reflected lines CE and DF,
produced as far as G.
It is very plain that the more elevated object A will strike the water,
and be projected from it more perpendicularly than the point B, and
therefore the shadow will seem inverted.
Q. When we see our shadow in water, why do we seem to stand on our head?
A. Because the line of the angle of incidence is always equal to the line and angle of reflection.
Suppose our head to be at A, and our feet at B; then the shadow of our head will be seen at D, and the shadow of our feet at C. (See last figure.)
Q. Why do windows seem to blaze at sun-rise and sun-set?
A. Because glass is a good reflector of light; and the rays of the sun (striking against the window glass) are reflected, or thrown back.
Q. Why do not windows reflect the noon-day rays also?
A. They do, but the reflection is not seen.
Q. Why is the reflection of the rising and setting sun seen in the window, and not that of the noon-day sun?
A. As the angle of incidence always equals the angle of reflection, therefore the rays of the noon-day sun enter the glass too perpendicularly for their reflection to be seen.
Here AB represents a ray of the noon-day sun striking the
window at B; its reflection will be at C:
But DB (a ray of the rising or setting sun) will be reflected to E (the
eye of the spectator).
Q. Why can we not see the reflection of the sun in a well, during the day-time?
A. Because the rays of the sun fall so obliquely, that they never reach the surface of the water at all, but strike against the brick sides.
Let BDEC be the well, and DE the water.
The ray AB strikes against the brick-work inside the well; and
The ray AC strikes against the brick-work outside the well.
None will ever touch the water DE.
Q. Why do we see the moon reflected in a well very often?
A. As the rays of the moon are not so oblique as those of the sun, they will often reach the water. (See next figure.)
Q. Why are the stars reflected in a well, although the sun is not?
Q. In a sheet of water at noon, the sun appears to shine upon only one spot, and all the rest of the water seems dark,—Why is this?
A. Because the rays (which fall at various degrees of obliquity on the water) are reflected at similar angles; but as only those which meet the eye of the spectator are visible, all the sea will appear dark but that one spot.
Here of the rays SA, SB, and SC, only the ray SC meets
the eye of the spectator D.
The spot C, therefore, will appear luminous to the spectator D, but no
other spot of the water ABC.
Q. At night the moon seems to be reflected from only one spot of a lake of water, while all the rest seems dark,—Why is this?
A. Because the rays (which fall at various degrees of obliquity on the lake) are reflected at similar angles; but as only those which enter the eye of the spectator will be visible, all the water will appear dark but that one spot. (See last figure.)
A. As the air absorbs and diminishes light, the higher we ascend, the less light will be absorbed.
Q. Why does the sun seem larger at his rise and set, than it does at noon?
A. Because the earth is surrounded by air, which acts like a magnifying glass; and when the sun is near the horizon (as its rays pass through more of this air), it is more magnified.
Here SC represents a ray of the sun at noon, and MC a ray
of the sun near the horizon. DEG represents the air or atmosphere around
the earth.
Because EC is longer than DC, therefore the rays of the sun at M pass
through more air than the rays of the sun at S, and the sun is more
magnified.
Q. Why does the rising and setting moon appear so much larger, than after it is risen higher above our heads?
A. Because the earth is surrounded by air, which acts like a magnifying glass; and when the moon is near the horizon (as its rays pass through more of this air) it is more magnified. (See last figure.)
Q. When candles are lighted, we cannot see into the street or road,—Why is this?
A. 1st—Because glass is a reflector, and throws the candle-light back into the room again; and
2ndly—The pupil of the eye (which has become contracted by the light of the room) is too small to collect rays enough from the dark street, to enable us to see into it.
Q. Why can’t persons in the street see into a well-lighted room?
Q. Why do we often see the fire reflected in our parlour window in winter time?
A. Because glass is a good reflector; and the rays of the fire (striking against the window-glass) are reflected back into the room again.
Q. Why do we often see the shadow of our candles in the window, while we are sitting in our parlour?
A. Because the rays of the candle (striking against the glass) are reflected back into the room: and the darker the night, the clearer the reflection.
Q. Why is this reflection more clear, if the external air be dark?
A. Because the reflection is not then eclipsed by the brighter rays of the sun striking on the other side of the window.
Q. Why is the shadow of an object (thrown on the wall) larger and larger, the closer any object be held to the candle?
Q. When we enter a long avenue of trees, why does the avenue seem to get narrower and narrower till it appears to meet?
Q. In a long straight street, why do the houses seem to approach nearer and nearer as they are more distant?
A. Because the more distant the houses are, the more acute will be the angle which any two make with our eye.
Thus in the last figure—
If A and B were two houses at the top of the street, the street would seem to be as wide as the line A B:
And if C and D were two houses at the bottom of the street, the street at the bottom would seem to be no wider than E F.
Q. In an avenue of trees, why do they seem to be smaller as their distance increases?
Q. In a long straight street, why do the houses seem to be smaller and smaller the further they are off?
A. Because the further any house is off, the more acute will be the angle made by its perpendicular height with our eye.
Thus in the last figure—
If A B be a house at the top of the street, its perpendicular height will be that of the line A B.
If C D be a house at the bottom of the street, its perpendicular height will appear to be that of E F.
Q. Why does a man on the top of a mountain or church spire seem to be no bigger than a crow?
A. Because the angle made by the perpendicular height of the man (at that distance) with our eye, is no bigger than the perpendicular height of a crow close by.
Let AB be a man on a distant mountain or spire, and CD a
crow close by:
The man will appear only as high as the line CD, which is the height of
the crow.
Q. Why does the moon appear to us so much bigger than the stars, though in fact it is a great deal smaller?
A. Because the moon is very much nearer to us than any of the stars.
Let AB represent a fixed star, and CD the moon.
AB, though much the larger body, will appear no bigger than EF; whereas
the moon (CD) will appear as big as the line CD to the spectator G.
The moon is 240,000 miles from the earth, not quite a quarter of a
million of miles. The nearest fixed stars are 20,000,000,000,000.
(i. e.. 20 billions.)
If a ball went 500 miles an hour, it would reach the moon in twenty
days: but it would not reach the nearest fixed star in 4,500,000 years.
Had it begun, therefore, when Adam was created, it would be no further
on its journey than a coach (which has to go from the bottom of Cornwall
to the top of Scotland) after it has past about three-quarters of a
mile.
Q. Why does the moon (which is a sphere) appear to be a flat surface?
A. It is so far off, that we cannot distinguish any difference between the length of the rays which issue from the edge, and those which issue from the centre.
The rays AD and CD appear to be no longer than the ray
BD; but if all the rays seem of the same length, the part B will not
seem to be nearer to us than A and C, and therefore ABC will look like a
flat or straight line.
The rays AD and CD are 240,000 miles long.
The ray BD is 238,910 miles long.
Q. Why do the sun and stars (which are spheres) appear to be flat surfaces?
A. Because they are such an immense way off, that we can discern no difference of length between the rays which issue from the edge, and those which issue from the centre of these bodies.
The rays AD and CD appear no longer than BD; and as B appears to be no nearer than A or C, therefore ABC must all seem equally distant; and ABC will seem a flat or straight line. (See last figure.)
Q. Why does distance make an object invisible?
A. Because the angle (made by the perpendicular height of the distant object with our eye) is so very acute, that one line of the angle merges in the other.
Here the tree AD would not be visible to the spectator C, even if he were to approach as far as B; because no visible perpendicular can be inserted between the two lines AC, DC, till after the point B is past; when the tree will appear like a very little speck.
Q. Why do telescopes enable us to see objects invisible to the naked eye?
A. Because they concentrate several rays within the tube of the telescope, and bend them upon the mirror or lens, which acts as a magnifying glass.
Q. When a ship (out at sea) is approaching the shore, why do we see the small masts before we see the bulky hull?
Q. What is meant by refraction?
A. The bending of a ray of light, as it passes from one medium to another.
Q. How is a ray of light bent, as it passes from one medium to another?
A. When a ray of light passes into a denser medium, it is bent towards the perpendicular. When it passes into a rarer medium, it is bent from the perpendicular.
Suppose DE to be a perpendicular line.
If AB (a ray of light,) enters the water, it will be bent towards the
perpendicular to C.
If (on the other hand) CB (a ray of light) emerges from the water, it
would be bent away from the perpendicular towards A.
Q. Why does a spoon (in a glass of water) always appear bent?
A. Because as the light of the spoon emerges from the water, it is refracted.
And the spoon looks like ABC. (See the last figure.)
Q. Why does a river always appear more shallow than it really is?
A. Because the light of the bottom of the river is refracted as it emerges out of the water: and (as a stick is not so long when it is bent, as it is when it is straight) so the river seems less deep than it really is.
Q. How much deeper is a river than it seems to be?
A. One-third. If, therefore, a river seems only 4 feet deep, it is really 6 feet deep.
N. B. Many boys get out of their depth in bathing, in consequence of this deception. Remember, a river is always one-third deeper than it appears to be:—thus, if a river seems to be 4 feet deep, it is in reality 6 feet deep, and so on.